***Useful Information For The Working Sparky***

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“ Rcd's “

Fire detection circuits must be supplied independently of other circuits and not protected by an Rcd protecting multiple circuits. 560.7.1 All circuits in a bath shower room must be protected by a 30mA RCD. P166, 701.411.3.3
Where applicable an RCD notice must be fixed on or next to the CCU 514.12.2 An RCD should not be used as a main switch 314.2 If the maximum Zs values for a circuit in a TN systems cannot be met, the circuit may be protected by a 30mA RCD. 531.3.1
If the maximum Zs values for a TN systems cannot be met, the installation may be protected by an 100mA RCD and treated as a TT systems . 531.3.1 : 411.5.1 : 411.5.2 : 411.5.3
Unless specifically labelled or suitably identified, all 13A socket outlets must be protected by a 30mA Rcd. 411.3.3
In a TN systems , the part of a lighting circuit in a bath or shower room is required to be 30mA RCD protected. 411.3.3 , 701.411.3.3

Where a cable is buried in a wall or partition at a depth of less than 50mm on either side it must be sufficiently mechanically protected against penetration OR RCD protected AND installed either horizontally within 150mm of the top of the wall or vertically within 150mm of the angle formed where two walls meet or run horizontally or vertically to an accessory, luminaire or CCU 522.6.6 , 522.6.7 . This applies to a cable in a partition constructed using metallic parts other than fixings irrespective of the cable depth. 522.6.8

Surface run cables do not require RCD protection. OSG p22

A single RCD protecting a TT systems must be installed at the origin of the installation unless the part of the system between the origin and the RCD fulfils the requirements for protection by Class 11 equipment or equivalent insulation 531.4.1

All Electrical equipment must be accessible for operation , inspection & testing maintenance and repair. 132.12

Rcd Test Procedure

Many RCD test meters have a facility where tests can be carried out during the positive or negative half of the supply cycle. For tests 1 & 2 the RCD operating time to be recorded is the longer of the two measured during the half cycle tests.

DO NOT press the test button on the RCD before testing as this can temporarily reset a faulty RCD

Test 1
Adjust the current setting on the test meter to 100% of the rated trip current of the RCD and perform a test. A general purpose BS4293 RCD should operate within 200mS . A general purpose BS-61008 RCD or RCBO to BS-61009 should operate within 300ms

Test 2
An RCD provided for Basic Protection should have a rated TRIP current not exceeding 30mA If the RCD is rated at 30mA , adjust the current setting on the test meter to 150ma ( x5 ) and perform a test. The RCD must operate in a time not exceeding 40mS.
Test 3
Adjust the current setting on the test meter to 50% of the RCD trip current and perform a test. The RCD should not operate within 2 seconds
The Test Button : Finally operate the RCD by pressing its test button

 

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Other Considerations

There are Additional Regulations and Codes of Practice that Need to be considered during the design of an
be considered during the design of an installation. These will affect the choice of consumer unit.
Division of Installation : Section 314.1 Calls for the Installation to be so Divided to:
( a ) Avoid Hazards and Minimize Inconvenience in the Event of a Fault
( b ) Reduce the Possibility of Unwanted Tripping of the RCD due to Excessive Protective Conductor Currents.

To Comply with these Requirements the Circuits of an Installation should Not be Connected to a Single RCD, as this could Lead to Loss of Supply to the Entire Installation in the Event of a Fault on One Circuit,
clearly Inconvenient for the User of the Building

All Circuits of an Installation should Not be Connected to a Single RCD :

● BS 5839-6:2004 Fire detection and Fire alarm systems for buildings
This Code of Practice has particular requirements for dwellings. This document makes reference to the power supply to such systems
and mentions RCD’s. The circuit supplying these systems should preferably not be protected by an RCD. This however is going to be
difficult to achieve if the circuit supplying these systems is buried in the walls and standard domestic wiring systems are used. Indeed the supply cables would need to be specially protected in earthed metal conduit etc. for RCD protection not to be used.

According to BS 5839, circuits supplying fire or smoke alarms in dwellings can be protected by an RCD provided that either:
(i) The RCD serves only that circuit. For example with the use of an RCBO ,
(ii) The RCD should operate independently of any RCD feeding socket outlets or portable equipment ,

Where RCD Protection is Needed for Smoke Detector Circuits it should Preferably be Supplying that Circuit ,

● The following options, each with their own benefits, can be considered by the installation designer. ( Electrician )

Consumer Unit Arrangements “ Not Permitted “
A consumer unit with a 30mA RCD main switch would not meet the requirements of the regulations for 3 main reasons:
Consumer Unit Arrangements “ Not Permitted “
A consumer unit with a 30mA RCD main switch would not meet the requirements of the regulations for 3 main reasons:
• The Fire detection circuit and the socket outlet circuits share a common RCD. This would be against the requirements of BS 5839.
• The cumulative effects of electronic equipment in the modern home, is such that some current is likely to flow in the protective conductor.
A 30mA RCD will trip between 15-30mA. This could cause unwanted tripping, regulation 314.1 (iv) refers.
• Any fault would result in the loss of all the lighting, this could in itself cause a hazard and the lack of power to the fridge / freezer
circuit for example would be very inconvenient. Regulation 314.1 (i)

A Consumer Unit with a 30mA RCD Main Switch would Not meet the Requirements of the Regulations

● Consumer Unit Arrangements Option 1
Main Switch with RCBO’s On All Circuits

A standard main switch disconnector controlled consumer unit could
be used with every circuit having individual RCD protection at 30mA.
This could be achieved by selecting RCBO’s for every outgoing
circuit instead of the usual MCB’s. A fault on any circuit would not
affect other circuits and hence all relevant regulations would be met by such a design.

Selecting RCBO’s for every outgoing circuit meets all relevant regulations

● Consumer Unit Arrangements Option 2
Split Load Twin RCCB plus Dedicated RCBO
This arrangement provides a dedicated 30mA RCBO for the smoke
detector circuit, but combines the rest of the circuits across two
further 30mA RCCB’s. Careful arrangements of the circuits can
reduce the likelihood of nuisance tripping, thereby limiting the
inconvenience or potential hazards that a loss of supply can cause by limiting the number of circuits affected.

This arrangement provides a dedicated RCBO for the smoke detector circuit

● Consumer Unit Arrangements Option 3

Split Load 3 RCCB Board
This arrangement provides a 30mA RCCB for the smoke detector
circuit which could also supply other circuits e.g. lighting, and
combines the rest of the circuits across two further 30mA
RCCB’s. Careful arrangements of the circuits can reduce the
likelihood of nuisance tripping, thereby limiting the inconvenience
or potential hazards that a loss of supply can cause by reducing the number of circuits affected.

This arrangement provides a RCD for the smoke detector circuit which could also supply other circuits e.g. lighting

● Consumer Unit Arrangements Option 4
Split Load Twin RCCB
This arrangement provides two separate 30mA RCCBs with the
circuits spread across both. The design of the circuit arrangements
must ensure the smoke detector is not fed from the same RCD as
socket outlets. Careful arrangement of the other circuits can reduce
the likelihood of nuisance tripping, thereby limiting the inconvenience
or potential hazards that a loss of supply can cause. However with
all circuits now over just two devices certain compromise must be accepted

The smoke detector must not be fed from the same RCD as socket outlets

● Consumer Unit Arrangements Option 5

Split Load Twin RCCB plus unprotected circuit
Under the 17th Edition requirements it is still possible to install some
circuits in domestic premises that are not fed via an RCD. Different
wiring systems would need to be used. The cost of installation could
rise considerably if most circuits were installed using armoured cable or earthed metal conduits.

The smoke alarm circuit could be installed in such a way to negate
the need for RCD protection, this may be possible by using one
of the other wiring methods described in 522.6.6 for the length of
run that the cable is in the wall (use of earthed metal conduit for
example). Or depending on the layout of the property there maybe an
attached garage for example where surface wiring might be possible.
The requirements of that regulation are therefore not applicable.

The smoke detector must not be fed from the same RCD as socket outlets

The level of compliance with the Regulations would therefore be the
same as option 2 Split Load Twin RCCB plus Dedicated RCBO.

If the smoke alarm circuit is not to be protected by an RCD it must be installed using

a method from (i) to (iv) of regulation 522.6.6

239- : Guidance for Recipients on the Recommendation Codes

Only One Recommendation Code should have been Given for Each Recorded Observation ,
Recommendation Code (1)
Where an Observation has been Given a Recommendation Code (1) ( Requires Urgent Attention )
The Safety of those Using the Installation may be at Risk ,
The Person Responsible for the Maintenance of the Installation is Advised to take Action without Delay to Remedy the Observed Deficiency
In the Installation , or Take Other Appropriate Action ( Such as Switching Off and Isolating the Affected Part(s) of the Installation )
To Remove the Potential Danger ,
The N----- Approved Contractor Issuing this Report will be Able to Provide Further Advice :

It is Important to Note that the Recommendation Given at Section 1 Next Inspection of this Report for the Maximum Interval until
The Next Inspection , is Conditional Upon all Items which have been Given a Recommendation Code 1 being Remedied without Delay ,

Recommendation Code (2)
Recommendation Code 2 ( Requires Improvement ) indicates that , Whilst the Safety of those Using the Installation may Not be at Immediate Risk ; Remedial Action should be Taken as Soon as Possible to Improve the Safety of the Installation to the Level Provided by the National Standard for the Safety of Electrical Installations BS - 7671

The N----- Approved Contractor Issuing this Report will be Able to Provide Further Advice :
Items which have been Attributed Recommendation Code 2 should be Remedied as Soon as Possible

Recommendation Code (3)
Where an Observation has been Given a Recommendation Code 3 ( Requires Further Investigation ) the Inspection has Revealed an Apparent Deficiency which could Not , Due to the Extent or Limitations of this Inspection , be Fully Identified , Items which have been Attributed Recommendation Code 3 should be Investigated as soon as Possible
The Person Responsible for the Maintenance of the Installation is Advised to Arrange for the N----- Approved Contractor Issuing
This Report ( or Other Competent Person ) to Undertake Further Examination of the Installation to Determine the Nature and Extent of the Apparent Deficiency

Recommendation Code (4)
Recommendation Code 4 ( Does Not Comply with BS-7671 ( as Amended ) will have been Given to Observed Non-Compliance(s)
With the Current Safety Standard which Do Not Warrant One of the Other Recommendation Codes ,
It is Not Intended to Imply that the Electrical Installation is Unsafe , but Careful Consideration should be Given to the Benefits of Improving these Aspects of the Installation ,
The N----- Approved Contractor Issuing this Report will be Able to Provide Further Advice ,

 

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Frequency of Inspections
The frequency of the periodic inspection and testing must be determined taking into account:
1. the type of installation
2. its use and operation
3. the frequency and quality of maintenance
4. the external influences to which it is subjected.
It is recommended that periodic inspection and testing is carried out at least every:
1. 10 years for a domestic installation
2. 5 years for a commercial installation
3. 1 year for swimming pools
Other instances when a periodic inspection should be carried out are:
1. When a property is being prepared to be let
2. Prior to selling a property or when buying a previously occupied property
Scope
The requirement of BS-7671 IEE Wiring Regulations, for periodic Inspection and Testing is for INSPECTION comprising careful scrutiny of the installation without dismantling, or with partial dismantling as required, together with a sequence of tests considered appropriate by the person carrying out the inspection and testing.

Electrical Testing - Legal Requirements 239-

Letting
You must maintain the electrical installation and any equipment provided by you, in a safe condition.
The Landlord and Tenant Act 1985 requires landlords to ensure the electrical installation is safe when the tenancy begins, and that it is maintained in a safe condition throughout that tenancy.
One way of ensuring safety is to undertake a regular visual inspection of the installation, looking for any obvious signs of damage such as damaged cables, socket-outlets showing scorch marks, etc.
In addition, the Institution of Electrical Engineers recommends that electrical installations are formally inspected and tested by a competent person on change of occupancy, and at least once every ten years.
Businesses
Any business premises from Guest houses, Shops, Offices or Hotels, are required under the Electricity at Work Regulations, that the electrical installation be checked. to satisfy this requirement.
Building Regulations
Part P of the Building Regulations (England and Wales) was introduced by the Government on January 1st 2005. It is designed to reduce accidents caused by faulty electrical installations and to prevent incompetent installers from leaving electrical installations in an unsafe condition.
Part P applies to the following situations:
• Dwelling houses and flats
• Dwellings and business premises that have a common supply eg shops that have a flat above
• Common access areas in blocks of flats such as corridors or staircases
• Shared amenities in blocks of flats such as laundries or gyms
• In or on land associated with dwellings – such as fixed lighting or pond pumps in gardens
• Outbuildings such as sheds, detached garages and greenhouses
Approved Document P is called ‘Electrical Safety’ and will be complied with if the standard of electrical work meets the ‘Fundamental Requirements of Chapter 13 of BS7671'.
Section P1 of Part P states: ‘Reasonable provision shall be made in the design, installation, inspection and testing of electrical installations in order to protect persons from fire and injury’
Section P2 of Part P states: ‘Sufficient information shall be provided so that persons wishing to operate, maintain or alter an electrical installation can do so with reasonable safety’
Part P applies only to fixed electrical installations that are intended to operate at low voltage or extra-low voltage which are not controlled by the Electricity Supply Regulations 1988 as amended, or the Electricity at Work Regulations 1989 as amended.
Note 1: The Workplace (Health, Safety & Welfare) Regulations (1992) apply in common parts of flats and similar buildings if people such as cleaners and caretakers are employed to work in them.
Note 2: The Electricity at Work Regulations (1989) cover all electrical work carried out professionally and the competence of the individuals carrying out that work
Note 3: Part P is concerned with safety and does not directly cover system functionality
Note 4: Part P does not specifically cover dwellings in places of work normally covered by the Electricity at Work Regulations (1989), such as caretakers flats in schools, MOD barracks etc.

The Periodic Inspection Report form is intended for reporting on the condition of an existing electrical installation.
You should have received an original Report and the contractor should have retained a duplicate. If you were the person ordering this Report, but not the owner of the installation, you should pass this Report, or a full copy of it, immediately to the owner.
The original Report is to be retained in a safe place and be shown to any person inspecting or undertaking work on the electrical installation in the future. If you later vacate the property, this Report will provide the new owner with details of the condition of the electrical installation at the time the Report.
The 'Extent and Limitations' box should fully identify the extent of the installation covered by this Report and any limitations on the inspection and tests. The contractor should have agreed these aspects with you and any other interested parties (Licensing Authority, Insurance Company, Building Society etc) before the inspection was carried out.
The Report will usually contain a list of recommended actions necessary to bring the installation up to the current standard. For items classified as 'required urgent attention', the safety of those using the installation may be at risk, and it is recommended that a competent person undertakes the necessary remedial work without delay.
For safety reasons, the electrical installation will need to be re-inspected at appropriate intervals by a competent person. The maximum time interval recommended before the next inspection is stated in the Report under 'Next Inspection.'
Accessories and Switchgear
It is recommended that a random sample of a minimum of 10 per cent of all switching devices is given a thorough internal visual inspection of accessible parts to assess their electrical and mechanical condition.
Protection against Thermal Effects
The presence of fire barriers, seals and means of protection against thermal effects will be verified.
Protective Devices
The presence, accessibility, marking and condition of devices for electrical protection, isolation and switching will be verified. It should be established that each circuit is adequately protected with the correct type, size and rating of fuse or circuit-breaker. The suitability of each protective and monitoring device and its overload setting will be checked.
Visual Inspections
Joints and Connections
It is not practicable to inspect every joint and termination in an electrical installation, nevertheless a sample inspection is made. An inspection is made of all accessible part of the electrical installation e.g. switchgear, distribution boards, and a sample of luminaire points and socket-outlets to ensure that all terminal connections of the conductors are properly installed and secured. Any signs of overheating and conductors, terminations or equipment will be thoroughly investigated and included in the Report.

239- Regs : page – 340 ( PS this will come up along the Line )
✔’
indicates that an inspection or a” test “ was carried out and that the result was satisfactory
‘✗’
indicates that an inspection or a” test “was carried out and that the result was Not satisfactory( Applicable for a Periodic Inspection Only )

N/A’ indicates that an inspection or a” test “was not applicable to the particular installation

LIM’ indicates that, that exceptionally, a limitation agreed with the person ordering the work prevented the Inspection being Carried Out ( Applicable for a Periodic Inspection Only )

prevented the inspection or test being carried out

 

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239- Test !! It is Big Exam , Good Luck for your examination , Amberleaf
Please have a Look at what am down Loading it will Help you Somewhere along the Line
PS some Old Notes

● Give THREE reasons for carrying out a polarity test on an installation, as required by BS-7671
● The resistance of an earth electrode is to measured. If a mains supply is unavailable, state the
(a) instrument to be used
(b) terms used to describe the TWO other test electrodes.

● State (a) why it may not be possible to obtain a measured value of loop impedance for a circuit protected by an RCD (b) how this value may be determined without the use of a special instrument
● A loop impedance test is to be conducted on a radial socket outlet circuit. State
(a) where in the circuit the test should be made
(b) TWO conditions that may affect the validity of the measured value as a comparison to the maximum value
● State THREE reasons for the use of a 20mA RCD
● In the formula IΔ n = 50/Zs (a) state what is represented by ‘50’ (b) the maximum residual current rating of the RCD required, when the value of Zs is 500Ω
● The PFC at the origin of an installation is to be measured. State the
(a) instrument to be used
(b) measurement units
(c) importance of the breaking capacity of the protective devices at the origin
● Section B – Answer ALL SIX questions Questions 21 to 26 all refer to the following scenario and Fig. 1 (attached).
Fig.1 shows the main intake and circuit arrangements for the new electrical installation in a refurbished dentist’s surgery. The heating, ventilating and air-conditioning unit (HVAC), located outside the building is supplied using p.c.v./p.c.v./s.w.a. multicore cable. All other circuits are wired using p.v.c. single-core cables enclosed in concealed p.v.c. conduit and surface trunking (c.p.c’s are the same size as the phase conductors). The lighting is a mixture of tungsten filament, fluorescent and PIR-controlled exterior security lights. The installation has been completed and is ready for inspection and testing.
● State the
(a) type of inspection and test to be completed (b) certificate that will need to be completed (c) signatures that are required on the certificate in b) (d) status of the signatories (e) documents that must accompany the certificate in b) (f) information regarding the incoming supply that should be recorded on ONE of the documents in e) (g) person who should keep the original completed documents

“ What types of electrical work DOES Part P cover “

Part P applies to electrical installations in buildings or parts of buildings comprising:
> dwelling houses and flats;
> dwellings and business premises that have a common supply – for example shops
and public houses with a flat above;
> common access areas in blocks of flats such as corridors and staircases;
> shared amenities of blocks of flats such as laundries and gymnasiums.

Part P applies also to parts of the above electrical installations:
> in or on land associated with the buildings – for example Part P applies to
fixed lighting and pond pumps in gardens;
> in outbuildings such as sheds, detached garages and greenhouses.

“ How will compliance with Part P be enforced ”

Electrical installations within dwellings are now covered by Part P (and others) of
the building regulations. As such, failure to comply with the building regulations is a
criminal offence and Local Authorities have the power to require the removal or
alteration of work that does not comply with the requirements.

As with other building regulations, they must be followed. This ensures compliance,
as well as regulated enforcement of standards and ultimately quality of workmanship

“ What benefits is Part P aiming to provide “

it is expected that bringing electrical work in dwellings under building regulations
control will reduce the number of deaths, injuries and fires caused by faults in
electrical installations. It is also expected that nationally, Part P will lead to an overall
improvement in the competence of electrical contractors and to an improvement in the quality of electrical work.

Insurance companies are not yet offering cheaper home insurance if your electrical
installation is checked compliant to latest regulations and Part P.

We are seeing 'required' demands from insurance companies that are let through
landlords - safety and Part P compliant against any recent updates / changes.

Longer term (2007 onwards) we expect additional movement through the government
and through insurance companies to begin to 'drive' Part P, requiring electrical
certification to be confirmed against building works since 2005 and creating a
framework for confirmation of safety upon change of ownership (conveyance).

Electrician : 2392-10 ←

As Part P is now a regulatory requirement for electrical installations within homes,
insurance companies will challenge claims when faced with related electrical issues,
with possible failure ( invalidation ) of the claim, if Part P was not followed.

Building Regulations

The Requirements of this Part Apply Only to Electrical Installations That are Intended to Operate at Low or Extra-Low Voltage – are :-
(a) in Dwellings
(b) in the Common parts of the Buildings Serving one or more Dwellings , but Excluding power Supplies to Lifts ,
(c) in a Building that Receives its Electricity from a Source Located within or Shared with a Dwelling , and
(d ) in garden or in or on Land Associated with a Building where the Electricity is from a Source Located within or Shared with a Dwelling

** Extra Low Voltage = 50v ac or 120v ripple free dc
** Low Voltage = not exceed 1000 V ac or 1500 V dc between conductors,
or 600 V ac or 900 V dc between conductors and earth.

Standard domestic 'mains' electricity = Low Voltage (as defined above)

Simply, Part P aims to tighten and enforce electrical regulations as defined by the
Institute of Electrical Engineers (IEE). These regulations now mean *most* electrical
changes, additions and updates within dwellings fall under Part P, and must comply
with the IEE, the BS-7671 wiring regulations

What is Periodic Inspection & Testing
The 'Electricity at Work Regulations 1989' in Regulations 4(1) and 4(2) lay down the requirement to ensure that an electrical installation should be designed, installed, constructed and maintained in a safe manner at all times. The basis for periodic inspection and testing is derived from Guidance Note 3 of the 'IEE Wiring Regulations BS7671:2008'.
Inspection & Testing of an electrical installation generates a Periodic Inspection Report which is a condition report supplemented by testing results. The Periodic Inspection Report will normally consist of:
• Client information and details of the installation
• Supply characteristics and Earthing arrangements
• Observation & Recommendations
• Extent & Limitations of Testing
• Summary of the Inspection
• Inspection & Test Schedules
'IEE Inspection and Testing Guidance Note 3' “ GN-3 “
The Testing of an electrical installation follows BS-7671:2008 and comprises of:
• C.P.C Continuity
• Ring Circuit Continuity
• Insulation Resistance
• Polarity
• Earth Fault Loop Impedance
• Prospective Fault Current
• Operation of Residual Current Devices
• Functional Testing
Testing is carried out as far as is reasonably practicable as defined in EAWR 1989.

 

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The Electricity at Work Regulations

Regulation 3: Persons on Whom Duties are Imposed by these Regulations
Regulation 4: Systems, work activities & protective equipment
Regulation 5: Strength and capability of electrical equipment
Regulation 6: Adverse or hazardous environments
Regulation 7: Insulation, protection and placing of conductors
Regulation 8: Earthing or other suitable precautions
Regulation 9: Integrity of Referenced Conductors
Regulation 10: Connections
Regulation 11: Means for Protecting from Excess of Current
Regulation 12: Means of cutting off the supply and for isolation ( Isolation from sources of energy )
Regulation 13: Precautions for work on equipment made dead
Regulation 14: Work on or near live conductors
Regulation 15: Working space, access and lighting
Regulation 16: Persons to be competent to prevent danger and injury :

The On-site Guide is intended to enable the competent electrician to deal with small installations (up to 100 A, 3-phase )

“ Inspection & Testing before into Service “ 2392-10 Electrical Installations should be Inspected and Tested as Necessary and Appropriate During and the End of Installation , Before they are taken into Service , to Verify that they are Safe to Use ,Maintain and Alter and Comply with Part P of the
Building Regulations and with any Other Relevant Parts of the Building Regulations

An HSE Inspector , whilst visiting a high-volume manufacturing plant , spots a serious problem with a complex machine –whilst it is running through a long “ cycle “ The machine IS under human supervision , which one of the following actions , available to him/her , is most appropriate in this case ,
( Issue a Deferred prohibition notice )

this will allow the machine to complete its “ cycle ( with safeguards’ in place ) after which time , it MUST NOT
Be used until the fault is repaired this option is available to Inspectors in this cases where shutting down in “ mid-cycle “
Can itself – cause danger / expensive damage

“ Apprentice “ Domestic Electrical Installation Condition Report :
A Visual Condition Report , which includes Inspection but Not Testing :

A Periodic Inspection Report ( Including Inspection & Testing )

Residual Current Devices ( RCDs ) are used Extensively in Installation to Provide Fault Protection and / or Additional
Protection Against Electric Shocks ,17th Edition of the IEE Wiring Regulations most if not all , Final Circuits in new or rewired
Installation in Domestic Premises need to be Provided with Additional Protection by an RCD having a Rated
Residual Operating Current ( I∆n ) – 30mA and an Operating Time Not Exceeding 40mS at ( 5 I∆n )

While RCDs Provide an Enhanced Level of Shock Protection , Precautions should be Taken to Avoid Unwanted Tripping of the Devices on Healthy Circuits , ( Repeated Unwanted Tripping is Likely to Damage User Confidence in RCDs , )

Unwanted Tripping of RCDs can be Caused by the Currents that may flow in the Protective Conductors of Circuits Supplying
Certain Items of Class 1 ( Earthed ) Equipment During their Normal Operation , Such Items Include Equipment Incorporating :

● Electrical Noise ( Radio Frequency )
Suppression Filters , such as Personal Computers , Hi-fi Equipment , TVs , DVDs
● Heating Elements , such as Cookers , Water Heaters or Radiant Heaters ,
● Motors , such as Fridges & Freezers ,

To avoid Unwanted Tripping , RCDs Should be so Selected and Circuits so Subdivided , that any Protective Conductors Current
Expected to Occur During Normal Operation of the ( Load(s) will be Unlikely to Cause Tripping of the Device ,
It is worth Noting that Product Standards Permit Certain Equipment such as Personal Computers , to Create up to ( 3 mA )
Of Leakage Current in the Protective Conductor ,
Tripping of RCDs the Number of Items of Protective Conductor Current-Generating Equipment per Circuit ,
And the Number of Circuits Served by each RCD , needs to be Sufficiently Small ,

As a Rule of Thumb , Tripping of an RCD may Result if the Total Protective Conductor Current in the Circuit(s)
It Serves Exceeds 50% of its Rated Residual Operating Current that is 15 mA for a 30 mA Device ,

“ Routine Inspection & Maintenance of Fire Safety Installations “

Once Fire Safety Installations , such as Fire Detection and Fire Alarms Systems and Emergency and Escape Lighting Systems ,
Are in Use , they should be Subject to Regular Checks and Maintenance , these Checks are Split into Daily , Weekly , Monthly ,
Three-Monthly , Six-Monthly , and Yearly Checks :
● the Indication of Normal Operation of the Control Panel of a Fire Detection and Alarm Systems or Emergency and Escape Lighting Systems ,

● Weekly Checks includes , Amongst other things , Standby Batteries are in good Condition and the Control Equipment is Able to Receive a Fire Signal and Initiate the Evacuation Procedure :

● Amongst other things , the Operation of Standby Generator Sets should be Checked Monthly , involving the Simulation of Power Failure and Allowing the Systems to be Energised for One Hour , Additionally , the Failure of the Supply to the Normal Lighting should be Simulated Monthly , and all Signs and Luminaires Inspected to Determine that they are Functioning Correctly ,

● Annual Checks by Competent Person(s)
Should be made amongst other things ,of the Fire Detection and Alarm Systems , Self-Contained Luminaires over Three-Years old and Sprinkler Systems ,

 

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City & Guilds Unit Breakdown

Unit 201
This unit is the first to complete and comprises of the following sections:
● Identify the legal requirements
● Identify occupational specialism’s
● Identify sources of technical information
This is a general Health and Safety unit that also covers some of the fundamentals of the industry, such as the use of drawings.
Unit 202
The second unit and the first one that involves the student thinking more and using their Math’s and Physics skills.
● Describe the application of basic units used in electro technology
● Describe basic scientific concepts related to electro technology
● Describe basic electrical circuitry and applications
● Identify tools, plant, equipment and materials
This unit introduces standard units as well as the theory and calculation behind resistance and resistivity and other technical concepts. Ohms Law is introduced at this point.
Unit 203
The third unit is an amalgamation of the first two. It splits Health and Safety and Electrical Principles into two but also requires both parts to be passed independently to achieve the certificate.
● Safe systems of working
● Using technical information
● Electrical machines and a.c. theory
● Polyphase systems
● Over current, short circuit and earth fault protection
Some of the more useful fundamentals of electrical installation are covered, such as protective devices and the purpose of earthing. Motors and a.c. theory also come into play.
This unit also contains Unit 204 which is a practical assessment involving the measurement of 3 phase supplies, both voltage and current and on and off load and the measurement of the different voltages and currents found in a standard fluorescent circuit. This is assessed internally but is needed to claim for your final certificate.
Unit 205
The unit that concludes the level 2 focuses on installation design and Regulations and covers the following subjects.
● Regulations and related information
● Purpose of specifications and data
● Types of installations
● Undertaking electrical installation
As part of unit 205 you have to complete Unit 206, you are expected to complete a large practical task (which can vary centre to centre) that consists of different wiring system such as steel conduit, trunking, swa and lighting and socket outlet circuits.
Unit 301
The start of the level 3 takes all the aspects of level 2 and increases the depth of content. It works in the same way as Unit 203 in that it is split into 2 parts, Health and Safety and Science and Principles and you must achieve a pass in both sections. Expect to cover the following:
● Comply with Statutory Regulations and organisationaI requirements
● Apply safe working practices and follow accident and emergency procedures
● Work effectively and develop competences
and for the second part
● Understand the functions of electrical components
● Understand electrical supply systems, protection and earthing
Unit 302
This unit concentrates on Inspection, Testing and Commissioning and takes the theory up to level 3 where you are expected to test live installations. It is made up of a practical assessment and a 2 hour written examination covering all aspects of Inspection and Testing.
Unit 303
Almost identical to unit 302, this unit concentrates on Fault Diagnosis and Rectification and finishes in a practical assessment and a 2 hour written exam.
Electrical Qualifications 2007

The actual AM2 is split into 4 sections:
• Section A involves a composite installation. It is a partially completed installation which you must finish. You need to know how to terminate SWA (armoured) cable, MIMS (Mineral Insulated Metal Sheathed or simply Pyro to many) and be comfortable with simple motor circuits, although you are given a wiring diagram. Once complete a visual and a functional test needs to be carried out.
• Section B involves an inspection and test of the installation
• Section C involves safe isolation procedures and a risk assessment
• Section D is fault diagnosis and rectification
AM2 Hints and Tips
Below are some ideas to help you pass the AM2 practical exam.
• Practice reading from a circuit diagram before you start
• Be especially comfortable with motor circuits and following the wiring diagram given to you beforehand
Be comfortable with lighting circuits in singles, make sure you know how to wire a landlords override switch and a Two-Way and Intermediate Lighting Circuit
•
• Take a padlock with you just in case and keep your tools locked
• Take a sharp knife such as an electricians knife sold by RS, Stanley knives or craft knives will not be permitted
• Fully understand the correct safe isolation procedure, make sure you keep the key in your pocket or locked away
• Know how to fault find, remember, continuity and insulation resistance tests are your friends
• Practice MIMS cable but leave it until the last task on the installation
• Practice doing back to back bends in steel conduit

Water heaters
As a general rule, any water heater over 15 litres should be fed by an independent circuit. Again, calculations should be made regarding the power consumption of the heating element although in most instances, it will be fed by a 16A protective device and 2.5mm2 cable.
OTHER CIRCUITS
Fire alarm
Introduced into the Building Regulations, specifically, Part B: Fire Safety, was the provision of a separate circuit for a fire alarm. It is thought that a separate circuit will not be isolated for any period of time hence the need to remove it from circuits such as lighting or socket outlet circuits.

 

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239- Inspection Testing & Certification of Electrical Installations Exam ( Old Notes )

The student must be aware of the correct use of terminology when answering questions, marks will be lost for incorrect use, some common examples include:
Using the term 'live' instead of 'phase'. Remember a live conductor includes the neutral also
Using the term 'completion certificate' instead of Electrical Installation Certificate
Using the term 'electrical inspection certificate' instead of Electrical Installation Certificate
Using the term 'initial' inspection certificate' instead of Electrical Installation Certificate
Using the term 'minor works certificate' instead of Minor Electrical Installation Works Certificate
Using the term 'Periodic Inspection Certificate' instead of 'Periodic Inspection Report for an Electrical Installation'
Incorrect use of the terms used to describe the 'Earthing Conductor' and Main Equipotential Bonding Conductors' For example using the incorrect term 'Main Bonding Conductor' instead of 'Earthing Conductor' and using the incorrect term 'Earth Bonding Conductor or even Bonding Conductor' instead of Main Equipotential Bonding Conductor
Incorrectly describing the 'Continuity of protective conductors' or 'Continuity of Ring Final Conductors' test as 'Open Loop' test when describing how to obtain R1 + R2
Incorrectly referring to 'Inspection Schedule' instead of 'Schedule of Inspections'
Incorrectly referring to 'Schedule of Tests' or Test Results Schedule instead of ' Schedule of Test Results'
It may sound petty, but you must learn to use the correct terms or you will lose marks - All the available past exam papers and the periodic inspection report check-list describe the correct terms to use
Naming documentation
When requested the documentation to complete during an initial or periodic inspection, many candidates will mention the Electrical Installation Certificate or the Periodic Inspection Report but forget to mention the 'Schedule of Inspections' and Schedule of Test Results'
Completing documentation
Many of the latest written exam papers include the requirement to complete an Electrical Installation Certificate or Periodic Inspection Report along with the accompanied 'Schedule of Inspections' and Schedule of Test Results' Often the question only asks the candidate to complete the schedule of test results for say a new ring final circuit. Many candidates waste valuable time and lose marks by completing the schedule of test results for every circuit given in the specification in part B of the question paper.
BIG TIP: READ THE QUESTION CAREFULLY:
Many candidates continue to get the value of Zs wrong for the circuit in question in part B of the exam. Check out my
A common error when completing questions involving schedule of test results is to forget to indicate functional tests have been performed and found satisfactory/unsatisfactory. To tick the tick box when completing details of ring final circuits, ensuring to indicate continuity of ring final conductors have been performed.
Failure to record the type of earthing system, i.e. TN-S, TN-C-S, TT,
Failure to record the value of Ze, PFC, Nominal voltage, Nominal frequency, are common errors.


Test procedures
A very common question is to explain in detail how to perform an insulation resistance test, often on a lighting circuit. Candidates regularly fail to state the instrument used which is an 'Insulation Resistance Ohm-meter' (Not a Megger! You will not gain any marks if you answer a Megger) Many candidates fail to identify the test voltage required for typical 230/240 volt installation which is 500 volts and the acceptable test value which is 0.5 Meg-ohm or greater (again due to change under 17th edition to a minimum acceptable value of 1.0 Meg-ohm) City and Guilds usually deliberately pick a lighting circuit that is stipulated as having two way switching, just to see if the candidate mentions to test the strappers in BOTH positions.
Failure to mention testing the insulation resistance of both strappers will lose you marks. Be warned!
Candidates regularly make mistakes when answering RCD questions. Often the question or specifications usually given in part B will make reference to a specific type of RCD for example a 30 mA RCD. Candidates are then asked to state the actual test current applicable to test this type of RCD. Candidates regularly incorrectly state the answer as x1/2 x1 and x5 instead of x1/2 = 15mA x1 = 30mA and x5 = 150mA
A frequent question which often occurs requires the candidate to explain how to perform a continuity test on a ring final circuit and to explain how R1 + R2 for the circuit is obtained. Many candidates regularly continue to get this procedure wrong check out my past exam papers and the Periodic Inspection Report check-list for a thorough explanation on how to perform these tests.
In the March 2007 paper -&- point out many exam candidates still cannot list the correct sequence of tests for a given scenario. One such scenario involves the testing of a lighting circuit, many candidates just listed the typical sequence including continuity or ring final circuit conductors, clearly they received 'Nil Pwa' for their effort
Many candidates could not correctly perform calculations involving the 'rule of thumb' or incorrectly applied the rule to the 'Measured' values instead of the 'Tabulated' values
On questions regarding insulation resistance, some candidates thought 0.00 Meg-ohms was an open circuit. Many candidates could not correctly describe the procedure for performing an external phase-earth fault loop impedance test. C&G are now requesting much more detail to such questions as well as RCD testing, requesting not only details of how to perform an RCD test but circuit preparation before hand as well as reasons why circuit preparation is required. See my March 2007 specimen paper for answers to all these questions including full colour diagram of how to perform the Ze test
October 2007 exam, part B scenario gave many candidates a difficult time. Here you were presented with a TN-C-S system installed in a domestic property, and an underground supply cable is being used to supply an external outhouse. However the electricity supply company will not allow you to use their means of earthing for the outhouse. So how do you provide a means of earthing for the outhouse? Where do you earth the supply cable? What checks must you make on the underground supply cable? Why can't you use the main house TN-C-S system as a means of earthing the outhouse? Many candidates have written to me seeking clarification on the part B scenario questions.
October 2007 Part B scenario
Solution to terminating the underground SWA supply cable
The above diagram is how the -&-'s question may lead you to terminate the SWA of the underground supply cable, since the question informs you. "You are NOT allowed to use the supply companies earthing system as a means of earthing the outhouse" However -&-'s mention nothing about earthing the supply cable itself. It is difficult to decide whether -&-'s are just testing the candidates knowledge of this situation VERY thoroughly or alternatively offering a red-herring to mislead the unwary candidate. Whichever is the reason for this question, it caused many exam candidates a great deal of difficulty and lost time trying to decide what the solution was.
The actual solution stems from BS 7671 regulation 542-01-09 part of this reg states
"If the protective conductor (i.e. the swa) forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device" This therefore has to be the main house end. See the only possible solution below

 

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But where manufactures recommend a method regulation 134.1.1 comes into play with:- Electrical equipment shall be installed in accordance with the instructions provided by the manufacturer of the equipment.

As if this should over ride regulations like 531.2.8 Where an RCD is used for fault protection with, but separately from, an overcurrent protective device, it shall be verified that the residual current operated device is capable of withstanding, without damage, the thermal and mechanical stresses to which it is likely to be subjected in the case of a fault occurring on the load side of the point at which it is installed. Is of debate but I can see anyone forcing you to change to all RCBO after a multi RCD board it fitted and in any case if going through building control direct you have to submit plans first so any objection would be before the work started.

Multi-RCD are required to comply with:-
314.1 Every installation shall be divided into circuits etc. ( only showing part of reg )
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.
531.2.4 An RCD shall be so selected and the electrical circuits so subdivided that any protective conductor current which may he expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device.

As I read it unless emergency lighting is used then lights should have there own RCD and not shared with any other circuit. The same would apply to fire alarm systems. Items like cookers which could cause other circuits to trip due to high earth leakage and sockets, likely used outside, should not really be on the same RCD as other items in the house. This could also apply to computer supplies.


“ Instructed person “
A person adequately advised or supervised by skilled persons to enable him/her to avoid dangers which electricity may create.:
“ Ordinary person “
A person who is neither a skilled person nor an instructed person.:

“ Wiring Systems “
To conform with the requirements of BS 7671, wiring systems must utilise cables complying with the relevant requirements of
the applicable British Standard or Harmonised Standard.
Alternatively, if equipment complying with Alternatively, if equipment complying with based on an IEC Standard is
to be used, the designer or other person responsible for specifying the installation must verify that any differences between that
standard and the corresponding British Standard or Harmonised Standard will not result in a lesser degree of safety than
that afforded by compliance :
A new series of Regulations ( 522.6.6 - 522.6.8 ) have been introduced in the 17th Edition of the IEE Wiring Regulations
concerning cables concealed in a wall or partition. These new Regulations introduce the concept of skilled person, instructed
person and ordinary person :
“ RCD Protection “
It is now a requirement to protect cables concealed in a wall or partition (at a depth of less than 50 mm) by a 30mA RCD where the
installation is not intended to be under the supervision of a skilled or instructed person where other methods of protection, including the
use of cables with an earthed metallic covering, earthed conduit/trunking or mechanical protection, can not be employed.
Irrespective of the depth, a cable in a partition where the construction includes metallic parts other than fixings shall be protected by
a 30 mA RCD. For example, this means that in a domestic installation, where insulated and sheathed cables are concealed in a
wall at a depth of less than 50 mm and have no mechanical protection, they need to be installed within the safe zones and protected
by a 30 mA RCD.
“ Skilled person “
A person with technical knowledge or sufficient experience to enable him/her to avoid dangers which electricity may create :

 

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Permitted cable routes :

with the British Standard. The effect of environmental conditions and general characteristics around various parts of the installation must be assessed to enable suitable electrical equipment, including the wiring system, to be specified.
For example, cables and equipment used in agricultural and horticultural premises should be installed away from areas or routes used by
animals or be of a type to withstand such attack. Any part of the fixed installation which may be exposed to a severe impact
must be able to survive it. In workshops, for example, where heavy objects are moved, installation of wiring systems in traffic
routes should be avoided or localised protection must be provided. Therefore, when designing a concealed installation, the designer
must select a suitable wiring system. Under the 17th Edition, depending on the type of wiring selected, the method of installation
and whether the installation will be under the control of a skilled person, or instructed person or ordinary person will depend
whether the concealed wiring will require RCD protection or not. For example, in a domestic installation, where insulated and sheathed
cables are concealed in a wall at a depth of less than 50 mm and have no mechanical protection, they need to be installed within
the safe zones and need to be protected by a 30 mA RCD. Regulations 522.6.6 and 522.6.8 are reproduced here for information.
522.6.6 A cable concealed in a wall or partition at a depth of less than 50 mm from a surface of the wall or partition shall:
(i) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective
conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or
(ii) be enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of these Regulations for a
protective conductor, or (iii) be enclosed in earthed trunking or ducting complying with BS EN 50085 and satisfying the
requirements of these Regulations for a protective conductor, or (iv) be mechanically protected against damage sufficient to prevent
penetration of the cable by nails, screws and the like, or (v) be installed in a zone within 150 mm from the top of the wall or partition or
within 150 mm of an angle formed by two adjoining walls or partitions. Where the cable is connected to a point,
accessory or switchgear on any surface of the wall or partition, the cable may be installed in a zone either horizontally or vertically, to
the point, accessory or switchgear. Where the location of the accessory, point or switchgear can be determined from the reverse
side, a zone formed on one side of a wall of 100 mm thickness or less or partition of 100 mm thickness or less extends to the reverse side.
522.6.7 Where Regulation 522.6.6 applies and the installation is not intended to be under the supervision of a skilled or instructed
person, a cable installed in accordance with Regulation 522.6.6 (v), and not complying with Regulation 522.6.6 (i), (ii), (iii) or (iv),
shall be provided with additional protection by means of an RCD having the characteristics specified in Regulation 415.1.1.
522.6.8 Irrespective of the depth of the cable from a surface of the wall or partition, in an installation not intended to be under the
supervision of a skilled or instructed person, a cable concealed in a wall or partition the internal construction of which includes metallic parts, other than metallic fixings such as nails, screws and the like, shall:
(i) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective
conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS 7846,
BS EN 60702-1 or BS 8436, or (ii) be enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of
these Regulations for a protective conductor, or (iii) be enclosed in earthed trunking or ducting complying with BS EN 50085
and satisfying the requirements of these Regulations for a protective conductor, or (iv) be mechanically protected sufficiently to
avoid damage to the cable during construction of the wall or partition and during installation of the cable, or (v) be provided with
additional protection by means of an RCD having the characteristics specified in Regulation 415.1.1.
NOTE: If the cable is installed at a depth of 50 mm or less from the surface of a wall or partition the requirements of Regulation
522.6.6 also apply. RCD Protection ,An RCD is a protective device used to automatically disconnect the electrical supply when an imbalance is detected between live conductors. In the case of a single-phase circuit, the device monitors the difference in currents
between the line and neutral conductors. If a line to earth fault develops, a portion of the line conductor current
will not return through the neutral conductor. The device monitors this difference, operates and disconnects the circuit when the residual current reaches a preset limit, the residual operating current (IΔn). An RCD on its own does not provide protection against overcurrents. Overcurrent protection is provided by a fuse or a circuit-breaker. However, combined RCD and circuit-breakers are available and are designated RCBOs. Unwanted tripping Unwanted tripping of RCDs can occur when a protective conductor current or leakage
current causes unnecessary operation of the RCD. An RCD must be so selected and the electrical circuits so subdivided that any
protective conductor current that may be expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device. Discrimination Where two, or more, RCDs are connected in series, discrimination must be
provided, if necessary, to prevent danger. During a fault, discrimination will be achieved when the device electrically nearest to the
fault operates and does not affect other upstream devices. Discrimination will be achieved when ‘S’ (Selective) types are used in conjunction with downstream general type RCDs. The ‘S’ type has a built-in time delay and provides discrimination by simply ignoring the fault for a set period of time allowing more sensitive downstream devices to operate and remove the fault. For example, when
two RCDs are connected in series, to provide discrimination, the first RCD should be an ‘S’ type. RCDs with built in time
delays should not be used to provide personal protection. Labelling Regulation 514.12.2 requires that where an installation incorporates an RCD a notice shall be fixed in a prominent position at or near the origin of the installation. The Regulation requires that the notice
shall be in indelible characters not smaller than illustrated in BS 7671, see fig. 4. Testing Refer to Regulations 612.8.1,
612.13.1 and 415.1.1 for requirements in terms of verification of installed RCDs.
“ Conclusion “ 1
Under the 17th Edition designers will now have to determine from the client whether the installation is going to be under the
supervision of a skilled person, instructed person or ordinary person.

Labelling requirement of 514.12.2

 

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Recognised devices ( Remember that BS- is Old / but still in Use )
RCDs are manufactured to harmonised standards and can be identified by their BS EN numbers. An RCD found in an
older installation may not provide protection in accordance with current standards. The following list identifies the applicable current standards:
BS 4293 : 1983 (1993) Specification for residual current operated circuit-breakers. (Replaced by BS EN 61008-1: 1995,
BS EN 61008-2-1: 1995 and BS IEC 61008-2-2: 1990). This Standard remains current :
BS 7071 : 1992 (1998)
Specification for portable residual current devices :
BS 7288 : 1990 (1998)
Specification for socket-outlets incorporating residual current devices. (SRCDs)
BS EN 61008-1 : 1995 (2001)
Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs)
BS EN 61009-1 : 2004
Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses (RCBOs)
Characteristics of RCDs
RCDs are defined by a series of electrical characteristics, three main characteristics are:
1. The rating of the device in amperes, ( I ) :
2. The rated residual operating current of the protective device in amperes, I∆n:
3. Whether the device operates instantaneously or incorporates an intentional time delay to permit discrimination. Such devices are called ‘S’ or Selective :
Devices are manufactured with different values of rated current and rated residual operating current but we will just consider the rated residual operating current of the protective device in amperes, I∆n:
Discrimination
Where two, or more, RCDs are connected in series, discrimination must be provided, if necessary, to prevent danger (Regulation 531-2-9 refers). During a fault, discrimination will be achieved when the device electrically nearest to the fault operates and does not affect
other upstream devices. Discrimination will be achieved when ‘S’ (Selective) types are used in conjunction with downstream general
type RCDs. The ‘S’ type has a built-in time delay and provides discrimination by simply ignoring the fault for a set period of time allowing more sensitive downstream devices to operate and remove the fault. For example, when two RCDs are connected in series, to provide discrimination, the first RCD should be an ‘S’ type. RCDs with built in time delays should not be used to provide personal protection.
Testing
RCDs must be tested. The requirements are stated in the following Regulations:
The effectiveness of the RCD must be verified by a test simulating an appropriate fault condition and independent of any test facility, or
test button, incorporated in the device (Regulation 613-13-1)
Tests are made on the load side of the RCD between the phase conductor of the protected circuit and the associated cpc. Any load or appliances should be disconnected prior to testing. RCD test instruments require a few milliamperes to operate; this is
normally obtained from the phase and neutral of the circuit under test. When testing a three-phase RCD protecting a three-wire circuit, the instrument’s neutral is required to be connected to earth. This means that the test current will be increased by the instrument supply current and will cause some devices to operate during the 50% test, possibly indicating an incorrect operating time. Under this circumstance it is necessary to check the operating parameters of the RCD with the manufacturer before failing the RCD.
Integral Test Device ( Functional Testing ) 612.13
An integral test device is incorporated in each RCD. This device enables the mechanical parts of the RCD to be
verified by pressing the button marked ‘ T’ or ‘Test’ :
Test Instrument
The test instrument used to test RCDs should be capable of applying the full range of test current to an in-service
accuracy, as given in BS EN 61557-6. This in-service reading accuracy will include the effects of voltage
variations around the nominal voltage of the tester. To check RCD operation and to minimise danger during the test, the test current
should be applied for no longer than 2s. Instruments conforming to BS EN 61557-6 will fulfil the above requirements.
● “ OLD RCDs “
General Purpose RCDs to BS-4293 & Protected Sockets-Outlets BS-7288 :
( 240V – 50Hz – 80A – Load BS-4293 RCD ) ↔ ( BS-Number : Only = 200mS ) 50% of operating current : Device should Not Operate ;
100% of Operating current : Device should Operate in Less than 200mS :
Where the RCD incorporates an intentional time delay it should trip within a time range from 50% of the rated time delay plus 200ms’
to 100 % of the rated time delay plus 200ms
● “ NEW RCDs “
General Purpose RCBOs BS-EN 61009-1 ↔ ( BS-EN = Only 300mS )
General Purpose RCCBs BS-EN 61008-1 ( BS-EN 61008-1 : 230V ( 80A – 30mA – 230V )
50% of operating current Device should Not operate :
100% of operating current Device should operate in less than 300mS :
unless it is of ‘Type S’ (or selective) which incorporates an intentional time delay. In this case, it should trip within a time range from 130ms to 500ms ( Appendix 3 , page - 243 )
Additional Protection : Test current at 5 I∆n : I∆n ≤ 30mA / Device should Operate in Less than ( 40mS )

What is an RCD and what does it do? “ Definitions “ p / 29
An RCD is defined, in BS 7671, as: ‘A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions’.

 

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“ 2392-10 “ 17th Edition of the IEE Wiring
Regulations on the 1st January 2008 has major implications for all electrical contractors, designers
and consultants. From 1st July 2008 all new electrical installations will have to be designed to comply with the new regulations : The biggest area of concern at present is the effect on an individual’s home being wired or re-wired.
there are five aspects that the 17th Edition identifies as requiring consideration for installations that are not under the supervision of skilled people (e.g. qualified electricians) or instructed people (e.g. facilities managers) – i.e. most domestic
installations,
● Socket outlets for general use in domestic installations must have the additional protection of an RCD Not exceeding 30mA (regulation 411.3.3 )
● All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA ( Regulation 701.411.3.3 )
● Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA ( Regulations 522.6.6–8 )
● To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD ( Regulation 314.1 )
● Separate circuits shall not be affected by the failure of other circuits ( Regulation 314.2 )
When designing the installation and selecting the correct assembly, the electrician will need to consider the above
five aspects, in accordance with the regulations. It is not solely the function of a particular consumer unit that has
one, two or three RCDs with an incomer isolator to solve the problem of meeting the 17th Edition regulations.
assembly to ensure that the installation complies with the 17th Edition regulations.
Everything starts ;
When designing an installation in a new build project, there are various options to consider, in order to satisfy
the requirements of the 17th Edition. For example, consider burying the cables more than 50mm into the
walls, or protect the cable with earthed metal, to remove the need for RCD protection. Once the wiring scheme has been finalised, the process of selecting how best to protect the circuits and the people can begin.
The regulations are open to interpretation, particularly regarding the division of installations, where acceptable levels of inconvenience resulting from a fault can be somewhat subjective. ( See page 11 for Regulations 314.1 and 314.2. )
● ( FCA ) Fully Compliant Assembly : ←
● ( PCA ) Partially Compliant Assembly : ←
● ( NCA ) Non-Compliant Assembly : potentially dangerous ; ←
to satisfy all aspects where an RCD has to be used for safety reasons but does not comply with all parts of regulation 314; a Non-Compliant Assembly is judged to satisfy neither safety aspects nor regulation 314. :
Customers are looking for a competitive offering, balanced with potential safety aspects and avoiding the
hazards and nuisance that a short circuit trip, overload condition or earth leakage may cause.
For “ example “ a consumer unit with an isolator and double RCD (see page 3 Example A) is often promoted as a way of meeting the 17th Edition regulations on the basis that every circuit is protected by an RCD, and the circuits split evenly between two RCDs. However, in the event of a fault on either set of MCBs, the RCD may trip as well. This creates an unwanted disconnection of the MCBs where no fault exists (contrary to section 314.1) and also causes unwanted disconnection of other circuits ( contrary to section 314.2 ).
A dual RCD split load board will meet the 17th Edition requirements for the following: :
● Socket outlets for general use in domestic installations must have the additional protection of an RCD not
exceeding 30mA.
● All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
● Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA :
However, depending on the installation design, it is unlikely to satisfy the regulations on:
*To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD. )
* Separate circuits shall not be affected by the failure of other circuits. In the event of a fault on either set of MCBs, the RCD may trip as well. This creates an unwanted disconnection of the MCBs where no fault exists (contrary to section
314.1) and also causes unwanted disconnection of other circuits (contrary to section 314.2 ).
Prior to the 17th Edition, lights have not been part of the RCD circuit as they tend to trip the RCD whenever an incandescent bulb fails, for example, causing unacceptable nuisance. Having the lights in the same circuit may cause nuisance disconnection to any other circuit supplied on the RCD. In addition there is a smoke alarm and a light circuit
on the same RCD, so every time the light circuit trips (when a bulb fails) the house and the alarm circuit could be disabled, putting the occupants at risk of a fire not being detected. The safety implications of this configuration by not
addressing the requirements of section 314 make this a Non-Compliant Assembly :

● ( PCA ) Partially Compliant Assembly : ←
A split load board with independent RCBOs will meet the17th Edition requirements for the following:
● Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
● All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
● Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by
appropriate earthed metal, must be protected by an RCD not exceeding 30mA :

However, depending on the installation design, whilst this layout takes into account the danger arising from the
failure of a single circuit (such as lighting) it does not fully comply with :
* To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
* Separate circuits shall not be affected by the failure of other circuits :
It may be advisable to have a socket circuit on a separate circuit. In larger residential properties it would be advisable to separate the downstairs circuit or the kitchen circuit from the other circuits supplied by a sole RCD to ensure that in the event of a fault there would be at least one power circuit available. Prior to the 17th Edition, it was common to have all
socket power circuits supplied from one RCD with no account of the implications for safety or significant inconvenience being apparent. However, the increasing number of electronic devices now found in a home results in a greater amount of earth leakage current that may lead to a tripping of the RCD and fall foul of 531.2.4.
( Note Also depending on the wiring scheme the smoke detector may not need to be an RCBO. )

Main switch with RCBOs on all circuits : ● ( FCA ) Fully Compliant Assembly : ←


A standard main switch disconnector controlled consumer unit with RCBOs for every outgoing circuit instead of the usual MCBs, will fully comply with the 17th Edition regulations. A fault on any circuit will not impact on other circuits, and so all aspects of the regulations are satisfied.:
● Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
● All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
● Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.
● To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
● Separate circuits shall not be affected by the failure of other circuits.

Main switch with RCBOs on critical circuits : ● ( FCA ) Fully Compliant Assembly : ←

By looking closely at the wiring scheme in an installation, the assembly can be made more cost effective by using MCBs to protect some circuits rather than RCBOs, without compromising compliance with the 17th Edition.
For example, at the point of cable entry to the consumer unit, incoming and outgoing cable runs are all encased in suitable ‘metal trunking’ or buried deeper than 50mm, so do not require RCD protection. As none of the cables are in the walls, the smoke alarms similarly do not require RCD protection. The burglar alarm system may have its own trunking or metal covering for the supply to the main control
console. If it does, and all the other devices are low voltage or cable runs in the ceiling, then this too could be MCB protected. This will depend on the wiring scheme employed for this ancillary piece of equipment. You could also decide that the immersion heater cable and cooker point (without socket) are in areas where they can be surface mounted and shrouded inside a short length of conduit, or buried deeper than 50mm in the wall.
This example illustrates that by analysing the wiring scheme, RCBOs can be replaced with MCBs, reducing the
costs whilst still providing a Fully Compliant Assembly ( FCA ).

 

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Split load board with RCBOs on critical circuits : ● ( PCA ) Partially Compliant Assembly : ←

By reviewing the wiring scheme employed with the split load assembly proposed the cost of the finished assembly can be reduced, and partial compliance with the regulations achieved. The split load board laid out here will meet the17th Edition requirements for the following:
● Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
● All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
● Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.:
However, depending on the installation design, it is unlikely to satisfy the regulations on:
* To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
* Separate circuits shall not be affected by the failure of other circuits. :
there is still the risk of one circuit failure impacting on another (regulation 314.2); however the level of inconvenience could be considered to be acceptable (only the sockets and showers are affected ), and no hazard or safety issues are inherent. Again, the installer will need to consider the amount of leakage in the installation, due to electronic devices in the house, and it may be a consideration to split the power sockets and have one of them on an RCBO.
The end result is that no base consumer unit exists that complies with the 17th Edition. The choice of
consumer unit and the configuration of devices within it can only be made after the wiring scheme has been finalised. :

Key extracts from 17th Edition of the IEE Wiring Regulations BS 7671 : 2008

314 Division of Installation
314.1 Every installation shall be divided into circuits, as necessary, to (i) avoid hazards and minimize inconvenience in the event of a fault(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that those circuits are not affected by the failure of other circuits, and due account shall be taken of the consequences of the operation of any single protective device.
411.3.3 Additional protection
In a.c. systems, additional protection by means of an RCD in accordance with Regulation 415.1 shall be provided fori) socket-outlets with a rated current not exceeding 20A that are for use by ordinary persons and are intended for general use.
531.2.4 An RCD shall be so selected and the electrical circuits so sub divided that any protective conductor current which may be expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device.
Section 701 Locations containing a bath or shower
701.411.3.3 Additional protection by RCDs
Additional protection shall be provided for all circuits of the location, by the use of one or more RCDs having the characteristics specified in Regulation 415.1.1 (30mA RCD)
( Note: see regulations 314.1 and 531.2.4. )

Note: There are exceptions when the socket outlets are used by skilled or instructed persons, but not relevant in residential property. For example 411.3.3 relates to socket outlets located anywhere in a home, including the socket in the cooker outlet. However RCD protection for the cooker outlet is required if any of the cables are buried in the wall and not deeper than 50mm, as indicated in 522.6

Fire Alarms :

The entire system should be tested to ensure that it operates satisfactorily and that, in particular,
automatic fire detectors and any manual call points function correctly when tested. Smoke detectors should
be smoke tested with a simulated smoke aerosol that will not damage the detector. Heat detectors should be
tested by means of a suitable heat source unless detector damage would otherwise result. The heat
source should not have the ability to cause a fire. A live flame should not be used.
It should be established that any interlinking works and that sounders operate correctly.
Manufacturer’s tests should be carried out.

Certification :
A certificate should have been issued to the user and this should be available for inspection. For Grade F
systems a certificate should be issued if installed by a professional installer.

User instructions :
The supplier of the fire alarm system should provide the user with operating instructions, which should be
sufficient to enable a lay person to understand, operate and maintain the system. Silencing and disablement facilities should be explained but it should be stressed that system readiness must not be compromised. Recommended action in the event of a
fire must stress the importance of all occupants leaving the building as quickly as possible and that
the fire service is summoned immediately regardless

Routine testing and maintenance :
instructions to users must stress the importance of routine testing. The system should be tested weekly by
pushing the test button. If the dwelling has been unoccupied for a period during which the supply (yes)
could have failed, the occupier should check that the system has not suffered total power failure and is still operable.

Maintenance :
Smoke alarms in Grade D, E and F systems should be cleaned periodically in accordance with the
manufacturer’s instructions. Where experience shows that undue deposits of dust and dirt are likely to
accumulate, so affecting the performance of the system before detectors are cleaned or changed, more frequent
cleaning or changing should be carried out.

Commissioning :

The system should be inspected. Electrical tests made to the mains supply circuit
should include earth continuity, polarity, and earth fault loop impedance. Insulation tests should be made
of all installed cables as required by BS 7671. Electronic equipment should be disconnected to avoid damage.

Supply to a Grade E system where the installation forms part of a TT system.
The 100 mA time-delayed RCD provides protection for the fire alarm system ( and other circuits )
and operates independently of the RCD protection for the socket-outlets


main switch
( 100 mA time delayed RCD )
S-type, double pole to BS EN 61008 :

interconnected by wiring should be connected on a single final circuit. Note that certain alarms are radio linked and such alarms
need not be on the same final circuit ,
Wiring systems :
All cables should be selected and installed in accordance with the requirements of BS 7671 and the recommendations of BS 5839-6.

“ RCCB “

Residual Current Circuit Breaker (RCCB). This is a term that does not appear in the current wiring regs, and does not have a consistent definition or usage. Some manufacturers use it to differentiate RCDs without overcurrent protection from those with it ( i.e. RCBOs ).
Nuisance trips :
A Nuisance trip is an unexpected operation of a RCD that does not appear to be related to an immediately obvious fault. There can be many reasons that these trips occur, some indicate that there is a latent problem with the electrical installation, some may indicate the presence of a serious but as yet unobserved fault, and others may be the result of a minor fault that in itself poses little or no risk ,
Tracing the cause of nuisance tripping can prove to be very difficult and time consuming.

What causes nuisance trips :
Using the wrong busbar
If you have a new circuit that trips the moment you attempt to draw power from it, the most likely cause is common wiring mistake in the CU. Split load CUs will have two or more sections, with a dedicated neutral bus bar for each. If you connect the live of a circuit to a MCB on a section of the CU protected by an RCD, but return the neutral to a bus bar not associated with that RCD, you will get an immediate trip sine the RCD can only "see" one half of the current flow. The same logic applies if using a RCBO, then the neutral for the circuit must be returned to the neutral connection on the RCBO ( and the RCBO's flying neutral wire in turn connected to the appropriate neutral bus bar in the CU).
Excess earth leakage
The RCDs operating principle is to measure the current imbalance between that flowing into and out of a circuit down live and neutral wires. In an ideal world the current difference would be zero, however in the real world there are a various different types of equipment that will legitimately have a small amount of leakage to earth, even operating normally. If the RCD is protecting too many such devices then it is possible that the cumulative result of all these small leakages will be enough to either
● trip the RCD
● or by passing most of the RCD's trip threshold current, make the RCD excessively sensitive to any additional leakage currents

 

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“ Domestic RCD “
This is a deprecated way of installing an RCD such that a single low trip threshold device ( typically 30mA ) protects all the circuits in a property. While counter to the advice given in the present wiring regulations. installations of this type are still commonly found. Whole house RCDs are very vulnerable to nuisance trips, and any such trips remove all power to the property
Wiring faults :
Fault : Neutral to Earth shorts ,
Mechanism :
A particularly problematic fault is a short between neutral and earth on a circuit. Since Neutral and earth are nominally going to be at a similar potential (especially in buildings with TN-C-S / PME . earthing
You can arrive at a situation where the current flow between neutral and earth is lower than the trip threshold of the RCD some of the time, however once the neutral current reaches a high enough level, its potential will be "pulled" away from that of the earth, and you get increased leakage current flow which may cause a trip. Needless to say this threshold will often be reached during transient current peaks caused by equipment being switched on or off.

Insulation breakdown or damage :
As cables and wires age, their Insulation can become less effective .

Faulty RCD :
One obvious possibility (and often overlooked) is that the RCD itself is actually faulty and not tripping at the correct current. A RCD that refuses to reset even when all output connections are removed is an obvious candidate for landfill. Swapping the device with a known good one, or using a RCD test facility , are other ways of finding faulty RCDs. Many RCDs include a "test" button that verifies the unit functions. This simulates a imbalance current internally, which causes the device to operate. Note however that because the test current may be several times the trip threshold, it does not test if the trip threshold has drifted too low or the mechanism has become slow - only that the trip detection and basic mechanics still work.
How to locate the cause of nuisance trips :
There are a number of empirical tests or experiments that you can try to narrow down the source of the problem. We cover some here. The first job is to identify which circuits the RCD is protecting. There is no need to concentrate efforts on examining circuits that are not connected and hence can not be affecting the outcome!
“ Techniques to try “
Do what : Turn off circuits in turn ,
Why : You may be able to identify which circuit is causing the problem by isolating circuits in turn, and seeing which prevents the trip from reoccurring ,

“ Remove appliances from suspect circuits “

Disconnecting appliances from suspect circuits can let you identify if the fault is in an appliance (the most common situation) or the circuits fixed wiring. If you still get trips with everything disconnected then you may have a wiring fault.
If it looks like appliances are to blame, you can apply the "binary chop" principle to narrow down the field quickly - i.e. unplug half of them and see what happens. If it still trips you know in which half the dodgy appliance probably is. The carry on in the same way - halving the list of remaining suspects, until you get close to the answer. (This method isn't bulletproof with RCDs.)
“ Check the likely culprits “
Identifying which appliances you have from the "high risk" categories listed above can help to take you to the cause of the trouble faster :

“ Check the likely culprits “
Identifying which appliances you have from the "high risk" categories listed above can help to take you to the cause of the trouble faster :

“ Identify coincidental factors “
Check for any patterns and relationships between trips and other events. Do they occur only in Damp
weather, or only at certain times of day, or only when the Freezer switches on, or the Central Heating
Pay particular attention to automated systems ( timers, thermostats etc ) that can be controlling significant bits of electrical equipment in your home without your manual intervention .

Notes on Schedule of Test Results ( 2392-10 ) Nice Wording for 2391 ←←

* (1) Type of Supply : is Ascertained from the Distributor or by Inspection ,
* (2) (Ze) at Origin : when the Maximum Value Declared by the Distributor is Used , the Effectiveness of the Earth must be Confirmed by Test , if Measured the Main Bonding will need to be Disconnected for the Duration of the Test ,

* (3) Prospective Fault Current ( PFC ) the Value Recorded is the Greater of Either the Short-Circuit Current or the Earth Fault Current
Preferably Determined by Enquiry of the Distributor ,

* (4) Short-Circuit Capacity : of the Device is Noted , see table 7.2 - OSG – table 2.4 – GN3
The Following Tests , where Relevant , shall be Carried Out in the following Sequence :
Continuity of Protective Conductors , including Main & Supplementary Bonding .
Every Protective Conductor , including Main & Supplementary Bonding Conductors , should be Tested to Verify that it is Continuous and Correctly Connected
* (6) Continuity :
Where Test Method (1) is Used , Enter the Measured Résistance of the Line Conductor Plus the Circuit Protective Conductor ( R1 + R2 )
See 10.3.1 – OSG – 2.7.5. – GN3 : During the Continuity Testing ( Test Method 1 ) the following Checks are to be Carried Out :
(a) Every Fuse and Single-Pole Control and Protective Device is Connected in the Line Conductor Only
(b) Centre- Contact Bayonet & Edison Screw Lampholders have Outer Contact Connected to Neutral Conductor
(c) Wiring is Correctly Connected to Socket-Outlet & Similar Accessories , Compliance is to be Indicated by a Tick in Polarity Colum ( 11 ) ,
( R1 + R2 ) need Not be Recorded if ( R2 ) in Column ( 7 )
* (7) Where Test Method (2) is Used , the Maximum Value of ( R2 ) is Recorded in Column ( 7)
See 10.3.1 – OSG – 2.7.5 – GN3
* (8) Continuity of the Ring-Final Circuit Conductors
A Test shall be Made to Verify the Continuity of Each Conductor Including the Protective Conductor of Every Ring-Final Circuit
See 10.3.2 – OSG – 2.7.6.- GN3
* (9) * (10) Insulation Résistance :
All Voltage Sensitive Devices to be Disconnected or Test Between Live Conductors’ ( Line & Neutral ) Connected together & Earth , The Insulation Résistance between Live Conductors’ is to be Inserted in Column ( 9)
The Minimum Insulation Résistance Values are Given tables : 10.1 – OSG – 2.2 – GN3 :
( All the Preceding Tests should be Carried Out before the Installation is Energised )
* (11) Polarity :
A Satisfactory Polarity Test may be Indicated by a Tick in Column ( 11 )
Only in a Schedule of Test Results Associated with a Periodic Inspection Report is it Acceptable to Record Incorrect Polarity ,
* (12) Earth Fault Loop Impedance ( Zs )
This may be Determined Either by direct Measurement at the Further Point of a Live Circuit or by Adding ( R1 + R2 ) of Column
6 to ( Ze . Zs ) is Determined by Measurement at the Origin of the Installation or Preferably the Value Declared by the Supply Company Used , ( Zs = Ze + ( R1 + R2 ) – Zs should be Less than the Values given in Appendix 2 – OSG or Appx 2 – GN3
* (13) Functional Testing :
The Operation of RCDs ( including RCBOs ) shall be Tested by Simulating a Fault Condition , Independent of any Test Facility in the Device . Record Operating time Column (13) Effectiveness of the Test button must be Confirmed ,
See : Section (11) – OSG or 2.7.15 / 2.7.18 – GN3 ,
* (14) All Switchgear & Controlgear Assemblies , Drivers , Control & Interlocks, etc must be Operated to ensure that they are Properly Mounted , Adjusted , and Installed : Satisfactory Operation is Indicated by a tick in Column (14) ,
( Earth Electrode Resistance )
The Earth Electrode Resistance of ( TT ) Installations must be Measured , and Normally an RCD is Required ,
For Reliability in Service the Resistance of any Earth Electrode should be below ( 200Ω ) Record the Value on form 1- 2 - 6 ,
As Appropriate , see 10.3.5 – OSG or 2.7.12 – GN3

 

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“ Isolation of individual circuits”

Where it is not practical to isolate a distribution board, individual circuits supplied from it can be isolated
by one of the methods described below, depending on the type of protective device used. However, bear
in mind the overriding advice to avoid energising any outgoing electrical distribution services, preferably
until the distribution switchgear and all connected circuits are complete and have been inspected and the
relevant tests carried out.
If any items required to carry out the procedures recommended below are not manufactured for the DB in
question or cannot be obtained through retail/trade outlets, it is acceptable to disconnect the circuit from
the DB as long as the disconnected tails are made safe by being coiled or insulated. Suitable labelling of the
disconnected conductors is important to prevent the supply being re-instated, particularly if other
electricians are present.
It should be remembered that work carried out inside a live DB is regarded as live working when there is
access to exposed live conductors. In this case the appropriate precautions should be taken as described in
HSG85 with respect to Regulation 14 of the Electricity at Work Regulations.
i. Isolation of individual circuits protected by circuit breakers
Where circuit breakers are used the relevant device should be locked-off using an appropriate locking-off
clip with a padlock which can only be opened by a unique key or combination. The key or combination
should be retained by the person carrying out the work.
Note
Some DBs are manufactured with ‘Slider Switches’ to disconnect the circuit from the live side of the circuit
breaker. These devices should not be relied upon as the only means of isolation for circuits as the wrong
switch could easily be operated on completion of the work.
ii. Isolation of individual circuits protected by fuses
Where fuses are used, the simple removal of the fuse is an acceptable means of disconnection. Where
removal of the fuse exposes live terminals that can be touched, the incoming supply to the fuse will need to
be isolated. To prevent the fuse being replaced by others, the fuse should be retained by the person carrying
out the work, and a lockable fuse insert with a padlock should be fitted as above. A caution notice should
also be used to deter inadvertent replacement of a spare fuse. In addition, it is recommended that the
enclosure is locked to prevent access as stated above under ‘Isolation using a main switch or distribution
board (DB) switch-disconnector’.
Note
In TT systems, the incoming neutral conductor cannot reliably be regarded as being at earth potential.
This means that for TT supplies, a multi-pole switching device which disconnects the phase and neutral
conductors must be used as the means of isolation. For similar reasons, in IT systems all poles of the
supply must be disconnected. Single pole isolation in these circumstances is not acceptable.

“ Electrical Permits-to-Work “
An electrical permit-to-work must be used for work on HV systems that have been made dead, and can
be useful in certain situations for LV work. These permits are primarily a statement that a circuit or item
of equipment is isolated and safe to work on. They must not be used for live working as this can cause confusion..

“ Caution Notices “
In all instances where there is a foreseeable risk that the supply could be reinstated as above, an
appropriate “caution” notice should be placed at the point of isolation. For DBs with ‘multiple
isolations’ a single suitably worded notice on each DB, such as the example shown below, would suffice:

* CAUTION: THIS DISTRIBUTION BOARD HAS A NUMBER OF CIRCUITS THAT ARE
SEPARATELY ISOLATED. CARE SHOULD BE TAKEN WHEN REINSTATING THE
SUPPLY TO AN INDIVIDUAL CIRCUIT THAT IT HAS BEEN CORRECTLY IDENTIFIED.

Question 1 :
State the necessary action that should be taken by an inspector on discovering a damaged socket outlet with exposed live parts during a periodic inspection and test :
GN-3 ( Required Competence )
Make an immediate recommendation to the client to isolate the defective part :
Question 2 :
State the documentation that should accompany an Installation Certificate or Periodic Inspection Report :
GN-3 ( Certificates & Reports )
1. Schedule of items inspected :
2. Schedule of test results :
Question 3 :
Why is it necessary to undertake an initial verification ?
GN-3 ( Initial Verification ) G
1. Confirm that installation complies with designers intentions :
2. Constructed, inspected and tested to BS- 7671:2008 :
Question 4 :
State the requirements of Part 6 of BS- 7671 with regard to initial verification :
GN-3 ( Initial Verification )
1. All equipment and material complies with applicable British Standards or acceptable equivalents :
2. All parts of the installation are correctly selected and erected :
3. Not visibly damaged or defective :
Question 5 :
Identify FIVE non-statutory documents that a person undertaking an inspection and test need to refer to
General Knowledge
1. BS -7671:2008 :
2. IEE On-Site Guide :
3. GS 38 :
4. Guidance Note 3 :
5. Memorandum of Guidance to The Electricity at Work Regulations :
Question 6 :
Which non-statutory document recommends records of all maintenance including test results be kept throughout the life of an installation ?
GN-3 ( Initial Verification )
Memorandum of Guidance to The Electricity at Work Regulations (Regulation 4(2) :
Question 7 :
Appendix 6 of BS 7671 allows the use of three forms for the initial certification of a new installation or for an alteration or an addition to an existing installation. State the title given each of these certificates :
GN-3 ( Initial Verification )
1. Multiple signature Electrical Installation Cert :
2. Single signature Electrical Installation Cert :
3. Minor Electrical Installation Works Certificate :
Question 8 :
Under what circumstances would it be appropriate to issue a single signature Electrical Installation Certificate ?
GN-3 ( Certificates )
Where design, construction inspection and testing is the responsibility of one person :
Question 9 :
State the information that should be made available to the inspector:
GN-3 ( Required Information )
1. Maximum demand :
2. Number and type of live conductors at the origin :
3. Type of earthing arrangements :
4. Nominal voltage and supply frequency :
5. Prospective fault current ( PFC ) :
6. External Impedance Ze :
7. Type and rating of overcurrent device at the origin :
Question 9 ( cont,d ) :
The following information should also be made available :
GN-3 ( Required Information )
1 Type and composition of circuits, including points of utilisation, number and size of conductors and type of cable :
2. Methods of compliance for indirect shock protection :
3. Identification and location of devices used for protection, isolation and switching :
4. Circuits or equipment vulnerable to testing :
Question 10 :
Where should the proposed interval between periodic inspections should be noted :
GN-3 ( Frequency )
1. On the Electrical Installation Certificate :
2. On a notice fixed in a prominent position at or near the origin of an installation :
Question 11 :
State in the Correct Sequence the first FIVE tests that would need to be undertaken on an A1 ring circuit during an initial verification :
1. Continuity of protective conductors :
2. Continuity of ring final circuit conductors :
3. Insulation resistance :
4. Polarity :
5. Impedance Zs :
Question 12 :
State TWO disadvantages of using Method 2 in order to verify the continuity of c.p.c.’s :
GN-3 ( Test Method 2 )
1. Long wander lead :
2. Gives R :
does not provide R :

 

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Question 14 :
State the British Standard number for a transformer used to provide electrical separation :+
GN-3 ( Electrical Separation )
Transformer complies with BS 3535 , Note: Transformer double-wound type :
Question 15 :
List FOUR types of external influence that affect the safety/operation of an electrical installation :
GN-3 ( Electrical Separation )
1. Ambient temperature :
2. Heat :
3. Water :
4. Corrosion :
Question 16 :
Identify the TWO procedures required when verifying the continuity of a ferrous enclosure used as a c.p.c. for a circuit
GN-3 ( Test Method 2 )
1. Inspect the enclosure throughout its length :
2. Carry out low resistance ohmmeter test :
Question 17 :
State in the Correct Sequence the Test required to verify the continuity of a ring final circuit :
GN-3 ( Continuity of Ring Circuit )
1. Identify and measure the resistance of each ring (end to end) r
1 / r :
2 / rn :
2. Apply figure of 8 (cross connection) between phase and neutral conductors at distribution board and then measure resistance between phase/neutral at each socket outlet :
Question 17 : ( con,d )
GN-3 ( Continuity of Ring Circuit )
3. Apply figure of 8 (cross connection) between phase and cpc at origin and measure resistance between phase and cpc at each socket outlet :
Note: where dead Tests are made the supply must be isolated before any work commences:
Question 18 :
The following measurements were taken at the origin of an A :
1 ring circuit. r 1 = 0.4Ω :
2 = 0.67Ω R n = 0.4Ω :
Determine the measured value of resistance at each socket outlet when the ends of the circuit are cross-connected to form a figure 8 :
GN-3 ( Continuity of Ring Circuit )
1. r , 1 + R n = 0.4 + 0.4 = 0.8/4 = 0.2Ω ;
2. r , 1 + r 2 = 0.4 + 0.67 = 1.07/4 = 0.267Ω :
Question 19 :
Identify ONE other test that is automatically performed when undertaking a ring final circuit test :
GN-3 ( Continuity of Ring Final Circuit )
Polarity :
Question 20 :
State FOUR items of equipment/components that may need to be removed prior to carrying out a test for insulation resistance on a circuit :
GN-3 ( Insulation Résistance )
1. Pilot or indicator lamps :
2. Dimmer switches :
3. Touch switches :
4. Electronic r.c.d.’s etc :
Question 21 :
State the test voltage and minimum acceptable value of insulation resistance for the following circuits :
1. 400V 3 phase motor
2. 760V discharge lighting circuit
3. 45V FELV circuit
GN-3 ( table – 61 )
1. 500V d.c. 0.1 MΩ
2. 1000V d.c. 1.0 M
3. 500V d.c. 0.5 M
Question 22 :
State the Correct Sequence for undertaking an insulation resistance test on a filament lamp circuit containing two-way switching :
GN-3 ( Insulation Résistance Testing )
1. Supply must be isolated : ← ← ←
2. All lamps removed :
3. Insulation test between live conductor :
4. Insulation resistance test between live conductors and earth :
5. Two-way switches operated during test
Question 23 :
State the type of test that should be applied where protection against direct contact is by site-applied insulation :
GN-3 ( Site applied Insulation )
1. Test at 3750V a.c :.
2. Apply test voltage for 60 seconds during which time insulation failure or flashover should not occur :
3. Instrument used: Site applied insulation3. Instrument used: Site applied insulation :
Question 24 :
State the THREE specific requirements for verification of polarity with regard to accessories : 612.6 ,
1. All single-pole devices are connected in the phase conductor
2. The centre contact of Edison screw lamps are connected in the phase conductor
3. All socket outlets : wiring has been correctly connected to socket-outlets and similar accessories :
Question 25 :
Identify the test that should be applied to verify polarity after the supply is energised :
GN-3 ( Polarity )
Test to verify correct polarity of the incoming live supply (PES ). Test made at the origin using approved voltage indicator :

Question 26 :
Identify the THREE electrodes used when used with a proprietary earth resistance tester:
GN-3 ( Earth electrode resistance )
1. Main electrode :
2. Potential electrode ( auxiliary electrode ) :
3. Current electrode ( auxiliary electrode ) :
Note: This method can be use for electrodes used for transformers, lightning protection systems etc.:
Question 27 :
State the action to be taken regarding the earthing conductor before measuring the resistance of an earth electrode
GN-3 ( Earth electrode for RCD’s )
Normal 100Ω ← ← ←
Special locations 50Ω
By calculation 50V ÷ 0.5A = 100Ω
Dry 25V ÷ 0.5A = 50Ω
Special Loc,
Question 27 :
State the action to be taken regarding the earthing conductor before measuring the resistance of an earth electrode :
GN-3 ( Earth electrode resistance )
1. Disconnect earthing conductor at MET to avoid parallel earth paths :
2. Do NOT disconnect any protective conductors before isolating the supply :
Question 28 :
State the maximum recommended value of resistance for an earth electrode :
GN-3 ( Earth electrode for RCD’s )
Electrodes having resistances in excess of 200Ω :
will require further investigation.
Note: Electrode resistances obtained in excess of 200Ω :
may indicate unstable soil conditions :
Question 29 :
State the formula used to calculate Impedance
Zs, at the furthest point within a circuit
Zs = Ze + ( R1+R2 ) Where Ze is by measurement or enquiry
and ( R1+R2) by measurement
State the formula used to calculate Impedance Zs, at the furthest point within a circuit :
GN-3 ( Earth Fault Loop )
( Zs = Ze + ( R1+R2 )
Where Ze is by measurement or enquiry and ( R2 ) by measurement :
Question 30 :
State TWO reasons why it is necessary to measure external earth fault loop impedance at the origin of an installation :
GN-3 ( Determining Ze )
1. To verify an earth connection :
2. The value is equal to or less than the value determined by the designer :
Question 31 :
State TWO methods by which the impedance of a circuit may be obtained without operating any r.c.d.’s protecting the circuit :
GN-3 ( Residual current devices )
1. Soft test ( 15mA )
2. By calculation Zs = Ze + ( R1 + R2 )
Question 32 :
Determine the prospective fault current given following information ( Three phase supply 1. Impedance between P and N = 0.25Ω :
2. Impedance between P and E = 0.5Ω :
( General knowledge )
1. 230V ( Uo ) ÷ 0.25 = 920A = 0.92kA
2. 230V ÷ 0.5 = 460A = 0.46kA
3. For three phase multiply P to N value by 2 ( 0.92 x 2 = 1.84kA )
Question 33 :
State the reason for undertaking a prospective fault current measurement at the distribution board at the origin of the installation :
( Prospective fault current )
1. To ensure the adequate breaking capacity of the overcurrent devices :
2. To ensure the adequate breaking capacity of the main switch :
Question 34 :
State the three required electrical Tests required to be undertaken on a 30mA r.c.d. complying with BS 4293
( Functional testing ) Old BS- Only – 200mS
1. 1/2 test - 15mA for 2 seconds - device does not trip ;
2. 1 x test - device tested at full rated current trips within 200mS ( 0.2 seconds ) :
3. 5 x test when tested at 150mS device operates within 40mS :
Question 35 :
State FIVE items of electrical equipment that would require functional testing
( Functional checks )
1. R.c.d.’s :
2. Circuit breakers :
3. Isolators :
4. Interlocks :
5. Switches :

 

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Basic applications MCBs : Apprentice ,
The essential distinction between Type B, C or D devices is based on their ability to handle surge currents without tripping. These are, typically, inrush currents associated with fluorescent and other forms of discharge lighting, induction motors, battery charging equipment etc. BS 7671 specifically refers to Types B and C, and the choice will normally be between these two types :
• Type B devices are generally suitable for domestic applications. They may also be used in light commercial applications where switching surges are low or non-existent.
• Type C devices are the normal choice for commercial and industrial applications where fluorescent lighting, motors etc. are in use.
• Type D devices have more limited applications, normally in industrial use where high inrush currents may be expected. Examples include large battery charging systems, winding motors, transformers, X-ray machines and some types of discharge lighting :
The classification of Types B, C or D is based on the fault current rating at which magnetic operation occurs to provide short time protection ( typically less than 100ms ) against short-circuits. It is important that equipment having high inrush currents should not cause the circuit-breaker to trip unnecessarily, and yet the device should trip in the event of a short-circuit current that could damage the circuit cables :
The tripping characteristics
• Type B devices are designed to trip at fault currents of 3-5 times rated current (In). For example a 10A device will trip at 30-50A.
• Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device ).
• Type D devices are designed to trip at 10-20 times In (100-200A for a 10A device ).
Normal cable ratings relate to continuous service under specified installation conditions. Cables will, of course, carry higher currents for a short time without suffering permanent damage. Type B and C circuit breakers can generally be selected to achieve tripping times that will protect the circuit conductors against normal surge currents in accordance with BS 7671. This is more difficult to achieve with Type D devices, which may require a lower earth loop impedance (Zs) to achieve the operating times required by Regulation 411.4.7 / 411.3.2.3 -
Overcoming unwanted tripping:
Sometimes failure of tungsten filament lamps can trip Type B circuit-breakers in domestic and retail environments. This is caused by high arcing currents occurring at the time of failure and is generally associated with inferior quality lamps. If possible the user should be encouraged to use better quality lamps. If the problem persists then one of the measures listed below should be considered :
A Type C device may be substituted for a Type B device where unwanted tripping persists, especially in commercial applications. Alternatively it may be possible to use a higher rated Type B MCB, say 10A rather than 6A. Whichever solution is adopted, the installation must be in accordance with BS 7671 :
A change from Type C to Type D devices should only be taken after careful consideration of the installation conditions, in particular the operating times required by Regulation 411.4.5 :
Other considerations:
Combined overcurrent and residual current circuit breakers (RCBOs) are available as integrated units or, in one case, as a modular device with a field-fittable clip-on RCD 'pod'. It should be borne in mind that if an RCBO trips it is not always clear whether tripping has been caused by an overcurrent or a residual current. Type B devices should only be used in domestic situations where high inrush currents are unlikely and Type C devices should be used in all other situations.

Short Circuit Capacity: Basic Calculations and Transformer Sizing : ( kVA ) ,

Short circuit capacity calculation is used for many applications: sizing of transformers, selecting the interrupting capacity ratings of circuit breakers and fuses, determining if a line reactor is required for use with a variable frequency drive, etc.
The purpose of the presentation is to gain a basic understanding of short circuit capacity. The application example utilizes transformer sizing for motor loads ,
Conductor impedances and their associated voltage drop are ignored not only to present a simplified illustration, but also to provide a method of approximation by which a plant engineer, electrician or production manager will be able to either evaluate a new application
or review an existing application problem and resolve the matter quickly ,

The following calculations will determine the extra kVA capacity required for a three phase transformer that is used to feed a single three phase motor that is started with full voltage applied to its terminals, or, "across-the-line."

Two transformers will be discussed, the first having an unlimited short circuit kVA capacity available at its primary terminals, and the second having a much lower input short circuit capacity available ,

kVA of a single phase transformer = V x A
kVA of a three phase transformer = V x A x 1.732, where 1.732 = the square root of 3.
The square root of 3 is introduced for the reason that, in a three phase system,
the phases are 120 degrees apart and, therefore, can not be added arithmetically They will add algebraically,

Transformer Connected To Utility Power Line ,

The first transformer is rated 1000 kVA, 480 secondary volts, 5.75% impedance. Rated full load amp output of the transformer is ,

1000 kVA / (480 x 1.732) = 1203 amps :

The 5.75% impedance rating indicates that 1203 amps will flow in the secondary if the secondary is short circuited line to line and the primary voltage is raised from zero volts to a point at which 5.75% of 480 volts, or, 27.6 volts, appears at the secondary terminals. Therefore, the impedance (Z) of the transformer secondary may now be calculated ,

Z = V / I = 27.6 volts / 1203 amps = .02294 ohms :

The transformer is connected directly to the utility power lines which we will assume are capable of supplying the transformer with an unlimited short circuit kVA capacity. The utility company will always determine and advise of the short

With unlimited short circuit kVA available from the utility, the short circuit amperage capacity which the transformer can deliver from its secondary is

480 volts / .02294 = 20,924 amps :

An alternative method of calculating short circuit capacity for the above transformer is:

1203 amps x 100 / 5.75% = 1203 / .0575 = 20,922 amps :

transformer and the value of the short circuit capacity The short circuit capacity is given as 20,900 amps.

Now we are ready to apply a motor to the terminals of the transformer secondary. We must determine the voltage drop which will be
We must determine the voltage drop which will be caused by the motor inrush on start. If the voltage remains within the rated voltage of the motor, then no oversizing ,

of the transformer is required. Motors rated for 460 volts are for use with distribution systems that are rated at
480 volts. The rating system allows a twenty volt drop in the distribution system which may occur along the feeder cables which connect the power transformer to the load.
The NEMA specification for a standard motor is that it requires the motor to be capable of operating at plus or minus 10% of nameplate voltage. Therefore, the voltage drop on inrush should not be allowed to drop below 460 volts less 10%, or, 414 volts ,

The transformer will be asked to supply power to a motor which has a full load amp rating of 1203 amps, which will fully load the transformer. Therefore, we will rate the motor at 460 V x 1203 A x 1.732, or, 958.5 kVA. We will assume that our motor will have an inrush of 600% of its full load rating which will cause an inrush of The transformer will be asked to supply power to a motor which has a full load amp
rating of 1203 amps, which will fully load the transformer. Therefore, we will rate the motor at 460 V x 1203 A x 1.732, or, 958.5 kVA. We will assume that our motor will have an inrush of 600% of its full load rating which will cause an inrush of ,

460 V x 1203 A x 600% x 1.732 = 5751 kVA :

The voltage drop at the transformer terminals will be proportional to the motor load. The voltage drop will be expressed as a percentage of the inrush motor load compared to the maximum capability of the transformer. [2] The transformer has a maximum kVA capacity at its short circuit capability, which is ,

480 V x 20,924 A x 1.732 = 17,395 kVA The voltage drop on motor inrush will be
5751 kVA / 17,395 kVA = .331, or, 33.1% :

The transformer output voltage will drop to 480 x .669, or, 321 volts. Thus, we can see that the transformer is much too small to use a motor that has a full load rating equal to the full load capacity of the transformer

The transformer must be sized so that its short circuit capabilty is equal to or greater than 5751 kVA times 10, or, 57,510 kVA in order to have a voltage drop of 10% or less. Therefore, the short circuit amperage capacity of the transformer to be used must be a minimum of ,



The transformer must be sized so that its short circuit capabilty is equal to or greater than 5751 kVA times 10, or, 57,510 kVA in order to have a voltage drop of 10% or less. Therefore, the short circuit amperage capacity of the transformer to be used must be a minimum of ,

57,510 kVA / (480 V x 1.732) = 69176 amps