Re-take - Useful Information for 2394 :

[amberleaf]

Based on Learning curve only .

Ambient temperature multipliers :book:
O.S.G. Expected ambient temperature (°C) & Multiplier values to Table 11
GN-3 . Expected ambient temperature (°C) (α) to table B1

As divider values GN-3 . table B2 . 0°C to 40°C
As divider values O.S.G. table 12 . 5°C , 10°C , 15°C , 20°C , 25°C .

This is because the table is intended to be used to calculate the résistance value of a conductor when you know the temperature that it is going to operate at.

In your case here , you are going to measure the temperature of the room, and calculate the résistance of the cable back to what it would be at 20°C.
Ambient temperature multipliers to be applied to R[SUP]1[/SUP] + R[SUP]2 [/SUP]values are shown in O.S.G. table 12

Example . You have measured an R[SUP]1[/SUP] + R[SUP]2 [/SUP]value of 0.84Ω
measured ambient temperature at 25°C .. R[SUP]1[/SUP] + R[SUP]2 [/SUP] ----- temp factor = the value of résistance at 20°C .

O.S.G. Table 12 @ 25°C - 1.02 (Cƒ*)
0.84Ω ÷ 1.02 = 0.82Ω résistance of the cable at 20°C

Having corrected the measured value to what it would be at 20°C the next step is to calculate what the résistance of the cable would be at its operating temperature .

O.S.G. table 13 .
Multipliers to be applied to table 11 to calculate conductor résistance at maximum operating temperature .
This is where you use the (1.2) multiplier
The résistance of the cable at its maximum operating temperature of 70°C .. 0.82 x (1.2) = 0.98Ω

This value can now be added to the measured Ze to provide a value of Zs , Which can be compared directly to the maximum Zs values provided in Chapter 41 of BS-7671: :icon_bs:

 

[amberleaf]

Useful junk .

The “ Add up method ”
When the fifteen-edition of the regulations appeared in 1981. :book:

It carried a lot of ((New & Controversial thinking)) One of the equations that appeared

Zs = Ze + ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )

In theory : BS-7671 is a design book . ( Regulation’s ) Earth fault loop impedance is given by: Zs = Ze + ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )

 

[Richard Burns]

For Amberleaf:
Learning curve
Resistances in parallel


These are resistances joined like the rungs of a ladder. Here the total resistance decreases the more resistances there are. The overall resistance of two or more conductors will also decrease if they are connected in parallel


The insulation between conductors is in fact countless millions of very high value resistances in parallel.
Hence an increase in cable length results in a decrease in insulation resistance. This value is measured in millions of ohms, i.e. megohms (MΩ).


The total resistance will be half of either one and would be the same as the resistance of a 2.0mm2 conductor. Hence resistance decreases if the conductor’s cross sectional area increases.


1 / R total = 1/R1 + 1/R2 + 1/R3 + 1/R4 . ( 1/R total 1/3 + 1/6 + 1/8 + 1/2 )




R1 ÷ 3 = 0.333333333 . R1 ÷ 6 = 0.166666666 . R1 ÷ 8 = 0.125 . R1 ÷ 2 = 0.5 . ( total = 1 ÷ 1.125 ) .. R total = 0.89 , round up.

 

[amberleaf]

To describe why the requirement for equipotential bonding is so important.

What is earth and why and how we connect to it ?

The thin layer of material which covers our planet - rock, clay, chalk or whatever - is what we in the world of electricity refer to as earth. So, why do we need to connect anything to it? After all, it is not as if earth is a good conductor.

The simple answer is, in our case, the definition of Earth, i.e. the conductive mass of earth, whose electric potential at any point is conventionally defined as zero (0 V).

This allows us to assign a standard reference for a potential difference. This is exactly what it says it is: a difference in potential (volts). In this way, two conductors having PDs of, say, 20V and 26V have a PD between them of 26 - 20 = 6V
The original PDs (i.e. 20V and 26V) are the PDs between 20V and 0V and 26V and 0V. This 0V or zero potential comes from Earth.

Thus, if we connect a T5-1000 electrical tester between a live part (e.g. the line conductor of a socket outlet) and earth, we will read 230V; the conductor is at 230V and the earth at zero. The earth provides a path to complete the circuit.

If the person were touching a faulty appliance (at 230V) and a gas or water pipe (at 0V) under fault condition’s there would be a potential difference (230V) that would lead to an electric shock.

One method of providing some measure of protection against these effects is to join together (equipotential bonding) all metallic parts that are at earth potential and connect them to the electrical point of earthing (main earth terminal).

Equipotential bonding: :13:
Electrical connection maintaining various exposed-conductive-parts & extraneous-conductive-parts at substantially the same potential.

This ensures that all metalwork in a installation is at or near 0V and, during a fault, all metalwork and the earthing of the installation will rise to a similar potential.

The intent is to ensure that the difference in potential between parts is less than 50V (the “safe” touch voltage, Ut).
So, simultaneous contact with two such metal parts would not result in a dangerous shock, as there would be no significant potential difference (PD) between them.

In summary, connecting metalwork to the installation earthing (bonding) means that during fault conditions it places all parts of the installation at a similar potential (equipotential).

Add to this, a low-resistance earth fault return path, which will enable the circuit protection to operate very fast (automatic disconnection of supply), and we have significantly reduced the risk of electric shock.

 

[amberleaf]

Safe Isolation is required ..................... lock off .

learning curve .

Main protective-bonding-conductor ... ( Gas service pipe )
Wander lead ( R[SUP]2[/SUP]) GN-3 tells us , must be below 0.05Ω

You are measuring something very low . ( in Ω )

4.3. Low résistance ohmmeters .
GN-3 tells us .
Instrument
(must be accurate) The measuring range should cover the span 0.02Ω to 2Ω ..

Wander lead used . 50m (( null out the leads )) one end into your Megger & the other onto the disconnected earthing-conductor

A good way of measuring it , ( Length ) 10mm[SUP]2 [/SUP]main protective conductor up to 25m would be ok . ( you should get a low reading )

This is bonding the Gas service pipe .
One end of test probe onto the disconnected earth-conductor . Green lead with crocodile clip
At gas meter :- to carry out the final check , to measure the resistance of the protective-bonding-conductor

Other lead from your Megger ( Red ) onto the Gas pipe .
this tell you that the BS-951 earth clamp is satisfactory and the clamp is secure and in good condition ( reading of 0.02 ) lower than 0.05Ω

if there was dirt or corrosion on the pipe , the résistance will be high .

 

[amberleaf]

Could calculations be used in Exams . Q/As 2394 / 2395

Still in use . by GN-3 . :book:

Rule of thumb 2008 . 2.7.14.
Rule of thumb 2011 . 2.7.15.
Rule of thumb 2015 . 2.7.16.

 

[amberleaf]

O.S.G.
A3 2015 . Guidance on initial testing of Installations . :book:

10.1. Safety and equipment
HSR 25 . EWR regulation 14 .
Electrical testing involves danger , The Electricity at Work Regulations 1989 .

State that :
a) it is unreasonable in all the circumstances for it to be dead ;
b) reasonable in all the circumstances for the work to be carried out ; and
c) that suitable precautions are taken to prevent injury .

Although live testing and diagnosis for fault finding may be justifiable, there could be no justification for any subsequent repair work to be carried out live.

612.1. 610.1. it is the test operative’s duty to ensure their own safety, and the safety of others, whilst working through test procedures. When using test instruments, this is best achieved by precautions such as .

2011: i , ii , ii , iv .
2015: a , b , c , d .

 

[amberleaf]

O.S.G.
A3 2015 :book:

Regulation 120.3.
1.4. Departures from BS-7671:2015

Where the designer decides to depart from BS-7671: the resulting degree of safety must not be less than that obtained by compliance with the Regulations. The designer is responsible for the safety of the design. Any intended departures from BS-7671: although the designer is confident regarding safety, must be recorded on the Electrcal Installation Certificate, There is a difference between an intended departure and a non-compliance .

 

[amberleaf]

(( Risk assessment is Electrical safety ))

Regulation .. A3 2015. P/417
( MEIWC ) Minor Electrical Installation Work Certificate

Part 1: Description of minor works
(1)
(2)
(3)
(4) Details of departures, if any, from BS-7671:2008 as amended
(5) Details of permitted exceptions ( Regulation 411.3.3. ) ........... Where applicable, a suitable risk assessment(s) must be attached to this Certificate .

Risk assessment attached [ ✓ ]

411.3.3. Additional protection . refer . :book: :icon_bs:

Part 3 : Essential Tests
RCD operation ( if applicable ) Rated residual operating current ( IΔn ) ...... mA
Disconnection time at IΔn ...... mS
Disconnection time at 5 IΔn ...... mS

Satisfactory test button operation ...... ( insert ✓ to indicate operation is satisfactory )

 

[amberleaf]

Regulation .. A3 2015.
( MEIWC ) Minor Electrical Installation Work Certificate :icon_bs:

(4) Details of departures, if any, from BS-7671:2008 as amended
Non departures are to be expected except in most unusual circumstances, see regulations 120.3 and 133.5.

(5) Details of permitted exceptions ( Regulation 411.3.3. ) ........... Where applicable, a suitable risk assessment(s) must be attached to this Certificate .

In non-domestic installations where a risk assessment has been carried out and the findings show that additional protection by RCD is not necessary, the assessment(s) must be attached to this Certificate .

 

[amberleaf]

Regulation .. A3 / 2015 :icon_bs:

Abbreviations used in the Standard ▼
P/41 ► IP International Protection Code .. 412.2.2.3.

Regulation 412.2.2.3.
Where a lid or door in an insulating enclosure can be opened without the use of a tool or key, all conductive-parts which are accessible if the lid or door is open shall be behind an insulating barrier ( providing a degree of protection not less than IPXXB or IPX2 ) preventing persons from coming unintentionally into contact with those conductive-parts, This insulating barrier shall be removable only by the use of a tool or key .

 

[amberleaf]

Abbreviations used in the Standard ▼ :icon_bs:
P/41 ► PE Protective conductor , meaning - Fig 3.8

TN- systems .. Fig 3.8
All exposed-conductive-parts of an installation are connected to this ( Protective-conductor ) via the main earthing terminal of the installation .

TN-C-S ( PME) system
All exposed-conductive-parts of an installation are connected to the (PEN) conductor via the main earthing terminal and the neutral terminal , (( these terminals being linked together ))

TT system
All exposed-conductive-parts of an installation are connected to earth electrode which is (( electrically independent )) of the source earth .

A3 / 2015

P/37 . re-cap 1,2,3,4, . Protective-conductor(s)
1) circuit protective conductor
2) main protective bonding conductor
3) earthing conductor
4) supplementary protective bonding conductor(s) .. Where required .

 

[amberleaf]

A3 - P/153
537 Isolation and Switching tables .

53.4 - Guidance on the selection of protective, isolation and switching devices
Device
Standard
Isolation (4)
Emergency switching (2)
Functional switching (5)

Yes = Function provided, No = Function not provided

(1) Function provided if the device is suitable and marked with the symbol for isolation (see BS EN IEC 60617 identity number S00288).
(2) See Regulation 537.4.2.5
(3) Device is suitable for on-load isolation, i.e. disconnection whilst carrying load current.
(4) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors. See Regulation 537.2.2.1.

(5) Circuit-breakers and RCDs are primarily circuit-protective devices and, as such, they are not intended for frequent load switching.
Infrequent switching of circuit-breakers on-load is admissible for the purposes of isolation or emergency switching. For a more frequent duty, the number of operations and load characteristics according to the manufacturer’s instructions should be taken into account or an alternative device from those listed as suitable for functional switching in Table 53.4 should be employed.

NOTE 1: An entry of (1,3) means that the device is suitable for on-load isolation only if it is marked with the symbol for on-load isolation
NOTE 2:In the above table, the functions provided by the devices for isolation and switching are summarized,

 

[amberleaf]

134.1 Erection :icon_bs:
134.1.1 Good workmanship by skilled or instructed persons and proper materials shall be used in the erection of the electrical installation.
The installation of electrical equipment shall take account of manufacturers’ instructions.

134.2 Initial verification
134.2.1 During erection and on completion of an installation or an addition or alteration to an installation, and before it is put into service, appropriate inspection and testing shall be carried out by skilled persons competent to verify that the requirements of this Standard have been met.

Appropriate certification shall be issued in accordance with Sections 631 and 632.

Ordinary person
Person who is neither a skilled person nor an instructed person .

Instructed person (electrically).
Person adequately advised or supervised by electrically skilled persons ( as defined ) to enable that person to perceive risks and to avoid dangers hazards which electricity can create.

Skilled person (electrically).
Person who possesses, as appropriate to the nature of the electrical work to be undertaken, adequate education, training and practical skills, and who is able to perceive risks and avoid hazards which electricity can create.

Note 1 : The term “ (electrically) ” is assumed to be present where the term “ skilled person ” is used throughout BS-7671:

 

[amberleaf]

Amendment 3: 2015 . :icon_bs:

Part 4
Protection for safety

Chapter 41 Protection against electric shock

411 Protective measure: Automatic Disconnection of Supply
411.3 Requirements for fault protection

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 for:
(i) socket-outlets with a rated current not exceeding 20A , and
(ii) mobile equipment with a current rating not exceeding 32 A for use outdoors.

An exception to (i) is permitted
(a) where, other than for an installation in a dwelling, a documented risk assessment determines that the RCD protection is not necessary, or
(b) for a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.

NOTE 1: See also Regulations 314.1(iv) and 531.2.4 concerning the avoidance of unwanted tripping.
NOTE 2: The requirements of Regulation 411.3.3 do not apply to FELV systems according to Regulation 411.7 or reduced low voltage systems according to Regulation 411.8.

NOTE 3: See Appendix 2, item 10 in respect of risk assessment.