Re-take - Useful Information for 2394 :

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For-Simplicity: ( L/N together ) as One , linked-together as the Regulations tells us L/N . & E

Between Live-conductors and Earth .

Minimum-insulation-résistance between Live-conductors , tested at 500V d.c. should not be less than 1MΩ .

Thismethod can be employed on a , periodic-test where installed-equipment is vulnerable to damage by thetest-voltage .

Main-switch in the On - position and all circuit-breakers closed .

• Between Live-conductor(s) separate - L . N . on there own .
Toensure that the readings obtained are , Accurate and therefore a true-reflection , of the condition of the conductors under-test .

 

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From Old-Notes , not for exam purposes . Useful-Junk

Verification of Test-Results :

Itis a common-mistake for electricians to compare-test-results directly against those in BS-7671:2008

Themeasured-values of ( Zs ) of each final or distribution-circuit has to be in compliance , the obtained-values have to be compared against :

- Rule of thumb figures .
- Tabulated-values in Table . 41.2 . 41.3 . 41.4 . in BS-7671:2008 , these-values will have to be corrected for temperature . Or
- Table provided in Guidance-Note 3 .
- The Earth-fault-loop-impedance-values , provided by the designer .

Rule of Thumb-Figures : ( 0.8- 80%)

(Zs ) value at the remote-end of a circuit should not exceed ( 0.8 ) of the a appropriate-value inBS-7671

A Note-under Tables 41.2 . 41.3 . 41.4 . 41.6. States that
“ the circuit-loop-impedance ( Zs ) given in the tables should not be exceeded when theconductors are at their normal-operating-temperature , if the cables are at a different-temperature when tested “ the reading should be adjusted accordingly, see - Appendix14 .

Therequirements are satisfied if the , Measured ( Zs ≤ 0.8 x Uo / Ia ) i.e. 80% ofthe tabulated-values in BS-7671:2008

Determine whether the measured-values of earth-loop-impedance foreach-circuit satisfy the requirements of BS-7671:2008

Shower-circuit: BS-7671:2008 earth-loop-impedance Ω - 0.62 x 0.8 = 0.49 : Measured-value Ω - 0.50 : Pass or Fail .
Ring-final-circuit: BS-7671:2008 earth-loop-impedance Ω - 1.14 x 0.8 = 0.91 : Measured-value Ω - 0.84 : Pass or Fail .
Radial-sockets: BS-7671:2008 earth-loop-impedance Ω - 1.85 x 0.8 = 1.48 : Measured-value Ω - 1.36 : Pass or Fail .
Immersion-heater: BS-7671:2008 earth-loop-impedance Ω - 2.67 x 0.8 = 2.13: Measured-value Ω - 1.36 : Pass or Fail .
Lights- ( 1[SUP]st[/SUP] floor ) : BS-7671:2008 earth-loop-impedance Ω - 18.5 x 0.8 = 14.8: Measured-value Ω - 13.9 : Pass or Fail .
Lights- ( Gnd floor ) : BS-7671:2008 earth-loop-impedance Ω - 10.0 x 0.8 = 8.0: Measured-value Ω - 0.74 : Pass or Fail .

 

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From Old-Notes , not for exam purposes. Useful-Junk

Verification of Test-Results :

It is a common-mistake for electricians tocompare-test-results directly against those in BS-7671:2008

The measured-values of ( Zs ) of each final ordistribution-circuit has to be in compliance , the obtained-values have to becompared against :

Tabulated-values in Tables - 41.2 . 41.4 . in BS-7671-2008 .

Two-factors , bothinvolving-temperature , must be taken into account when comparing any measured-value of ( Zs ) with the values obtained form BS-7671 :


i) Cable-operating-temperature-factor ( Assumed to be 70°C ) &
ii) The actual-temperature of theconductors when the measurement was taken , ( Usually-deemed to be the Ambient-temperature )

Conductor-operating-temperature-factor
The tabulated-value in BS-7671: are used for design-purposes and assume that the cables are attheir maximum-operating-temperature .

i.e. 70°C for thermoplastic - PVC/PVC . cables .

for comparison-purposes with BS-7671: the ambient-temperature at the time of the test is assumed to be 20°C .

i.e. using the temperature-coefficient of résistance for copper of , 0.004Ω/°C , then the factor to adjust the measured-loop-résistance-value to what it would be at 70°C .

= 1 + [ 50 x 0.004Ω/°C ] = 1.2 . for PVC - insulated-cables .

This factor is applied as a , multiplying-factor to be measured ( Zs - value ) then compared against the tabulated-value in BS-7671:2008:
Alternatively - the 1.2 can be used a divisor and applied directly to the tabulated-value in , BS-7671:2008: The measured-value can then be compared directly against this reduced-value .

Example : Determine whether the measurement taken at the socket-out for thecircuit shown complies with BS-7671:2008:

[ Table 41.3. type-B . BS-EN-60898 - 32A = 1.44Ω ] three-sockets , reading furthest-socket - ( 1.1Ω )

Using the method stated : Measured-value , x 1.2 = 1.1 x 1.2 = ( 1.32Ω )

Note : therefore , as the adjusted-measurement is less than , 1.44Ω it complies .

 

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From-old-notes . 2008 .

Actual-conductor-temperature-factor

If themeasured-earth-fault-loop-Impedance-value , is taken at ambient-temperature Other than 20°C then it must be adjusted for the temperature-difference .

Example : Determine whether the measurement taken at the socket-outlet for thecircuit show-complies with BS-7671:2008:

32A -BS-EN-60898 Maximum- Zs 1.44Ω -

The ( 1.3Ω ) reading on the instrument has been Obtained at an , Ambient-temperature = 25°C .

Using the temperature-coefficient of résistance for copper of , 0.004Ω/°C , thenthe factor to adjust the measured-loop-résistance-value to what it would be at , 70°C

= 1 + [ ( 70 - 25) x 0.004Ω/°C ]
= 1 + [ 45 x 0.004Ω/°C ] = 1.18

From the table-provided :

Therefore , Reading at 70°C = 1.30 x 1.18 = 1.53Ω

As the measurement taken is greater than 1.44Ω then the circuit Does-Not-Comply .

Where the designer has provided test-values , they should have been compensated for the temperature-differences, thus enabling the Inspector to make a direct-comparison .

 

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Earth-fault-loop-impedance:

State : four-points as described in Guidance-Note - 3 , where the measurement of earth-fault-loop-impedance is required to be taken .

i) at the supply-intake - ( Ze )
ii) at each distribution-board - ( Zdb ) GN-3
iii) at theremote-end of all finial-circuits - ( Zs)
iv) at each-socket-outlet - ( Zs )

indentify from the Four-points indentified above , reasons why these measurements are necessary .

i) at the supply-intake .

- to ensure the installation is effectively connected to Earth . -&-s big-time . 2394.
- to confirm that the actual ( Ze ) is equal to or less than what the designer has used in his design-calculations .

ii) at each-distribution-board ( Zdb )
- to enable the , prospective-earth-fault-current to be calculated .
- to confirm that the , protective-device supplying the distribution-board under earth-fault-conditions in thetime required by BS-7671:2011:
- in conjunction with either a visual-inspection , an ( Ra ) or a ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )test , an earth--loop-impedance-test will confirm-polarity .

iii) at the remote-end of all-final-circuits ( Zs )
- to confirm that the protective-device supplying the final-circuit-disconnects under-earth-fault-conditions inthe time required by BS-7671:2011:

iv) at each-socket-outlet .
- to confirm that the protective-device supplying the socket-outlets disconnects under-earth-fault-conditions inthe time required by BS-7671:2011:

Identify ,Two-factors that the Inspector will have to consider when confirming his ,earth-fault-loop-impedance-measurements , are in compliance with the requirements of BS-7671:2011:

i) the normal-operating-temperature of the cables .
ii) the temperature of the cables at the time of test .

typical :electrical-installation(s) require an effective-connection to earth as part of the protective-system .

identify the Three most common , earthing-systems available to the District-Network-Organiser - ( DNO ) and state the , maximum-earthing-fault-loop-impedance at the supply-terminals for each-system .

i) TN-S . 0.80Ω
ii) TN-C-S . 0.35Ω
iii) TT - . 21Ω , from the DNO suppliers earth-electrode - Only . 21Ω + the consumers electrode-résistance .

you have an independent - earth-electrode - Ω , from the-supplier

iii) TT - . 21Ω , O.S.G. p.11
TT - arrangement ,21Ω is the usual-stated-maximum-résistance of the distributions , earth-electrode at the supply-transformer . Etc .

Note: the résistance of the ( Consumers-installation-earth-electrode ) should be as low as practicable and a value exceeding 200Ω may not be stable .

Type of Protective-device : p.23 -O.S.G. RCBO - it can serve , under-fault-conditions . Overload : Short-circuit : Earth-fault. written-exams.

30mA RCD - is for Additional-protection only , theprotect the User . so its Additional . :gettree:

 

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Values of (R1+R2) are measured :gettree:
Using an ohmmeter with a low range, such as 0to 20Ω, having a no-load voltage of between 4 V and 24 V, d.c. or a.c., and a short-circuit current ofnot less than 200mA (Regulation612.2.1 refers). The Q/A came up on the 2391 .

 

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2394 : Written-exams .

Additional-protection:

Additional-protection by Residual-current-devices.

Theuse of an RCD whose rating ( IΔn ) does not exceed 30mA and disconnects within 40mS when tested at 5 x ( IΔn )

Is recognised as providing :

• Additional-protection against the failure of Basic-protection .
• Additional-protection against the failure of Fault-protection .
• Additional-protection against Carelessness by the User .

RCD-tester, must comply with BS-EN-61557-6
Leads to be in compliance with - GS-38

 

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( G) non-delay RCD :

Q) for a 100mA non-delay RCD toBS-EN-61009-1 the residual-currents and maximum-permitted-disconnection-times , are .

a) 100mA - 300mS : 500mA - 40mS
b) 100mA - 130mS : 1500mA - 150mS
c) 100mA - 0mS : 100mA - 40mS
d) 100mA - 400mS : 500mA - 50mS

Q) The protective-measure , automatic-disconnection of supply . ( G) non-delay RCD :
a) is only permitted if the installation is under effective-supervision .
b) is a method of reducing thermal-effects .
c) is acombination of basic and fault-protection .
d) is a combinationof under and over-voltage-protection .

Q) An earth-fault-loop-impedance-test determines the actual-value of : a) Zs , b) Ze , c) R[SUP]1 [/SUP], d) R[SUP]2 [/SUP], The word - external was not Used . :gettree:

Earth fault loop impedance (Zs) is a characteristic vitalto the protective measure Automatic Disconnection of Supply (ADS),

The meaning of earth fault loop impedance
Earth-fault-loop-impedance (Zs) is theimpedance of the intended path of an earth fault current ( known as the earthfault loop )

 

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GN-3p.66 . Requirements for Inspection & Testing .

Written-exams .
Q) What is the reason for a periodic-inspection. ? :christmaswreath:

3.8.1. Scope :
Thepurpose of periodic-inspection and testing is to provide an engineering-view on whether or not the installation is in asatisfactory-condition where it can continue to be used in a safe-way .

The periodic-inspectionand test-comprises a detailed-examination of the installation together with appropriate-tests .
The inspection is carried out without taking-apart or dismantling-equipment as far as is possible .

GN-3 are reminding us .
The tests made are mainly to confirm that thedisconnection-times-stated in Chapter -41 . are met , as well as highlighting-other-defects. Etc .

Q) On completionof periodic-testing an old-domestic-installation , theinsulation-résistance is found to belowthe requirements of BS-7671:2011: The immediate-action to be taken is to . ?

Notify thelocal-supply-authority
Notify the person-ordering the work
Put awarning-notice on the supply-intake-position
Protect thecircuits with smaller-sizes of fuses

Q) The Electricity at Work-Regulations 1989 concern’s its self with . ?
All aspects of electrical-systems
System up to 1000 volts
Special-locations only
High-voltage-systems only

2392-10 - 2394 :
Q) it is important to test a new-installation in the correct-sequence because . ?
The sequence is stated in GN-3
Each test relies on the previous to be correct
It is more-convenient
It is easy to remember

 

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Old-notes 2002

Insulation-résistance, By calculations .

An installation comprising six-circuits have individual-insulation-résistance of : 2.5MΩ . 8MΩ . 200MΩ . 200MΩ . 200MΩ . 200MΩ.

Q ) So thetotal insulation-résistance will be .

1/Rt = 1/2.5 + 1/8 + 1/200 + 1/200 + 1/200 + 1/200 .

= 0.4 + 0.125 + 0.005 + 0.005 + 0.005 + 0.005 .

= 0.545

Rt = 1/0.545

= 1.83MΩ …… 1.0MΩ minimum but less than 2MΩ . ◄◄

This is clearly greater than the 1.0MΩ minimum butless than 2MΩ .

lessthan 2MΩ . ) are recorded then this might indicate a latent but not yet visible-fault in the installation. which would require further-investigation .

• If a value of less than 2MΩ is recorded it may indicate a situation where a fault isdeveloping .

Themore-resistances , there are in parallel, the lower the overall-résistance ,and in consequence , the longer a cable the lower the Insulation-résistance
Addto this the fact that almost all installation-circuits are also wired in parallel

 

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this is the Question your likelyto get . 2392-10 / 2394.

Q) if each of the three-circuits had been tested individually and gave readings of - 80MΩ , 60MΩ , & 30MΩ respectively , what would be the expected overall insulation-résistance .

80MΩ , 60MΩ , 30MΩ ,

Q ) So thetotal insulation-résistance will be .
1/Rt = 1/80 + 1/60 + 1/30 .

= 0.0125 + 0.0166 + 0.033

= 0.0621 .

Rt = 1/ 0.621 = 16. , doingyour Calculation . where your decimal-point here . Yeah


a) 0.0625MΩ
b) 15MΩ
c) 16MΩ
d) 160MΩ

 

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Q) the metal-partsof an building-structure are called : ? conductive-part liableto introduce a potential
earthing.
equipotential-bonding
exposed-conductive-parts
extraneous-conductive-parts

Regulations- p.28 .
Extraneous-conductive-parts A conductive-part liable tointroduce a potential , generally Earth-potential , and notforming part of the electrical-installation.

Q) the metal-parts of an electrical-installation notnormally-live are called : ?
earthing.
equipotential-bonding
exposed-conductive-parts
extraneous-conductive-parts

Regulations- p.27 .
Exposed-conductive-part Conductive-part of equipment which can be touched & which is notnormally-live , but which canbecome-live-under-fault-conditions .

Q) the process which maintains a potentialof zero-volts between all extraneous-exposed-conductive-parts is called : ?
earthing.
equipotential-bonding
exposed-conductive-parts
extraneous-conductive-parts

Q) the act of connecting exposed-conductive-parts to the earthing-terminal ofan installation is called : ?
earthing .
equipotential-bonding
exposed-conductive-parts
extraneous-conductive-parts

Q) the trunking & conduit of an electrical-installation are called : ?
thegeneral-mass of earth
thecircuit-protective-conductor - ( CPC)
exposed-conductive-parts
extraneous-conductive-parts

Exposed-conductive-part Conductive-part of equipment which can be touched & which is notnormally-live , but which canbecome-live-under-fault-conditions .

Q) an electrical-connectionwhich maintains exposed-conductive-parts & extraneous-conductive-parts atthe same-potential is called : ?
(CPC ) - circuit-protective-conductor
earth-conductors
protective-equipotential-bonding : equipotential-bonding for thepurposes of safety .
supplementary-bonding

 

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Re-cap. 2392-10 , 2394.

Résistance in Series :

These are resistance joined , end to end in the form of a chain .

The total-resistance-increases as more-resistances are added .

Hence: if a cable-length is increased , its résistance will increase in proportion .

Example: 100m length of conductor has twice the résistance of a 50m length of the same-diameter .

1/R[SUP] total [/SUP] = 1/R[SUP]1[/SUP] + 1/R [SUP]2[/SUP]+ 1/R[SUP]3[/SUP] + 1/R[SUP]4 [/SUP]

1Ω - ( R[SUP]1[/SUP]) 2Ω - ( R[SUP]2[/SUP]) 10Ω - ( R[SUP]3[/SUP]) 4Ω - ( R[SUP]4[/SUP] ) Ω .

R[SUP] total [/SUP] = 1 + 2 + 10 + 4 = 17Ω

Résistance in parallel .

These are résistance joined like therungs of a ladder . here the total-résistance-decreases .

1/R[SUP] total [/SUP] = 1/R[SUP]1[/SUP] + 1/R [SUP]2[/SUP] + 1/R[SUP]3[/SUP] + 1/R[SUP]4 [/SUP]
[SUP] [/SUP]
3Ω - 6Ω - 8Ω - 2Ω

1/R[SUP] total [/SUP] = 1/R[SUP]1[/SUP] + 1/R [SUP]2[/SUP] + 1/R[SUP]3 [/SUP] + 1/R[SUP]4 [/SUP]

= 1/3 + 1/6 + 1/8 + 1/2

= 0.333 + 1.167 + 1.125 + 0.5

Therefore: = 1/1.125 .

High-value-resistances’ inparallel . hence : anincrease in cable-length-results in a decrease in insulation-résistance.

This value is measured in millions of ohms . - megohms ( MΩ )

Watch you Questions shown in words . :mad2:

• If youthen do the full-calculation using the ( Un-round up figures ) 1.125 .
• In exams , you will have to ( Roundit up ) exactly 16M . 100M .

 

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BS-7671:2008/ 2011: Basic principles of : Protection against !!

Protection against Electric-Shock . Part-4 . ( So in theory , Principles of Electric-Shock protection )

Electric-Shock occurs when a person becomes a part of the electrical-circuit .

Methodsused , keep-people away from Live-electrical-equipment. it is called - Basic-Protection . 131.2.1.

► Consumer-Unit , ( 2392-10 . Why do we use Class - 11 enclosure on all Installations now . Protection for the User ) N/A, Touching exposed-conductive-parts .

Basic-Protection . Protection against Electric-Shock. under fault-free-conditions .

By- Definition : p.23. Basic-Protection.
Note : for low-voltage-installations , systemand equipment , basic-protection generally-corresponds to ( Protection against ) direct-contact, that is

“ contact of persons or livestock with live-parts “

Re-cap: Indirect-contact:
Touching exposed-conductive-parts , such as themetal-work of an appliance . whichhas become live as a result of a fault . The potential-voltage on this metalwork rises above earth-potential , electric-shock may occur when someone-touches the metalwork .

By- Definition : p.28. Fault-Protection.
Note : for low-voltage-installations , systems and equipment , fault-protection generally corresponds to ( Protection against ) indirect-contact. mainly with regards to failure of ,basic-insulation ,

Indirect-contactis , “ contact of persons or livestock with exposed-conductive-parts which have become-live under fault-conditions “

Protection against touching something made live as a result of a fault , is called fault-protection , and is achieved by protective-equipotential-bonding & automatic-disconnection of the supply - ( ADS ) in the event of a fault-occurring , 131.2.2.

O.S.G. p23 . Types of protective-device .

RCBOs- will minimize-inconvenience in theevent of a fault and is applicable toall systems , TN-S , TT -
Theconsumer-unit contains devices for the protection of final-circuit(s) against : Overload , Short-circuit , Earth-fault . ( Electronic to trip the contacts )

RCBO- Small-white-flylead which connect to Earth , either D.C.. sensing or A.C. sensing .
Modern - RCBOs have a cream-wire which connects to earth known as a functional-earth .
Modern - RCBOs some - facts , generally the ones with a functional-earth use ( Electronic to trip the contacts ) as Opposed to just using ( Coils )

RCD- is used to provide Additional-protection, against electric-shock and the earthing-system is there to keep all ( Exposed / Extraneous-conductive-parts - ( Surfaces ) at or about the same-potential in the event of a fault , & to clear the fault-quickly byoperating the Protective-device .

Protective-device(s) Genetic-name for : :gettree: :17:
Fuse .
Circuit-breaker .
RCD . to improve-safety .
RCBO - MCB /RCD .

 

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Protective-equipotential-bonding

Is equipotential-bonding for the purpose of safety , theapplication of protective-equipotential-bonding to earth is one of theimportant-principles for safety .
Regulation- 131.2.2.

p.17. inconnection with fault-protection , theapplication of the method of protective-equipotential-bonding isone of the important-principles for safety . :santa5: