Re-take - Useful Information for 2394 :

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Extracts BS-7671: 2001 (2004) ◄

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 (Regulation 531-02-04 refers). Such tripping can occur on heating elements, cooking appliances etc., which may have elements that absorb a small amount of moisture through imperfect elementend seals when cold. When energised, this moisture provides a conductive path for increased leakage and could operate the RCD. The moisture dries out as the element heats up. Although not precluded in BS 7671, it is not a requirement to use an RCD on such circuits if other satisfactory means of protection are available. Providing an RCD with a higher rated residual operating current may solve the problem but the requirements of the Regulations would still have to be met.

 

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Extracts BS-7671:2008(2013) :icon_bs:

Unwanted operation :svengo:
Unwanted operation of RCDs can occur when a :aureola: protective-conductor-current causes the RCD to operate under non-fault conditions, i.e. the accumulative of protective conductor currents developed by the switch-mode power supplies of computers, e.g. too many computers on one circuit. 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 operation of the device (see Regulation 531.2.4). Such operation can occur on circuits with heating elements of cooking appliances etc., where elements can absorb a small amount of moisture through imperfect seals when cold. When energised, this moisture provides a conductive path for current to flow and could operate the RCD. The moisture dries out as the element heats up. Although not precluded in BS 7671, it is not a requirement to use an RCD on such circuits but the requirements of the Regulations would still have to be met, i.e. cables in walls, Regulation 522.6.101.

 

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Ring-final-circuit .
Learning curve .

This also has the benefit that successful test results in Step 2 & Step 3 will confirm that the socket has been correctly wired and the polarity is correct.
End-to-end
r[SUP]1[/SUP] = 0.6Ω
r[SUP]N [/SUP] = 0.6Ω
r[SUP]2 [/SUP]= 1.0Ω

R[SUP]1[/SUP] and R[SUP]N[/SUP] should be the same as the ( CSA ) areas of neutral & line-conductor(s) should be the same in a single-phase-circuit
( T&E ) 2.5mm[SUP]2[/SUP] & circuit-protective-conductor 1.5mm[SUP]2 [/SUP] ... ( CSA ) of the CPC is indeed smaller than the ( CSA ) Line & neutral

The reading expected in Step 2 .
i) ( R[SUP]1[/SUP] + R[SUP]N[/SUP] ) / 4
ii)
iii) ( 0.6 + 0.6 = 1.2 ) ÷ 4
iv) = 0.3Ω

The reading expected in Step 3 .
i) ( R[SUP]1[/SUP] + R[SUP]N[/SUP] ) / 4
ii) ( 0.6 + 1.0 = 1.6 ) ÷ 4
iii) = 0.4Ω

The actual readings obtained in Step 2 & 3 have been recorded .
Socket-outlet (1) Step 2 0.31Ω , Step 3 0.41Ω
Socket-outlet (2) Step 2 0.3Ω , Step 3 0.4Ω
Socket-outlet (3) Step 2 0.32Ω , Step 3 0.41Ω

like any test result , very close to the expected values .

 

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2394
Q/As

A circuit breaker to BS-3871 is marked “ M6 ″ explain what this marking means and describe how the same information would be marked on a circuit breaker to BS-EN-60898-1.

M6 means the circuit breaker has a breaking capacity of 6kA. On a BS-EN-60898-1 circuit breaker it would be marked as the figure 6000 inside a rectangular box.

 

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2394
Q/As
Earth fault loop impedance can be calculated from measured values of Ze, ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )
a) state the equation to be used
b) explain why a measured value of earth fault loop impedance may be less than a value calculated as above.

a) Zs = Ze + ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )
b) Ze is measured with the earthing-conductor disconnected from the main earthing terminal. Zs is measured with all bonding connected and this may provide parallel paths which reduce the apparent value of Zs.

 

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2394
Q/As

An insulation resistance test is to be carried out on a lighting circuit. State the action to be taken in respect of :
a) lamps
b) light switches
c) two-way light switches

a) remove lamps or switch off locally if removal is not possible
b) put light switches ‘on‘ unless the switch is being used to isolate lamps which cannot be removed
c) operate two-way switches alternately while applying the test voltage to ensure that both strappers and the switch wire are tested


​

 

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2394
Q/As
An insulation resistance test is to be carried out on a lighting circuit. State the action to be taken in respect of vulnerable equipment including smoke detectors and self-contained emergency lighting luminaries.

Isolate the vulnerable equipment or disconnect it. If this is not possible then only test between all (( live conductors )) connected together and earth.


​

 

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2394 / 2395
Q/As

Explain briefly why it is desirable to minimise the amount of dismantling during a periodic inspection and test.

Dismantling introduces the risk of:
i) damage
ii) incorrect reassembly
iii) forgetting to reassemble

 

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Q/As

State the maximum rating of a RCD used for:-
a) protecting a 63A socket outlet on a construction site
b) fire protection in an area with a high risk of fire due to the materials
c) protecting a socket outlet supplying a caravan.

a) 500mA
b) 300mA
c) 30mA​

 

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Q/As Yeah they do ask this one .

Describe briefly how to measure the prospective fault current at a three-phase distribution board.

Measure the prospective short-circuit current between each line and neutral. Take the highest reading and double it to get the prospective fault current.

 

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2394
Q/As

1) List two statutory documents applying to the conduct of inspection and testing and one statutory document requiring installations to meet the standards set out in BS 7671.

2) List two main things to be agreed with the client prior to inspecting and testing an existing installation and state where these should be recorded.
3) List five things to check before using a test instrument and its leads.
4) State the instruments which would have the following ranges:-
0-2Ω
0-200 MΩ
0-20 kA

1) Two statutory documents relating to the conduct of inspection and testing:
Health and Safety at Work etc. Act 1974
Electricity at Work Regulations 1989

Electricity Safety, Quality and Continuity Regulations 2002 requires installations to meet requirements of BS 7671.

2) Two things to be agreed with client:
Extent
Limitations
Record these on the Electrical Installation Condition Report form.

3) Five points should be given. See GS-38.
Instrument :​
- in good condition
- calibrated
- battery ok
- suitable for voltage it is to be used on
- suitable ranges for the measurement to be taken.

Leads:
in good condition
adequately insulated for the voltage they are to be used on
fused and/or fitted with resistors to limit current
probes fitted with finger guards
probe tips with bare minimum of exposed metal (2mm or less) or
else covered by retractable spring loaded shrouds
adequate length
coloured to aid identification

4) a) low resistance ohmmeter,
b) insulation resistance ohmmeter,
c) prospective fault current meter

 

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List two factors affecting the measured value of earth fault loop impedance of a given circuit.

a) State a simple rule for taking account of the factors in a) above when comparing measured values of
b) Zs with the maximum values given in BS 7671.

a) circuit loading and ambient temperature
b) Measured values of Zs should not exceed 80% of the maximum given in the tables in Chapter 41 of BS 7671. (Appendix 14 of BS-7671)​

 

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a) State the practical advantage that measuring Rl + R2 has over the alternative method of measuring c.p.c. continuity.
b) Say when it is necessary to use the alternative method.
c) State the percentage rise in the resistance of a copper conductor between 20°C and 70°C.

a) Measuring R1 + R2 requires meter leads of standard length whereas the alternative long lead method, measuring R2, requires a lead the length of the circuit.
b) The alternative ‘long lead’ method must be used for checking the continuity of bonding conductors.
c) 20%
​

 

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2394
Q/As

Outline the dangers which could arise during the following tests:
a) insulation resistance test
b) earth fault loop impedance test
c) RCD test

a) The use of high voltages creates a risk of shock and the risk of damaging vulnerable equipment.
b) If the c.p.c. is broken then every exposed conductive part downstream from the break will be live during the test and present a shock risk.
c) Danger is the same as for b)

A SELV circuit in zone 0 of a bathroom is being inspected. State:-
a) the BS number of the safety isolating transformer
b) the maximum voltages allowed ( both ac and dc )
c) where the transformer should be located

a) the BS-EN number of the safety isolating transformer
b) 12Vacor30Vdc
c) Outside zones 0, 1 and 2

The continuity of the conductors of a ring final circuit is checked using the method given in IET Guidance Note 3. List four things which are confirmed by satisfactory readings. Assume that testing has been done through the front of the sockets using a test lead with a plug.

1) the ring is continuous
2) the value of ( R[SUP]1[/SUP] + R[SUP]2[/SUP] )
3) there are no interconnections in the ring
4) socket outlets are correctly connected

 

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2395 Practical assessment

You will do an insulation résistance test on a single-phase consumer unit , Here you would have to identify 2 faults which would either be a short-circuit , low-résistance , or and earth-fault .

This is very straightforward and should not pose a problem if you know , how to work with insulation résistance tester .

Points to remember . (( Examiner ))
This might differ from examiner to examiner . if they tell you , :yes: do it in the sequence they tell you , you will be better off doing it their way . if you do not then they will fail you , and challenging them on it will probably not get you very far . :35: