sum usefull stuff to study 2391 pt 3

[bobby101]

Q36 state two reasons for confirming the value of Ze for a new installation

A36 (i) to verify the existence of a supply to earth
(ii) to verify that the Ze is not greater than the value used in any of the design calculations, this value can be added to R1+R2 values to give a Zs value for individual circuits

Q37 A circuit is protected by a TYPE C circuit breaker and is just over the maximum allowed earth fault loop impedance value. List three possible solutions to get the required disconnection time

A37 (i) try a TYPE B circuit breaker if load characteristics allow the maximum Zs for a TYPE B circuit breaker
is twice that for a TYPE C
(ii) use a RCD or RCBO with suitable ratings
(iii) use circuit conductors with a larger C.S.A to reduce Zs

Q38 list six things to be checked before using a low resistance ohmmeter and leads

(i) Check condition of meter
(ii) Check the meter is within calibration date
(iii) Check the battery
(iv) Check the instrument is zeroed
(v) Check the condition of the leads and probes
(vi) Check the resistance of the leads and probes is nulled out

Q40 state the BS number of those Edison lampholders to which the polarity check does not apply in normal circumstances

A40 The check is not required for E14 & E27 lampholders to BS EN 60238

Q41 State the maximum allowable disconnection times for the following 230V a.c. circuits
A. 6A lighting circuits in a TN- S installation
B. Distribution circuits in a TN-S installation
C. 32A ring final circuits in a TT installation

A41 A. 0.4 s
B. 5 s
C. 0.2 s

Q42 state the maximum allowable rating for a RCD used for:-
A. Additional protection
B. Fire protection on a farm

A42 A. 30mA
B. 300mA

Q43 state the maximum recommended value for the resistance of an earth electrode for a TT installation protected by a RCD

A43 200Ω

 

[bobby101]

An installation fed from a public supply has a TT earthing system the resistance of the earth electrode is 100Ω.
A. Estimate the maximum value of Ze
B. Explain why it is necessary to protect all the circuits with an RCD
C. Calculate the maximum allowable rating of the RCD protecting the whole installation
D. State the maximum rating for a RCD protecting a circuit in the installation with socket outlets which might be used to supply portable equipment used outdoors
E. Explain how discrimination can be achieved between the RCD protecting socket outlet circuit and the RCD protecting the whole installation



A. The maximum value of Ze is 21Ω plus the resistance of the of the installations earth electrode.

Ze = 21 + 100
Ze = 121Ω

B. With a Ze of 121Ω the earth fault loop impedance of the circuits in the installation will be far too high to allow earth fault protection by overcurrent devices

C. The following condition must be met:

I∆n x Ra ≤ 50V

Ra is the resistance between an exposed conductive part and earth. Assuming that the resistance of the cpc is negligible by comparison with the resistance of the earth electrode, the maximum rating of RCD is approximately:-

I∆n ≤ 50 / 100

= 0.5 A

A 500mA RCD is therefore the very largest which could be used assuming that the required disconnection time was met.

D. 30mA

E. Discrimination between two RCD’s in series can be achieved by using a time delayed RCD in the upstream position



A. a domestic TT installation is to be protected by a 30mA RCD. Calculate the maximum
Theoretical value of earth electrode resistance

B. a circuit in a milking parlour fed by a TT installation is to be protected by a 100mA RCD. Calculate the maximum theoretical value of earth electrode resistance

C. describe the circumstances in which an earth fault loop impedance meter may be used to measure earth electrode resistance

D. explain why earth electrode resistance readings above a certain value should be investigated and say what the value given in GN 3 and (BS 7671) is


A. Assuming that the resistance of the cpc is negligible by comparison with the resistance of
the earth
Electrode, Ra is approximately equal to the resistance the earth electrode:-

Ra ≤ 50V / I∆n

Ra ≤ 50 / 0.03

Ra ≤ 1667Ω

B. as in (a) above

Ra ≤ 50V / I∆n

Ra ≤ 50 / 0.1

Ra ≤ 500Ω

C. in a TT installation where the earth electrode resistance is expected to be very much higher than 21Ω it may be assumed that the earth electrode resistance is very close to Ze. Taking a measured value Ze as the value of the earth electrode resistance errs on the side of safety
D. if the resistance of an earth electrode is higher than 200Ω it might not be stable the resistance might become unacceptably high due to the soil drying out or freezing etc

Earth electrode resistance should be as low as possible