B
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Q44 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
A44
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
Q45 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
A45 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
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
A44
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
Q45 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
A45 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
