Measurement of earth fault loop impedance – consideration of the increase in conductor resistance with the temperature rise due to load current 49-50 The importance of correct cable sizing in providing satisfactory cable life 52 Cable sizing for normal load and overload 53-54 Factors affecting the accuracy of loop impedance measurements 57-58 snags and solutions: A practical guide to everyday electrical problems 60-61 Responsibility for earthing of an electrical installation 62-63
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Measurement of earth fault loop impedance – consideration of the increase in conductor resistance with the temperature rise due to load current Circuit conductors that were not carrying sustained load current just before a measurement of earth fault loop impedance (Zs) is carried out on the circuit are liable to be at about ambient temperature during the measurement process. Where this is the case, it must be taken into account that Zs will increase above the measured value when the circuit conductors carry load current, since the conductors will then be at above ambient temperature.
T
his is true irrespective of whether Zs is measured directly (using an earth fault loop impedance test instrument) or indirectly (by measuring the R1+ R2 of the circuit conductors with a continuity test instrument and adding this value to the external earth fault loop impedance (Ze)). Guidance in Appendix 14 of BS 7671 Appendix 14 of BS 7671 gives an easily applied approximate method of accounting for the increase in Zs with the rise in conductor temperature caused by load current. The method, which uses condition (a) below, assumes that the ambient is about 20 oC, the conductor operating temperature under
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p49-50 loop impedence.3.indd 1
sustained load conditions is about 70 oC, and the major part of Zs consists of the series resistance of the line and protective conductors of the circuit (R1+ R2). If condition (a) is satisfied, the measured value of earth fault loop impedance is considered to meet the requirements of Regulations 411.4.5 (TN system) or 411.5.4 (TT system) for automatic disconnection. (a) Zs (m) ≤ 0.8 x
U0
Ia
Where: Zs (m) is the measured value of earth loop impedance in ohms U0 is the nominal a.c. rms line voltage to earth, in volts
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