Technical

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

TECHNICAL 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 (Z s) 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 Z s will increase above the measured value when the circuit conductors carry load current, since the conductors will then be at above ambient temperature.

This is true irrespective of whether Z s 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 (Z e)).

Guidance in Appendix 14 of BS 7671 Appendix 14 of BS 7671 gives an easily applied approximate method of accounting for the increase in Z s 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 sustained load conditions is about 70 oC, and the major part of Z s 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 I a

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

Ia is the current in amperes causing automatic disconnection of protective device under earth fault conditions.

Condition (a) can be rewritten as condition (b), below, because the values of maximum earth fault loop impedance given in Tables 41.2, 41.3 and 41.4 of BS 7671 were obtained using the formula Z s = U0/I a

(b)

Zs (m) ≤ 0.8 x Zs tab

Where:

Zs tab is the appropriate value of maximum Z s given in Table 41.2, 41.3 or 41.4.

Notes. 1. The measure value Z s(m) used in condition (a) or (b) should be that at the most distant point of the circuit. 2. Possible inaccuracies in the value of Z s(m) arising from the test instrument and the measurement process need to be taken into account. The factors affecting the accuracy of measurements made with an earth fault loop impedance test instrument are discussed in the article on page 57 of this issue of Connections.

Worked example of using condition (b) Find the highest acceptable measured value of earth fault loop impedance for a Type B circuit-breaker for a circuit of nominal voltage (U0) to Earth of 230 V.

The value of maximum Z s given in Table 41.2 of BS 7671 for the above circuit-breaker is 5.11 Ω.

Therefore, using condition (b), the highest acceptable measured value of earth fault loop impedance for the device is 0.8 x 5.11 Ω = 4.08 Ω.

NICEIC table of limiting measured values NICEIC has for many years published a table of limiting values of measured earth fault loop impedance, derived using conditions (a) or (b) above. The table is shown in Fig 1.

A copy of the table is included with every pad of NICEIC Electrical Installation Certificates,

iii) measure the R1+ R2 of the final circuit. iv) correct the resistances obtained in steps (ii) and (iii) for conductor operating temperature. v) add the corrected values obtained in step (iv) to the external earth loop impedance obtained at step (i).

The use of the above method will be covered in more detail in a future issue of Connections.

Fig 1 – Illustration of the table included with pads of NICEIC certificates and reports

Minor Electrical Works Certificates, Periodic Inspection Reports, and the domestic and computer-friendly versions of these.

Borderline values of Z s(m) There can be situations where a measured value of earth fault loop impedance, Z s(m), (taking account of possible inaccuracies, as mentioned earlier) is slightly higher than the limiting value given in the table in Fig 1 (or obtained using 0.8 x U0/I a or 0.8 x Zs tab ).

Where this is the case, the person responsible for checking the measured value for compliance with the requirements of BS 7671 may decide to make more precise assessment of the value. Appendix 14 of BS 7671 gives the following method of carrying out this more precise assessment. i) Measure the external earth fault loop impedance (Z e). ii) measure the combined line and protective conductor resistance (R1+ R2) of the distribution circuit , if any.

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