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Earth fault loop impedance by calculation / maximum disconnection time

simonturner83

Hi

I have a fairly rudimentary question regarding earth fault loop impedance by calculation to determine maximum disconnection time requirements per BS7671.

Some calculations I have come across simply calculate Zs from the combined resistance of the cables within the installation, it strikes me that this omits the impedance associated with the source (e.g., a supply transformer) and load (e.g., an AC motor) devices, where the latter should have some influence in the case of a fault to earth in the motor windings.

This warrants further consideration in the case of >100A applications, where the guidance of GN6 also incorporates reactance in the fault current equation.

Are source/load (i.e., devices and not cables) impedances commonly accounted for in earth fault loop impedance calculations? And if so how (e.g., the case for a 415V transformer feeding an AC motor)?

Or would measurements on the as-built installation always be preferable when applying such considerations? With reactance coming into play via LCR measurement.

Many thanks in advance.

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    simonturner83 said:
    Some calculations I have come across simply calculate Zs from the combined resistance of the cables within the installation, it strikes me that this omits the impedance associated with the source (e.g., a supply transformer) and load (e.g., an AC motor) devices, where the latter should have some influence in the case of a fault to earth in the motor windings.

    That approach does not conform to BS 7671, which requires you to take into account "worst-case" conditions.

    You are correct, that the source impedance Ze or Zdb is also required ... and also that the (R1+R2) used should also be calculated for 70 degrees C (this is the purpose of the 0.8 factor rule of thumb if comparing "as-measured" values with Tables 41.2 to 41.5 of BS 7671) ... or a higher value of temperature for cases where conductor temperatures under load are higher (not recommended unless you can prove that terminals of equipment connected to a 90 or 110 degree conductor can withstand that temperature).

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    simonturner83 said:
    Some calculations I have come across simply calculate Zs from the combined resistance of the cables within the installation, it strikes me that this omits the impedance associated with the source (e.g., a supply transformer) and load (e.g., an AC motor) devices, where the latter should have some influence in the case of a fault to earth in the motor windings.

    That approach does not conform to BS 7671, which requires you to take into account "worst-case" conditions.

    You are correct, that the source impedance Ze or Zdb is also required ... and also that the (R1+R2) used should also be calculated for 70 degrees C (this is the purpose of the 0.8 factor rule of thumb if comparing "as-measured" values with Tables 41.2 to 41.5 of BS 7671) ... or a higher value of temperature for cases where conductor temperatures under load are higher (not recommended unless you can prove that terminals of equipment connected to a 90 or 110 degree conductor can withstand that temperature).

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