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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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    Some calculations I have come across simply calculate Zs from the combined resistance of the cables within the installation

    They shouldn't. The basic formula is Zs = Ze + R1 + R2 - where R1+R2 are the "internal" resistances, but Ze is the "external" (i.e. source plus impedance of the public supply wiring).

    Faults inside equipment are trickier - windings can add very considerably to the loop impedance and often the installation installer can have no knowledge of actual numbers (e.g. when designing to supply a socket outlet into which anything could be plugged). Generally BS 7671 designs cover the installation and then it's down to appliance standards to work out how to provide protection for internal faults. In general though, the larger the internal resistance, the lower any resulting touch voltages, so longer disconnection times become tolerable, so often it not quite as bad as it might appear at first sight.

      - Andy.

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  • 0

    Some calculations I have come across simply calculate Zs from the combined resistance of the cables within the installation

    They shouldn't. The basic formula is Zs = Ze + R1 + R2 - where R1+R2 are the "internal" resistances, but Ze is the "external" (i.e. source plus impedance of the public supply wiring).

    Faults inside equipment are trickier - windings can add very considerably to the loop impedance and often the installation installer can have no knowledge of actual numbers (e.g. when designing to supply a socket outlet into which anything could be plugged). Generally BS 7671 designs cover the installation and then it's down to appliance standards to work out how to provide protection for internal faults. In general though, the larger the internal resistance, the lower any resulting touch voltages, so longer disconnection times become tolerable, so often it not quite as bad as it might appear at first sight.

      - Andy.

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