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Thread: Loop Impedance

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    Loop Impedance

    So glad to find a place where I can ask questions.

    Is there a chart or table that can help me determine the acceptable loop impedance and PSU results on an existing installation.

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    From SANS 10142-7 : 8.7.5, At the main switch, the impedance shall be such that an earth fault current double the rated current (or higher) of the main protective device automatically disconnects the supply to the installation. The formula
    for a 60 amp supply : voltage over 2 x current ( V/2I ) ie 230 divided by 120 = 1.92ohms. A reading less than 1.92ohms is acceptable. (p.s. post again if you need this test described in laymans terms)
    Your second question - I presume you mean PSC (prospective short circuit)(rating). What is the rating of your breakers?
    Probably 5kA or 5000amps (marked on the breaker) - your reading must be less than that.

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    Dave A (27-Apr-12)

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    Thanks! I was recently told by the council we can't accept readings above 1.4ohms. Got a bit confused.

    On 3 phase - would I then use 380V or 230V.

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    If your reading is high, (too high for your Council's liking), consider installing an earth rod.
    Pretty sure you would use 230V for a 3 phase installation as you are testing the earth resistance back to the transformer where the suppliers earth is supplied.

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