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Topic Title: Earth Loop Impedence and PSCC
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Created On: 16 July 2017 12:43 PM
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 16 July 2017 12:43 PM
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Ragh6402

Posts: 11
Joined: 28 July 2015

Hi ,

I recently attended an earth loop impedance and PSCC measurement test. The test was conducted live, the operative used three leads , one connected to the phase, second to neutral and the last to earth. The test was carried out at the DNO incomer fuses. Now my question , is it not required to isolate the supply and disconnect the means of earthing to get the external loop impedance?. I understand if it is not disconnected will it not result in measuring the impedance of the whole installation including the downstream circuit rather than measuring Ze ( impedance external to the network). I do agree if the test was carried out at the final outlet it would be acceptable as it would yield Zs , i.e sum of Ze + R1 (phase conductor ) + R2 ( CPC).

My second question- I understand PSCC is the maximum short circuit current , which is the current resulting from shorting all three phases together. But what the operative did was just to double the value that was obtained between phase and neutral, is this correct???. Is there any more multiplication factor to be applied to arrive at the actual PSCC value??
 16 July 2017 01:21 PM
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geoffsd

Posts: 1651
Joined: 15 June 2010

Originally posted by: Ragh6402

I recently attended an earth loop impedance and PSCC measurement test. The test was conducted live, the operative used three leads , one connected to the phase, second to neutral and the last to earth. The test was carried out at the DNO incomer fuses. Now my question , is it not required to isolate the supply and disconnect the means of earthing to get the external loop impedance?.

Yes. That is how Ze is measured.
He may have used three leads but set the meter to L - N.

I understand if it is not disconnected will it not result in measuring the impedance of the whole installation including the downstream circuit rather than measuring Ze ( impedance external to the network). I do agree if the test was carried out at the final outlet it would be acceptable as it would yield Zs , i.e sum of Ze + R1 (phase conductor ) + R2 ( CPC).

Correct for three wire test - that will be Zs at the incomer including any bonding -
but if the meter is set to L - N that will be what he is measuring.



I understand PSCC is the maximum short circuit current ,

It is. L - N.

which is the current resulting from shorting all three phases together.

No. PSCC is the result of the L - N loop. (set meter to L - N, or N and E leads both connected to N, or two wire test)
Prospective Earth Fault Current is the result of the L - E loop.
Prospective Fault Current is the larger of the two.

But what the operative did was just to double the value that was obtained between phase and neutral, is this correct???.

That sounds like a quick estimate of PSCC for L - L in three-phase installations.
No need on a single phase installation.
 18 July 2017 03:45 AM
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spinlondon

Posts: 5490
Joined: 10 December 2004

To measure Ze, you would disconnect the installation earthing and bonding.
However to measure Prospective Earth Fault Current the installation earthing and bonding has to be connected.
Doubling up the highest measured Prospective Short Circuit Current is one method of determining the PFC on a 3 phase installation.
Other methods are to add the two highest measured values together or preferably to use a 3 phase PFC tester.
Of course the values measured today are likely to be different to those measured tomorrow or next week.
Values can differ even when measured half an hour apart.
Statistics

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