Neutral fault loop test

**Firepool** · 06-Apr-22, 11:19 PM

Originally posted by skatingsparks

I got the new SANS 10142 detailing the new test for the Neutral loop impedance test.

Note 2 of 8.6.5.2. says "At no stage may neutral ever be higher than that calculated for the earth"

Currently doing a COC on an old building.

In a big old building with loads of metal conduit buried in concrete and bonding on the pipes etc - surely it wouldn't be unusual for the earth reading to be lower than the neutral reading? Main switch on each local DB is off during the test and is done from the top of the mains switch.

I know it says calculated but how are you supposed to calculate the effect of lots of metal work buried in the fabric of a building?

AIA inspector has been out and is happy with that set up so covered my own a%$# as much as I don't like just having just conduits as the earth.

Every Sub DB in every flat the Neutral loop impedance is higher (not by much, but every flat) than the earth fault loop.

Worst reading so far (top floor flat furthest from main DB in basement) earth fault loop 0.24 ohms and neutral loop 0.28 ohms.

But I'm not surprised by that - am I misunderstanding something?

Good question .My question is how would you even fix it if was too high.Rewire the entire block of flats? Do you agree that anything over 0.35 ohms on the earth loop test is high or do you know what sans says about the earth loop test reading.

**skatingsparks** · 07-Apr-22, 06:42 AM

Originally posted by Firepool

Good question .My question is how would you even fix it if was too high.Rewire the entire block of flats? Do you agree that anything over 0.35 ohms on the earth loop test is high or do you know what sans says about the earth loop test reading.

Even if you pull new cables it will only improve the earth loop reading as I would pull earth wires with it as its now a new install so that would make the earth reading even lower.

All the book asks for is that the loop impedance will allow a fault current "double the rated currrent (or higher) of the main protective device automatically disconnect the supply from the instalation"

So with a 63 amp breaker loop impedance could be as low as 1.76ohms. Wouldn't like it but can't fail it.

minimum fault current = Volts/(2xbreaker rating) = 230/(2*63) = 1.76ohms

How you supposed to lower the neutral reading to be lower than the earth reading in this case?- you can't.

Or am i missunderstanding this?

**Isetech** · 07-Apr-22, 07:43 AM

I have a similar situation, old building, rusted pipes and bad earth continuity. I thought simple, just pull in an earth wire. Well that' where the problem started.

Its seems the pipes have rusted in the concrete, making it impossible to remove the wires, never mind trying to pull in an earth wire.

The only solution for me is to rewire.

**Dylboy** · 07-Apr-22, 09:42 AM

As for the calculated thing, I feel as long as the neural loop trips the CB at the double rated then that's good. Otherwise as you say how would you improve the neutral.

I guess could add a resistor in the earth loop to make it worse... hahaha I am joking

Now I am big into the UK regs and the BS 7176 is only a gudience... I.e used as a guide ans a defence in case taken to court as you are only going to court if you break the EAWR act stuff... bascially following the BS7176 keeps you in line with the EAWR.

So if you don't follow the book but can prove that all is fine to the EAWR then you ok as fbe BS7176 is not Law, only guidence

So is our Sans the same thing or is it Law here ?

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Neutral fault loop test

Neutral fault loop test

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