DIY Doctor

House Supply Earth Loop Impedance High

Postby willsie01 » Tue Sep 03, 2019 1:38 pm

Hi,

After remodelling work on my house the electrician tells me he is unable to issue a certificate because the earth loop impedance of the incoming supply is out of the accepted range. He also checked a neighbour's house and found the same issue so said this meant the problem was off our properties. UK Power Network sent an engineer who checked my supply and the local substation and confirmed this. He tells me the next step is a crew to come and dig a hole. Problem for me is that UK Power aren't treating this as a priority although it is for me! Do I have any recourse against UK Power?
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Postby ericmark » Wed Sep 04, 2019 11:52 pm

In a word no, the DNO are not required to supply an earth, they normally do, but where the earth is not below 0.35 ohms (TN-C-S) or 0.8 ohms (TN-S) then they can require you have a TT supply where their earth can be up to 15 ohms. And you need to fit an earth rod.

Since today all circuits are RCD protected if the earth is below 200 ohms it is safe, above 200 ohms it is considered as unstable it would still comply but is not considered safe.

In the main with a 100 amp supply with a TN-C-S supply to keep within the volt drop the line - neutral impedance needs to be 0.35 ohms or better, so in the main they have to have an earth better than 0.35 ohms to satisfy volt drop requirements.

I know my son had a problem with a house with no supply at the time of re-wire. He phoned up DNO and asked what type of supply, and was told TN-C-S so he left three tails line, neutral and earth to be connected by the DNO, however when they came to connect they supplied a TT supply, so there was no earth to the house.

There has been a move to require an earth rod as well as the DNO earth, well technically it's not an earth electrode it an "extraneous-conductive-part" but idea is should the DNO earth be lost, then you still have an earth, it is important it is not too good, or excessive current can flow, so normally aim at around 60 ohms which is enough to ensure a RCD will trip but should the DNO earth be lost it will not pass excessive current.

However with RCD protection for earth to be too high of an impedance so it will not pass an EICR it would be over 200 ohms, as to what to do if you don't have RCD protection not so sure, the old 100 or 300 mA RCD used before 30 mA versions were fitted on every circuit are not stocked as they were, and to use a single 30 mA on all circuits it will likely trip, so it could need a consumer unit change to comply.

The more 30 mA RCD's you have, the less likely any one will trip, I have 13 RCD's in this house, they are combined with the MCB so called RCBO's but these are longer than the standard MCB so will often not fit in old fuse boxes or consumer units.

However as an electrician with domestic he has a real problem, he is not permitted to make the house uninhabitable, which it would be classed as if he leaves the power switched off, but also he is not permitted to turn on the power if the circuits don't comply. He is required to find you alternative accommodation however he is not required to pay for it, so in real terms he has a problem, as he should have not turned the power back on.

If I had the problem I would fit a stand alone RCD even if only temporary.

This is from 17th Edition I don't have 18th.

314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

Fitting a single RCD (iv) is where the problem lies. There has been arguments among electricians as to what is a circuit, does a RCD produce a circuit, or does only a MCB produce a circuit? if former then having just two RCD's is hardly enough, if latter it is, but the 30 mA RCD in my old house was a problem, there were two, but we would get batches of tripping, it may trip 6 times in a week then not trip again for 2 years. The new consumer units have filters in them, that may reduce RCD tripping?

But nitty gritty it is down to your electrician to do some thing. Be it fit an earth rod, or RCD's he has to do some thing to make the system comply, unless there is also a volt drop problem the DNO are not forced to do anything.

Do wait for further answers, my option may be wrong, as I don't have 18th to check for updates.
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Postby willsie01 » Thu Sep 05, 2019 11:57 am

Thank you for info.
We do know the system is TNS.

It sounds like your bottom line is our domestic wiring electrician should provide a solution? The impression I get is he thinks it's the DNO who should rectify it.

Regards

John
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Postby ericmark » Thu Sep 05, 2019 5:20 pm

He is likely correct the DNO should rectify the problem, however if they don't then it's down to him.

If he can show there is a second problem, like volt drop, then their hand can be forced, but the DNO do not need to supply you with an earth.
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Postby willsie01 » Tue Sep 17, 2019 2:32 pm

UK Power sent a crew to follow up their earlier check who disagree with the findings of my electrician and their own engineer who made the first check. Contrary to the first tests they're now saying the problem is not in the supply but in the house wiring.
There is an Earth block mounted next to the incoming supply and this is connected via a short loop to an earth strap on the incoming cable. From this block three other earth cables run as part of the internal house system. The UK Power crew have identified one of these as being the problem. One cable I can see connected to a strap on a gas pipe and another I can see going behind the board on which the CB Board is mounted so I guess this is where it is connected. The remaining problem earth disappears under the suspended floor and I don't know where it terminates yet. If this is the problem cable what will need to be done to diagnose the problem?
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