Earth grounding problems and RCDs (Strictly a UK scenario, I think)

[sophiecentaur]

TL;DR Summary

Unusually for UK, we have an overhead supply to our semi-rural house. The Earth resistance is below spec . . . . .

The story is that I did some wiring additions ` couple of years ago. I extended a Ring Main (cover your eyes dear US readers) and bought one of those socket checkers for a few quid. I have got this far in my life without one but I though why not splash out. The connections, of course, checked out OK but it warned of an inadequate Earth Impedance. Same result all over the house.

I got a local 'man' to look at the wiring and he confirmed the Earth Impedance was out of spec. But he said that most houses with overhead supply have out of spec values. So I bought a standard copper rod and hammered it into the ground, a few feet from the house. I then got a younger, fitter man to connect a fat Earth wire to the company board and, at the same time, he added a good Earth wire to the Gas Meter (we couldn't find one in the meter box).

But the Earth Impedance is still out of spec - hardly any different, afair (can't remember the values). I can't find any obvious sign of an old Earth spike but there have been two extensions so it may have been covered up and also it could have dried out. There is no company Earth so I have to assume that the Earth Impedance Test is actually including the resistance of the (long) Neutral supply cable, rather than the nice armoured sheath that you find in an underground supply.

Any thoughts about this? I have an RCD so I don't think I have any worries.

 

[Guineafowl]

What earthing system does your house have - TN-S, TN-C-S, or TT?

TN-S - Earth all the way from the transformer, usually via cable’s lead sheath - obvious at the service head/main fuse. Max Ze 0.8ohm.

TN-C-S - Earth spilt from neutral at service head. Max Ze 0.35 ohm.

TT - no Earth from supply - Earth rod or mesh. Max Ze about 200 ohm, preferably <100.I guess the latter. If your Ze (external loop impedance) is too high, it’s usual to install another Earth rod, or a deeper one (you can join two together) or go for mesh and other things, like a disc.

You can measure the Earth rod’s contribution to Ze using an Earth stake method and a multifunction tester with Re function. Some can estimate from the Ze test itself.

You can assess the L/N contribution by comparing the Ze with the L/N loop impedance.

A poor Earth loop impedance reading is not a particular problem with whole-house RCD protection, but anything over 200 ohm is considered unstable and likely to worsen with time.

 

[DaveE]

TIL what a Ring Main is, never saw or heard of one before. Also learned RCD = GFCI.

AFAIK, ground impedance isn't measure in the US (normal residential applications). You just build it like the building codes say and assume it's OK. OTOH, I'm an EE, not an Electrician.

 

[Svein]

Welcome to the world of "scoured rock" (the last ice age did the scouring). You just discovered the reason why "null" and "protective ground" are two very different things.

 

[Rive]

There is no company Earth so I have to assume that the Earth Impedance Test is actually including the resistance of the (long) Neutral supply cable, rather than the nice armoured sheath that you find in an underground supply.

Guess this means you have 'TT'?
Is TNCS an option there?

 

[sophiecentaur]

TIL what a Ring Main is, never saw or heard of one before. Also learned RCD = GFCI.

HAHA that's why I said my question was for UK members only. It's a brilliant invention and gives you a virtually unlimited number of outlets with individually fused plugs in an area. Read this.

 

[sophiecentaur]

What earthing system does your house have

It looks like T-T as we only have two cables from the Pole.
The 200Ω requirement figure seems to be the relevant one. I have to assume that the resistance must be measured between the N input to the house and the local Earth structure. That has to include the N cable, all the way back to the transformer Ground, I suppose.
With just a DMM, I don't see how I could measure the N/E resistance myself as there are always a few N Volts to Earth. Putting and taking a resistive load and measuring the change in volts would give a way into finding the R. I guess that must be what the 'tester' meters do. My cheapo one fails my system because it assumes it's looking for 0.35Ω.

My daughter has bought an inflatable Hot Tub with a 13A mains lead and an integral RCD. It fails to start up and an error code comes up to say the supply Earth is inadequate. Same problem, I guess but with no excuse as it's an underground supply. I'm hoping that a new consumer unit and improved Supply Connection would sort that. She doesn't have the problem of a two conductor supply. As a side note, that RCD is rated at 10mA and not the usual 30mA tripping current.

 

[sophiecentaur]

Guess this means you have 'TT'?
Is TNCS an option there?

I could always ask the Company if that would be acceptable but I don't see how there would be any safety advantage in connecting the N to Earth locally. It would help my installation to pass the low Resistance requirement, though. I have read labels on company supply terminations that forbid NE connection in the home.

 

[Baluncore]

A ring main.
That explains why the plugs I remember as a child in the UK had a fuse.
There is no fuse in Australian plugs.

 

[sophiecentaur]

A ring main.
That explains why the plugs I remember as a child in the UK had a fuse.
There is no fuse in Australian plugs.

The 13A plug could be thought of as too bulky but they tend to be high quality and only the really cheap ones get overheated when supplying a 3kW appliance. Assembling them is a thing of the past because the moulded ones tend to last the life of the appliance.

 

[Rive]

I have read labels on company supply terminations that forbid NE connection in the home.

That's the same for (official) TNCS too. The connection is (and: only!) within the main switchboard, with the required equipment.

I like TNCs since it gives PE a good (copper) local loop to trip the fuses in case of problems. You are also independent of the return current of the neighbourhood since it's shorted to ground outside.

With a good RCD it does not matter that much, I think.

 

[Guineafowl]

You may notice that the neutral (usually lower) overhead wire is tapped to Earth at each pole. This is known as PME, protective multiple earthing.

You could call the DNO and request a TN-C-S earth, but there are problems with those - if the neutral (which becomes a combined protective E+N, or PEN) breaks, this will cause earthed metal objects to rise to mains potential. Also not favoured for EV charging points, looking to the future, or metal framed buildings and caravans, for the same reason.

The external loop impedance, Ze, is the loop from Tx - Line - Earth rod - tx. You need a special tester for this.

I would get it checked and sorted properly, keeping in mind the Earth is the foremost protection from fire and shock, RCDs or not.

 

[sophiecentaur]

You may notice that the neutral (usually lower) overhead wire is tapped to Earth at each pole.

This caused me to go outside and look again. There are four twisted, insulated conductors running between poles and I guess there's some supporting steel inside the insulation. From what I can see, two (L and N) emerge and are strung from pole to house roof with more steel. I was assuming that the steel is just for support but it could be a ground conductor. My neighbour (same pole) gets a different L feed for his. I guess I will need to ask the Company what's actually up there. If it's not a TT system then, despite what 'my man' said about it often failing out here in the country, should I not be expecting a low Earth resistance?

More homework needed for me I think.

 

[Guineafowl]

This caused me to go outside and look again. There are four twisted, insulated conductors running between poles and I guess there's some supporting steel inside the insulation. From what I can see, two (L and N) emerge and are strung from pole to house roof with more steel. I was assuming that the steel is just for support but it could be a ground conductor. My neighbour (same pole) gets a different L feed for his. I guess I will need to ask the Company what's actually up there. If it's not a TT system then, despite what 'my man' said about it often failing out here in the country, should I not be expecting a low Earth resistance?

More homework needed for me I think.

I imagine those four are 3L + N from a wye secondary, all the same colour but with different rib patterns for identification.

Take a good pic of the service head/cutout area and we should be able to identify the earthing system. Most likely TT in rural and older homes.

Look up the specs of your socket tester - see what Earth loop value it will ping at.

 

[sophiecentaur]

Take a good pic of the service head/cutout area and we should be able to identify the earthing system. Most likely TT in rural and older homes.

Hand held with a very bright sky - this is the best I could do.

Our supply is the nearer of the two. There seem to be two conductors going into the top left cable. But I also spotted a pair of bare conductors, strung from pole to pole beneath the supply. Could that be an Earth?
Note: I could see no connection between this extra pair and the power lines (on our pole or any nearby pole. And what could that box, connecting the pair together, be doing?

 

[Guineafowl]

I actually meant the service head, ie where the supply comes into your house, where the large 100A (or whatever) fuse is.

Having said that, it certainly looks like you are not provided with an earth, therefore you have a TT supply. (Or maybe IT, if you’re in Norway;))

I can’t say for sure what the bare conductors are - but they certainly are at Earth potential, being tacked straight to the pole and uninsulated. Lightning conductors (possibly not)? Follow them to their origin!

Where the DNO provides a separate Earth conductor (TN-S), it’s usually derived from the lead sheath of the cable, rather than a dedicated wire.

If your Earth rod has a threaded top, you can buy a coupler, connect on another rod, and bang the lot down into the ground.

 

[sophiecentaur]

No sign of a third conductor emerging from the fuse holder block so you're probably right about TT.
Cop a load of the smart meter! Some people seem to have trouble but I've ben ok for about five years with ours.