Earthing

[Toom Tabard]

My electric wiring was done 25+ years ago, professionally, has given no problems, and is still in good nick. Earthing, not now compliant, is by bonding to the incoming pipe containing power supply cable and that seemed to work. British Gas heating advisor came to estimate for new boiler 2 weeks ago, tested my power sockets with 'socket and see' device and everything ok including earthing. Engineer came to install boiler yesterday and his 'socket and see' showed earthing totally inadequate. Electrician then did more detailed measurement and got earth reading of 5ohms (should be under 1). Boiler installation now postponed, and can't really expect anyone to work on faults on existing boiler The one possibly relevant thing in the two weeks has been gas mains in street and up to house renewed with plastic pipes. Meter is now outside with no earthing. Equipotential bonding is on copper pipe pipe from meter just after it enters house. It used to be on consumer side of internal meter. Could it be that the iron supply pipe (now disconnected) on other (supply) side of the meter was providing some of the earthing for my electricity circuits through the equipotential bonding? Power distribution is responsibilty of Scottish Power who are coming to look at fitting fused cut out and mains earthing terminal at consumer supply point. Apparently so many people are finding their houses non-compliant for various reasons that I may join a 6-8 week queue. Anyone know of any other compliant answer to getting earthing to required level?

Thanks

Toom

[cynic]

Convert the installation to TT.

[Invisible Man]

Which will mean RCD protection and an earth spike.

[Peter Crosland]

After 25 years you should seriously consider getting the entire wring system checked by NICEIC qualified electrician.

Peter Crosland

[Robin]

NICEIC are not the only game in town (albeit they try to give the impression people they are). Other "competent persons schemes" for installers who can do all electrical installation work are BSI, EC Certification Limited / ELECSA and NAPIT

(And yes, I dislike monopolies and wannabe monopolies.)

[js.b1]

Conflict of interest between those doing the PIR and those wanting to do remedials :-)

It sounds like the OP has 2 issues. #1 - Supplier earth is not up to specification. A DNO provided earth must be maintained once provided. If the supply is TN-C-S (PME) or TN-S the Ze figure should be

[John Rumm]

That sounds like TN-S earthing, and is still an acceptable method today.

Does not tell you much - most plug in devices like that would report anything under 10 ohms as fine.

This is a serious fault in the supply.

Indeed.

The main EQ bond to the gas supply path may indeed have been creating some "fortuitous earthing" - hard to tell now.

I would strongly suggest to them that they they come and fix it immediately, since you are at serious risk from what is a dangerous supply fault. If your external earth loop impedance is genuinely 5 ohms, and you don't have *all* your circuits RCD protected, then you have a problem as a result of lack of prospective fault current.

A minimum of 5 ohms earth loop will limit your fault current to 46A. If you look at the response curve for a type B MCB (the most common type):

will see that a 32A protected ring could remain energised indefinitely even if you drove a nail into the cable and caused a live to earth short! Something is going to get very hot....

[Toom Tabard]

Thanks to all who replied - that info is very helpful. I'm not an electrician and don't understand all the terminology used. But, if a gas meter can allow a continuous circuit - can it? - metal pipe to metal chassis to metal pipe? - then that might explain the earth working the other way into the iron supply pipe and maybe helping earth the whole house, and why the change to unearthed external meter/ box with plastic supply pipe has made a difference. I called in a senior technician from the gas supply renewal company, who are still working in the area - he seemed unaware that that could happen and is interested to find out if it is the cause - I suppose few folk happen to have their earthing tested just before and then just after gas supply replacement.

The distribution company - Scottish power - did send someone within hours. They checked it out, but merely said I should phone their installation dept and arrange for them to come fit their standard earthing. He did mention the possibility of getting an electrician to fit a front end RCD meantime but said they'd need to check if that was regs 17? compliant - he wasn't sure.

Neither boiler company nor distribution company electrician gave any indication of it being an emergency situation, and there is this mention of 6 - 8 week queue for rectification of many similar situations - mainly where water mains and water supply pipes have been replaced with plastic. Scottish Power are sending someone to look at rectification in detail - but not for several days.

I'll look into it further, since a more urgent temporary or permanent fix seems needed, both for safety and to get boiler installation moving.

Thanks again to all

Toom

[ARWadsworth]

I have come across quite a few TN-S supplies with a high Zs.

Usually cleaning the incoming supply clamp up fetches the supply back into spec.

I would suggest that the OP leaves it to his supplier to fix if he has not got access to an earth loop tester.

If a supply is out of spec and I am unable to fix the problem then an engineer usually comes around within an hour of them receiving the phonecall.

[BruceB]

The senior technician is just being dozy, either deliberately or otherwise. In other areas, where utility companies work on gas or water mains replacing metal with plastic they send postcards out to affected consumers warning them of the possible effect on earthing and that it is the customer's responsibility to sort out.

You have done well to get the supplier to do something about it, particularly if they will sort out the earthing for free. In many areas, DNOs seem to try to deny they provided the original earth and so it becomes the consumer's problem via his electrician.

Regards Bruce

[John Rumm]

Most ought to be covered in the FAQ or wiki, however just ask if unsure.

Details on earthing:

equipotential bonding:

gas meter can allow a continuous circuit - can it? - metal pipe to

Indeed this can happen. The difficulty that clouds the area here is that you don't actually have proper measurements of the earth impedance made at each stage - some of the tests were just go/nogo tests with a plug in socket tester.

Ideally you would need measurements made with a proper earth testing meter, e.g. see:

and after to be able to get enough useful information.

Surprising that he was so clueless!

17th Edition of the wiring regs probably, aka BS7671. Just adding a single RCD to the front end of the installation would not really comply since it offers no "discrimination" - i.e. in the event of a fault, it trips the supply to all circuits regardless where the fault is. Protective devices ought to discriminate and only isolate the faulty part of the system.

If you are going to live with a high earth impedance, then the who electrical system needs to be designed to take this into account. Folks in rural areas often encounter these problems when the supplier can not provide and earth at all. These are called TT systems:

Neither boiler company nor distribution company electrician gave any

Which suggests they have not thought through the implications!

What sort of consumer unit do you have? Fuses, MCBs etc? A photo might help so we can give you some idea of what options are open to you. They might range from "you can make it safe with a small change here" or "you need a complete new consumer unit!"

Not that this really ought to be your problem!

[Toom Tabard]

Many thanks for all that info/pointers, and to others who've given info and advice. I'll read up on it and am trying try to locate some reliable expert locally. I'd leave it to the line distribution company if they'd come quickly, give a ballpark figure and get on with it. So far have had a both phone call and a letter saying they'll contact me to arrange a time to examine system and 'produce a quotation'.

Thanks again,

Toom

[Toom Tabard]

Electricians and Scottish Power have both mentioned the problems when earthing is to water pipes and these are replaced with plastic. Senior technician for gas pipe replacement said they leave cards when they notice the equipotential bonding on internal gas pipes isn't there, or isn't correctly positioned. Position of mine isn't quite correct but that wasn't mentioned at the time or by card. It does seem they could be a bit more consumer friendly about it, but general approach seems to be they do their bit and don't want to get involved in the complexities which exist or which they cause in individual houses. In my case, if equipotential bonding was incidentally giving earthing to my electrics through old metal gas pipe, then that seemed something technician didn't know about, but wanted to find out more. Said he knew all about equipotential bonding for gas regulations, but did know domestics electrics.

I gather from engineer that what is supplied and fitted at consumer unit is at customers expense and it's a good earner for the company. Any problem with earthing from there to distribution system is company's problem/expense.

Regards

Toom

[Toom Tabard]

Sorry should read Said he

[Clot]

After 25 years you should seriously consider getting the entire wring

I disagree. Provided that the circuits have not been modified in the time being, there is no need to have it checked.

Be aware, the Elfins are about.

[js.b1]

Main Equipotential Bonding (MEB)? That is required by the DNO ("company") under ESQW, and is something you can have done privately. Some companies offer a flat rate fee (=A3250) others charge based on time/materials because some jobs are very quick.

Earth provided by the DNO. Where a DNO has provided an earth such as TN-C-S (PME), and if I recall correctly TN-S, they are henceforth legally bound to maintain it. However there is probably a loophole whereby if it is "not reasonably practicable to do so" they can withdraw the facility.

I have a growing suspicion that some maintenance work has been outsourced in the past, with requirements for TN-C-S (PME) not properly acknowledged and without legal redress if the contractor company has closed. This might have been deliberate (as occurs with planning permission) because all DNO would "very much like" people to move to a TT supply because it removes the cost burden of maintaining a PME network. Due to difficulties (cost) in maintaining a sufficiently low Ze supply for a 100A fuse many DNO are routinely downgrading people from 100A to 80/60A main fuse. This is occurring even if people have previously applied for an upgrade, but not charged (Northwich).

At present the problem still remains - if you have a 45A MCB protecting a 11kW shower then with a 5ohm figure you will not get L-E fault disconnect - because only 48A can flow to earth when it takes

220A to trip a 45A Type-B MCB. Thus something else will act as the "fuse" - a piece of copper pipe or wire - hence the real chance of a fire.

[PeterC]

Ah, that explains why the chap wanted to put in an 80A fuse when he changed my meter last year. He muttered something about old wiring, but I persuaded him to use a new 100A fuse as I still have a couple of storage heaters as standby.

[ARWadsworth]

Due to difficulties (cost) in maintaining a

Changing the fuse will not alter the Zs.

The limits are 0.35 ohms for PME and 0.8 ohms for a TN-S supply regardless of the size of the fuse.

[ARWadsworth]

Due to difficulties (cost) in maintaining a

Changing the fuse will not alter the Zs.

The limits are 0.35 ohms for PME and 0.8 ohms for a TN-S supply regardless of the size of the fuse.

[Toom Tabard]

Thanks again - I'm now more aware of the possibilities, which I'll follow up if DNO can't fix quickly. I appreciate the dangers - fortunately don't run anything remotely approaching power requirements for an electric shower; wiring and our electrical devices all otherwise good. Large DNO backlog in our area (Edinburgh) rumoured to be largely because they've been digging up miles of main road to shift all utilities from under the new, over-budget, behind schedule and mired in litigation, intended tram system. All done in plastic (utilities I mean, not the tram lines) and folks in many old tenement flats have found their earthing affected/removed

regards Toom