Garage circuit earthing

[robgraham]

I think I've done enough searching of the archives and FAQ's to be satisfied that I have a reasonable knowledge of the topic now. Burt one thing I would like some guidance on please.

My house earthing is TT - overhead 2 wire distribution. The garage is some 20m from the house and even further from the house earthing point so is definitely a candidate for its own earth spike. A new CU with MCB's and RCD is being fitted. The garage has recently been extended to include space for a metal and wood working lathes, etc hence the rewiring.

Could someone confirm the following for me please. On the basis that there is an rcd to detect the relatively low earth fault current, due to the earth spike resistance, the earthing cable if is mechanically protected, can be 2.5mm. I've picked up the instructions for testing the earth resistance from the archive so I will be able to test that.

A further question is just how far from the CU can the earth spike be ? On the basis that the resistance of the cable is minimal in comparison to that of the earth spike, the distance can be the length of the garage at least ?

Rob

[Andy Wade]

Yes, a separate earth electrode or system for the garage may be a good idea. Although the house is TT earthed you could find that its main bonding connects the main earth terminal to the supply neutral via metal water and gas pipes and the main bonding of other consumers who have converted their installations to PME, thus effectively making the system TN-by-stealth. In this case there is a risk of raised earth potential occurring under fault conditions and you might not want that risk in the garage, if there isn't a well-defined equipotential zone.

If the above isn't a risk (all bonded services are plastic) and the house earth system is good, and provided there's no intention to convert it to PME, there's probably no need to provide the garage with its own earth electrode. The house earth could be exported via the CPC of the sub-main with no concerns over the earth-fault-loop impedance which is is dominated by the earth electrode resistance. However an additional electrode for the garage is still a good idea. If you're confident that the house is pure TT you can parallel the two electrodes via the CPC giving a degree of redundancy and lowering the Ze for both installations. Alternatively you can separate the two earths, giving two separate TT systems.

Yes, but if buried it needs to be protected against corrosion _and_ mechanically protected for that to apply. Also any future conversion to PME would require an increase in the earthing conductor size, so I'd be inclined to stick to the usual 16 mm^2 and avoid any doubts.

At least. Unless you're worried about externally created potential gradients along the ground (any substations or electric railways nearby?) and touch voltages to local ground, or about lightning protection, the distance is not really an issue.

[robgraham]

Thanks Andy

I think it pretty unlikey there will be any change to PME in the near future, and I'm sure that if it is done it will be done professionally and it is then up to them to make themselves aware of what the current earthing arrangements are.

I hadn't considered running the two earth spikes in parallel, which is obvious when one considers it. It also now comes to mind that it wouldn't be a bad idea to check the existing house earth - I'm not sure whether this is regarded as a necessary maintenance activity but nature being what it is, it is probably not a bad idea to check such an earth every so often for corrosion.

Cheers Rob

[Andy Wade]

One of the arguments sometimes used against TT is that it does demand a higher level of maintenance, as so much depends on the earth system. In any case BS 7671 _requires_ you to check any existing earthing and bonding on which new work will rely for safety, so if you're using the house earthing as part of your extensions, it should be checked.

[Christian McArdle]

Personally, I would always specify new work to be up to PME standards. Then one day, when the electricity board turn up unexpectedly with a new cut out, it can be used. This is particularly important when specifying main bonding. Just use PME compatible CSAs and be done with it.

Many rural supplies have been converted to PME, even on overhead lines.

You are mistaken when you say it is up to them to make themselves aware of your current earthing arrangements. This is not the case. They simply provide an earth terminal and have no responsibility if you decide to use it, or whether or not your system is compatible with it.

Christian.

[Andy Wade]

But for every consumer's supply they should have accurate records of (a) the type of earthing (if any) they offer, i.e. TN-S, TN-C-S, PNB or none, and (b) whether or not a TN earth connection is in use.

[Frank Erskine]

Northern Electric wouldn't tell me what my earthing arrangements is. They said that they would have to come out to check, but that they don't do this, and I would have to contact my "electrician".

As it happens, I found a label inside the meter box saying that it's PME (TN-C-S), and I confirmed this with a Northern Electric engineer working around the corner on the 11kV substation.

[Andy Wade]

Quote the law at them:

Statutory Instrument 2002 No. 2665 The Electricity Safety, Quality and Continuity Regulations 2002

to be provided on request

1. A distributor shall provide, in respect of any existing or proposed consumer's installation which is connected or is to be connected to his network, to any person who can show a reasonable cause for requiring the information, a written statement of -

(a) the maximum prospective short circuit current at the supply terminals;

(b) for low voltage connections, the maximum earth loop impedance of the earth fault path outside the installation;

(c) the type and rating of the distributor's protective device or devices nearest to the supply terminals;

(d) the type of earthing system applicable to the connection; and

(e) the information specified in regulation 27(1),