Mains earthing

[Set Square]

ISTR from discussions in the past that there are various alternative earthing schemes used with regard to mains earthing - some of which use an earth provided by the electricity supplier, and some of which don't.

Can some kind soul please either remind me of the salient characteristics of each scheme - or point me to an on-line source of said information, with particular reference to:

• how I determine which sort I've got

• which schemes (if any) continue to provide an earth in the event of a power cut

Many thanks.

[Chris Oates]

[ARWadsworth]

characteristics

Adam

adamwadsworth@(REMOVETHIS)blueyonder.co.uk

[Owain]

"Set Square" wrote | ISTR from discussions in the past that there are various alternative | earthing schemes used with regard to mains earthing - some of which use an | earth provided by the electricity supplier, and some of which don't. | Can some kind soul please either remind me of the salient characteristics of | each scheme - or point me to an on-line source of said information, with | particular reference to: | * how I determine which sort I've got

TT - earth rod TN-S - supply protective conductor / cable armour - often a clamp round the supply incomer before it goes into the service fuse. TN-C-S - supply neutral used for earth (also called Protective Multiple Earth) - an earth wire will come out of the service fuse along with the meter tails. TN-C-S can have an earth rod as well sometimes.

| * which schemes (if any) continue to provide an earth in the event of a | power cut

TT

Owain

[Set Square]

system is TN-S.

I have what looks likes SWA cable coming out of the ground and connected to the main fuse. There is a thick black cable (presumably containing L & N conductors) from the top of the main fuse to the meter. On the side of the main fuse block are screw terminals to which several earth wires are connected - to the consumer unit and to pipes etc.

Is it fair to assume that earthing is provided via the armour of the SWA cable - which is connected both to phase neutral and to a true earth at the local sub-station? If so, is it fair to assume that this true earth connection will continue to exist even during power cuts?

[-]

In article , Set Square writes

Not if the power cut is due to a damaged cable, i.e. JCB digs up cable.

[Dave Liquorice]

Seems reasonable from that description.

No, the power cut could be due to a JCB slicing through the cable...

I don't think you can rely on anything provided by the electricity supplier during a failure. This is where it gets tricky when installing a generator and its earth... B-)

[Lurch]

A more common method would be TN-C-S but they both effectively present themselves at the cutout in the same manner. During a power cut it is quite possible that the earth could become disconnected either by the original problem of a line coming down for instance or in the process of repairing a fault and having to dig a cable up. Quite a few people I have worked with in the past have prefered to use a TT system in order to be able to provide an earth themselves. I could go into quite a bit of detail but if you really wanted to know I could probably find some info for you somewhere. Oh, and FYI the cable coming in from the REC is more than likely concentric, not SWA. When you say thick cable, do you mean like 15-20mm dia. or 25-30mm dia. region. If the latter it could be TN-S, the former more likely TN-C-S

[Lurch]

Actually, in the event of a powercut if there was no power there wouildn't be a need for the earth. Having said that, if the earth was required for the purposes of the generator you would need to reference the generator earth to the same potential as the TT arrangement.

[Set Square]

The cable coming out of the ground is about 15mm and the cable from the fuse to the meter is about 13mm.

Yes, perhaps it is TN-C-S because there is no clamp with an earth connection around the incoming cable - the earth connection is higher up, alongside the fuse cartridge.

You may have seen my other thread about using a genny as an emergency power supply for central heating electrics and freezers in the event of a power cut. I am still trying to bottom out what to do about earthing such an arrangement - which is why I wanted to know whether I can rely on any sort of earth from the mains installation.

It looks like I can't. My current thoughts are that I should plant a metal spike in the ground and connect the genny earth to it - and also power all the critical items via an RCD when I unplug them from the ring main and connect them to the genny instead. Does this sound reasonable?

[Lurch]

I personally just chuck the genny in the back garden and plug a lead in to power the important stuff, TV, digibox, PC etc... but that's just me! Depending what size genny you use and what you're connecting to some come with a earth stud on the frame, connect this to an earth spike a la TT method. Also switch to TT on the main earth at the mains position. I have done this in the past with the installation of earth links to be able to bring in and out the two different methods of earthing, (that is TT and TN-C-S). If you're just running the CH and a couple of lights its probably not really required so yes, your original idea of supplying what you want via leads would suffice, as I said, it works fine for me.

[Chris Oates]

certainly can't - one failure mode is loss of neutral and with TN-C-S that can put everything 'earthy' at 230volts.

Normal method is a 'transfer switch' which does the job for you but that does entail turning off loads that the genny won't power

[Dave Liquorice]

And ensure that what the genny says is "neutral" is also connected to the genny frame and you spike. Do a bit of googling on the hows of planting a spike as well. It's not quite as straight forward as buying an earth rod and hammering it into the ground, though it can be.

As you are going the unplug and plug to generator extension wiring method it seems very reasonable and side steps that knottty earth problem.