Cable down to the shed

[ex 2 Base]

My neighbour has run a  3 core, 4mm steel wire armoured cable down to his shed, about 80 feet from their split way consumer unit ( two elcb, I'm not familiar with the modern term). There is a 32 amp spare circuit breaker in the consumer unit which he intends to use. The cable will feed a ring socket outlet circuit and a couple of lights. He has a  small metal consumer unit with two circuit breakers in already mounted in the shed, one for lighting the other for the sockets. The question is should he install an earth electrode, as I seem to remember that without an electrode he is exporting the earth protection and some special precautions should be taken. Any advice will be much appreciated.Ted   

[Yorkie]

I'd be intersted in the answer because I did likewise fifteen years ago!

[ppppenguin]

We'll ignore the fact that this work needs to be certified by somebody in a registered scheme (NICEIC, NAPIT ELECSA etc) and assume he's reasonably competent to do a decent job. Strictly speaking he shouldn't be doing this work himself.

Exporting earths is a slightly tricky area. This can often be done safely, usually if there are no extraneous conductive parts in the shed. Generally this means no water pipes or large lumps of metal connected to local earth. There are other issues too. You have already told us that the circuit is protected by an RCD (the modern term for ELCB) so that's good. A 32A MCB will protect 4mm cable though I haven't done the volt drop and earth loop calculations to ensure that the MCB will in fact trip satisfactorily in case of short circuit. Assuming that the earth is being exported I hope that one core of the cable is being used for earth. The armour should be bonded at both ends.

Next is discrimination. A 20A MCB at the shed will just about give discrimination wrt the 32A MCB at the house. A 32A MCB at the shed is not recommended.

The installation must be tested to ensure low enough earth loop impedance and correct RCD tripping.

If you decide not to export the earth then all sorts of things change. Won't cover that in this post.

As an example my own detached garage is fed via a dedicated 30mA RCD and a 20A MCB via 4mm 3 core armoured cable. Sockets are fed directly, as an ordinary 20A radial circuit. The lighting is via a FCU (fused spur in old terminology) with a 5A fuse. There is no signifcant earthed metal in my garage so I reckoned it was OK to export the earth.

[pwdrive]

What a coincidence, after a long delay (years) I finally connected the caravan in the garden to our house supply two days ago, I should mention at this stage that the caravan is not a metal one, it's of timber construction, the water supply comes in via the usual blue plastic water pipe buried underground, it rises out of the ground to connect to the caravan, from then on in it's copper pipe inside.

The feed cable is armoured 3 core (10mm square conductors) fed from the split box consumer unit in the house, rightly or wrongly I've earthed the protective galvanized multi wire sheath at the consumer unit end only, two of the cores are live & neutral, the other core is earth (exported) that earth is exported from the earth bus bar in the house consumer unit.

In the caravan the armoured cable feeds a four way consumer unit and an electric shower, the four light/socket circuits from the consumer unit are RCD protected and all have MCB's, the feed to the electric shower is directly from the the armoured cable input to the consumer unit via a separate shower RCD/MCB unit, all the copper pipes in the caravan are electrically bonded with 6mm earth cable which is connected to the caravans consumer unit earth (the exported earth from the house) I did all the main wiring some years ago, I've got some 10mm earth cable spare, should the 6mm bonding earth cable within the caravan be changed to 10mm?

The caravan feed from the house consumer unit is via an MCB and RCD within the consumer unit, the MCB and RCD are suitably rated.

The incomer earth from the DNO is PME.

I've consulted various sources as to whether or not it should be TT'd, some say do, some say no need, anyone know for sure given the system described?

Anything else I need to think of?

Lawrence.

[Terry]

I think you'll find that this earthing problem is a complex one.

I would suggest that you post a request on the  uk.d-i-y Usenet news group where you will find lots of helpful advice from professional electricians.

I think it will be useful if you can also provide details of how the existing supply is earthed as, from memory of previous such requests, I believe that has a bearing on the answer.

There is some help in identifying that on their excellent Wiki:  http://wiki.diyfaq.org.uk/index.php/Earthing_Types

You may find other advice regarding your query if you investigate the Wiki in depth.

[Ed_Dinning]

Hi Gents, try the IET Forum, it is open access and is great for situations like this.
I see there are now, cheaply available "garage units " with ELCB and 2, MCB's all fitted in one unit, this is probably ideas for the OP's application.

Ed

[ppppenguin]

The main question is to TT (fit an independent earth rod) or not to TT the remote installation. Everything else follows from that. If you do TT the remote end then don't use a metal consumer unit down there as it will inevitably be connected to the cable armour and hence export the earth.

RCD already in house, so unless there is some good reason for taking the remote fed from a circuit that isn't covered by an RCD then don't duplicate the RCD at remote end. It won't be unsafe but it will be annoying when there's a fault and either or both can trip. There are good arguments for having the RCD at either end but ultimately it's not that important which end it's at.

For PWdrive's case where there are water srvices in the caravan I have a feeling that the earth should not have been exported. Especially if the supply is PME (TNC-S).

Ultimately you can always safely TT a shed etc installation but you've got the hassle of fitting an earth rod and making sure the earth is genuinely not exported. Exporting the earth is convenient if it's safe to do so.

[pwdrive]

Jeffery wrote: "For PWdrive's case where there are water srvices in the caravan I have a feeling that the earth should not have been exported. Especially if the supply is PME (TNC-S)."

Thanks for the reply Jeffery, can you tell me a bit more about the above eg: reasons etc, I want to get this right from a safety aspect.

I said earlier that the DNO incomer was PME, that was the case at one point as I saw an ID tag/sticker at the incomer but a couple of years or so ago the meter had to be replaced and also the incomer cable and the overhead supply cables from the DNO's transformer, the designation sticker at the incomer is now marked CNE, is PME & CNE one of the same?

Lawrence.



Lawrence.

[ppppenguin]

PME, formally known as TNC-S, is where the incomer to your home has the neutral also acting as the earth. Under certain fault conditions the neutral and hence earth potential can rise alarmingly. To minimise this, the neutral is earthed at many points (the ME of PME) along its route from the transformer to your house. I've not heard of CNE but it must mean combined neutral and earth. There is a special case of PME where the transformer only supplies one property and this may be the case for you. There is less risk of the neutral/earth getting to a high potential with this arrangement.

This document explains a lot of stuff about PME etc:
electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf

In particular a PME supply must not be used to provide the earth for a caravan. You need to give the caravan an earth rod and its own RCD.

[pwdrive]

Thanks Jeffery, I'm still unsure as to why an exported earth from the house can't be used in the caravan, to be clear again this is not a metal caravan eg: metal body/chassis etc, it's all timber construction, I built it to avoid any planning malarky with with planners, it meets the legal definition of a caravan even though it looks like a large wooden chalet. I'll take some pics......


Pics taken and shown...

The "caravan" wot I built, needs gutters and some timber cappings doing....sometime:

   

The shower/toilet unit I fitted, there are curtains for it:

   

Under sink 10 litre water heater, sink at taps to come at some point:

   

Main electrical stuff, the armoured cable cores are black, brown and grey, I've used brown for live, black for neutral and grey for earth, not colour sleeved the black and grey yet but will do, the armoured cable's box is earthed to the consumer unit shown, the armoured cable's protective sheathing is not earthed at the caravan end but is at the house end, all that lot seen will be enclosed in a small cupboard:

   

The shower feed unit:

   

The living room, the cats stay there at night, I made the wooden cradle for our first born....awe:

   

I found this that I posted some years ago, page 4 describes one method using TT, the bit after that suggests another way (similar to what I have done so far)

Comment's/suggestions welcome.

Lawrence.