Cable down to the shed

[ppppenguin]

As I said before TT, correctly done, is always safe.

PME is a bit of a compromise. It gives the convenience of a low impedance earth terminal. Plus no need for an earth rod. The downside is that neutral faults elsewhere on the system can cause the "earth" potential to rise. If you're the only user on a PME system then the risk of a neutral fault is very low.

The object of the regs is to ensure that there's never a large potential between 2 conductors that you might touch at the same time. Hence all the equipotential bonding in a house.. In a caravan there's the nearby presence of genuine earth. Even more so in a boat. It's harder to ensure that an exported earth is close to true earth under these conditions.

If there's any doubt then TT the outbuilding etc and make sure that any exported earth such as the cable armour is kept well insulated. Which end for the RCD? At the source you could rely on an existing RCD in the CU. But if it trips it's a walk in the dark back to the house. At the destination is convenient but it doesn't protect the cable. IMO the cable can look after itself. The classic spade through cable will trip an MCB or blow a fuse on a PME system.

Cascaded RCDs? Don't do it. There's no discrimination unless the upstream one has a time delay. So when you get a trip you won't know which of them has tripped. Could be one, the other or both. Even if one is 100mA and the other 30mA they still don't discriminate, a 100mA leakage will trip whichever moves faster.

Anyway, that's my understanding and I could be wrong.

[pwdrive]

Whoops...forgot to post the link referenced to post #10, here it is:

http://golbornevintageradio.co.uk/forum/...p?aid=5906

Lawrence.

[ppppenguin]

That document predates current wiring regs but still looks pretty much OK. One point I overlooked is that if you export a PME earth to an outbuilding with extraneous metal parts (water pipes etc) then they must be bonded back to the main earthing terminal with a 10mm^2 cable. The cable armour doesn't do this, nor does the 4mm core used as earth. If you export a TNS earth to an outbuilding I'm not sure how extraneous metal parts should be bonded. May still need to be 10mm but could be less.

All in all, if there are water pipes etc I'd almost certainly TT an outbuilding. If it's a caravan or boat then you MUST TT it. If it's on a farm there are additional requirements which I haven't read up. One particular problem on farms is that livestock are mor vulnerable than humans. Partly because they don't wear wellies (no rude jokes please) but mainly because their feet are further apart. So a voltage gradient in the earth due to a fault that would be harmless to a biped could be harmful to a quadruped. I think this means that PME is rarely used on farms, or if used it's only for the farmhouse. Everything else would be TT. You don't get much TNS in rural areas.

[ex 2 Base]

Very many thank for your informative reply Jeffery. I was OK on the wiring regs up to the 14 edition but never got passed that, and to update, not as though I need to, would apparently cost an arm and a leg. I was never one for domestic wiring to hard on the knees so let things slip by.Ted

[pwdrive]

The core in the armoured cable that I'm presently using for earth is 10 mm so far as I know, I would TT the caravan/chalet but the ground here is not "normal" ground, it's the site of a former metalliferous mine and a big one at that, the top soil if you can call it that is very shallow below that is tons of mine deads (Killas, Quartz, Peach etc) which sit on top of bedrock (Killas) I might not get a spike down, a mat is out of the question for various reasons, if I can use PME it would be a lot easier for me but it needs to be acceptably safe.

Lawrence.

[ppppenguin]

I thought you said you were using 3 core 4mm armoured. Hence all 3 cores are 4mm. One core certainly won't be 10mm. Don't rely on the armouring for earth continuity. It can be done but you need to be very sure of your clamps. Calculating the equivalent copper area of SWA is a faff too, though I'm sure there are tables somewhere.

If you're exporting the PME then you could always run a length of 10mm green/yellow alongside it to give a decent earth bond.

[pwdrive]

Jeffery I believe Ex 2 Base's is 4 mm, mine is 10 mm as per post #4.

I wouldn't be using the galvanized wire armour for any earth due to a risk of any plastic outer sheath penetration (rats, mice, other subterranean beasties etc) the mine waste the cable is buried in contains lots of Iron Pyrites etc which can become very acidic and might dissolve the galvanized and eat through the steel strands.

Lawrence.

[ppppenguin]

Sorry, got confused. 10mm armoured isn't cheap and not mormally needed for a few sockets and lights. Unless it's a very long run and there's a volt drop problem.

The trouble with using the 3rd core for earth bonding is that the CPC (circuit protective conductor, earth to most of us) isn't the same thing as an equipotential bonding conductor. AFAIK you can't use the same bit of wire for both. If you had 4 core armoured it would be different. You could then use separate cores for CPC and bonding.

[pwdrive]

Sorry, got confused. 10mm armoured isn't cheap and not mormally needed for a few sockets and lights. Unless it's a very long run and there's a volt drop problem.

The trouble with using the 3rd core for earth bonding is that the CPC (circuit protective conductor, earth to most of us) isn't the same thing as an equipotential bonding conductor. AFAIK you can't use the same bit of wire for both. If you had 4 core armoured it would be different. You could then use separate cores for CPC and bonding.

No worries on the mix up.

Yes, 3 core 10 mm armoured is what we have, I believe the electric shower is around 8/9 Kw, I'll check the actual KW out if I can, there is likely to be an electric radiator as well if the caravan/chalet is used in the winter probably a 1 KW job at most, the under sink water heater is 2 KW, the structure is well insulated, the ceiling/roof is 300 mm Rockwool, the walls are 100 mm Kingspan/Celotex along with 50 mm of Rockwool so that's 150mm in the walls with that combination, the plywood floor is insulated underneath with Kingspan/Celotex.

I believe the armoured cable I laid is rated at 15 KVA, I could run a separate bonding conductor back to the house via the plastic pipe duct mentioned in the Internet Connection Question thread which I started a day or so ago, it might be able to run along side the Cat 5 cable which has been proposed inside that duct...not sure if that's regs proof though?

If not I can dig another trench alongside the one that the armoured cable was buried in and put another duct in for the bonding return to the house.

Lawrence.

[ppppenguin]

I see no reason why the earth bonding cable can't share with comms cables. It's not a live conductor within the meaning of the regs. Incidentally the term "live" in the regs includes both phase and neutral conductors. Actually comms cables can share space with armoured cable since there is an earthed barrier between live conductors and aything else.

Not sure if I've interprested this corerctly but are there cascaded RCDs in the feed to the shower? I explained the problem of cascaded RCDs and discrimination in post #11.

I think you may also have discrimination problems between the MCB for the shower and the MCB at the house end of the armoured cable. This is mainly a convenience issue rather than a safety one, except that if the house end MCB trips due to a fault on the shower you're left in the dark.