Any Electricians ?

[Skywave]

A few comments on my arrangements for a similar situation may be helpful here.

I have a similar arrangement for an electrical power feed to my outside workshop (which was originally a stand-alone garage: a 1930s style semi-detached house): I installed 3-core armoured cable for that feed. The incoming mains cable (L, N and lead casing for earth) to the house is fed to the house consumer unit and is also fed to an additional small consumer unit that contains an RCD and one MCB: this is connected to the armoured cable that feeds the workshop. The house incoming earth is bonded to the armour of the outgoing cable and that cable also carries a dedicated earth line that feeds the garage. At the workshop end I have another small consumer unit that contains an RCD and 3 MCBs: one for workshop lighting, one for workshop power and one for a feed to the greenhouse (adjacent to the workshop) for lighting only in that greenhouse. The workshop power is configured as a ring-main with conventional 3-pin rectangular sockets, conventionally connected to L, N and E.

The thinking behind having two consumer units (with MCBs and RCDs) was simply that with that arrangement I have effectively provided protection against possible cable faults only between the house and the workshop and also separate protection against faults that occur within the workshop (and green house) itself. All of that was done about twelve years ago, and apart from the occasional 'ker-lunk' from an MCB or an RCD when a mishap has occurred (therefore effectively testing the protection system), I have experienced no problems at all.

I have also installed a low-power emergency lighting system that drops in automatically should the lighting circuit MCB or either RCD trip. Since the workshop has no windows, this is an absolute necessity, especially if the lighting only fails when I am using a power tool!

Al.

[pwdrive]

Thanks Al, that is basically how I have wired mine up so far, the small consumer unit in the caravan feeds a small ring main, lights and an electric shower, all RCD/MCB protected. My armoured cable is three core + steel wire sheath I am wondering if it is ok to use the spare wire in the three core as an earth as well as the sheath.
As Dr Mark has already pointed out the regs seem bit confusing when it comes to exporting power to an outbuilding which is effectively what the caravan is.

Lawrence.

[AlanBeckett]

I am wondering if it is ok to use the spare wire in the three core as an earth as well as the sheath.

That's what I've done. There's no shortage of local Earths 'cause at the Greenhouse all the aluminium is bonded to the incoming Earth as well.

Alan

[pwdrive]

I am wondering if it is ok to use the spare wire in the three core as an earth as well as the sheath.

That's what I've done. There's no shortage of local Earths 'cause at the Greenhouse all the aluminium is bonded to the incoming Earth as well.

Alan

G'd mornin Alan, sounds logical, thanks.

Lawrence.

[Mark Hennessy]

Hi,

My above reply was dashed off in a bit of a rush, so perhaps I ought to try and clarify things a bit...

First, I agree that if you are exporting the earth, then yes, do use the 3rd core. I did write that in my first reply, but took it out after deciding that exporting the earth was A Bad Thing.

Regarding the issue of exporting the earth, I'll explain my background with this:

I'm obviously not an electrician, but I have some experience in these matters because we have run courses for OB engineers, who have to plug their vehicles into a range of different types of supplies. From first principles, we've had to explain the different types, and what the risks are with each. And of all of those, PME supplies are considered to be the worst.

The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.

This page explains this, with a diagram: http://www.tlc-direct.co.uk/Book/5.6.3.htm

Interestingly, BBC OB vehicles are regarded as caravans in the eyes of the law!

Caravans, hence BBC OB vehicles, can not be connected to PME supplies. In our case, quite how the rigger establishes what type of supply the venue has is another matter

This is a risk with your house as it currently stands, but of course what you're doing is new work, and has to comply with the standards. If you export your earth, the new work also has this risk of earths becoming live in the event of a neutral O/C ahead of your incoming RCD. It's almost certainly not allowed.

So, by providing an RCD feed in the house, with a local earth spike at the caravan, and not exporting your earth, you have converted the supply from the house to a TT supply. Should the incoming neutral to your house fail O/C, then at least the exposed metalwork of any Class 1 appliances in the caravan will be at ground potential.

Of course, your earth spike results in a fairly high earth-loop impedance, so a live-to-earth fault might not pass enough current to blow the fuses, which is why an RCD on the feed is essential.

Does that help to make things clearer? Now that I've slept on it, and double-checked a couple of things here at work, I'm much happier that I understand this and can try to explain it better if needed.

Cheers,

Mark

[pwdrive]

Thanks Mark, been reading up also, some of these internet forums are lethal!! According to IEE (I think) PME can be exported to a domestic outbuilding but if there are any conductive pipes within the building then they must be bonded and connected to the suppliers earth back at the house using 10 mm/2 (cross sectional square area) earthing cable. But apparently exporting PME to say caravan pitches and livestock buildings is a no no. Just to re iterate my caravan is not a old style aluminium caravan, it's basically a wooden chalet but constructed so as to satisfy the legal definition of a caravan just in case the planners have anything to say!
I have just been outside to check the SWA cable it's three core (3x 10mm square) So does that mean that I can use the spare 10mm conductor as an exported PME earth, I will not hold anyone to their answers, just trying to get a general consensus at the moment.
Have spent the morning searching through various forums on this subject, it's alarming to read so many cavalier responses to questions on electrical safety.
I will try and find the IEE link to that PME article and post it, as I am sure many of us have outside sheds/ workshops etc that have or thinking of having electricity supply feeds.

Lawrence.

[pwdrive]

PDF links:

IEE Earthing systems.

IEE Outbuilding Supply.

Lawrence

These were written a few years ago so not sure if current regs, use as general guidance.

[Joe]

I'd do as you'd suggest, ie use the black SWA core as neutral, the brown as live and the white as earth. Extraneous Conductive Parts should be earthed full stop.

That said I just do things to my satisfaction and sod the Regs. That is my Opinion and not Advise.

- Joe

[pwdrive]

I'd do as you'd suggest, ie use the black SWA core as neutral, the brown as live and the white as earth. Extraneous Conductive Parts should be earthed full stop.

That said I just do things to my satisfaction and sod the Regs. That is my Opinion and not Advise.

- Joe

Hi Joe, sometimes I think sod everything.. full stop, as I am trying to finish my porch roof but it keeps raining.
I think I'm on the right track with the wiring etc, I will wait to hear of any more responses etc.

Lawrence.

[Skywave]

The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.

This page explains this, with a diagram: http://www.tlc-direct.co.uk/Book/5.6.3.htm

Mark

Interesting what you say there: it may explain a scenario I witnessed many years ago on a new-ish build house - presumably, therefore, with PME.

The householder told me that when she touched the radiator in the hall, she received a shock. As her hall floor carpet fibre was a nylon mix, I suspected nothing more than static, but since I was consulted, I had a look at it anyway. I casually touched the radiator at a place where there was no paint on it and - OUCH! That is 'hot'! Clearly not static! The electricity authority were informed and duly attended: fault was confirmed by them. The upshot was that they had to dig a large trench in the road - about 20 metres long, back to a 'T' junction in the road - before they found the fault; it went on for about 3 days and nights. Being curious, one evening I spoke to the person supervising the work: "Real headache, this one. We've just got to keep digging back from the house until we find the trouble." Which they eventually did: an O/C neutral in a bitumen-filled junction box, so I was told. Peculiar thing was that several adjacent houses should have been also affected, but only this one house appeared to experience the fault. Perhaps the occupants of the other houses hadn't noticed, but I find that difficult to believe.

Al.