Another electricity supply question

[Graeme]

Bugger. Fixed :-)

I have added a few photos of the house end. Not the best photos, so will try for some better ones, and the TN business, probably tomorrow.

[ARW]

Afraid not. It's deffo a TN supply.

But does it matter if it TN-S or a TN-C-S? If you treat it as a TN-C-S then you have covered the TN-S rules.

So it seems the shop CU is the one providing the water bonding not the house CU. Therefore this would require a 10mm earth from the house CU to the earth bar next to the shop CU.

[Graeme]

In message , ARW writes

What I don't understand is what was the position prior to the shop installation? The shop was not built until 1979, but the house (1880s) must have had electricity before then.

The shop and house, although having independent electricity supplies, share a common water supply. What I need to check, I think, is whether there is a water bonding point in the house.

[ARW]

It's not always easy. You are supposed to bond where the pipe enters the building.

So if the shop supply is removed you need to bond from the house CU to the earth bar at the shop.

[Graeme]

Thanks Adam. The water supply enters the house from the water mains, the water supply in the shop, where the earth is bonded, is merely a branch off the house supply. I'll need to investigate further. One thing I am sure of is that there is no plastic piping, apart from overflows.

[John Rumm]

Yup, fair point...

Looking at the extra photo's of the house end now, it looks like the cutout has two cables on the supply side - the second one likely to be the feed that goes to the shop.

With that cutout it looks like they have split out the conductors of the supply cable outside of the enclosure, and I can't see (although the photo is not that clear) a separate earth connection from the supply cable into the cutout, but I can see several earth connections to its neutral block. So I am leaning toward TN-C-S on that end - which would strongly suggest the same at the other end.

So yup, work out the total load on the shop end, apply some diversity, look at the length of cable and select a suitable sub main size - you may only need 6 or 10mm^2 since some of the shop end circuits are no longer loaded by the sounds of it. Slap in an additional 10mm^2 single bonding connection as well, and you should be mostly sorted. Ideally the main bond to the water pipe should then be from the point of pipe entry, and to the main earth terminal in the house end. (although if its to the shop end and connected to the shop CU, that's going to be nearly as good so long as you have adequate bonding between the CUs)

[ARW]

Agreed

Again I agree - with one small suggestion - if you take the 10mm to the earth bar next to the shop CU (say it's a lot of effort to replace the earth all the way to the stop tap) and change it's name from "earth bar" to "earth marshalling terminal" I would say you have fulfilled the regs:-)

[Graeme]

In message , ARW writes

Thank you all, particularly John and Adam. I'm not ignoring the advice, but am not in a position to proceed further yet, although the issue is still live (ha ha).

One factor is change of supplier to the house, which has resulted in an increased fixed daily charge, which makes amalgamating the two supplies less attractive economically. What I do intend to do is take better pictures of the earth of both supplies, just to establish the earthing types.

Cheers,

[alan_m]

Is this true? Combining two supplies means you lose one of the higher cost daily fixed charges.

[Graeme]

Indeed, but the question remains, having paid for the conversion to one supply, how long does it take to recoup that amount? Yet to be determined. It was ?95 a year at my old rates, but new (higher) rates from next month.

[ARW]

When you are ready post the photos.