Supply Earthing

[Vic Water]

Hi

Just about to have our Water Mains changed to plastic (Thames Water renewal of Victorian mains). Their 'help' booklet says they have a team of specialist contractors and a phone number to help decide whether a new Electical Earth is needed when the plastic pipes are fitted - absolutely useless trying to get any help from them - they say to call an electrician - some help.

Anyway - all I want to know up front is whether it is necessary to call an electrician.

We had a new supply fitted from the street to the main fuse in 1989 when some builders digging the foundation of our off-street parking sliced though the old cable. This new (black) cable is about as thick as my thumb and is a sheathed armoured cable (I guess). It goes into the main fuse at the bottom and the tails then come out of the top and to the meter etc.

Also coming out of the bottom of the main fuse is some thick bare copper wires that go to an earthing stud on the wooden mounting board. From this stud other green/yellow cable go down under the floor and are clamped to the water mains in pipe. I can't tell from that whether the earth is supplied from the water main pipe or the electicity supply.

Can someone tell me whether the supply cable IS supplying earth also? and the water pipe then providing the earth strapping?

Where does an electicity company's earth get supplied from nowadays?

Thanks.

 

[plugwash]

from your description it sounds like your ok but could you post a picture.

 

[JohnD]

The wire coming out of the service head is probably an earth provided by the electrcity network.

Look for a label near the meter saying "PME" or "Protective Multiple Earth".

If you can post a pic of the cable head and ?earth wires? and fusebox/consumer unit we can probably tell you more.

There should be a green & yellow earth cable going into your fusebox/consumer unit from the earthing point on the wooden board or direct from the service head.

 

[Vic Water]

Thanks Plugwash/JohnD

Here's a photo of the supply....

Ignore the yellow object in the tails - just a pencil.

The bare copper wires come out of main fuse and to the earthing stud. From there, the Green/Yellow earth going down goes to water pipe; The green cable going to the left is to consumer unit earth.

 

[JohnD]

can't see the pic

 

[Vic Water]

How about this then?

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[JohnD]

Yes, I can see that, but if you can stand back a bit and give us a wider view, that will be interesting.

Is there any sign of a PME label?

The copper wire is very likely the supplier's earth. it's odd to see it in bare copper though. Perhaps they re-used an existing wire when fitting the new service head. It might even have been made up from scrap strands. Can you see a terminal that it is attached to? Usually on the side of the service head.

The grey plastic box is almost unfamiliar to me...

I would have preferred to see, in place of that stud, one of these:

(this is an 8-way, should cost you a couple of pounds). It gives a good secure fixing, and you can remove any one cable (or add more) without disturbing any of the others. I am sure it would be better to have a more secure fixing like that (each wire is gripped by two large screws). Sleeving could be slid up the bare copper at the same time.

Preferably it would have (usual figures) a 16mm G&Y earth wire to the service head; a 16mm G&Y to the consumer unit/fuse box (if it is an old one it may be difficult to fit this size) and 10mm G&Y to the water and gas pipes.

How near is your current installation to that? The green wire to the CU is probably an old one and under current size, this may not be a bid dealk though.

 

[RMS]

my guess is would be a pme earthing.

RMS

 

[RMS]

The earth connections looks like its been robed from a 30amp joint box.

RMS

 

[Vic Water]

Ok, well the earth stud on the wood was always there. There is no connector as you describe JohnD on the grey box, the copper wires just come out of the bottom behind the supply in. Since the box is sealed, I cannot see where the wire comes from - my guess was the casing of the supply cable.

A wider angle wouldn't give any clearer view as that is about all there is to see. The grey box continues upwards and has a plug in type big fuse that is held in place by the seal. The two grey covered cables are the tails that go to the meter and then a black and a red come out of that.

 

[Vic Water]

sorry, a GREY and a red come out of that.

 

[JohnD]

... There is no connector as you describe JohnD on the grey box, the copper wires just come out of the bottom behind the supply in. Since the box is sealed, I cannot see where the wire comes from - my guess was the casing of the supply cable...

Usually, it is connected to the neutral of the incoming supply (before it joins your installation) which is earthed at frequent intervals in the street and at the substation, and this gives you a very sound and reliable earth. But in this case there ought to be a PME label fixed near the meter. AFAIK the supplier is obliged to provide such a label if it is PME.



Tthe reason i was hoping for a wider view was to get an idea of the general age and tidiness of the installation.

 

[plugwash]

does one of the two cables seen leaving the earth stud in the picture go to the consumer unit and the other to the water pipe?

if so then apart from the fact the earth stud looks dodgy (should really be replaced with a proper earth block but its not urgent) i can't see any real problem, bringing out the earth cores from a (presumablly split concentric) supply cable directly is a bit untidy but perfectly

 

[Lectrician]

It is split concentric - and therefore a TN-S supply. They have used the incorrect service head for a split concentric cable, hence the bodge with the cable leaving the bottom.

As long as an earth cable leaves the stud and goes to the CU, and also water/gas, you are fine. I would change the stud for a henley.

 

[ban-all-sheds]

Would it be an idea to measure Ze, just in case those wires aren't actually attached to anything inside the service head?.....