Ze at infinity

[martin43]

Hi all

Discovered today that my installation isn't earthed! This came about as I sorted out what needs to be done for a new CU. The existing CU main earthing conductor is connected to the copper water supply pipe under the floor, which unfortunately becomes plastic before it leaves the premises, hence no earth. The cable head (see photos) looks as though its the original when the house was built (1922) and the i/c cable appears to be a tar wrapped lead sheath 2 core. I rang Scottish Power to find out what type of supply I had, apparently they have no one who is able to tell me until Monday, the terms phase, earth, and fault didn't arouse the same trepidation in them as it did me. My queries are

1 It looks as though the installation was earthed via the water pipe and as such is TNS, can I reasonably assume that this is still the case?

2 If so can I cut off the tarred covering of the cable and fit an earth clamp there?

3 I presume an RCD would not work under these conditions as when an earth fault occurs there would be no earth current and hence no imbalance in the RCD?

Thanks in advance for any thoughts
Martin

 

[Adam_151]

1 It looks as though the installation was earthed via the water pipe and as such is TNS, can I reasonably assume that this is still the case?

Earthing via the water pipe isn't allowed anymore (not since the '60s IIRC) and even if it was then I'd presume it'd count as TT as its acting as an electrode in the ground, unless the power companies used to have some agreement with the water companies to use metal pipes as earthing conductors, but this seems unlikely as the earth would be lost if the water company needed to remove a section of the pipe to carry out work

2 If so can I cut off the tarred covering of the cable and fit an earth clamp there?

The cable is the property of the DNO and you should not touch it, anyway a normal BS951 clamp is not suitable because it'll crush and deform the cable, and also in doing so will make poor contact. The correct way for an earth to be connected to to 'sweat' it onto the lead, of course only the DNO should do this, but it is not unknown for people who know how to do this to do it themselves, but thats certainly not adviseable . Also you don't know the state of the connection to the lead at the other end of the cable, the DNO do

3 I presume an RCD would not work under these conditions as when an earth fault occurs there would be no earth current and hence no imbalance in the RCD?

Well it would to a certain extent, if you come into contact with something live, the fault current through you into the ground might trip the RCD, so RCD makes it safer than no RCD, but yeah without the earth you have lost the protetcion against indirect contact, and are relying on the RCD to protect you against direct contact against things that wouldn't become live anyway if the earth was there

Best advice in the meantime is an RCD and an electrode sunk into the ground, and have it as TT at least until the DNO decide what kind of earth they can give you

 

[martin43]

Adam

Thanks for your thoughts.

Bearing in mind this installation dates from the 1920's, and was possibly DC originally, earthing via the water pipe was almost the universal way of doing things then. Also all water pipe then was cast iron so a section removed wasn't that much of a problem. I suspect the discontinuance of earthing via the water pipe came in when plastic pipe first became used.

Secondly the response from Scottish Power today was that they no longer provide earth connections and that my electrician would do it. This explicitly allows a third party to do things to the DNO's equipment, assuming a TNS supply. It may of course vary between Scotland and England.

I did post a photo via admin, it's a hefty old cable, I wouldn't have thought a cable clamp could crush it.

Since the DNO aren't going to give me an earth connection anyway, I only wanted confirmation of the type of supply so I can sort it out.

Regards
Martin

 

[baldelectrician]

martin

SP told me (over the phone) to put a clamp on a lead cable to get an earth .

They came out to another customers house and failed to repair a sweated on earth, but instead put on a clamp themselves. This seems to be the method they favour now (cheaper and quicker), but i personally refrain from touching their equipment- if anything goes wrong they will pass the buck firmly to someone else.

A clamp is not a preferred method of obtaining the earth on a lead sheathed cable (doesn't do the cable any good at all- remember the cable is already years old so a clamp won't do anything to extend it's life)

 

[RF Lighting]

If you have a photo (which admin have not yet posted), then you could upload the picture to an image hosting site such as Photobucket. This is a free service.

 

[securespark]

Yes, the use of a water pipe to provide an earthing facility was stopped in the 14th (1966). The use of a gas pipe for such has never been allowed.

Personally, I frown upon EC14's being used on TN-S cables, but I have rung the org. representing DNO's and they say it is acceptable.

As far as an RCD is concerned, a 30mA unit will only guarantee disconnection in the required time if the earth loop is a maximum of 1666 Ohms.

So, higher than 1666, it may still trip, but not quickly enough.

 

[martin43]

Hi RF

picture is here http://i62.photobucket.com/albums/h113/martin43_2006/IMG_0999.jpg

 

[martin43]

SP told me (over the phone) to put a clamp on a lead cable to get an earth .

I guess this is the accepted albeit bad practice. I must admit the thought of sweating an earth connection onto a live cable, given the melting point of lead and solder isn't much different, isn't that attractive.

but i personally refrain from touching their equipment- if anything goes wrong they will pass the buck firmly to someone else.

No doubt about that, but the other option of TT isn't much of a goer because of the physical conditions and getting the Ze low enough.

Thanks for the input baldelectrician


I'm going to speak to SP tomorrow and see what they think there supply nominally is supposed to be, but given their insistence that they no longer provide earth terminals it looks like the clamp is reluctantly the way forward.

Regards
Martin

 

[RF Lighting]

Looking at the age and style of the cutout, you may be able to persuade SP to upgrade you from what will probably be a 30A BS3036 fuse to a new cutout with a 100A BS1361 fuse. They might even provide an earth terminal as part of the replacement. AFAIK they should not charge for the upgrade, but I could be wrong. It is certainly worth making the enquiry.

BTW nice cutout. First one I have ever seen like that. (guess that it is a regional style). The ones round here of similar age are made by Lucy Oxford and are painted green. They are also a little bit smaller that yours looks to be.

 

[martin43]

RF wrote

BTW nice cutout. First one I have ever seen like that. (guess that it is a regional style).

Yes isn't it. On the front is Aeroflex Fluvent Hope's Patents. It also has cast on the front G.C.E.D, Glasgow Corporation Electricity Department, which suggests it was made to order for the corpy. Since the South of Scotland Electricity Board and I came into existence in 1954 it's older than me. I've no reason to think its not the original installation.

Heres another photo of the interiorhttp://i62.photobucket.com/albums/h113/martin43_2006/img_1002.jpg. looks suspiciously like a fused neutral although I haven't pulled either of the links to check. I'll certainly be asking SP for an upgrade.

Thanks
Martin[/quote]

 

[Adam_151]

Do NOT pull the fuses, there is a chance with some old metal service heads that the insulators will break away and the incomming live will be pulled onto the metal case and it'd make a pretty big bang

 

[RF Lighting]

looks suspiciously like a fused neutral although I haven't pulled either of the links to check.

I does look a bit that way. You may find that a solid link has been fitted as neutral fuses are nothing but bad news. (but then again it might just be a standard fuse wire )

I would advise that you do not pull the fuses unless you are absoloutly certain of what you are doing. It is not unknown for fuse holders of this age to break / disintigrate when you try to pull them, putting you in direct contact with the incommer. (and if you do, switch off all loads first and pull the live first and put it back in last)

Also the wires coming into the bottom of the fuse holders will probably be
uninsulated so care needed again.

Oh and the tails look to be cloth covered so there is a good chance the insulation will split or disintigrate if these are moved or flexed.

Oh and the cutout case could come live if some insulation fails (with it not being earthed) so again exercise great caution when playing with it.

OH and there may be asbestos within the fuse holders (for flash supression if a fuse blows) but the cutout looks be too old for this.


But apart from that, I can't see anything wrong with it




<edit> By 'eck he's quick at typing is that Adam (btw it won't go bang as the cutout isn't earthed it'll just kill you instead)

 

[martin43]

Thanks Adam and RF for your words of caution, I have heeded them. When I speak to SP tomorrow I'll stress the age, need to upgrade etc, and see where that gets me. With a bit of luck they'll decide it needs changed, here's hoping. I'll let you know the outcome.

Thanks for your help

Martin

 

[ebee]

I thought that they are not obliged to supply an Earth terminal but that if they had then they were responsible for it integrity

 

[martin43]

Hi ebee

Trouble is they never have supplied an earth terminal, ever, which was really the cause of the problem in the first place. We'll see what happens.

Cheers
Martin