Mains power connection: mysterious black substance

[Jonelle]

Hi all

Background: We recently had new CH installed. It required an electrician to come and do the controls wiring. He looked in the cupboard housing our Consumer Unit and mains connection, said the earthing was wrong, and fitted a new earthing system (connecting CU, gas, etc all to the mains water in-pipe under the sink). (He has charged £79 labour and £13 parts for doing this; £109 inc VAT) (The CU and gas were previously earthed to a proper earthing stake, hammered 2' into the ground under the cupboard, by me,

15 years ago: he said he wasn't "getting a reading" from this.)

Question: Having done this, the guy pointed out to my wife (I was at work the whole time he was here, else I'd have quizzed him a bit) that a black substance was oozing out of the mains cable, at the point where it joins the electricity supplier's fuse box (can't provide a picture sorry).

This stuff, which looks like tar, is solid, and cold; however it has the appearance of "flowing", and indeed it's already flowed over the earthing strap that the electrician fitted, three weeks ago. It's either coming out of the cable pipe (it's the one about 1" wide, up through the ground) or from the box it connects to (which precedes the actual mains fuse box, before that connects to the CU).

I had never noticed this stuff before (ever, in the 22 years we've been here). It could have been there for years, or ... maybe it started when the electrician was working there?

The electrician said that it was the wrong side of the fuse box for him to do anything about it, but "we might want to get the Supplier" to look at it.

I'm wary of doing that, because they are absolutely bound to find something wrong (now, aren't they?!) and we'll end up with a bill at least as big as the CH (£2800) to have the place "properly" rewired. (Maybe the electrician was hoping he'd get the job!)

We rewired the house 15 years ago: I did most of the work, but was advised by a competent electrician from my place of work.

Any thoughts, anyone?

Cheers John

[Andrew Gabriel]

It's pitch, which was often used to fill (and hence waterproof) old mains connection boxes. Is the box (or whatever) it's coming from warm or hot? That would speed it up.

You might want to read up on the Pitch Drop Experiment:

I had never noticed this stuff before (ever, in the 22 years we've been

I would ignore it unless it's warm or hot.

[cj]

Hi It's probably the sealant from the mains incoming cable . The cable usually came into a spreader box of cast iron or similar where it was terminated to the supply tails of the main fuse ,the whole lot was then filled with a black "goo" similar to Tar or pitch. You may be seeing the original overspill or a slight leakage which occurs as the load increases and the joint warms up under peak demand. Not a cause for worry unless you see strands (caused by molten sealant) hanging from the box or the tar is hot. Try checking the temperature of the joint box if its warm or hot contact your supplier as it's their responsibility. Otherwise don't worry.

HTH CJ

[The Wanderer]

Hmm, I'd suspect the sparkie disturbed the cable termination when he fitted the earthing clamp.

More correctly, bitumen, aka A57 Compound.

No problems with that, but I'd get back to the sparkie anyway and make sure he knows it's only been apparent since he did his work. The cable termination may well have wooden bungs in the bottom, one of these will have had a hole made in it when the cable was first installed and the sparkie will probably have disturbed the cable when he fitted the clamp. It's not unknown for the bungs to split in half.

Unlikely. You tell the local distribution company that you have bitumen leaking from the main fuse, it's their equipment. The guy(s) they send to coreect it won't be 'wiring' electricians but a cable jointer or possibly overhead linesman.

Don't ignore it, not if it's already oozed down to cover the earth clamp in a matter of three weeks. You might just have a loose connection in the main fuse that heats up slightly when you happen to have plenty of load switched on, just enough to get the bitumen running out.

What they'll probably do is take off the old termination and fit a new one. Shouldn't cost you anything. But, if they try to say that fitting the earth clamp was the cause, then you tell them to charge electrician.

[Colin Wilson]

IIRC all iron-clad cutouts are supposed to be replaced by about 2012, so call it through now and get it booked in.

[newshound]

I had a (non-technical) neighbour once in Bristol who asked me about something similar; I told him his electricity must be coming from Pembroke power station (which is oil fired). Should explain I'm in the nuclear business.....

[cynic]

A "proper" earthing stake would have a comparatively high resistance path through the ground for a current to flow if you had a fault situation in your house. Typical values of ground loop resistance lie within 20 to 200 ohms so for a dead short between a line and earth you would see currents of 11.5 to 1.1 amps Unless you have an RCD (earth leakage circuit breaker) this will not cause disconnection of the supply as a 6amp or higher circuit breaker would simply allow the current to flow for an extended period, during which exposed metalwork may be at a high(ish) voltage to earth and you or your family be at risk of electrocution. Your electrician has bonded the main exposed metalwork systems together which will minimse the voltage difference between them in the event of a fault. If he has connected this metalwork to the supply earth also then a low resistance fault path should now be present (less than 1 ohm, probably much less). This should provide a path for sufficient fault current to cause operation of a fuse or circuit breaker, disconnect the supply and achieve safety. He should also have given you a certificate with test results thereon of the situation after he had carried out the works. This does of course assume that the supply earth is present and securely connected. Earthing is a complex matter but I have tried to simplify it above.

This is not uncommon but it may be that it is a relatively recent development. If so you may have a problem which requires attention quite rapidly

No comment

If the pitch is flowing this shows that it is being heated up, thus softening it. Was there any notable added load to your installation recently? The central heating would be a negligible addition unles you were going for a number of electrical heaters, however "controls Wiring" suggests you have gone for gas or oil? You sound unlikely to have means of measuring current in your supply conductors but the electricity meter would be a good way of estimating what current is flowing. Take a reading, wait 60 minutes and take a second reading. Deduct reading 1 from reading 2, divide the difference by 230 and the result is the average current in amps over the period. This will have errors if some of the loads are under thermostat control etc but it will give a broad picture

I would be concerned that a high resistance joint may be developing in the service head. At best you may lose supply, at worst your house could burn down. Call in the supplier to check it as you have been advised

[Jonelle]

Dear all

*Many* thanks for the prompt and reassuring advice here. Slightly worried (naturally) by Cynic's contribution -- partly because I don't understand it, and we all fear that which we do not understand!

I will now keep a close watch on it, and if things get worse, will take the advice to call in the supplier.

John

[Andy Wade]

Which I see, just out of interest, is still available:

[Andy Wade]

Well you originally said

There's nothing at all proper about that. An earth electrode for 'TT system' earthing needs to be at least 4 ft. long in most cases and should be outside, not under the house floor. This type of earthing is only effective when used in conjunction with RCD protection (or the older type of ELCB 'trip switch').

It seems to me the electrician here has acted with good intent, but not necessarily full competence. He's converted the earthing to the TN-S system by making a connection to the metal sheath of the distribution network operator's (DNO's) cable. However this wasn't really for him to do, as only the DNO should make such a connection. As pointed out in another recent thread there are potentially serious dangers in fitting certain types of earth clamps to old service cables. That thread also revealed that at least one DNO no longer offers TN-S earth connections and insists on conversion to the PME system (protective multiple earthing, aka TN-C-S). Again the point is that the provision of either type of metallic earth facility (or none) is at the DNO's discretion, not your electrician's.

Follow The Wanderer's authoritative advice and call the DNO anyway.

[tony sayer]

See copper earth tape is now POA.. Hate to ask how much;!..