Earth Terminal ?

Spoke to Soon :-(

Eon sent their contractor to change the cut-out, which he completed with the added bonus of checking the Earth Loop imp (0.26 ohms) so now I don't have to use the default value of 0.8 when I'm calculating it for the ccts. Now the not so good:---- I asked him about providing a better clamp on the earth sheath of the cable ready for when I need to connect my new 16mm earth tail. (The existing one is small and only has a 4mm earth tail) He said he couldn't do that and that I would be better off applying for a PME.. I pointed out that a PME would cost me =A370 and since I have a TN-S supply with a perfectly good earth why should I spend =A370 instead of Eon making proper provission for a 16mm earth tail.

your views appreciated ....

Andy

Reply to
ac1951
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You aren't supposed to connect _your_ earthing conductor to _their_ cable. _They_ connect _your_ earth wire. Usually you present a 16 mm^2 earth along with the meter tails and they will connect it to the cable sheath or to the sealed PME terminal, as appropriate. In this case you should probably have mounted an earth block near the isolator position and left a short length of 16 mm^2 attached and he might have connected it - but too late for that now.

You could ask explicitly whether they'd come back and connect a TN-S earth. If not the only options are to take up the PME conversion offer (£70 doesn't sound too bad), or to sink your own earth electrode and go for TT. With everything needing 30 mA protection anyway under the 17th ed. (unless you use special cables), TT could be the cheaper option - but beware that it rules out using a metal CU, unless the manufacturer offers an 'RCD insulation kit' or some such, to achieve Class 2 insulation on the live incoming tails ahead of the RCDs.

Don't be tempted to make your own TN-S earth connection using a BS 951 earthing clamp. There's a known risk of squashing the old lead sheathed cable and making a big bang. Yes I know the old electricity boards and RECs used to use them, and still sometimes re-use them, but it's a deprecated practice now. If they do it it's at their risk, but if you blow their cable up ...

Reply to
Andy Wade

Anyone done that?.. Bloody great big bang or does it arc and burn and either burn itself out and/or take the substation transformer fuse with it?..

Reply to
tony sayer

tony sayer coughed up some electrons that declared:

From the stories my late father told me (ex LEB), assuming a short happens, it will most likely be a *bloody* big bang, followed by a section of cable around the fault disappearing, followed by your mate dialling 999 because the plasma from the ex-cable section had an argument with your face. Typical dead-short fault currents at that point are likely to be measured in many kA. This all changes if the installation is rural on the end of a bit of string and a weedy transformer on a pole, but it would still be unwise to invite trouble.

My father was been consigned to hospital for a fair time back in the late

50's due to an substation breaker exploding on him. He met many co-victims of electrial burns.

I found the medical photos when clearing out his house. The results really weren't pretty. To be fair, his injuries were exasperated by the breaker's arc quenching oil going up, but plasma burns can still be pretty horrific.

As to the breaker at the substation, for a typical urban substation, probably not. As for the fuse in the link box down the road (depending on the REC and system), maybe, maybe not. 300A fuses were used in London 30 years ago, no idea what they use now.

The real problem with old cable is that some of it is paper/oil insulated and over time, the oil can drain and dry out of the end section leaving the paper brittle and prone to failure by the slightest disturbance which includes fiddling with earth clamps, bending or otherwise crushing the cable.

Cheers

Tim

Reply to
Tim S

Yes, the fuse is likely to be in a link box or distribution pillar - 300 or 500 A in my limited experience of such things.

Reply to
Andy Wade

On Tue, 17 Jun 2008 01:39:10 +0100 someone who may be Andy Wade wrote this:-

Depending on cable type the lead sheath should be covered with steel reinforcing wire, assuming it was terminated properly.

Obviously putting a standard earthing clamp directly on the lead sheath of a cable is likely to end in tears.

Reply to
David Hansen

It happened in the Kentucky Fried Chicken in Dunstable a few years ago. The resulting smoke damage closed the place for some time, but unfortunately, it did eventually reopen...

Reply to
Andrew Gabriel

Met and interesting one the other day. A mate needed some minor electrical work done, and asked me to have a look at the existing setup to see if there was anything else that needed doing. I figured the main EQ bonds may need attention, and so had a look at the head end. It revealed a TN-S system with 10mm^2 earth single connecting an earth block to the cable sheath. The sheath seemed to have a steel pipe appended to the end of it, and a bog standard BS 951 earth clamp wrapped round it. The thing that caught the attention was that single wire was the only thing connected to the earth block. The CU wasn't!

Investigation with a loop tester did however reveal a decent enough impedance of about 0.22 ohms.

After a bit of hunting about we eventually found a bare stranded single running from the CU into a metal box used to terminate some conduit (old wylex insulated fuse board), from there it ran hidden behind a bit of conduit (odd wiring - done in the 60's, some in PVC Twin (but not always with E), and some in steel conduit) disappearing under the floor. Eventually found it connected to the lead incoming water main somewhere under the floor.

Indeed, I am nervous enough making a clamped connection to a lead water pipe... at least all that happens there if you break it is you get wet!

Reply to
John Rumm

Current 4mm earth wire is attached to cable sheath using a shell clamp that is obviously designed to meet the outside diameter of the sheath. Looking at it I could crimp or solder a 16mm lug (standard lug comes

6mm hole) onto the earth tail and attach this to one of the shell clamp bolts.

Thought I would call Eon and asked them to send someone to carry out the work ;-) They said, earthing is your responsibility and whilst there may currently be an earth provided via the cable sheath we offer no gaurantee that this is adequate. The only earth service we now offer is PME and if that cannot be delivered you have to make your own arrangements. With regard to the PME we will only make it available and you will have to get your own electrical contractor to make the connection onto it.

So, looks like I'm about to kiss goodbye to =A375.

Reply to
ac1951

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