Upgrading PME with earth spike.

[hairyben]

Any thoughts on this from the sparks here? I've met one or two chaps over the years who said they always did it for extra protection, not fully trusting the PME system.

Never gave it too much thought, but a few years ago I got sent to a house where the customer was complaining of shocks from the bath to upgrade the earth bonding. When I got there, I soon realised there was no PNE at the cut out, which explains everything. (the only reason the electrics were working was through a bizarre fault concerning some rodent damaged cables in the kitchen ceiling which involved the entire current of the house passing though the kitchen switch. You couldn't make it up)

Yeah, I know failure of the PNE is "not supposed to happen". But it did.

Called the leccy board, who after the usual "we don't want to come, how about next week, if it's not our fault we'll charge you loads of money money money etc" agreed to come and sort it, I left the customer with no power to await their presence.

Following day I'm back, leccy board have been but customer is still getting electric shocks.

Leccy board had left her with wrong polarity at the meter.


My reason for posting this is the NIC inspector chap pointed out a while ago I should be doing something the all-too-common "clamp on lead sheaf" earths. The normal result of this is a PME earth, and I can't help but feel this is less safe...

 

[Taylortwocities]

You don't want to do that.
I know its rare, especially with concentric cable but consider what might happen if the neutral got cut in the feed - your spike and its conductor would then be the return path back to the sub station....+

 

[hairyben]

You don't want to do that.
I know its rare, especially with concentric cable but consider what might happen if the neutral got cut in the feed - your spike and its conductor would then be the return path back to the sub station....+

Is that better or worse than every earthed metal part in the house being at 230v, untill a customer becomes "the return path"?

 

[tSpark]

Part1 EARTHING AND BONDING 3RD EDITION PUBLISHED BY..........NICEIC

"Earthing clamps must not be used on paper-insulated/lead sheathed or steel or wire armoured (swa) cables."

 

[Adam_151]

Not sure I'm following your post correctly... but for what it is worth, you certainly shouldn't be trying to connect up your own TNC-S earthing terminal, the network might not be upto PME standards (which require sufficent electrodes to tie down the neutral conductor, and also that the integrity of neutral conductor is not too much at risk (which it could be if it were overhead and passes through trees, etc)

If you are talking about fitting your own electrode to tie a DNO suppied TNCS terminal down... then I can't see much wrong with it, as long as you size the cable appropriately(table 54H in the 16th, don't know what it is now). Not sure whether it'll help a great deal in the event of a DNO fault though... now perhaps if everyone had them and they were a lot longer as well.... (hold on, haven't I seen that idea somewhere before )

 

[GaryMo]

I assume he's talking about supplementing the PME earth by installing an earth rod and connecting this to the MET. Belts and braces so to speak.

 

[RF Lighting]

I don't see the point unless the whole installation is configured as a TT.

In the event of a failed PME earth or earthed neutral, there will be far too much current trying to flow to earth for the earth rod to deal with.

This is why TT supplies are RCD'ed.

I can see the thoery of tying down to earth to stop the CNE potential rising, but how often does this happen?

 

[kai]

On my system, when the mains gas pipe was replaced with a new one, the redundant disused metallic gas pipe buried in damp soil, was used as an earth electrode, and connected to the MET, as a backup to the PME system earth. (and there is a dual RCD protected consumer unit!).

 

[hairyben]

I can see the thoery of tying down to earth to stop the CNE potential rising, but how often does this happen?

Not often, admittedly.

But like said, I have seen it happen, and the worst part was the leccy boards lapsidasical attitude to it.

And lets not forget one thing- the only reason leccy boards like PME systems is cost, or lack of. Smaller cables, less conductors. There's a really good reason you or I can't install a TNC system in someones house, however the electric board effectively write their own regs.

 

[securespark]

....fewer conductors.....

 

[ricicle]

In the event of a failed PME earth or earthed neutral, there will be far too much current trying to flow to earth for the earth rod to deal with.

This is why TT supplies are RCD'ed.

TT systems have RCD protection because there is not enough Earth fault current due to usually higher impedance than TN systems!

 

[RF Lighting]

Thats the point I was trying to make, though perhaps didn't word it very well.

too much current trying to flow to earth for the earth rod to deal with

Should have really said something more like "too much current trying to flow to earth for the earth rod to deal with because its high impedence will not allow it"

Or something like that...

 

[GaryMo]

My initial thoughts would be higher impedance, lower current flow.

 

[RF Lighting]

I'm not doing well am I?

The point I was trying to make is that a earth rod will not work as a replacement for a lost CNE.

The rods impedence will be too high for all the current of the installaztion to flow through.

The protection offered from the earth part of the CNE will also be lost as the earth rod will not proform any protective duties unless the whole installation is setup as a TT installation.

I see the point for trying to stop the CNE potential drifting up a bit, but installing a rod as a backup on a PME supply will just not work.

 

[hairyben]

I see the point for trying to stop the CNE potential drifting up a bit, but installing a rod as a backup on a PME supply will just not work.

I dunno, with a proper PME system - which is spiked down regularly (protective multiple earth) you could get a fairly low reading with just a spike.

but like you say my point was in the intrest of protection, not function.