EICR unearthed an unearthed TT

[mole42uk]

We were doing an EICR on a TT house installation the other day and measured a Ze of 408Ω .... so we took the lid off the earth rod terminal box, only to find it full of - earth. The 10mm PE was fitted with a suitable crimped terminal, but it wasn't connected to the the clamp on top of the earth rod. I can only think that someone thought it was amusing to "Earth" the system is this way.

[Graham G3ZVT]

A bit like connecting a radio set earth terminal to a rod in a plant-pot. Such plant-pot earths were actually sold to the gullible.

[Refugee]

If it was for an audio reason the earth wire should have been routed to a coal bunker surely

[mole42uk]

Originally Posted by Refugee

If it was for an audio reason the earth wire should have been routed to a coal bunker surely

The house is in South Wales - no need to look far for coal, a working earth rod would do it by default. Maybe the house could be sold, when the earth fault is corrected, as an audiophile listening chamber?

[pmmunro]

Originally Posted by mole42uk

We were doing an EICR on a TT house installation the other day and measured a Ze of 408Ω ....

Probably the person who was (ir)responsible for this wouldn't even understand the terminology of your first sentence. There's a short article in the August edition of the "E & T" magazine about this sort of thing.

I wonder what they would make of C & G 2382, 2392 and 2393, just the names I mean, let alone what they stand for.

PMM

[mole42uk]

Unfortunately I think you might be correct. I have C&G 2392 and I was an accredited tutor on C&G 2399 where my experience did not fill me with confidence.

[locknut]

Reminds me of something TT related I saw at a neighbour's. We are all on TT here, and they had just had a solar panel installation. He was showing me the installer's handywork in the cubby hole built around the meter and fusebox (still on Wylex rewireables), when I noticed something odd.

The cubby had been built over the main earth block, making it inaccessible - so the installer had put the earth for the solar inverter and panels into the neutral...

[Lucien Nunes]

If the supply is TN-C-S that's naughty but not very dangerous. If it's traditional TN-C with no nearby CNE, or TT, it gets more dodgy.

Working mainly on large non-domestic installations we don't tend to see bodges on the service equipment like DIY TN-C-S, BS951 clamps on PILC, etc. We did find a 315A intake at a college earthed by a loose M6 roofing bolt.

[locknut]

Originally Posted by Lucien Nunes

If the supply is TN-C-S that's naughty but not very dangerous. If it's traditional TN-C with no nearby CNE, or TT, it gets more dodgy.

Certainly gets interesting if a TT neutral to the building becomes accidentally disconnected - certainly not beyond the realms of fantasy when the two conductors are fed from the pole by singles.

[Lucien Nunes]

If it's traditional TN-C with no nearby CNE

Oops, typo here, this should of course read TN-S

gets interesting if a TT neutral to the building becomes accidentally disconnected

I think you've done it as well, I guess you meant to say TN-C-S neutral. Working with true TN-C (CNE from origin to load) is an eye-opener when the CNE isn't a high-integrity conductor like earthed-concentric. I first did this in Germany in the 1980s - it's called 'klassische Nullung' - where the old grey PEN conductor in a lighting circuit had to become both neutral and earth at a metal fluorescent batten. Disconcerting until one's initial jiitters about the thing going open-circuit calm down.

My comment about the retrospective TN-C-S for the solar inverter being potentially dangerous (see what I did there?) ws referring to two different 'versions' of earth, one from the rod and one from the neutral, existing side by side in what is supposed to be an equipotential zone. The TT earth becomes an unbonded extraneous conductor relative to the TN earth.

[locknut]

Originally Posted by Lucien Nunes

I think you've done it as well, I guess you meant to say TN-C-S neutral.

My comment about it being potentially dangerous (see what I did there?) ws referring to two different 'versions' of earth, one from the rod and one from the neutral, existing side by side in what is supposed to be an equipotential zone. The TT earth becomes an unbonded extraneous conductor relative to the TN earth.

Specifically thinking about our situation here, which is all TT.

If the neutral was disconnected from the building, the neutral inside the building (and indeed the solar panel earth) would rise to live, back fed through any connected appliances.

Under normal circumstances, no neutral would not trip the incomer RCD... but with the neutral connected to the solar panel earth?

[Lucien Nunes]

our situation here, which is all TT. If the neutral was disconnected from the building, the neutral inside the building (and indeed the solar panel earth) would rise to live,

But the solar side of things isn't TT, it's sneaky TN-C-S, as the incomer neutral is serving as an unofficial PEN. Of course it might be OK in that role, at least as OK as any overhead CNE can be, but it is definitely up to the DNO to decide that (e.g. based on existnce of PME nearby) and to make the connection at the head.

Since the RCD is in the neutral only, downstream of the neutral-earth link, it does nothing to protect against open CNE. This is a big deal with equipment used outside the equipotential zone and new monitoring devices are on the market to deal with the scenario by disconnecting all three conductors in the event of a CNE loss.