PME Ze - Confused!

[Graeme Harrold]

Been to a property recently that has a TN-C-S supply installed, but the innstallation earthing is via a spike. As the supply has recently been installed I was going to move the earthing back to the supplier (Scottish Power) unfortunately when tested, the Ze was 0.79. An engineer has just been round had a look and is going to measure, however he said I could have used their earth as the Ze was below 1 ohm because the installation was covered by an RCD........is he correct as this is along way of the max 0.35 in the regs???

 

[Sintra]

If the maximum Ze value for a TN system cannot be met, the installation may be protected by a an RCD and treated as a TT system. 531.3.1, 411.5.1, 411.5.2, 411.5.3

 

[eskimo39]

If the maximum Ze value for a TN system cannot be met, the installation may be protected by a an RCD and treated as a TT system. 531.3.1, 411.5.1, 411.5.2, 411.5.3

My god man, you are on fire tonight. You have beaten me twice with the regs. I only just managed to get the book from the van this time.

 

[Sintra]

I'm hard to beat when i'm on a roll. Better luck next time lol.

 

[eskimo39]

Have you memorised the regs!!! I'm dealing with bloody Rain Man

 

[WayneL]

If the maximum Ze value for a TN system cannot be met, the installation may be protected by a an RCD and treated as a TT system. 531.3.1, 411.5.1, 411.5.2, 411.5.3

Unfortunately, it doesn't say that.

In fact it doesn't mention Ze at all.

If the earthing system is supplied by the distributor, then they have to guarantee the max Ze.....TN-S....0.8 and TN-C-S.....0.35

The regs that you have quoted refer to parts of an installation where Zs figures cannot be met.

 

[Spudnik]

Which in turn, is affected by Ze.

 

[WayneL]

Which in turn, is affected by Ze.

Yes, but they are referring to you not meeting Zs on particular parts of the installation for whatever reason - it is never mentioned that it would be due to Ze being too high.

You can't just stick an RCD on because the supply earth Ze is too high - they probably wouldn't let you use it in that situation, you would have to change the installation to a TT system. (which seems to have happened in the OPs case)

531.3.1 In a TN system, where, for certain equipment in a certain part of the installation, the requirement of Regulation 411.4.5 cannot be sattisfied, that part may be protected by an RCD.

411.4.5 is the section for meeting max Zs figures in order for the protective device to operate.

Really, nothing to do with the Ze

 

[Graeme Harrold]

Thanks for the replies, just confirmed what I thought. DNO now has Ze down to 0.33, so good to go back to PME from current TT.

Just got to rip out a load of VIR cable now!!

 

[WayneL]

Been to a property recently that has a TN-C-S supply installed, but the innstallation earthing is via a spike. As the supply has recently been installed I was going to move the earthing back to the supplier (Scottish Power) unfortunately when tested, the Ze was 0.79. An engineer has just been round had a look and is going to measure, however he said I could have used their earth as the Ze was below 1 ohm because the installation was covered by an RCD........is he correct as this is along way of the max 0.35 in the regs???

Hi Graeme,

As an additional thought, this statement by the DNO engineer is a bit worrying to me.

A Ze of 0.79 ohms, on a TN-C-S system, gives you a PSCC of 291 Amps - not enough to trip even a 60A BS1361 cutout in 5 seconds, let alone anything bigger.

The fact that the installation is covered by an RCD is irrelevant.

So unless they have different disconnection times for their supply fuses, I think he might have got it wrong