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Discuss Multiple distribution boards and earthing arrangements in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Hi all, first post, and it relates to quite a challenging installation.
The short version is that I'm working through remedial work on a 75 page EICR on a site with 24 distribution boards, hit a very nasty C1, have resolved it, but now have max Zs issues that were actually there before but not listed on the EICR. The following is the relevant info:

The supply, with TNS earthing has a nice Ze of 0.09 ohms. It's right by the sub station. The supply room has a fused isolator, feeding a run of 3 core SWA underground to DB2 (distribution board 2). Zs at DB2 is 0.22 ohms.

On DB2 is a three phase C63 (60898) MCB feeding some 25mm sq. 3 core SWA underground to DB5 in a separate building, approx 30m away. Max Zs is listed as 0.27 ohms for that circuit, I guess using the 80% figure.

Zs at DB5 is 0.42 ohms. This isn't explicitly mentioned as a problem on the EICR, but subsequent similar downstream problems are. As well as using the armoured sheath as the cpc, DB5 has an earth electrode, and I've not measured it independently yet. As we're already at 0.42 ohms here, then life from this point gets very interesting as DB5 feeds 3 further buildings, all of which are a further 30 to 50m away. The description of the original supply to these three is nicely encapsulated in the following C1:

C1: DB5 - circuits 4L1, 4L2, and 8L1 are supplied from the same SWA cable, therefore sharing the same neutral. Somewhere there must be another joint because once the supplies get to the distribution boards they are wired in two core SWA. 4L1 and 4L2 are on the same TP circuit breaker making 4L3 unavailable. The phases are unbalanced and the neutral must get overloaded because of this
So basically there was a 4 core SWA containing phase 1 twice, phase 2, and shared neutral, feeding three boards, with unknown joint or joints underground. This was looking more and more glorious.

Over the last week I've found the two underground joints, ran a new 25mm csa 3 core SWA to one of the outbuildings, and re-used the original 4 core as phase and neutral twice to the other two, leaving one single (documented!) underground joint with an inspection pit. I also added an earth electrode at the underground joint as well as bonding all three of the armoured sheaths together.

At two of the outbuildings I changed the split-load boards ordinary isolators to 100ma RCD incomers, and added local earth electrodes as common sense suggested that there are 3 underground cable runs and 2 distribution boards between the supply and the final board. This probably wasn't strictly required but just felt like the right thing to do.

In theory I'd hoped this would resolve matters, but the Zs at DB values remain an issue. I have
DB21 - was 0.64 ohms, now 0.54 ohms, but max Zs is 0.35 ohms (fed from C63 MCB)
DB22 - was 0.58 ohms, now 0.35 ohms , max Zs is 0.35 ohms (fed from C63 MCB)
DB23 - was 0.81 ohms, now 0.78 ohms, Max Zs is 1.37 ohms. (fed from B32 MCB)
and DB5, supplying all of the above, has 0.42 ohms but max Zs is 0.35 ohms (fed from C63 MCB)

So even though the C1 is resolved in one sense, DB21 is still a problem, DB22 is right on the limit and I'm now suspicious of the reading and will re-measure, and upstream DB5 has a problem. On the plus side everything now has it's own neutral conductor, and the earth impedance has seemingly improved everywhere but I can't really use that to justify the remaining issues!
Questions please:
1 - Improving Zs at DB5 seems to be the next logical step as everything downstream would benefit too - I can only think to be running a new supply to DB5 with appropriately sizes cables (which is a significantly difficult task), OR adding more earth electrodes at DB5, OR consider using fuses to supply DB5. I'd appreciate thoughts on the merits of these, or other better ideas.
2 - Are there any circumstances you can legitimately have a TNS earthed supply and a board feeding distribution to a remote building and use only TT earthing at that building? I'd rather not be thinking in these terms, as it feels like I'd be increasing the impedance and reducing the safety just to suit the regs, but this is rather a difficult situation. I'm curious if it's allowed in the first place though.

Many thanks for any comments.
 
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Simon47

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