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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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Is DB22 picking up parallel paths because it's Zs is lower than DB5 from where it is supplied. Are Type B not an option.
 
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  • #7
Is DB22 picking up parallel paths because it's Zs is lower than DB5 from where it is supplied. Are Type B not an option.
Yes, I'm suspicious of that reading for the reason you give and will re-measure tomorrow. Could be parallel paths. More testing needed.
I'll certainly think about Type B's.
 
If the Cs are not there for a specific purpose fit Bs or you could maybe get away with 50s, selectivity is all to pot anyway.
 
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  • #9
Thank you, investigating the loads on the boards seems like the next thing to do and reassessing the breakers in use, as well as double checking that suspect/curious measurement.
 
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  • #10
Just an an update, changing to type B breakers has bought DB's 21-23 within the max permitted Zs values. One previous reading was incorrect too as suspected.

The remaining issue is the supply to the upstream board DB5. The issue is that the supply comes from a C63 MCB on DB2, so max permitted Zs is 0.35 ohms, and my Zs at DB5 is 0.42 ohms.
It's a seriously old cable, and really wants replacing. Triangular conductors and Red, White and Blue phases, anyone want to date that?! Don't think I was born when that went in.
Replacing it is a large and costly project as there is a road and tennis court on top of it now - built since it was installed as far as I can tell.

I think my only throw of the dice on this will be trying a C50. Swapping it to a fused supply would technically satisfy the regs, but increase the disconnection time, and it grates a bit to increase the disconnection time in the name of compliance, for 0.07 ohms. Seems just wrong.
Unless anyone has any other bright ideas.
 
1950ish I should imagine. No reason why is should be a problem unless somehow the armour has degraded.
 
Any main bonding required in these areas.
Definitely improved situation,but really a poor design originally.
Tt may be the way to go if feasible
 

Lucien Nunes

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Mentor
Arms
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The R/W/B cable could be 1960s, agree there is no particular reason to assume it is U/S.
Also agree that changing the earthing configuration and in particular decreasing the functionality of the installation to deal with a marginal failure would be a pity. Could an alternative type of protective device, such as an adjustable MCCB that could be installed in a separate enclosure if necessary, provide a better compromise between In and max. permitted Zs? Personally, I would have no issue with protection by a fuselink though.
 

Simon47

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Arms
Replacing it is a large and costly project as there is a road and tennis court on top of it now
Just an FYI, there are contractors that do directional drilling. Some years ago at a previous job we looked into using one to link the network across two customer's sites with a road in between. The guy would just dig a pit at both sides, park the drill at one side and put you a duct between the pits - then traditional trenching across the grass from there. As he wasn't interfering with the road structure, no need for a permit etc - I assume he made careful checks of where the other utilities were located before drilling ! I suspect he might not be able to do across a tennis court though.
 

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