Thank you for this very in depth answer!
I did some fault finding to try find a nuisance trip of the house which I will be changing the CU. I’ve narrowed it down to fridge freezer, fish tank filter or pump as I put them in an extension lead for the time being on a socket fed by the RCD that wasn’t tripping and it tripped same day.
Having a current clamp meter that goes down to 1mA or so is useful here to find out if it is an accumulation of AC leakage from capacitance (so you see around 10mA-ish all the time) or a fault (usually near zero but something occasionally spike enough to trip it).
This is good value for that job (but like most affordable ones, AC-only):
Basically you clamp L&N of each circuit to see what is leaking. Clamping the CPC can give bizarre high readings due to parallel return paths in TN-C-S cases.
It is currently a RCD split board installed by Mr bodger. I’ve recommend they get it redone and changed to RCBO board. They also use so many extensions. Every socket has a fully occupied extension lead minimum 4 plugs. Anyways I did IR test with everything unplugged and it was clear.
Probably too much on one RCD.
More sockets are always handy if there is space to fit them. Or at the very least having good quality extension blocks for the likes of TV/AV/Hi-Fi and IT stuff.
Olson Electronics (London based company) make really good metal cased ones, but at a price. We have some dating from 1991 still in good working condition!
Will need to do R1 + R2 tests tho.
I did end to end and like you mentioned about the split ring being fed by 2x 32a MCB that’s exactly what I found. I also found another ring which had been split but this ring had one of its LNE leg just connector blocked floating inside the CU.
OK, you know that well!
Can I just check the Ze or does Zs need to be carried out too before the CU change?
Ze (i.e. supply earth tested with installation off and it separated) is most fundamental as it is the only one you ought to rely upon for fault disconnection:
- If TT it should be under 200 ohms, and definitely find and inspect the rod(s) it to see if in sound condition.
- For TN supplies you would normally see under 0.8 ohm (TN-S) or 0.35 ohm (TN-C-S) but that is not guaranteed. Those TN values are used in the OSG for guidance on circuit size, etc.
Measuring Zs at the DB to get max PFC, and measuring max PSCC, is easier and the higher of the two tells you if the OCPD can safely disconnect, etc. But with modern domestic CU you are
very unlikely to ever see an issue as they can disconnect to 6kA at least. Old rewirable can be a risk though, at 1kA or 2kA typical max PFC.
But checking Zs is not telling you Ze is good - it might be relying on service pipe bonding that can change externally and without warning due to repairs when metal pipes are replaced with plastic.
When finding out if there are borrowed neutrals, I’m guessing I would check continuity between neutral and other neutral conductors at the board. For example disconnect 1 neutral and test between that conductor and the neutral bars?
Typically you need to IR one circuit to another, and with two-way switches in all permutations, as typically it is L of one circuit returning via N of another so only in some positions is it connected, and it depends on the lamp (which could be LED or CFL) being driven in to conduction which might need tens of volts at least.
I think the ROI regs demand that circuit-circuit isolation is measured, basically this sort of test.
