Measuring the L-L and N-N resistance on a ring circuit is giving me 0.7 and 1.1 ohms respectively. Does this point towards a poor/loose neutral connection in the back of a socket somewhere? If so, is there a good way to locate where through testing as opposed to removing each socket for direct inspection?
Quite possibly, or something else not quite right.
You could go for a "binary chop" type approach. Disconnect the ends of the ring at the CU, and short L & N & E of one end together. Now using a plug as a test point, plug it in somewhere on the circuit near to where you think the middle might be. You can now check if the L to E and N to E match. Whether they do or don't, you have just eliminated half the circuit. Go half way again in the appropriate direction, and you can get rid of half the remaining ones again. Should take no more than Log_2(n) tests, where n is the number of sockets on the circuit.
Something like one of these makes the job easy:
I should've said that this is a 2007-built house and so hopefully the issue is not a result of some bizarre circuit design/layout. Also, the E-E resistance is 0.9 ohms and all these readings are with a non-zeroed meter (lead resistance measures at ~0.5 ohms) so whilst they are all low it is the L-L/N-N inequality that raised my suspicions rather than the absolute values, and given the still-lowish N-N reading that's what led me towards it being a contact point issue rather than a break or similar.
Okay that sounds doable - thanks. To be fair I think there are only 13 sockets so it's not that big an installation, and one or two of those are spurs and thus wouldn't have contributed to the measured resistance (their point of attachment would of course be - and perhaps that's where it is quite likely to be?).
Hopefully this will be a one-off so a conventional plug with a removable back should equally suffice, particularly given the power will be off?
In which case, yup, probably just a loose screw etc.
Yup, I would agree.
Yup the test sockets make it a bit easier (and safer), but working in a controlled environment and with care, a normal plug and lead is fine. If in doubt, you can always turn off everything at the main switch.
Sounds quicker to just unscrew the sockets and retighten the screws.
NT
Visually check each socket's face, for signs of any over heating.
With the power off, you could make up a pair of (naughty) plugs with bare wires coming out, then work your way around the ring, comparing L to L and N to N resistances across sockets in the ring, until you localise it, then delve into the socket for the cause.
Measuring such low resistances has always been a tricky problem.
First of all measure the resistance of each of your test leads too.
Well John, it looks like it really was 'something else not quite right'! ....
I was all set to tackle this at the weekend and disconnected the ends of the ring. Measuring the L-L and N-N resistances again I found them both to be 1.1 ohms with the E-E at 1.5 ohms. Factoring in the 0.4 ohms test lead resistance gives me 0.7 ohms and 1.1 ohms respectively.
I see that copper cable is 7.41 mohms/m for 2.5mm2 and 12.1 mohms/m for the 1.5mm" CPC so with my measurements that suggests my circuit is ~90-95m long which sounds feasible for a 45m2 house footprint, a dozen or so sockets over two floors and 2.4m ceiling drops.
My conclusion is that I'd screwed up the original measurements somehow - perhaps measuring the L-L resistance of another ring or already taking off the test lead resistance; either way it seems like user error and that I don't have a problem afterall. I can't help but feel a bit disappointed as I was looking forward to finding and fixing the 'fault'!
Thanks anyway John for the suggestion and I'll bear it in mind should I ever have to do this again.
Oh well, that seems like a reasonable solution then... (either that, or you have found a really tricky intermittent poor connection! :-))
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