On Wednesday, July 17, 2019 at 1:43:15 AM UTC+1, John Rumm wrote:
On 16/07/2019 23:05, Mathew Newton wrote:
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?
Quite possibly, or something else not quite right.
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.
If so, is there
a good way to locate where through testing as opposed to removing
each socket for direct inspection?
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.
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?).
Something like one of these makes the job easy:
https://www.amazon.co.uk/Kewtech-KEW.../dp/B0058HZLTK
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?