In a fit of conscientiousness I tested a ring circuit I had interpolated a few things in. Resistance was alright (0.33, 0.33 and CPC 0.38 ohms) and live and neutral were reasonably isolated from earth. But there was less then one megohm between live and neutral, probably because of a wired-in timer spur which (obviously) has it's load isolating switch *after* some of the electronics. Should I have disconnected this spur (which obviously might have introduced a new fault on reconnection of the permanent wiring), or is this good enough? Should there be an isolating switch before the timer - this would surely be overkill?
Testing a socket ring circuit.
Jun 28, 2024
Last reply: 2 years ago
8 Replies
No one want to offer an opinion? I usually act on reasoned opinions here!
Assuming 2.5mm^2 T&E, isn't the r2 value compared to r1 and rn somewhat low? Would expect closer to 0.55 ohm
That makes sense. How could that arise? Maybe two or more spurs were to equipment separately earthed. The original 'electrician' ran 10mm earth wire all over the place, he seems to have misunderstood equipotential bonding and thought it applied to the whole house. Doesn't sound too much like a problem though?
Connections (other than spurs) from one point of the ring to another?
Ah, that sort of thing would do it.
I don't suppose being "over-earthed" causes any danger ...
Regarding your insulation resistance, I think the measurement that counts is the one you said was OK (L+N together) to E, were you measuring with a multimeter or a megger/mft etc? What numbers did you get?
Possible damage to equipment left in circuit from 500V is a reason to disconnect it before IR testing, or if you're seeing lowish resistance from L to N.
Doesn't sound as though the installation new enough to be likely to have surge protection fitted in the CU?
There is a fan radiator on a spur (with copper water pipes) in about the middle of the ring. And a cooker hood.
A rather inappropriate 100V Megger (what was it even made for ?old telephone equipment) gave 130Megohm. Again higher would have been better but there was at least one electronic device. Not likely to be harmed by 100V DC.
I've got a [decades out of calibration] wind-up Robin I/R tester ...
Calibration would cost a lot more than what any of my equipment is worth! If it isn't actually broken it's probably within X3 or /3 of the right answer.
I too have an old Robin tester and built my own calibration checker from
1% resistors. For checking small resistance values using common small values in parallel. Actually as the max voltage for the resistor may have only been 350V so using 2 in series before making parallel combinations. For large resistance values putting, say, 10ff 10Mohm 1% in series.I put the combinations of resistors between terminal posts so that I could check each of the Robin's switchable ranges at full scale and a mid scale points.
The high output voltage from the Robin was measured using a DMM.
While this calibration checking method is not for any form of certification it does give a good indication of accuracy for my checks.
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