Hello folks,
Any assistance or suggestions on where to go next on a potential high Zs problem much appreciated:
THE PROBLEM:
Subject is the ring final circuit (one for the whole house – 4 bed, 2 storey, 20~25 sockets – all wiring in the downstairs ceiling so long drops)
TN-C-S system, one 30mA RCD for whole CU
Type 2 32A breaker
R1+R2 of 0.519ohms (4 wire measurement with 3 amp PSU)
Assuming Ze is 0.35 ohm (I don’t know, can’t measure it) that gives a Zs of 0.869ohms which I understand is greater than the limit for this type of installation, which would be...leafing through OSG…0.79 ohms? I believe? – please correct me if I’m wrong.
I have an old EIC from 03/2011 which does align with my R1+R2 measurement, but it incorrectly identifies the breaker as Type B, and records a Zs max of 1.44ohm, which I guess is from BS7671 at that time (can’t find it in 18th OSG), so it all passed.
So it’s probably worth getting someone in to measure Ze/Zs properly, of course, but it still seems unduly high and worth investigating – I think.
WHAT I RECKON:
After spending a while with a drill, endoscope and AC pen the RFC is definitely served by a pair of cables for the most part, that travel basically the same path around the house with either(?) one of those cables serving an outlet as they pass.
This would be a bizarre waste of cable in my view, or I’ve missed something important here.
Theory 1: The RFC is horribly long and inefficient – i.e. it resembles two radial circuits dotting around the house outlets following very similar paths that are joined at the CU and one other socket at the end of them. I think I’ve found which socket this would be but haven’t dismantled that corner of the house to confirm 100%. It’s also about 2 metres from the CU, so just makes no sense whatsoever that the ring wouldn’t have been completed just running one cable from there back to the CU.
Theory 2: the house had an upstairs and a downstairs ring in the past. For some reason in the past these were combined into one massive ring that effectively does 2 laps of the house. There’s not really any strong evidence for this in the CU though, although connections could have been made in the ceiling above of course.
Theory 1 just sounds silly. Theory 2 is unlikely as there are 3 spare breaker positions on the CU so why would somebody have a need to combine 2 RFCs. Theory 3 doesn't exist yet.
I was rather hoping I’d figure out what I’d need to do next by writing this down, but I haven’t, so I’m posting it!
Thanks in advance for any thoughts!
Cheers
++++++++++++++++
Moderator's note:
The OP says:
"AlEightyFour Yesterday at 10:45 PM Hi I think I've solved this issue or at least thought up a few things to test so it can be deleted. I can add it up again in a better form when I get more info. Cheers."
Any assistance or suggestions on where to go next on a potential high Zs problem much appreciated:
THE PROBLEM:
Subject is the ring final circuit (one for the whole house – 4 bed, 2 storey, 20~25 sockets – all wiring in the downstairs ceiling so long drops)
TN-C-S system, one 30mA RCD for whole CU
Type 2 32A breaker
R1+R2 of 0.519ohms (4 wire measurement with 3 amp PSU)
Assuming Ze is 0.35 ohm (I don’t know, can’t measure it) that gives a Zs of 0.869ohms which I understand is greater than the limit for this type of installation, which would be...leafing through OSG…0.79 ohms? I believe? – please correct me if I’m wrong.
I have an old EIC from 03/2011 which does align with my R1+R2 measurement, but it incorrectly identifies the breaker as Type B, and records a Zs max of 1.44ohm, which I guess is from BS7671 at that time (can’t find it in 18th OSG), so it all passed.
So it’s probably worth getting someone in to measure Ze/Zs properly, of course, but it still seems unduly high and worth investigating – I think.
WHAT I RECKON:
After spending a while with a drill, endoscope and AC pen the RFC is definitely served by a pair of cables for the most part, that travel basically the same path around the house with either(?) one of those cables serving an outlet as they pass.
This would be a bizarre waste of cable in my view, or I’ve missed something important here.
Theory 1: The RFC is horribly long and inefficient – i.e. it resembles two radial circuits dotting around the house outlets following very similar paths that are joined at the CU and one other socket at the end of them. I think I’ve found which socket this would be but haven’t dismantled that corner of the house to confirm 100%. It’s also about 2 metres from the CU, so just makes no sense whatsoever that the ring wouldn’t have been completed just running one cable from there back to the CU.
Theory 2: the house had an upstairs and a downstairs ring in the past. For some reason in the past these were combined into one massive ring that effectively does 2 laps of the house. There’s not really any strong evidence for this in the CU though, although connections could have been made in the ceiling above of course.
Theory 1 just sounds silly. Theory 2 is unlikely as there are 3 spare breaker positions on the CU so why would somebody have a need to combine 2 RFCs. Theory 3 doesn't exist yet.
I was rather hoping I’d figure out what I’d need to do next by writing this down, but I haven’t, so I’m posting it!
Thanks in advance for any thoughts!
Cheers
++++++++++++++++
Moderator's note:
The OP says:
"AlEightyFour Yesterday at 10:45 PM Hi I think I've solved this issue or at least thought up a few things to test so it can be deleted. I can add it up again in a better form when I get more info. Cheers."
Last edited by a moderator:
