Confused about a fault with downstairs lighting, any ideas?

jonnyb · Oct 4, 2018

Hi chaps I have a query and wouldn't mind any suggestions. I've been preparing a consumer unit swap, ran the usual tests and no major worries except a low insulation resistance test on a ring. Going to swap the board tomorrow morning. Customer asked me to swap some lighting switches whilst I was there prepping today. Pulled the fuse for the downstairs lighting and this reversed the polarity on the ring sockets. Double checked and happened every time I pulled the fuse. Will investigate further in the morning but has anyone ever had this happen?

Octopus · Oct 5, 2018

Dave OCD said:
I'm just wondering if this could be something to do with a 'borrowed neutral' from the RFC connected to a light, can't really get my head round it though.

Like this one of mine ?

L-E 199v, N-E 199v ............... real readings - https://www.electriciansforums.co.uk/threads/l-e-199v-n-e-199v-real-readings.168792/

Pete999 · Oct 5, 2018

Post 24 question, "what type tester are you using" to prove polarity?

alasdairp · Oct 6, 2018

My best guess: Telephone skt fed by lighting circuit L but has N borrowed from RFC. Removing lighting cct fuse would mean that if any incandescant bulbs were 'on' they would provide a low resistance path to the lighting circuit N. So, as there is a L-N problem on the RFC - that may produce just enough (reverse) voltage for the very low power requirement telphone to try to power-up via the N connection. This would not show up as a reverse-phase skt using a proper cct tester. The OP has not answered what meters (etc) he is using. The installation needs a full EICR, methinks. Something is very wrong.