interesting fault

telectrix · Mar 10, 2012

came across this yesterday. called out to a park home to reported fault, RCD tripping with RFC fuse inserted. client was using socket outlet on cooker point to power fridge/freezer since night before. opened CU ( 6way wylex with 4 HRCs and 2 MCBs, 30mA standalone RCD front end). got out the trusty megger 400, IR readings on RFC of 0.1M, L-E, but, in addition, no continuity end/end on L and E. 4 years ago, he'd had an "electrician " install some extra sockets, 3 in total, spurred off RFC, every one had been fitted with no cpc sleeving, just bare cpc floating about behind sockets. first thought was possible S/C so sleeved and re-terminated all these, faults still there. now time to split RFC. guessed mid point and split. m1 leg IR >99M. other leg 0.1M. next S/O, same. next one , think i've found it. leg between 2 S/O's down. traced leg under home ( 2' cavity under, pulled it out suspecting rodent damage. IR tested loose cable-definitely low IR of 0.1M. closer inspection of cable revealed approx 6" from 1 end, L conductor broken and arcing damage burnt through cpc and char marks inside sheathing. 5m length of 2.5T/E installed, back to CU, >99M on RFC and E/E reinstated at 0.45 L, 0.46 N, 0.65 E.

just goes to show how easily you can go down a wrong path looking for the most obvious.

needasparks · Mar 10, 2012

Lucky to find the actual damaged part, anywhere else most would have to write off that section of leg.
Goes to show the wiring that is installed without testing.
I used to go nuts when i see the builders hanging their coats and stuff on my first fix cables.

telectrix · Mar 10, 2012

no, this was the original wiring. guess around 30 years old. 2.5mm solid . would never have failed it it had been good old 7/029. funny thing is, the client is thinking of replacing the CU soon and as all sockets are on 1 RFC, will probably remove the new cable and split into 2 radials to give circuits split between 2 RCDs.

ringer · Mar 10, 2012

Any ideas what caused the split/damage to the cable?

telectrix · Mar 10, 2012

think might have been stressed on installation, subject to large temperature fluctuations over the years ( the cable is clipped under the park home so subject to outside conditions) just finally gave way. no excessive loading.