RFC LN-E IR 0.15 ohm - why was the RCD not tripping?

[S P]

Small job adding a few sockets, while doing some preliminary checks I found this reading on the socket circuit!

After hours of fault finding -
Searching for any possible connected loads -
Questioning if the SFS's were DP then disconnecting the load N (found SP SPS's online) -
Breaking circuit in halves each time left me between 3 sockets....

The one in the middle was a radial!
Lifting floorboards searching for a JB, the spur from the middle socket disappeared into a bedroom towards under a large wardrobe!
(FFS, quick smoke)

Sticking my head and torch into the floor voids tracing the leg from the previous socket I noticed the cables appeared to go down to GF.
Found a socket behind a radiator!
Above that was a picture..., tested from nearby socket to picture hook SAME READING!
Removed picture hook and reading dramatically improved

It was already very late and I still had to put back all I'd taken apart so will chop the wall section tomorrow to repair.
Client was very happy the problem had been found but I couldn't explain why It hadn't caused the RCD to trip??

BTW - RCD test have been carried out and all ok.

 

[NDG Elecs]

Was the 0.15 continuity or IR? My testers IR resolution would never go down as far as 0.15 ohms, so am wandering if you meant Megohms?
And if it was Megohms, then thats still way above what it would take to trip a 30mA RCD. It's late and am tired but think around 7000 ohms is the region in which a 30mA will trip.
Good find by the way, sounds like you had fun in the process!!

 

[S P]

Thank you NDG
Yes I did mean M.ohm, so the reading would have to be less than 0.1 M.o... Wow

And thanks but no it wasn't much fun, but a relieve when I did find it

 

[westward10]

This type of fault will not necessarily trip an rcd if the current decides to be more biased toward the neutral path thus not creating a sufficient imbalance to trip the device.

 

[telectrix]

basics. ohm's law. for a 30mA RCD, R=V/I, so R = 230/0.03 = 7,667 ohms. ssssimplesss.

in the real world it's 240V so R = 8k ohms. theroretically, but will be a bit more, as most RCDs trip around 23 -25mA.

 

[littlespark]

Did the customer never mention he got a tingle every time he straightened the picture?

Socket behind a radiator? typical heating engineer, making things difficult... grumble grumble

 

[buzzlightyear]

Did the customer never mention he got a tingle every time he straightened the picture?

now this post takes me back a few years .when a customer had been shoving nails in a wall for pictures ,Vir cable no earths and lighting cable in the walls it was like a pub dart board,go on another go!