help.. insulation test problems..intermittent fault

[vitoboy]

hi all
im testing a new house wired by my self..intended from the start to be one for my nic inspection.
all is as it should be ...but for one circuit.
so when testing a ring for lounge,bed 1,bed 2,bed 3, (16 dsso in total) i found i had a n/e fault around .64 M ohms.
so normal checks split the ring test both sides...found a fault and followed it then it just was gone.
so removed all fittings and did i/r test oneach bit of cable...reconnecting as i go testing what ive reconnected and the next leg before connecting it.
at socket no 7 i found i had a fault n/e about .62 M ohms between the n of the previously done leg and the e of the next..?????
just after this i removed 2 x dsso that i had checked and reconnected and the fault was gone.
so i reconnected all dsso testin after each one..all good till the last one.
it was ok till i screwed the dsso to the back box(metal)then fault..
for about half an hour i screwed it on...fault then off...no fault...
change the dsso (mk)
still the same...
take the back box off the wall(not easy)..back box not on wall...no fault...back box back on wall...fault....
check back box screws...ok...put shorter ones in for the hell of it....but i know no other cables near that screw..
so if i leave the dsso off the back box no fault??!!.

decide to put back all fittings but not the sus one one at a time i/r testing each one in turn
put two back on(not the sus one)and have the fault again.
remove the two ive just put on....ha fault still there...
ive even i/r tested the socket...no joy.

im thinking i must just have a cable thats....just..only just been winged by a nail scew soldering gun ect...maybe i need some load to find it
any ideas...#
anyone...
who can be bothered reading all this ....
ta
vitoboy

 

[vitoboy]

ps.
im using a megger 1552
and ive got two...and they both give the same readings...

 

[telectrix]

have you double checked that you've not crossed N and E in the socket? cable entries into back boxes grommeted, no cable damage?

 

[vitoboy]

yes bud
all cables have been inspected for damage...all gromits in place..
it a house with a small 1st floor..big eves all round..i can look down the jost spaces and see all the cable runs..only the drops behind the dot and dab or in the stud thats not visable..

 

[telectrix]

stud walls, joiners, nails . time to get the hammer out,

 

[vitoboy]

this was my fear..
i was wondering about removing the rcbo and fitting a 64A c rated mcb then pluging in all power hungry stuff and have a look to see if i can make the fault bad enough to find.....carpenters....no use moaning..my dads one....

 

[olly w]

Have you linked the cpc at the sockets to the back boxes when connecting them up?

 

[vitoboy]

no
i have an nic eic guide note that says no need
but i kn ow...how can a metal back box thats not connected to anything...when i screw a socket to nit cause a fault...and its not a pressure fault ..
as i say each bit of wire bells out 100%
its like the neutral from one side of the ring is shorted to the earth on the other side..if you get my drift

 

[vitoboy]

thr only way i can see is that its screwed where both sides of the ring pass through a hole...
how else??

 

[gpz900jim]

i had something very similar to that. in the end i had to rewire the circuit to get it to work. i never found out what was wrong, but i wondered if the wire was some of that bad stuff from turkey. The customer supplied the materials so one never knows??

 

[olly w]

Did you use steel capping...?

 

[gpz900jim]

another thought, could the cable be wet inside the sheathing?

 

[DurhamSparky]

or wet plaster!!

 

[vitoboy]

well
i supplied the cable
2 types are used on this circuit
the standard smooth type
and the one with a dip between the L and N
i am not! rewiring the whole thing..no way...no how
how can we charge the money we do and
spend the money on test gear we do and
spend the money on qualifications we do and
do things right and still
not be able to say how an insulation fault has happend and where.
?

 

[telectrix]

welcome to the land of sod's law. it overrules ohms law and kirkoff's law. not too sure about parkinson's law. think that one's to do with shaking hands.