IR fault tripping MCB???

[telectrix]

called out yesterday to house built 1992, split wylex CU. 3 ring finals. the problem one feeds around 10 sockets and central htg. 32A MCB tripped while cust was away. only pointer is an IR rdg. od 0.7M. remembered going there 2 years ago to RCD tripping and found water in an outside socket. no probs. since. can't understand why the MCB is tripping. with the boiler alone connected, the circuit is pulling 1A. she's phoned this morning to say it tripped yesterday and in middle of last night, both MCB and RCD tripped. going there at around 11.00 to try and sort it. one thing i did find yesterday was , when resetting the RCDafter testing, the MCB tripped as the RCD was switched on. i am thinking it could be a dodgy MCB. obviously the poor IR reading needs addressing, but any thoughts on possibilities would be welcome.

 

[PC Electrics]

Maybe the fault is intermittant? Depending on the temp of the conductors there may be a split in insulation somewhere which touches a back box when either warm or cold?

Neither would I rule out a faulty MCB, I've had a few of those. For what they cost I'd change it anyway.

 

[malcolmsanford]

Look for a short on the PCB in the boiler mate first off, so isolate that and IR.

The RCD may not have tripped first off Tel as it may have been stuck, I bet it as not been tested on the trip switch method since you went 2 years ago. Since then it as freed itself and is tripping.

I think you may find that once you have completely disconnected the boiler your IR should be ok, if not then it's fault finding time mate and you know better then most what that entails.

 

[telectrix]

thought that. will have a look at the outside socket later. maybe split the ring and try and track the IR fault. even so, a 0.7M will not cause a problem if it doesn't come lower. if it were the RCD tripping i could understand it, this one's doing my head in.

 

[telectrix]

Look for a short on the PCB in the boiler mate first off, so isolate that and IR.

The RCD may not have tripped first off Tel as it may have been stuck, I bet it as not been tested on the trip switch method since you went 2 years ago. Since then it as freed itself and is tripping.

I think you may find that once you have completely disconnected the boiler your IR should be ok, if not then it's fault finding time mate and you know better then most what that entails.

will have to crawl in airing cupboard to get at boiler FCU. i think the 0.7M is either the boiler or the outside socket. the RCD was replaced 2 years ago. will take a 32A MCB with me.

 

[PC Electrics]

Testing IR with the boiler FCU on then off should prove whether it's the boiler or not as the FCU is DP. At least it's an easy test to do first before getting stuck into a tiny space.

 

[telectrix]

think the boiler FCU is single pole and it's low level at back of airing cupboard. real pain to get to. might be easier to disconnect at boiler. think boiler may have been moved from airing cupboard to garage, quite a way away from FCU.

 

[PC Electrics]

You've learnt me summat there - I thought that ALL fcus were double pole and that it was a requirement of BS1363.

 

[telectrix]

most are single pole. try on with a meter. switch in off position, you read short across N terminals.

 

[telectrix]

Result : IR tested cct again this morning -- 0.7M. reset MCB, out goes RCD. hmm. after a smoke, thinks look at outside socket again. it's one of those metal box jobs with a screw cap over the socket front. pulls the socket front off, no signs of moisture. but noticed very little space behing socket and 2 legs terminated and rather squashed. IR reading now up to 5M. ( global on 7 circuits ). close inspection of the cables revealed insulation damaged on entry to the metal back box and line conductor of 1 leg showing copper, a bit of a tug and it snapped. conclusion -- expansion and contraction causing leakage sufficient to trip RCD, with occasional Dead Short giving rise to fault current sufficient to trip MCB. back tomorrow with a IP65 plastic socket. luckily there's enough cable comimg through the wall to strip back and rear entry into new socket. would have expected IR test to have given a much lower rerading though. bloody megger.

 

[malcolmsanford]

I now hope that everyone on here that doubted you will apologise ........................

 

[telectrix]

that's 2 hours took me to track down the fault, must be slipping in my old age.

 

[Mr Mike]

Result : IR tested cct again this morning -- 0.7M. reset MCB, out goes RCD. hmm. after a smoke, thinks look at outside socket again. it's one of those metal box jobs with a screw cap over the socket front. pulls the socket front off, no signs of moisture. but noticed very little space behing socket and 2 legs terminated and rather squashed. IR reading now up to 5M. ( global on 7 circuits ). close inspection of the cables revealed insulation damaged on entry to the metal back box and line conductor of 1 leg showing copper, a bit of a tug and it snapped. conclusion -- expansion and contraction causing leakage sufficient to trip RCD, with occasional Dead Short giving rise to fault current sufficient to trip MCB. back tomorrow with a IP65 plastic socket. luckily there's enough cable comimg through the wall to strip back and rear entry into new socket. would have expected IR test to have given a much lower rerading though. bloody megger.

Well done that man. You found fault and reasons for the interuptions- result.

 

[telectrix]

careful, you'll get me going big-headed. and some buxom wench will want to have my babies

 

[malcolmsanford]

careful, you'll get me going big-headed. and some buxom wench will want to have my babies

Oh no ....................that as now put the mental picture in my mind ...................and I was gonig to have a swarma for tea