Discuss Cooker dedicated RCD tripping under zero load in the Electrical Forum area at ElectriciansForums.net

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BikeNick

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Hi, I am new to the forum, and a DIYer. For a couple of weeks our cooker circuit has been tripping the RCD in the Breaker dedicated to this circuit at the CU. The cooker is a Falcon 90cm induction hob unit, it is installed with a fused dipole isolation switch. The tripping has always happened (so far) while there is no load from the cooker, which has no LED clock or timer. Tripping has not occurred while the cooker is in use. The wiring is within an internal (party) wall where there is no sign of damp.
I checked, and the tripping also occurred (overnight) while the cooker was isolated using its dipole switch.
The cooker is c.10 years old. The breaker / RCD is a Contactum B63 (lhs CPB63 and rhs CPR63) separated by a test button (picture included). It was installed at the same time as the cooker. The test button correctly activates the RCD when pressed.
Based on the above, I am assuming that there is a problem either in the Breaker/RCD, or in the wiring up to and including the dipole switch, and that the cooker itself is OK.
Have I missed anything obvious, or will this need a professional to troubleshoot it?
Thanks in advance for any advice.
 

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The circuit will need testing using the correct equipment. Is the cable run a void in the party wall?
 
Any recent DIY activity that might have damaged a buried cable?

I suspect the best action is simply to get someone in to test the RCBO and check for cable faults, etc. There might be someon on this forum that covers your area which would be good.
 
No DIY activity in that general area that I am aware of (on our side of the party wall...). That CU is (a bit over)due for its first 10 year check. Looks as if I'll have to get it done sooner rather than later. Thanks for the input.
 
Hi - if the RCD trips overnight with the cooker DP isolated then it’s hard to see the cooker being the problem. But there is something causing the trip and it’s best found with testing.
 
It is quite likely that there is a neutral-earth fault between the CU and the cooker switch (double-pole, not dipole!) This can cause exactly the symptoms you describe, i.e. tripping unrelated to the load on the protected circuit. If the switch is operating properly, and tripping occurs with it off, the cooker is not a suspect in this case. An insulation test should reveal what, specifically, is wrong with the wiring.
 
It is quite likely that there is a neutral-earth fault between the CU and the cooker switch (double-pole, not dipole!) This can cause exactly the symptoms you describe, i.e. tripping unrelated to the load on the protected circuit. If the switch is operating properly, and tripping occurs with it off, the cooker is not a suspect in this case. An insulation test should reveal what, specifically, is wrong with the wiring.
It could also be a live to earth leak in the cable to the cooker switch, although the apparently random intermittent tripping would indicate that neutral to earth is more likely.
Switching off the MCB would eliminate the live to earth scenario if the tripping continued, and virtually confirm the neutral to earth one.
 
It could also be a live to earth leak in the cable to the cooker switch, although the apparently random intermittent tripping would indicate that neutral to earth is more likely.
Switching off the MCB would eliminate the live to earth scenario if the tripping continued, and virtually confirm the neutral to earth one.
Thanks all. It looks as if a test with the MCB and the isolation switch both off is called for tonight.
 
63A MCB seems a bit over kill for a domestic hob,would be surprised if the cable is suitably sized for this - and if it is the connections at the double pole switch could be interesting
 
With that size MCB you'd be expecting a 16mm cable to the DP switch, good luck terminating that nicely. l;;;;;;;;;;;;;;;;;;;; - EDIT The effing cat decided to walk straight over my laptop keyboard. ?
 
With that size MCB you'd be expecting a 16mm cable to the DP switch, good luck terminating that nicely. l;;;;;;;;;;;;;;;;;;;; - EDIT The effing cat decided to walk straight over my laptop keyboard. ?
No RCD tripping (so far) with both the MCB and the DP isolator off. The cooker has a dual oven, and 5 induction rings, hence the rather industrial cable, fuse, and isolator (pictures included).
 

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Looking at the cable in the first pic., the problem could well be there. The cable has been bent in a 'flat bend', which will be pushing the cpc hard into the insulation of the conductor on the outside of it.
 
Looking at the cable in the first pic., the problem could well be there. The cable has been bent in a 'flat bend', which will be pushing the cpc hard into the insulation of the conductor on the outside of it.
Hmmm. It is possible that the cooker cable has been moved a (fractionally) recently, when I was checking the best route into the CU for a couple more cables (in 16x25mm conduit) for a planned new upstairs ring. Is it feasible to check that section of cable from the outside, or would it be simpler to disconnect it from the CU, re-terminate in a junction box, and go from the new junction box to the CU with appropriate equivalent cable?
Not sure its a job I'd want to take on myself though, given the thickness of that cable! It would have to go into the list of electrical jobs that is slowly accumulating for someone qualified to deal with.
 

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