Rcd tripping fault

[SparkySteve4]

Hi, hope someone can help on this one since it has me stuck.
A lady had a new kitchen put in 2 years ago with a new range master induction cooker. Everything was fine for a year. Then the rcd covering the cooker, 2 socket circuits and 4 lighting circuits started tripping every time she put the kettle on the induction center ring. She called out range master thinking it was a fault with the cooker and they said it is fine and to put it on an rcbo. This is where I got involved.
I have put the cooker circuit from the mains switch to a rcbo with the rcd covering the other 6 circuits.
the fault occurs intermittently, it can happen several times a day or not for 2 days.
when I was there it was fine so I thought it was fixed. A week later I got a call. The rcd covering the 6 circuits was tripping when the center induction ring was used on the kettle.
Now these circuits are totally separate, rcbo from mains switch doing cooker, rcd doing another 6 circuits. It's a 17th edition board and the 2nd rcd is totally unaffected.
There was an extension lead behind the tumble dryer with a bad l-e ins res reading. This got removed off one of the socket circuits. The rcd tripped again today.
I have ins res tested between cooker cable and rcd busbar / neutral bar and >100mohms and live to earth ok. Have checked the rcbo and the operates fine 21/18ms and rcd 121/24ms. ramp test 22.5ma on rcbo
Have you any ideas why when the center induction hob is turned on it trips the rcd not the cooker circuit rcbo? It occurs when the cooker knob is actioned on intermittently, but only occurs with the cooker operation.

 

[bigspark17]

Have you IR the cooker at the outlet plate.?
is all bonding inplace.?
what Amp rating is the rcd.?

 

[Octopus]

Have you clamped the tails to see what the leakage is?

I suspect you have 2 faults

 

[Richard Burns]

A neutral earth fault on one of the two socket circuits (or something plugged into them) or on one of the four lighting circuits.
If the fault is intermittent then this causes problems, but using a high current appliance on any circuit can cause the RCD on the faulty circuit to trip.
It would be worth checking if there are any similarities with any of the circuits at the time of tripping e.g. plugging in the toaster or such like.
Possibly swapping a circuit at a time over to the other RCD (which if that one then trips it is the moved circuit and if it does not then it is one of the others)
A bit slow and steady but possible to find. Obviously insulation resistance testing may help a lot toward identifying possible candidates for relocation.

 

[SparkySteve4]

The rcd is a 63amp 30ma.
all bonding is in place
have Insulation resistance tested all circuits together on rcd at 4mohms
thing is the rcd and related circuits have nothing to do with the cooker rcbo circuit. Why would the cooker rcbo circuit trip the rcd unrelated to it?

 

[petlac]

This may sound weird, but I had a similar issue with a new Bosch oven. Bosch sent out an engineer after I had done all the obvious changes, RCBOs etc. They then told the customer to turn the control knob clockwise only. that includes when switching off. They say it is a known fault but like to keep it quiet. My customer now turns the control knob as suggested and has not had any problem since. Over 9 months now.

 

[Octopus]

The rcd is a 63amp 30ma.
all bonding is in place
have Insulation resistance tested all circuits together on rcd at 4mohms
thing is the rcd and related circuits have nothing to do with the cooker rcbo circuit. Why would the cooker rcbo circuit trip the rcd unrelated to it?

I

You NEED to clamp the tails

 

[Richard Burns]

The rcd is a 63amp 30ma.
all bonding is in place
have Insulation resistance tested all circuits together on rcd at 4mohms
thing is the rcd and related circuits have nothing to do with the cooker rcbo circuit. Why would the cooker rcbo circuit trip the rcd unrelated to it?

Your Insulation resistance results do not seem to show a fault bad enough to trip an RCD, this may be why it is intermittent, it may be a fault after a light is switched on or an is item plugged in for instance.

If the fault is of sufficiently high resistance so that normal use of the faulty circuit generates <30mA to earth, when the high current from the cooker also passes through the fault this may be enough to trip the RCD.

 

[SparkySteve4]

Ok, I will get them to only turn the knob clockwise between now and Tuesday see if it makes a difference. I will go back next week and clamp the tails and get everything on. In ten years I haven't had a fault I can not find but there has to be something there. Your explanation is useful Richard, just finding it being so intermittent. Thank you all for quick replies, will keep you posted.

 

[SparkySteve4]

Ok, someone personal messaged me with a suggestion of using a hager rcd because they have filters built into them. 2 weeks on and no tripping. To do with voltage transients being impressed by capacitance leakage when the induction hob is turned on. Many thanks to that person. Hope this might help others in the future