Induction hob possible fault!

[Jimathome]

Hi all,

can anybody help me out?
ive been investigating an intermittent fault that causes the bottom rcd of a 20 way dual rcd cu to trip approx every 8/10 days. (Really annoying fault to trace)..
i used a mili amp clamp meter today to measure the leakage to earth from the bottom row and got a reading of near to 30ma (strange as the rcd trips on ramp test at about 25ish)
however when I started to isolate circuits to see if I could see if a circuit was excessively leaking to earth nothing much seemed to change on the metre reading... I then started to disconnect the cpcs one at a time to see if I could cause the reading to drop...
when I disconnected the cpc for the induction hob circuit (40 amp 10 mm feed) I saw an arc from the cpc to the earth terminal bar (I know you shouldn't disconnect cpc when live but scratching my head here!) I then went to put the cpc back in and as I touched the copper to straighten it I received one of the finest belts in my history as a spark!!! I then carried out IR test on my feed and everything was clear.. I then removed the isolator under the hob and tested here ...and as I clamped my leads on my cpc and then the earth from the hob my meter showed 192 v and 50 hz??? ( meter was in series so in theory I shouldn't of had a reading?) is there some serious capacitors inside the hob? And if so why when I splashed the cpc didn't the arc stop it was continuous? Hope this makes sense any ideas greatly appreciated!!
thanks in advance

jamie

 

[Plonker 3]

Silly reply

 

[Plonker 3]

Also have you carried a IR on the offending circuit?

 

[Jimathome]

If you isolate the hob via 40 amp double pole switch the voltage and arcing goes away...

 

[Jimathome]

Yeah all IR are over 500M its a new install!

 

[HandySparks]

Welcome to the forum.

Are you saying that if you clamp round the L & N feeding the bottom RCD and then turn off the MCB supplying the induction hob, that the differential current measured doesn't change significantly?

 

[44ALLAN]

mmmmmm

 

[Jimathome]

Welcome to the forum.

Are you saying that if you clamp round the L & N feeding the bottom RCD and then turn off the MCB supplying the induction hob, that the differential current measured doesn't change significantly?

Sorry...I'm using a mili amp clamp metre around the earth of the install

 

[HandySparks]

Sorry...I'm using a mili amp clamp metre around the earth of the install

If you're trying to look at why an RCD is tripping, you need to be measuring the differential current that the RCD itself is seeing. Clamping an earth conductor is likely to give all sorts of odd readings, depending on the earthing system, parallel paths, etc.

What is the earthing system?

 

[Plonker 3]

Have you carried any tests on the cooker? is the current present when the cooker is disconnected if thats not possible?

 

[Lucien Nunes]

EMI filters in induction hobs can cause high leakage due to capacitors from lives to CPC, this is normal. Whether it is too high will be determined by test as per Handysparks above. Capacitors do not influence IR test as that is DC.

E2A: Don't energise a circuit with its CPC disconnected. There is always a safer, alternative way to test it, whatever might be wrong with it.

 

[Jimathome]

Thanks for the advice....
asuming that the induction hob is (normal) and its some form of internal strange going ons how would you go about trying to locate the fault that intermittently takes out the rcd?
i was thinking of moving a circuit onto a rcbo above? To try and reduce the leakage on the bottom row of circuits?

 

[telectrix]

might be an idea. put the hob on it's own RCBO, or, as a temp. measure, a non-RCD protected MCB. if the hob is the problem, then you'll have it verified after a while with no RCD trips.

 

[Donkey_Dave]

Had a hob doing the same thing .The manufacturer said that these hobs leak about 11 milliamps. Solution was to have it on its own RCBO.