Cooker hood tripping RCBO - head scratcher

[Rob190]

Hi,

Been back in the trade a few years now & a regular lurker here (236 I & II & C, 2391, time served etc, 17th) - I have always managed to track down answers using the search however need some help on this one.

New kitchen install in a 2 bed bungalow involving a board swap to RCBO's as only three circuits (lights, ring & cooker).

2nd fix day went fine, all tests carried out with satisfactory results - cooker hood flicked off & on to check function before leaving - job done.

Call via kitchen company I'm subbing for that RCBO is tripping out when fan speed increased on cooker fault. Go back & double check IR tests (all over 200 MOhms), test RCBO again (X1 19.7 X5 7.8 ramped at 27 mS).

Earth leakage clamped & when fan speed increased see a jump to 70 mA - aha I think faulty cooker hood.

Replacement arrives but fitters call to say same thing happening.

Go back again & found that if I connect hood to short leg of ring back to board it trips - connect it to other longer side of ring it doesn't! So remove all other circuits from board & same thing happens. Then swap over to similar length cooker circuit (2M) & it trips! Connect to short leg via extension lead it doesn't trip. This all after double checking IR etc. Swapped RCBO's , double checked connections in consumer unit.

I have installed numerous kitchens recently & never seen this fault. I realise that the common factor here is the short length of the circuit but can't work out why a common Bosch cooker hood would generate 70mA leakage because of it!

I spoke to Bosch technical who after unhelpfully suggesting removal of the RCBO have asked me to email them & will send an engineer out - just wondering if anyone has any ideas?

Cheers,

Rob

 

[Midwest]

Blimey. What if OP switched off hood at socket, moved hood switch to fast, then turned socket back on. Would this negate the problem and prove you right Marconi?

 

[AndrewMgan1976]

Is the cooker hood fitted on a metal stud partition wall? Could be the fixing screw is through one of the studs and is causing a parallel earth

 

[marconi]

Midwest, yes, according to my reasoning that is my expectation. I think it is the electrical disturbance/transients caused by the action of switching the cooker hood control (whether up in speed or down) after the motor is run up that precipitates the RCD trip via the short leg of the ring final circuit. It may be though that the transients are of different strengths and forms depending on whether one steps up or down in speed. The only real test is to do it and to to connect up test equipment to analyse why.

All that said, I am conscious that these types of problem are nightmares for the busy electrician and wasteful in time and money. That is why I tried to offer a practical solution first - the filtered socket and maybe transient tolerant RCBO. Be interesting to know if one or other solved the problem.

Rob190's idea of connecting the hood 13A socket further around the ring ( - in the ring or as a spur) was the simplest and cheapest and thus best way to proceed. It could be easily trialled first too.

 

[Midwest]

We'll have to wait for Rob to get back to us. Can't think it might be something simple, but willing to listen to any explanation.

 

[Rob190]

Thanks guys for all your input - great resource here.

Big thanks to Marconi for his detailed explanation & solution - hager RCBO sorted it.

Very useful to know for the future.

Rob