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

 

[Plonker 3]

Have you tried connecting the cooker hood to a designated circuit in the DB and checked the EL then?

 

[Rob190]

Hi,

Well disconnected all other circuits and was testing from a 2M run 2.5 T&E that is the first leg of the ring to the socket for the hood - even replaced this piece of twin (even though previous tested fine) & replaced single socket. Done the same again with the socket on the CCU with only the cooker connected in board. Tripped both times.

I didn't wire the hood directly into the RCBO as moulded plug.

When connected via extension lead or longer side of ring the fan operates correctly with an EL of 1.58 mA - the whole installation between 4-6mA total.

Cheers

 

[Plonker 3]

Did you clamp the CPC at the DB or on the extractor lead?

 

[Richard Burns]

Have you tried IR testing the cooker hood? maybe at 250V. It certainly sounds like the cooker hood fan is vibrating against an earthed part at speed, it may be a design fault that is not enough to cause problems in factory testing.
possibly there is enough capacitance in a long length of cable to mitigate the effect of a transient earth leakage fault.

 

[Rob190]

Clamped the fan CPC via a stripped extension lead & got same readings - fine when connected to a longer circuit but spikes when fan speed increased on short leg.

 

[Octopus]

Hi,

Well disconnected all other circuits and was testing from a 2M run 2.5 T&E that is the first leg of the ring to the socket for the hood - even replaced this piece of twin (even though previous tested fine) & replaced single socket. Done the same again with the socket on the CCU with only the cooker connected in board. Tripped both times.

I didn't wire the hood directly into the RCBO as moulded plug.

When connected via extension lead or longer side of ring the fan operates correctly with an EL of 1.58 mA - the whole installation between 4-6mA total.

Cheers

Thats a bit of a clue where to look!

 

[Plonker 3]

back to basics now, but have you ramp tested the RCBOs and both your RCD tester and EL tester are both calibrated?

 

[Rob190]

@Richard - was thinking along those lines though I'm not aware of the method to prove this theory - ring circuit tests were r1 0.63 rn 0.57 & r2 0.97 - average 2 bed bungalow so around 80 square meters.

Trouble is I know when the Bosch engy comes out he will say it works on one socket so must be 'lectrics'!

 

[Rob190]

MFT calibrated - EL clamp meter six months old. RCBO's ramp tested (25mA -27mA) Even though RCBO's were new swapped for new ones.

Murdoch - I'm looking believe me but trying to understand the science behind it.

 

[Octopus]

MFT calibrated - EL clamp meter six months old. RCBO's ramp tested (25mA -27mA) Even though RCBO's were new swapped for new ones.

Murdoch - I'm looking believe me but trying to understand the science behind it.

Mate, of the leakage is low on 1 leg and an extension, but high on the other............

 

[Richard Burns]

I think you have proved the theory quite well with your testing!!:smile5:
Put a nice tightly wound coil of cable behind the socket, problem solved, though possibly causing other problems!:sick:

80m of ring on a 2 bed bungalow sounds quite high really, and if you are testing on a 2m length then on a 78m length there is a major difference. Though the extension lead is a smaller change, but possibly higher resistance joints.
The only other thought would be a loose connection arcing somewhere, but with all the changes you have made this sounds unlikely.

Take all the covers off the cooker hood and put tape around any metal where the motor /fan rotates and see if the tape gets scraped when it runs.

@Richard - was thinking along those lines though I'm not aware of the method to prove this theory - ring circuit tests were r1 0.63 rn 0.57 & r2 0.97 - average 2 bed bungalow so around 80 square meters.

Trouble is I know when the Bosch engy comes out he will say it works on one socket so must be 'lectrics'!

 

[Rob190]

yeah, I know mate. So as above replaced short leg 2M with IR tests of > 900 Meg - no other circuits connected in board.

Duplicated fault on cooker ciruit with similar length.

So if the longer leg is absorbing 70mA it doesn't compare with other faults I have dealt with such as leakage on a long ring from garden supply?

Appreciate the help & will atke it on the chin if i am missing the obvious!

 

[Rob190]

@Richard

Yeah, pretty straightforward now at home - straight 2M run to board = trip, longer run = no trip.

Would open hood up but will leave it I think until Bosch have had a look.

These faults are sent to test us lol.

 

[Lucien Nunes]

if the longer leg is absorbing 70mA

I don't buy this. A longer cable won't make 70mA of leakage 'disappear', that's enough to run a 15W lamp. If the leakage from the hood were from L-E then it's going into one of two cores at almost the same potential - the leakage added by the cable's capacitance L-CPC will be hundreds of times more than that removed by the capacitance N-CPC. In any case, when the long and short legs are both connected the RCBO wouldn't trip.

Much more inclined to think it's an N-E fault. Then with the short cable it's much lower resistance and diverts more current. In that case the 70mA is probably quite arbitrary and you will tend to get different numbers with different loads on elsewhere.

Suppose there is a stock fault on the hoods that causes N-E fault on high speed? Have you tested the hood leakage on high speed when isolated from the outside world, e.g. PAT with it sat on the worktop?