Induction Hob & RCCB

[John B]

Would appreciate your thoughts on this please.

Wylex split load consumer unit with one rccb, new circuit installed for induction hob on non rccb side with 32 amp rcbo.
When the hob is switched on the rccb trips, not the rcbo. The circuit has been tested out and is fine and the neutral for the rcbo is in the correct neutral bar, both rcd and rccb pass on the rcd tester.

Don't know if anyone has come across this before, will change rccb tomorrow just in case it has an intermittent fault but would appreciate any other ideas.

Thanks
John B

 

[James]

Would appreciate your thoughts on this please.

Wylex split load consumer unit with one rccb, new circuit installed for induction hob on non rccb side with 32 amp rcbo.
When the hob is switched on the rccb trips, not the rcbo. The circuit has been tested out and is fine and the neutral for the rcbo is in the correct neutral bar, both rcd and rccb pass on the rcd tester.

Don't know if anyone has come across this before, will change rccb tomorrow just in case it has an intermittent fault but would appreciate any other ideas.

Thanks
John B

A quick diagram posted as a photo would help a lot.

 

[John B]

A quick diagram posted as a photo would help a lot.

Thanks for your reply James, don't really need a diagram if we are both familiar with split load consumer units, it's just that it's a strange fault, almost like the neutrals are in the wrong bar or a circuit from the rccb is connected into the rccb side which of course it isn't.

 

[Deleted account]

You’ve probably put the neutral lead from the RCBO in the wrong neutral bar.

 

[James]

Thanks for your reply James, don't really need a diagram if we are both familiar with split load consumer units, it's just that it's a strange fault, almost like the neutrals are in the wrong bar or a circuit from the rccb is connected into the rccb side which of course it isn't.

I wasn't trying to patronise, sometimes getting the configuration down on a piece of paper will clear things up, you might have even spotted the fault yourself.
also, i was having a little trouble visualising it, split board with rcd, rccb and rcbo.

go back to basics, all of these devices are looking for a difference between L + N currents.
the one that trips is seeing a difference.

 

[Jon Bateson]

What’s feeding the RCBO ?

 

[John B]

You’ve probably put the neutral lead from the RCBO in the wrong neutral bar.

Was my first thought but no, the neutrals are all in the correct places.

 

[John B]

What’s feeding the RCBO ?

The rcbo is installed on the non rcd side of the split load consumer unit, neutral from rcbo into correct neutral terminal and rcbo does not trip when induction hob switched on, its the rccb on the rcd protected side of the coard which trips.

 

[John B]

I wasn't trying to patronise, sometimes getting the configuration down on a piece of paper will clear things up, you might have even spotted the fault yourself.
also, i was having a little trouble visualising it, split board with rcd, rccb and rcbo.

go back to basics, all of these devices are looking for a difference between L + N currents.
the one that trips is seeing a difference.

Thanks James, wasn't implying you were trying to patronise.

 

[jackhammerJIM]

Its possible there is a fault to earth on the rccb side .

 

[John B]

Its possible there is a fault to earth on the rccb side .

Thanks for your reply, the rccb only ever trips when the induction hob is switched on which is not on the rccb circuits but supplied by an rcbo on the non rccb side and does not trip trip?

 

[SparkyAndGeorge]

What was your neutral to earth IR results on the RCCB circuits?

 

[John B]

What was your neutral to earth IR results on the RCCB circuits?

Over 100 mega ohms so ok

 

[Lucien Nunes]

Re-check these IR's including anything plugged in. if possible, measure differential current through RCD under normal operating conditions to see whether it's already near the limit.

Obvious possibilities are:

N-E fault on a circuit on the RCD. This is very common, I seem to post this answer nearly once a week. The heavy load of the hob causes enough drop in the supply N to drive enough current back through the neutral of the RCD and to earth at the fault, even though the hob is not on the RCD. If this is the cause, any heavy load is likely to cause a trip, it just happens to be the hob that has shown the problem up.

Harmonics / noise generated by hob electronics finding AC paths to earth through filters in loads on the RCD side. On an old split load install where the lighting is all on the non-protected side and the sockets are on the protected side, the bulk of the capacitance to earth within appliances will be on the protected side, so any common-mode hash the hob-puts out that the filters shunt to earth will show up as imbalance at the RCD. If the RCD is already near tripping point with total leakage, this might be enough to push it over the edge.

Fault between hob circuit and an RCD-protected circuit, that has not been revealed by your particular sequence of IR tests. Unlikely, and would probably have been tripping the RCBO too unless the RCD is significantly faster or more sensitive.

I know you've tested the IR but so many of these turn out so be some sneaky N-E fault it's worth checking again.

 

[John B]

Re-check these IR's including anything plugged in. if possible, measure differential current through RCD under normal operating conditions to see whether it's already near the limit.

Obvious possibilities are:

N-E fault on a circuit on the RCD. This is very common, I seem to post this answer nearly once a week. The heavy load of the hob causes enough drop in the supply N to drive enough current back through the neutral of the RCD and to earth at the fault, even though the hob is not on the RCD. If this is the cause, any heavy load is likely to cause a trip, it just happens to be the hob that has shown the problem up.

Harmonics / noise generated by hob electronics finding AC paths to earth through filters in loads on the RCD side. On an old split load install where the lighting is all on the non-protected side and the sockets are on the protected side, the bulk of the capacitance to earth within appliances will be on the protected side, so any common-mode hash the hob-puts out that the filters shunt to earth will show up as imbalance at the RCD. If the RCD is already near tripping point with total leakage, this might be enough to push it over the edge.

Fault between hob circuit and an RCD-protected circuit, that has not been revealed by your particular sequence of IR tests. Unlikely, and would probably have been tripping the RCBO too unless the RCD is significantly faster or more sensitive.

I know you've tested the IR but so many of these turn out so be some sneaky N-E fault it's worth checking again.

Ok Thanks, harmonics and neutral drop was some of the things I was considering with it being a induction hob. Hope to find answer tomorrow.