Oven Tripping RCD help!

[GarryJ1990]

Hi all,

Was wondering if anyone has come across this before and could maybe help - Its a new one to me!

Customer recently altered layout of their kitchen slightly and bought a new AEG single oven and a new AEG micro / grill oven after doing away with some gas appliances.
1 of the new electric appliances is 3kW and the other is 3.2kW. Both are situated at a far corner area of the kitchen where access was difficult to get to and below the floor was quite hard to get new cabling in also hence the 1 supply cable.
Anyway to cut a long story short, we have run a new 6mm twin and earth cable from the consumer unit to supply the 2 new electric appliances.
Both appliances came pre-flexed up.
The run is approx 6m from the consumer unit. We have used deep pattrice boxes and then fitted 2 x 20 Amp double pole switches - 1 to isolate and supply each oven where the ovens flex connects into each separate one.

The problem I have is a strange one - I can turn on any 1 of the 2 ovens and they work fine, the second I turn the 2nd ovens isolator on the main RCD trips.
It does not matter what sequence you do it in - as soon as the 2nd one is immediately turned on it trips.

It is an older MK consumer unit with the older Siemens sliding mcb's? There is an 80 Amp 30mA RCD main switch.
My 6mm is off a 32 Amp type 2 MCB.

I have fully bench tested the RCD main switch - all readings at x1/2 etc as expected.
I have fully disconnected and insulation resistance tested the 6mm supply cable - all is as clear as day.
I have disconnected / swapped 20 amp double pole isolators just because it was easy to do an eliminate that aspect.
I have clamped main earth in consumer unit with each oven on to measure earth leakage - its about 2mA.
I have clamped the L+N / E at the individual isolator for each oven to measure earth leakage just doing it the core balance CT way which
is obviously exactly what the RCD is doing.
I have insulation resistance tested the ovens at 250v (L+N) - E = all is clear.

It is 100% not overload of the 2 ovens - the supply circuit (even though in an ideal world would be 2 individual circuits) is more than capable
for this and each appliance has local accessible double pole point of isolation.

Im left thinking it is harmonics or DC power supply in 1 of the ovens but can not think what else to try or test?

Any help greatly appreciated!

 

[Markyd]

Have you tried turning them on with all other breakers off, also an earth leakage on whole system L N tails not main earth, Ramp test on rcd....

 

[littlespark]

Cumulative earth leakage across ALL circuits.

Might be an overly sensitive rcd mainswitch.. (ramp test, as suggested above)

Turning off other circuits might not help as neutral would still be connected.


I had same in a barn conversion. Old MK board with mainswitch rcd… Couldn’t have all 4 lighting circuits on at same time… but had to disconnect each one completely in turn to diagnose.

Ended up with a new all RCBO board.

 

[GarryJ1990]

Have you tried turning them on with all other breakers off, also an earth leakage on whole system L N tails not main earth, Ramp test on rcd....

I have not tried 2 ovens on by themselves with every other circuit off but certainly will try this and then do as you suggested.
Thanks for the help!

 

[Pretty Mouth]

Have you insulation tested all the other circuits? Possibly a N-E fault on another circuit?

Current from the ovens that should be making its way back home through the RCD, could make its way up the neutral of a faulty circuit and go to earth at the fault. 1 oven may not be enough to trip the RCD, but 2 together might be enough to tip the balance.

 

[GarryJ1990]

Have you insulation tested all the other circuits? Possibly a N-E fault on another circuit?

Current from the ovens that should be making its way back home through the RCD, could make its way up the neutral of a faulty circuit and go to earth at the fault. 1 oven may not be enough to trip the RCD, but 2 together might be enough to tip the balance.

Hi there,
Thanks for your input and thought!
I did think about this to be honest - Customer has since contacted me to say he has been trialling things in my absence.
He says if he turns off the downstairs lighting MCB both ovens will operate fine! Next Task......

 

[telectrix]

Have you insulation tested all the other circuits? Possibly a N-E fault on another circuit?

Current from the ovens that should be making its way back home through the RCD, could make its way up the neutral of a faulty circuit and go to earth at the fault. 1 oven may not be enough to trip the RCD, but 2 together might be enough to tip the balance.

I'll go with that. had similar fault . the fault was on the shower circuit on RCD2 but when switching on cooker or kettle on RCD1, it tripped.

 

[Gavona]

I have a similar issue. In my case part of the problem is the cumulative residual current across different circuits protected by a single rod. I improved things by replacing a heating element in the oven but residual currents remain close to my RCA’s tolerance - so I still need a sparky to resolve properly.
Anyhow in my research I did see reference to the fact that new ovens can have high leakage until the elements have been run hot for a while to dry them out. So anyone having this problem could try connecting their ovens one at a time and running it for an hour. Might improve things.