UK RCD trip without appliance powering on

Markbebbs · Jul 1, 2023

I’m going to call an electrician out on Monday, but curious if anyone has any ideas about this fault:

I have a new kitchen fitted and two existing ovens connected to existing Cooker circuit. Both ovens have been working fine.

new 7.4kw hob has been just been fitted to replace a small 13a one and it’s been put on its own circuit.

If the cooker MCB is on, the RCD trips as soon as Hob MCB is switched on. If the cooker MCB is off and the hob MCB is on, RCD stays on. Hob works fine if cooker MCB is off.

Wondering if this could be a faulty RCD although I hear that’s pretty rare. CU is about 25 years old. RCD is hager u762 20470.

Thanks

timhoward · Jul 1, 2023

It's unlikely to be a faulty RCD.
If CU is 25 years old I'm guessing there is only one RCD protecting half of it? A photo of the consumer unit would help.

It is possibly that the hob is RCD protected and the oven isn't, and there is some interconnection between the circuits.

Markbebbs · Jul 1, 2023

timhoward said:
It's unlikely to be a faulty RCD.
If CU is 25 years old I'm guessing there is only one RCD protecting half of it? A photo of the consumer unit would help.

It is possibly that the hob is RCD protected and the oven isn't, and there is some interconnection between the circuits.

Hi Tim

There is only 1 RCD on the CU. Hob doesn’t have its own. Seems the hob has been temporarily connected to an old unused shower circuit, which I have been told will be replaced with a smaller 32a MCB by the kitchen guys. It’s only been connected to test the hob because it was bought as ex display, not new.

Markbebbs · Jul 2, 2023

Markbebbs said:
I’m going to call an electrician out on Monday, but curious if anyone has any ideas about this fault:

I have a new kitchen fitted and two existing ovens connected to existing Cooker circuit. Both ovens have been working fine.

new 7.4kw hob has been just been fitted to replace a small 13a one and it’s been put on its own circuit.

If the cooker MCB is on, the RCD trips as soon as Hob MCB is switched on. If the cooker MCB is off and the hob MCB is on, RCD stays on. Hob works fine if cooker MCB is off.

Wondering if this could be a faulty RCD although I hear that’s pretty rare. CU is about 25 years old. RCD is hager u762 20470.

Thanks

I read about earth leaks so unplugged some appliances - kettle, toaster, washing machine and the RCD isn’t tripping with oven and hob MCB’s on. Would this mean I should be talking to an electrician about either new CU or getting an additional RCD fitted?

Thanks

snowhead · Jul 2, 2023

Is the cable from the new hob to the consumer unit new, or is the hob connected to the cable that originally went to the shower?

SJD · Jul 2, 2023

Cumulative leakage from several faulty appliances, which on their own don't trip the RCD, but added together might?

You would be better off having individual RCD protection for each circuit, i.e. a new all-RCBO consumer unit, rather than trying to add a second RCD. So don't have anyone suggest to fit a dual-RCD consumer unit.

timhoward · Jul 2, 2023

Markbebbs said:
I read about earth leaks so unplugged some appliances - kettle, toaster, washing machine and the RCD isn’t tripping with oven and hob MCB’s on. Would this mean I should be talking to an electrician about either new CU or getting an additional RCD fitted?

Thanks

Some testing is needed to verify the new circuit is correctly installed, and to find out which appliances are faulty.
It should be possible to get your current setup working. In due course, as suggested above, an RCBO board would allow 30ma leakage per circuit as opposed to 30ma for half the house.
Something like a Fusebox board with RTAM RCBOs that switch both live and neutral.

Markbebbs · Jul 2, 2023

snowhead said:
Is the cable from the new hob to the consumer unit new, or is the hob connected to the cable that originally went to the shower?

It’s a new cable

mainline · Jul 2, 2023

Markbebbs said:
It’s a new cable

An IR check for N to E faults would be a good start.

An N to E fault could be on any of the circuits or appliances.
The fault will remain when turning off single pole mcbs.

rewireIT · Jul 2, 2023

As mainline says, my first thought with instant tripping is N-E fault. Such a fault on any circuit will trip the RCD.

Markyd · Jul 3, 2023

Have you had new circuits for sockets or have the sockets been extended from existing install, either way RCD will need upgrading to a Type A and if your new cooker \ hob has been installed with new cable from CU then will need protection....looking at the board i would say it needs some remedial work carried out for the new kitchen to conform to regs