RCD tripping when switching oven isolator off

[streamer]

Hi all.
I was called to a house recently where the occupier reported that when they turned the oven isolator off the RCD would trip.
So I tried it and that's exactly what happened. There was nothing switched on on the oven at the time. When the oven is on it works fine.
It is an integrated oven on a dedicated 16 amp circuit. Circuit tested fine in all respects and a ramp test on RCD showed it tripping at 24 mA.
They have to turn the isolator off when not using the oven as they have a son with learning difficulties who sometimes turns the controls on the front of th oven.
Any ideas what the cause might be?
Thanks.

 

[MJPD29]

Did you try an earth leakage clamp.on the circuit? It is possible that there is a spike when turning it off which is pushing an already close RCD over the edge, it's a long shot but I have seen it happening

 

[littlespark]

Faulty isolator switch?

Ooh it’s dark!..... Stabby stabby!

 

[Yozza19]

Had this before caused by a faulty dp switch not disconnecting at the same time causing an inbalance

 

[telectrix]

as post #4. try replacing the isolator before the head scratching.

 

[Midwest]

Edit as previously mentioned. Have a look at the Beama RCD guide, section 9.3. Could be a possibility

 

[marconi]

Replace the DP switch with one which makes the neutral before the line and breaks the line before the neutral. I think MK cooker switches operate this way.

 

[Midwest]

Replace the DP switch with one which makes the neutral before the line and breaks the line before the neutral. I think MK cooker switches operate this way.

I've had some DP switches that don't isolate the live at all.

 

[Wilko]

Another cooker switch vote here

 

[streamer]

Thanks everyone. I'll try changing the isolator and let you know what happens.

 

[Charlie_]

Is it a cooker switch or 20 amp dp switch?

 

[streamer]

Is it a cooker switch or 20 amp dp switch?

It's a 20 amp DP switch.

 

[streamer]

Hi folks, sorry for not getting back with the outcome. Changed the 20 amp double pole switch and problem solved. Thanks for all your replies.

 

[Bobster]

Replace the DP switch with one which makes the neutral before the line and breaks the line before the neutral. I think MK cooker switches operate this way.

Understandable for a 3ph DB main switch for example.

However, this shouldn't in theory effect a single phase load. There isn't a difference in current draw, by switching line or neutral.

Unless the switch is faulty and is sending some current elsewhere.

 

[Midwest]

Understandable for a 3ph DB main switch for example.

However, this shouldn't in theory effect a single phase load. There isn't a difference in current draw, by switching line or neutral.

Unless the switch is faulty and is sending some current elsewhere.

Beama talk about 'Double pole switching within the fixed wiring is known to produce a strange phenomenon whereby switching OFF a double pole switch supplied through an RCD can cause the RCD to trip.' in their RCD Handbook.

So there must be some substances to it.

 

[Bobster]

Beama talk about 'Double pole switching within the fixed wiring is known to produce a strange phenomenon whereby switching OFF a double pole switch supplied through an RCD can cause the RCD to trip.' in their RCD Handbook.

So there must be some substances to it.

The only thing I can think of, is it causing some from of capacitive spike in the cable.

 

[marconi]

Re #14: The sequence of switching of the line and neutral does affect the changes of potential differences of the line and neutral conductors with respect to earth - and because of this there is the possibility of transient current flows through the line and neutral conductors distributed capacitance to earth which does create a differential current through the RCD. Plot out the changes of potentials/currents for the equivalent circuit and you will see what happens - which is why in earthed neutral systems single phase DP switches are make N first break N last - so N conductors remains at earth potential - ie do not momentarily jump up to Line potential then down to zero to avoid nuisance tripping of RCDs.

 

[Bobster]

Re #14: The sequence of switching of the line and neutral does affect the changes of potential differences of the line and neutral conductors with respect to earth - and because of this there is the possibility of transient current flows through the line and neutral conductors distributed capacitance to earth which does create a differential current through the RCD. Plot out the changes of potentials/currents for the equivalent circuit and you will see what happens - which is why in earthed neutral systems single phase DP switches are make N first break N last - so N conductors remains at earth potential - ie do not momentarily jump up to Line potential then down to zero to avoid nuisance tripping of RCDs.

Yes, as soon as I stopped an actually thought about it, I realized what could happen.

 

[Dustydazzler]

Had this on a board change I did a while ago.
Customer reported rcd tripping when they turned the cooker off.
I must say this was the first time I have come across this particular problem