RCD trips occasionally when outside lights switched off.

[Hawk]

A customer has been in contact to say that her RCD is tripping intermittently whenever she turns the outside lights off. There is two consumer units in the house each protected by a 30mA RCD.
Has anyone ever had this problem? Going to it tomorrow to do a bit of head scratching!! Lol

 

[Leesparkykent]

I wonder if the switch is double pole.

 

[Hawk]

No single pole

 

[Hawk]

It's a 2 gang switch one for utility lights and one for 28w cfl's under the soffit of the bungalow

 

[Richard Burns]

Vague thoughts, possibly when the light is switched off there is an arc drawn that is permitting an apparent imbalance to be seen at the RCD, this could be due to slightly worn switch contacts making the arc worse or perhaps the fluorescents are stop starting as the switch goes off causing the RCD to suffer.

I might be aiming for a simple switch change or an RCD change as a test, but if it is intermittent it can be hard to track down.
Alternatively disconnecting one of the fluorescents and leaving temporarily to check but a bit long winded.

 

[Marvo]

Could be there's a large amount of standing leakage on that RCD and the arc when switching is just taking it over the tripping threshold. I'd start by doing IR tests on the circuits and a live leakage test with a clamp meter around the L+N at the RCD. You may find that changing the switch sorts it out but the switch may not be the main problem so it might reoccur at a later date.

 

[alanl]

I had the same recently.
SON floodlights, when turned off, the RCD would trip.
It was a poor design, with all lighting in a pub on the one RCD. Double pole switch was exasperating things.
There was a spike coming back to the RCD when they were turned off. SP switch sorted that one.
There are ways of fixing it with the current lighting, but easier to change the light fittings for a standard halogen/filament fitting.

 

[Hawk]

So I changed the switch and it tripped once when I tried it. Done a ramp test and it trips at 19mA which I thought was low. Tested the other one and it trips at 25mA. So think my next step is to replace the rcd. I was going to swap them but customer didn't want me to.

 

[Leesparkykent]

So I changed the switch and it tripped once when I tried it. Done a ramp test and it trips at 19mA which I thought was low. Tested the other one and it trips at 25mA. So think my next step is to replace the rcd. I was going to swap them but customer didn't want me to.

did you disconnect all loads when ramp testing the RCD?

 

[Simonslimline]

did you disconnect all loads when ramp testing the RCD?

If not then test it again with the loads dissed, the difference in the two results will be the earth leakage on the RCD.

 

[Hawk]

No some of the load was on when I tested.

 

[Leesparkykent]

No some of the load was on when I tested.

Before replacing the rcd i would ramp test it again with loads disconnected. If you test it with the loads on/connected and again without the difference will be the accumulative leakage on that RCD as Simon pointed out. Just ramp testing it with loads connected as you have done may not have given you a true indication if its on the low side.

 

[Hawk]

Cheers guys much appreciated. It's the first time I've had the problem of rcd tripping when lights turned off. Is it possible that there's a back emf as it is 5 28W 2d fittings?