The Wrong Rcd Keeps Tripping!

[Bessie]

My customer's house has two consumer units connected in parallel. The first one supplies several ring circuits and already had an RCD. The second supplies several lighting circuits, washing machine, microwave, boiler and immersion heater - until I upgraded it, there was no RCD, just a 100A switch.

I checked insulation resistance on both boards before fitting the RCD - all circuits tested out OK, although one circuit on the board with the new RCD was a tiny bit under 2MOhm. Also tested the new RCD, likewise OK. I thought the job was done however customer rang the next day to say the new RCD keeps tripping.

Sure enough, half the times he turns on the microwave the RCD trips. Fair enough, maybe the RCD is faulty after all and a bit over sensitive, but what I can't understand is that when he boils the kettle on the kitchen ring circuit (remember, this circuit is connected to the other board via its own RCD) the new RCD trips even though it doesn't protect this circuit and belongs in a different board!

Is this a recognised phenomenon? Does anyone now what is going on here?

Thanks.

 

[sparkzalot]

Must be interconnected somewhere. FI required.

 

[westward10]

Must be interconnected somewhere. FI required.

This is the most obvious cause of this, is it just the kettle or does any appliance cause the tripping.

 

[Richard Burns]

Try an IR test from neutral on one board to earth on the other board and vice versa when they are both isolated.
Running a high current item that trips an unrelated RCD tends to indicate a fault on one of the circuits the RCD protects and the high current is passing enough through the fault to trip the RCD.

 

[Bessie]

This is the most obvious cause of this, is it just the kettle or does any appliance cause the tripping.

The iron also does the same thing - but neither of these appliances trip their own RCD, just the RCD in the other board.

 

[Leesparkykent]

Richards hit the nail on the head IMO.

 

[ED17]

Might be a borrowed neutral.

 

[Richard Burns]

Might be a borrowed neutral.

The tripping would be every time the circuit was used, there would be no maybe about it.

 

[Wilko]

A cct fault is likely. If nothing found then perhaps a hunt through the affected ccts to secure no borrowed or crossed neutrals (result of some diy?). No problem till RCD installed. Just a thought. Cheers.

 

[Deleted member 93172]

sounds like a borrowed neutral from the other board to me, or the ring main has been connected across the two boards by error, good luck in finding the fault

 

[Bessie]

Try an IR test from neutral on one board to earth on the other board and vice versa when they are both isolated.
Running a high current item that trips an unrelated RCD tends to indicate a fault on one of the circuits the RCD protects and the high current is passing enough through the fault to trip the RCD.

Thank you Richard and everybody who posted advice - it all helped! After reading your comments, the solution was quite obvious - I had forgotten to mention that, before I could install the RCD, I had had to bypass one of the original lighting circuits which had an earth-neutral short in the supply cable. I had disconnected the live feed at the CU but left the earth and neutral still attached, so current still leaking from neutral to earth - I just disconnected them at the CU. As you pointed out, it just seemed to be the high current devices on any circuit which caused the RCD to trip - lighting circuits weren't a problem.

 

[Deleted member 93172]

Glad you found it, this old fault finding lark can lead you up many a garden path the wrong way