RCD question

[Gib Bogle]

A circuit in my house trips the RCD whenever a device on the circuit is switched on. The circuit is intended for the waste disposal unit on the kitchen sink. The bench was not installed when the wiring was done, and the operation of the circuit apparently was not tested by the sparky. First test with the disposal unit tripped the RCD, so I tried plugging a lamp into the socket instead - it still trips. The under-sink area is completely dry (new cabinets etc.) so I don't think moisture is the issue. I have tested the resistance between all the terminals with my multimeter, and it shows open circuit between earth and both phase and neutral.

Since I don't really understand how an RCD works (beyond that it detects leakage current to earth) I'm stumped. I will get the electrician back, but would also like to know what's happening. I suspect my simple multimeter is not adequate for checking the circuit for leakage.

[Gib Bogle]

In case I didn't make it clear, the RCD trips when the lamp is turned on, not when it's plugged in.

[Dave Osborne]

Assuming it is a dedicated circuit, it may well be that the neutral wire for the circuit has been connected to the non-rcd neutral block in the consumer unit. This will only cause the RCD to pop when a load is connected to the circuit.

[Dave Plowman (News)]

If your CU is a split load type, first thing I'd do is make sure the neutral of that circuit goes to the correct busbar. Everything protected by that RCD must use the same one.

[John Rumm]

back, but would also like to know what's happening. I suspect my simple

The use of the wrong bus bar is the most likely fault:

[Gib Bogle]

Thanks to all for the neutral bus suggestion. I will report back.

[Roger Mills]

That's certainly the most likely culprit. Another possibility - albeit not very likely - is that the neutral and earth connections are reversed in the socket.