wrote:

In uk.d-i-y, wrote:

................................................. When you join the N to
the E, you provide an alternative path back for some of the N current, ...

(in case it wasn't clear) I should have written "... an alternative path
back for some of the N current *in the other circuits on the RCD which
are still live".

The N voltage to earth may only be a 100mV (hardly measurable - or less)
but the path is usually less than 1 Ohm so you get the 30mA you need to
trip the breaker.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html