Does it trip at the crossing point (when the current is effectively zero)

It trips when the current is zero. Anything else would torch the contacts.
Even a manual switch will "let go" at a crossing point if there are any.
This is why a normal switch is rated higher for currents at AC than DC.

Note that the voltage being zero will only coincide with current being zero
if the circuit is power factor corrected. But switching voltage is easy.
It's current that hurts.

That's usually nine tests in all, and the nearest convenient socket at
which to plug the tester in is rarely within arm's reach of the RCD :-)


I fitted a double socket next to the consumer unit for one ring circuit. The
other ring circuit is for the adjacent room anyway...

Christian.