In message ,
"Christian McArdle" wrote:

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.


There's a difference between "letting go" and "actuating". MCBs have arc
extinguishing chambers precisely because they are highly likely to trip
just about anywhere *except* at a zero point - they are largely
mechanical devices after all with absolutely no electronics. Are RCDs
any different? My gut feeling was that the coil would be actuated at a
certain current - which is likely to be other than at the zero voltage
point - and that at that point the thing is mechanically "tripped".
Quite how long it takes for the arc to extinguish is another matter. I
have never opened up an RCD but would be surprised if there is any
electronics in there to "hold off" until the zero point, merely in the
interests of extending contact life.

Note that the voltage being zero will only coincide with current being zero
if the circuit is power factor corrected.

Obviously, but for domestic use it's usually good enough to say that the
pf is reasonably good unless there's a lot of fluorescent lighting
about.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Since you're going to die anyway, can we use you as a shield?