On Sun, 21 Sep 2008 02:05:32 -0700 (PDT), wrote:

a) latching - if power is restored after a power failure, the RCD will
remake the circuit
b) non-latching - if power is restores after a power failure, the RCD
will NOT remake the circuit

I think you'll find that the latching type stay set, like a CU RCD if the
power fails rather than make/remake the circuit on power/power loss. The
non-latching ones drop out (trip) when the power fails, they may well
consume a little bit of power to hold the latch in. I think that all
plugin type RCDs these days are supposed to be the non-latching type, it's
safer. Means that if are using a portable power tool and the power goes
off it doesn't come back on with the power.

The non-latching variety I have used require a button to be pushed to
make the circuit - it seems to arm a spring loaded mechanism. The
latching variety I have do not have a spring loaded mechanism, but a
pair of microswitches, one to test, and one to make the circuit after
testing or detection of a residual current.

All the ones I have are spring loaded set mechanisiums, one I have looks
like a small microswitch push button with only a small amount of movement
but it still sets a spring loaded latch.

The description you have of the phase and neutral being wound round a
core may be a variant of the latter

A possibly inaccurate a sense coil around the conductors is much more
likely from the enginerring POV, the size of wire required to carry, say
80A, is not conducive to making small compact coils... I've not had dead
RCD to disect. B-)

(Idle thought - would it trip if there were an excess of current in
the neutral?).

They should, any imbalance between the send and return should trip an RCD.

--
Cheers
Dave.