On 21 Sep, 12:45, "Dave Liquorice"
wrote:
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.

I have three latching RCDs that interrupt and remake the circuit on
power loss. They are very useful (but not on power tools, obviously).
If all plug-type RCDs are meant to be non-latching these days it
explains why I can't find more of them!

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.

Could be. None of mine have died and I haven't succumbed to the
temptation to take them apart to see how they work.


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-)

That is what I would have thought too, but 'The Natural Philosopher'
thinks otherwise in this thread.


(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.

'Should' is a great word. I may be being simplistic, but an excess of
current on the neutral conductor rather than a deficit will presumably
reverse the direction of the current output of the sense coil, which,
if is being fed into a solenoid, would reverse the direction of the
generated force. This can be accommodated mechanically with suitable
mechanism design, but without tearing a few RCDs apart and testing, I
don't have an easy way of determining what's going on. I would hope
that when people write 'imbalance' that is what they mean, rather than
'excess of phase over neutral', but sloppy thinking and wording are
rife (I'm no exception).

Although we are dealing nominally with AC here, the target disconnect
time of 30ms not much different to one standard mains AC cycle of 20
ms, so 'steady state' assumptions and calculations are not necessarily
valid. The inductance of the sense coil and solenoid, and the effect
of any electronics probably needs to be taken into account, and I'm
definitely not qualified to work out exactly how it all works.

Thank-you very much for taking the time to write your previous
considered reply.

Cheers,

Sid