wrote:

Ahh - we might be talking about different things. I'll try and
clarify.

All the RCDs I have used are not consumer unit mounted ones, but
either

Ah, yup that does make a difference.

Normally when discussing RCD for circuit protection we are talking about
the DIN rail mounting type (last two pictured here):

http://wiki.diyfaq.org.uk/index.php?title=RCD

a) Unit placed between a 13A plug and socket
b) Unit replacing 13A plug

I have found that they can be obtained with a feature of being either

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

The NVR type often include a solenoid to hold the device "on" once
powered up. I could certainly understand these getting a little warm in
operation - even NVR switches without a RCD function can do this under
load.

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. This latching variety (of
which I have many examples) gets warm even when passing no load. The
non-latching do indeed have to be reset after a power failure - which
would be a right royal pain if such an RCD were feeding a freezer and
you happened to be away on holiday.

Indeed. The standard CU type latch - but the latching is done with a
permanent magnet.

Now, my understanding of RCDs (which is imperfect) is that the sense
coil is wound around the (straight) phase and neutral conductors, and

My understanding also...

some RCDs electronically process the output of the sense coil to allow
features like delayed activation, activation when the current is not
sinusoidal, and to prevent activation on normal inrush currents (and
possibly ofr other reasons). Some RCDs use the output of the sense
coil directly to activate a solenoid which triggers a spring loaded
mechanism to disconnect the circuit. The description you have of the

The solenoid when activated is in effect reducing the mechanical
attraction of the permanent magnet, thus allowing the spring loaded
mechanism that is working against the magnet to pull the contacts open.

phase and neutral being wound round a core may be a variant of the
latter - possibly used in consumer unit RCDs.(Idle thought - would it
trip if there were an excess of current in the neutral?). AFAIK, they

As in greater neutral current than phase? Yes that will trip it.

are only guaranteed to work within specification on sinusoidal
currents.

Depends on the type you specify. MK for example have a number in their
range designed for different conditions like pulsing DC etc:

http://www.tlc-direct.co.uk/Technica...entry/RCDs.pdf

So, my imperfect knowledge of consumer unit RCDs may be leading me to
make the wrong conclusion. My understanding of consumer unit RCDs is
that they are routinely of the latching variety (i.e. will remake a
circuit on restoration of power after a failure), and that therefore

yup...

they will be dissipating power at zero load, as my assumption is that
_all_ latching RCDs disspate power at zero load. Either or both of my

nope, see notes above and also:

http://www.tlc-direct.co.uk/Technica...CD%20Works.htm

(they may dissipate a very small amount due to the electronics in them -
however none of the data sheets I have seen so far give any indication
if this is so, and if so how much).



--
Cheers,

John.

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