On 20 Sep, 23:30, "Dave Liquorice"
wrote:
On Fri, 19 Sep 2008 15:03:22 -0700 (PDT), wrote:
Yes! - At least every single one I have used has done so. They get
even warmer when passing load. My _guess_ is that all the ones I have
come across use a solenoid to keep the contact closed, and some power
is used in the solenoid.

They are magnetic and like TNP said the Live and Neutral are woud around
the same core when there is suffcient current imbalance the resultant
magnetic field pulls the latch set when the device is set to "on" and a
spring causes it to trip off. AFAIK they are not held on by power
otherwise everytime you had a power cut you'd have to reset them...

The only CU RCD here is the same temperature as the MCBs next to it, at
least by touch, ie cold.

I have no idea why they warm up under load. I hope its not ohmic
heating across the contacts!

Quite simply ohmic losses in the coils.

--
Cheers
Dave.

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

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 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.

Now, my understanding of RCDs (which is imperfect) is that the sense
coil is wound around the (straight) phase and neutral conductors, and
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
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
are only guaranteed to work within specification on sinusoidal
currents.

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
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
assumptions could be wrong, and I would be grateful if someone with
better knowledge (and hopefully URLs to demonstrate this) could
elucidate. Most of my Google searches end up at Wikipedia, which can
hardly be regarded as authoritative on this matter.

Cheers,

Sid