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.