Set Square wrote:
In an earlier contribution to this discussion,
fred wrote:

In article , Set Square
writes

Sounds like a secondary safety device which trips if the main stat
fails to operate for some reason.

snip
I'm sure you're right. It seems a *bit* odd for it to be re-settable though
because if the main stat has welded contacts you shouldn't be able to turn
it on again! [An electric kettle is different because some fool might turn
it on with no water in. When it's re-set and re-filled, it's fine.]

If for example there are a couple of heaters in one cylider, then it's
quite possible to trip the device with out it being faulty itself.

Alternatively, it is of course possible if the water fails, to run the
heater dry.