circuit breaker/fuse for relay switched fail safe temperature sensors

[vjekobalas]

I would like to use a uC + relay with 230V output to control a 230V input/400V three phase output contactor.
The contactor is connected to a 3kW water heater/3 elements).
At the moment, the setup is not controlled by a uC+relay but rather a 230V timer, so I'm using two
230V bi-metal temperature sensors and an automatic circuit breaker/fuse in series in the 230V circuit for safety.

When I replace the timer with a uC+relay, what is the best fail-safe mechanism and "automatic fuse" to use
and how can I keep the uC alive in case of worst case failure (e.g. both bi-metal sensors are shorted)
and sense the failure ?

[rooppoorali]

I think opto isolated relay is an option. Let's see what others have to say.

[vjekobalas]

At the moment, the bi-metal temperature sensors are in the 230V circuit
which drives the contactor. I also thought of using a uC driven opto-isolated
relay with NO contacts to again switch the same 230V circuit.
My aim is to have the uC to communicate with a more powerful uC/pc etc.
for central processing.

The thing I was not sure about was the "usual" setup/circuitry used for the following
assuming the more powerful uC/pc needs to be informed:
- fuse/safety in 230V circuit for the case both bi-metal temp sensors short circuit and
how to sense the failures
- check that the 230V circuit power is on/off, check each phase of 400V circuit is on/off
- check that the uC power is on/off