Help with wiring zwave switch for water heater (240V)

If UK Mains, try the newsgroup uk.d-i-y

I'm from OZ so we have different codes but I would use the L3 terminal as the common neutral and run the wire from there to the remote switch and the coil of the contactor. There is no need to have protection in the neutral, I suppose you could put a 5amp fuse inline for the Fibaro but it would be overkill IMHO and would not protect the Fibaro just the wiring to it.

I'll do my best as I am in Greece and you obviously aren't... 1.yes, the N is the common and are all wired together and they aren't allowed to be disconnected while the live is connected, because the N will become live...there are no fuses or circuit breakers needed in the neutral. 2. usually you use the same as hot (brown) or red at least here. you can get a special contactor for rail mounting in a enclosure with just 2 poles, for 20 A AC1 because the one you picked is 3 phase which is overkill and might be longer so the cover of the enclosure might not fit.here they are called storage heater relays (another word for contactor)because they were used in house enclosures for that purpose.for instance-schneider A9C20732, with nominal current 25 A.2 pole and with the size of a circuit breaker. their cost is ~20 euros.

BTW, you don't need to ground the rail, as its only purpose is holding the elements (circuit breakers etc.) and usually might not become live or come into contact with humans. for ground purposes you use the ground rail (ditto for neutral) of the enclosure...