Many of us connect 240V to our rigs day or so before we head off to get the fridge/freezer down to temperature ready for transferring food.
In my case I leave my houseboat for a week or so and wish to leave the fridge on but if for some reason either the RCD or the circuit breaker chucks out there is no way I'll know till I return and smell things going green.
If I could rig up a flashing light in a front window if the external power goes off then my neighbours would see it & either reset the breaker or phone me.
I have a 12V supply onboard so was thinking a relay that sits in the 240V line & triggers the 12V to turn on a light when the power goes off. I figure it'd have to be an electro/magnetic relay as I'd need the NO (normally open) contacts.
I'd prefer a light that comes on rather than goes off. (could even connect to a siren?

Any ideas?