I have two mains power inputs (in case one fails, the other is likely to still work), but the device only has one power supply, so I decided to use a relay to switch between the inputs (the device should be able to cope with a momentary interruption during the switching), like this:
In theory and for low voltages this should work just fine. When the primary input fails, the relay switches to the backup input. Is there any potential problem I should be aware of when using this for 230V?
Connected to this: is there a way to add a fuse to this circuit so if something (the relay or the device) shorts out that fuse blows instead of tripping the circuit breaker? Maybe some resistor to limit the short circuit current?
Best Answer
The relay coil must be for 230 V AC. Most relays are for voltages less than 24 V, and also DC. An AC relay will consume less power.
The contacts must be for 250 V AC minimum. But your demands are different than for a common 250 V contact. If there's a spark over an open contact in a normal serial switch application then no harm is done, because the circuit is designed to allow current through the contacts anyway. But that's not the case here: a spark between two contacts may cause a short circuit.
Also if the second power source is a different phase of the 230 V then you can have 400 V between the two contacts. It would be better then to have the neutral and the phase of V1 on one contact and the phase of V2 and the neutral to the other contact. So not the two "-" together, but the "-" of one to the "+" of the other one.
In any case I would recommend using a 500 V relay.