Answer / kingshuk dey

I'm not sure what you mean by "both relay contacts have live supply". I mean, relay contacts ALWAYS have live supply.

Case 1: One side of contacts have live supply. When you give supply to relay coil, the contacts close/open (NO/NC logic), hence this "live supply" continues on to next part of the circuit.

No supply to relay coil: Live---[ ]---0 (NO contact)
Supply to relay coil given: Live---[/]---Live (NO to NC)

Case 2: Let's say the relay has only 1 set of contacts (SP), and both contacts have supply, then nothing will happen. It's like two live wires in a junction point.

Live supply 1 ----o---- Live supply 2
|____(lamp)--Neutral

Any supply switches on (or both) the lamp glows (the "o" point shares both supplies, think of it as mating relay contacts!)

However, if Live supply 1 and 2 are AC & DC, there's a shorted circuit.

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