stupid question about electrical, nothing relate to plc !

[TomekPLC]

To prove that red wires are the coil wires check them with ohm meter, should be a low resistance (possibly below 200ohms). Then check resistance between green wire and one of the red wires, should be in the high mega ohm range.
Tom

 

[OkiePC]

I have some DC coils with two red wires and a green wire. Tom has it right. Ohm it both ways to see if there's a difference, if not don't worry. I do not believe the ground is necessary in all cases. In some cases, it is part of a connector to which multiple different coils connect and may be dead-ended in a 24VDC coil with a plastic shell, or connected to case with an 240VAC coil in an aluminum case depending on which coil you connect. In either case, its a green wire so just ground it and remove all doubt.

(after you ensure it's not in the coil circuit)...