I have a 16 channel SainsMart relay board and I'm controlling 120VAC on one side and DC on the other.
I tested with DC by wiring the source to the common pin, and the load on the NO pin. I intend on doing the same thing for the 120V rail. However, it's common convention to wire the source to the NO and the load to the common. This way, only one PCB trace and screw is energized while the relay is open.
My only question to my fellow engineers is if they are aware of any downside of wiring the source to the common pin and the load to the NO pin. Essentially, it makes no difference as far as I can see. The relay contacts would seem to have less chance of arcing the way I'm wiring it, does anyone agree or disagree?