A voltage-free contact is any kind of switching circuit that is isolated, does not introduce any potential into the user circuit, and is not referred to ground or a supply rail. This allows the user to connect it to any circuit potential of choice, within reason.
The most obvious example is a pair of relay contacts, which are truly volt-free. A common sort of output is the open-collector transistor stage. This is not volt-free since its base and emitter are referenced to the driver circuit and one supply rail, usually ground. Opto-isolators are arguably semi volt-free as they provide high voltage isolation between input and output, but the floating transistor can only drive certain kinds of other circuits and not an a.c.load directly.
hi...
if you use a meter whether in AC or DC scale these contact has no potential at all with respect to ground or any part of a circuit ... but this contact is still being actuated by an electromagnet..
another word use in industry for this is "dry contact"..
