The contacts of an electromechanical relay are potential free until you wire up something that applies a voltage.
Some devices, like pressure or flow switches, use electromechanical micro switches or reed switches as their output and as a free-standing field device, would be an example of "potential free".
Typically, the digital input on the receiver device is setup to source some amount of voltage/current which it senses through the switch.
"dry contacts" is another name for "potential free" contacts.
A device with "potential free contacts" by itself cannot be classified "intrinsically safe" because one cannot determine what the circuit the device will go into.
It is certainly possible to use an appropriate intrinsic safety barrier with a potential free contact device and an appropriate low voltage digital input to achieve an instrinsically safe circuit.
No, it just means a set of contacts that are not provided with power from the circuit by which they are controlled. They're also called "dry contacts". The information consumer must wet the circuit in which the contacts are used. For example if you have a machine which has a certain bit of information needed by another machine with a separate source of power, you might provide outputs as dry contacts (either mechanical or solid state), and the other machine would send a power common via one wire and receive the information back on another wire. Since the power in the signal wire originates in the signal receiving machine it can be connected to a PLC or relay, etc., whose return is from that power supply.
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Steve Myres, PE
Automation Solutions
(480) 813-1145
