Difference Between Free Contact and Dry Contact?

Potential free contact, voltage free contact and dry contact, they all are same.

The dry contact is the contact which is physically operated with the main device, but not electrically connected to it. It is a voltage free on/off contact and can be used in any system. The auxiliary contact of the motor contactor used for the motor run feed back is the example of the dry contact. The contacts of the interposing relays are also dry contacts.

- MS

Perhaps you mean "dry circuit"? Contacts which need to work in such circuits are generally gold-plated, since the current and voltage levels are too low to break down the contaminant layer on conventional contacts...

Generally that means the open circuit voltage is 20mV or less and the short circuit current is 100mA or less.

Potential /Volt free" or "dry contacts" are used when the equipment being controlled has its own power supply e.g.,PLC's have a 24 VDC power source in them .

The external contact used to start the motor/equipment is a dry contact, and it switches the voltage supplied by the PLC's .The output modules in PLC uses Interposing relay connected to MCC , the dry contact connects the drive input to the PLC's 24 VDC and powers the drive circuit input. The internal devices sense the voltage, start the drive, and the current and voltage is returned to the PLC's own power supply through internal circuitry.

All Good Answer are already made.