Interposing Relays

When the power which needs to be handled exceeds the capacity of the product in question, an interposing relay is used.

An example - let us say you have a very large motor to be switched on, and the contactor which should switch on this motor is say 800A. Its electromagnet will require possibly 1kW of power to switch on. This cannot be handled by the small pushbutton that is on the front panel of the starter. So, an interposing relay is used, which can handle the power of 1kW or more, and the pushbutton is wired to switch on the interposing relay's electromagnet, which should be very small.

This is just one application, there are many others, you can search on google.

Interposing Relay is the auxiliary relay which is used to isolate the two different systems (such as high current and low current or 24V control and 120V control or AC voltage and DC voltage etc.) in the control circuits. For example, if the motor's contactor coil voltage is 120V AC but PLC signal is 24V DC and it is required to operate the motor from the PLC signal, an interposing relay of 24V DC is used in the PLC circuit loop and its dry contact is wired in the 120V AC circuit to operate the motor. Its use is more common in between the instrument system (PLC or DCS) and the electrical system. However it is also used within the electrical system as well. It isolates the two different systems and so it is also called isolating relay. Some people think it couples two different systems and call it coupling relay.

- MS

I also use them to reduce MTTR (mean time to repair). In electronically controlled systems, any relay contact in a PLC or other I/O device is going to wear out over time. If you use an interposing relay, the output relay inside the I/O device can last virtually forever, and a plug-in interposing relay can be replaced in a matter of seconds.