There are broadly two major applications, and they may overlap in some cases. Terminal Block Relays are used between a controller (e.g. PLC) and a high-power load (a motor), mainly for 2 reasons:
1. When a controller is not able to drive the load directly (due to different voltage requirements and insufficient driving current)
2. For electrical isolation
Here, our application is to remotely operate a 440V 3-phase motor through a contactor. The contactor requires a 230V AC supply across its coil. Now, because the PLC can only provide a 24V DC signal against the requirement of 230V, a Terminal block relay is used as an interposing relay. It is energized using PLC output, the contacts of the relay then switch the contactor coil to drive the motor. In this case, we are not only operating a motor through PLC but also isolating the low-voltage controller and control circuit from the higher-voltage power circuit.
