Why do we have a shunt trip breaker or Isolation contactor in the enclosed Altistart Softstarts (ATS23, ATS46, ATS48, ATS22)?
We install either a Shunt trip breaker or Isolation Contactor when building an enclosed or MCC installed softstart. These options are also shown in our recommended wiring diagrams in the product documentation.
This is for line isolation.
All softstarts use SCR's to switch the voltage during operation.
Since the solid-state switches (SCR's) may be incapable of completely blocking incoming power should a failure occur (they will almost always fail shorted), auxiliary isolation on the line-side of the controller is required.
The isolation device must be capable of operation via command from the Fault relay of the controller.
An isolation device consisting of either a circuit breaker incorporating a shunt trip coil or an electromagnetic contactor can be used to open the controller power circuit in the event of a controller fault.
This prevents voltage and current from passing through a damaged SCR and single phasing the motor.