In this application you are looking for a sensitive gate SCR, as they require a lower trigger current to turn on, and the device is probably only a 200mA 400V rated device, with a gate requirement of 1V5 0.5mA to trigger it. Older units used a Gate Controlled Switch, which is a SCR with all 4 internal junctions brought out, and they typically were used with the anode gate triggering the trip relay instead of the cathode gate like almost all SCR units. They typically used a BRY39 GCS device to do the detection and triggering all in one package.