Having a short circuit would cause the output current to increase to a high level, hence the Immersun unit shuts down the output to protect both the unit and the external circuitry. The cause could be a short circuit within the wiring or the immersion heater, or possibly within the Immersun unit itself.

If you have a isolation switch between the Immersun unit and the immersion heater then switch this off and see if the fault clears. This would indicate that the fault is at the immersion heater or the wiring connected to it.

If the fault is still present then switch off the power supply to the Immersun unit and disconnect the output cabling. Switch the power back on to the Immersun unit and if the fault is cleared then the short circuit is on the wiring from the Immersun unit to the isolation switch, or the isolation switch itself.

If the fault is still present then the problem is within the Immersun unit.

@derek-m, I left the unit off overnight (disconnected from power).

I turned it on this morning with the immersion connector removed, so only the Immersun was starting up. Within 2 minutes the same is back. Looks like I'll have to send a message to Immersun.

As an update to this, Immersun's customer support was excellent. I ran some diagnostics at our end and the error persisted. Immersun organised for the unit to be collected by DPD – a week later it was returned, and it's working perfectly. So the error was related to the Immersun unit, not the immersion heater.