Molybdenum disilicide (MoSi2) heating element
Loads such as transformer fed heaters and heaters with very low cold resistance such as Super Kanthal, can take a severe high inrush current when first powered up as the elements resistance is nearly a short circuit until its temperature rises. This can be as much as 15 times its nominal current resulting in blowing fuses and tripping breakers.
The solution is to use the soft-start function whenever power on or power change is called for. The soft-start rations power by restricting the firing angle then advancing it over several cycles. The gradual increase of power prevents surge currents and avoids the possibility of saturating inductive loads like transformers.
By the time the soft-start is over, a transformer load should be past the threat of inrush current, as the elements should be up to temperature where its own resistance limits the current. Slower responding heaters need a current limit as well as soft-start.
Current Limit is useful to help prevent large current surges during start-up due to loads with low resistances when cold. This feature, only available with phase angle firing, monitors the current and throttles back the firing angle to hold the current below the safe limit. It can typically hold the output RMS current to within 1% with +-10% line voltage change and resistance changes up to 5 times.
Here the circuit Soft-start can play its part here too by restricting the firing angle at power on until the current limit takes control. Suggested thyristor controller would have phase angle firing, current limit and soft-start set at a minimum of 2 seconds.