text: The three phase systems are fed via contactors similar to a three phase motor direct on line starter which is controlled by a thermostat somewhere in the line. This ensures that the temperature is kept constant and the line does not overheat or underheat.
  If a line becomes frozen because the heating was switched off then this may take some time to thaw out using trace heating. This thawing out is done on the three phase systems by using an auto transformer to give a higher voltage, and consequently higher current, and make the trace heating elements a bit hotter. The boost system is usually on a timer and switches back to normal after a period of time.