Garrette 731 engines use hot engine bleed air for nacelle heating. The system is quite simple. One switch/light assembly, one solenoid controlled valve, and a pressure switch.
The important aspect of this system is the fact that the valve's solenoid is powered "on" to keep the valve closed. Which means the solenoid gets very hot with the aircraft sitting on the ground using external power. This valve is a no-go item on most aircraft. With the engine running (on the ground), the maximum time for having nacelle heat on is 10 seconds. Any longer...... the nacelle turns a pretty shade of blue as the hot air quickly overheats the metal.
The pressure switch used for indication comes off a "T" fitting downstream from the valve. (This one is shown with the cannon plug off)
If the "NAC" light is illuminated during or after engine start, the crew should immediately shut down the engine. (20 PSI is all that is needed to make the switch.)
Some corporate operators make a practice of pulling certain circuit breakers for ground maintenance. Including the nacelle heat breakers to that group is a good idea.