Prognostic health monitoring for heater

A method of monitoring a health of a heater having a resistive heating element, electrical insulation surrounding the resistive heating element, and a metallic sheath surrounding the electrical insulation, the heater connected to a power supply by a first power lead conducting an inlet current and a second power lead conducting an outlet current, the method comprising:

• generating, with a signal generator, a time-varying injection signal;
• supplying, to an injection transformer comprising a plurality of primary turns that are inductively coupled to the first power lead, the time-varying injection signal;
• receiving, by a signal reader, a diagnostic signal from the heater;
• passing, by the signal reader, a frequency associated with the time-varying injection signal;
• passing, by a band pass filter, the frequency associated with the time-varying injection signal while attenuating frequencies less than and greater than the frequency associated with the time-varying injection signal;
• and
• producing, by a capacitance measurement circuit, the heater capacitance signal in response to a capacitive reactance of the heater that depends at least in part on a condition for the electrical insulation;
• and
• producing, by a capacitance measurement circuit of the signal reader, a heater capacitance signal in response to a capacitive reactance of the heater, the capacitance signal indicative of a capacitance value of the heater;
• wherein:
• the capacitive reactance depends at least in part on a condition of the electrical insulation, and
• the heater capacitance signal is indicative of the health of the heater.