The second philosophy deals with transmission of the measured field parameters back to a central control mechanism utilizing existing cabling. When looking at transmission of measured field parameters, review the signals required to monitor and control a heat tracing system and where these signals can be generated. You will see that temperature, current and ground fault reflect the basic signals. With regards to any temperature responsive cable, current monitoring is not as beneficial a factor as with constant wattage style cables. The temperature of the system being protected is the one signal that must be measured in the field, under insulation.
Typical measurement is with three-wire RTDs.
This signal must get from the pipe or vessel back to the device making the decision about out of tolerance alarms and control. Typically, this is done with dedicated shielded triad cabling. Approximately 10 years ago, two controller manufacturers tried to utilize power cabling to transmit the measured temperature back to the decision device. One firm met with major failures and the other firm met with limited success. At the distribution center, the signal was stripped off and interpreted in relation to the actual field-measured temperature.