During miswiring to field power source, a current continuously flows through the burden resistor. PTCs allow a continuous current up-to I_HOLD without tripping. As the current increases above I_TRIP ­ and PTCs temperature go above switch temperature or Curie point, PTCs increase resistance and add drop to loop in normal operation. I_TRIP and I_HOLD vary significantly (~50%) with the ambient temperature. Figure 3-2 provides the variation of I_TRIP and I_HOLD ­with temperature of a typical PTC with I_HOLD = 55mA at 25°C.

To operate with current inputs up-to 24mA, The PTC needs to be selected such that I_HOLD is more than 24mA over operating temperature range. With PTC having I_HOLD more than 24mA across operating range, I_TRIP is typically more than 100mA at 25°C.

PTCs have an operating time in order of few seconds to bring down the current to the residual leakage current. Figure 3-3 provides the typical operating time of a PTC with I_HOLD = 55mA.

With an operating time of few seconds, a significant amount is power dissipated in burden resistor and Zener Diode. Figure 3-4 provides the power dissipated in Burden resistor and Zener Diode during miswiring to Field Supply.

Due to the increased power dissipation in Zener and Burden resistor, the power rating of Zener has to be increased more than 2W and power rating of burden resistor has to be increased more than 300 mW for a 50-Ω burden resistor.

With increased power rating of Zener Diodes and Burden resistor, a Zener diode with larger package has to be used and multiple resistors in parallel are required for R_Burden to dissipate the power during miswiring event.

Figure 3-5 shows a typical PCB layout with an estimated area of 142 mm² for current input circuit with PTC, Zener and multiple 0805 (125 mW) burden resistors. Figure 3-6 shows a typical PCB layout with an estimated area of 102 mm² for current input circuit with PTC, Zener and a 1W MELF resistor.