of the transformer secondary winding are connected to the input circuit 50 which includes a full wave rectifier 312 connected to a variable resistance 314 and associated capacitors 316 and 318 to scale the output of the full wave rectifier 312 to the desired range the preferred range to interface with conventional control panels 20 is 0 volts when no current within the power cable 23 c is sensed to 5 volts when the maximum desired level within the power cable is sensed referring to fig4 current sensor 320 provides either an open circuit or short circuit at its output terminals 41 a and 41 b depending on whether the voltage signal produced in response to the current sensed by the transformer 34 surpasses a predetermined threshold level a variable resistor 326 sets the threshold level referring to fig5 current sensor 340 provides a 4 u2013 20 ma variable output signal at its output terminals 41 a and 41 b when the current sensed in the power cable 23 c is 0 then the sensor circuit 340 puts out a 4 ma signal when the current sensed in the power cable 23 c is equal to a desired maximum when the variable resistor 341 is correctly set then the sensor circuit 340 puts out a 20 ma signal referring to fig6 an electrical schematic of a switch circuit 60 is shown a.