<p style="text-align:justify"><span style="font-size:14px">A protective relay is a black box whose inputs are connected to C. T. and/or P. T. and/or any kind of transducer output and whose output is a dry potential free contact (or contacts) which changes its status based on required characteristics (instantaneous, definite time delayed, inverse current-time and many more complex characteristics which we will try to study). The black box can use electromechanical elements in electromagnetic relays, it may use discrete components like P-N junction diodes, transistors, op-amps and other integrated circuits, or further talking about evolution of protective relay technology, numerical relay as a black box can use 8-bit or 16-bit microprocessors (like 8086) or microcontrollers, or as per latest developments, it can use digital signal processors (DSP), surge filters, multiplexers, ADCs (Analog to Digital Converters) and other communication peripherals.</span></p>