Differential protection is a power system relay method that compares current entering and leaving a protected zone. One of the fundamental laws of electric circuits is Kirchhoff's Current Law, which. Differential Relay Definition: A differential relay is defined as a device that responds to the difference between two or more similar electrical quantities, such as currents or voltages, to detect faults. This is possible by direct comparison of instantaneous values or by vector (phasor) comparison. Relays are classified into different types like latching, reed, solid state, automotive, timer delay, differential relay, etc.