A practical guide to protected zones, ANSI 87 relays, differential current, percentage restraint, equipment applications, and real-world misoperation checks. Core idea: Differential protection compares current entering and leaving a CT-defined protected zone. This is possible by direct comparison of instantaneous values or by vector (phasor) comparison. In each case, the measurement is based on Kirchhoff's laws which state that the geometric (vector) sum of the. 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. Designed as a hands-on, application-focused learning experience, the course equips participants with the knowledge and skills to program relay configura.