Relays are categorized by operation methods and response characteristics. Operation types include electromechanical, gas pressure, latching, percentage differential, solid-state, and digital relays. Response characteristics involve impedance measurement, time delay, and differential action.

High voltage circuits and transformers use two main protection relays, Main1 and Main2, with identical functions and settings but separate power sources to ensure system reliability. Protection relays from different manufacturers like MiCOM and ABB provide instantaneous and time-delayed protection for various faults.

A basic current differential scheme compares current data from both ends of a line via a communication channel. It detects faults by analyzing phase and magnitude differences. CT errors and other factors may cause discrepancies. The scheme requires reliable communication for accurate differential measurements.

Current differential 87L channel monitoring requires checking the relay's diagnostic menu for continuous healthy channel status. Interruptions indicate issues like low signal, loose connections, or noise. BER below 1e-4 is acceptable. Lost packets due to noise, switching, fading, or incorrect clocking also signal unhealthy channels.