What must be protected first: equipment, continuity, personnel, or system stability? How much fault energy can be tolerated, and where? How quickly must a fault be cleared to prevent cascading consequences? Those decisions form the protection philosophy, and the selection. Relay systems protect high-voltage equipment and transmission lines to ensure safe, stable systems. Although failure of a protective relay system may have severe local or regional impacts, most protective relay systems are not required to operate to prove they are in working order. It emphasizes selectivity, coordination, fault response, and system behavior rather than individual relay devices. This handbook covers the code of practice in protection circuitry including standard lead and device numbers, mode of connections at terminal strips, colour codes in multicore cables, dos and donts in execution. In HV (High Voltage) and MV (Medium Voltage) substations, relay protection safeguards critical assets such as transformers, circuit breakers, and lines.
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