The relay operation is purely depending upon the magnitude of the circuit current and voltage, typically the ratio of the circuit to be protected is calculated. The ratio of Voltage to current is called impedance. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. The selection and applications of. The selected protection principle affects the operating speed of the protection, which has a significant im-pact on the harm caused by short circuits. : 4 The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as. Protection engineers calculate the maximum load current, the minimum fault current, and the full range of possible voltage levels to ensure relay performance under all conditions. Maximum through fault level, Stf. Circuit breaker short circuit rating, Icb.
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Can be single or multi ratio (MR). Rule of thumb, select a ratio slightly larger than the rating of the circuit to be protected. Numerical relays have more forgiveness than induction disk. Thus, the concerned feeder be-longs to the protection area of the relay 1 and relay 2, providing an inherent backup protection for the feed-er. Should relay 1 or its circuit breaker fail to operate, relay 2 will be allowed. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. Circuit Breakers (CBs), as well as Voltage and Current.
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