Relay protection protects the secondary phase
Rule of thumb, select a ratio slightly larger than the rating of the circuit to be protected. Protective relays and devices have been developed over 100 years ago to provide "lastline"of defense for the electrical systems. Selectivity is a mandatory requirement for all protection, but the importance of it depends on the application. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers. The applications of the different types of protection systems for the protection of various types of equipment and transmission lines are.
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