What is the principal protective device in distribution substations designed to operate in as little as 10 cycles?

The principal protective device in distribution substations designed to operate in as little as 10 cycles is the circuit breaker. Circuit breakers are automated devices that interrupt the flow of electrical current in the event of a fault, such as a short circuit or overload. Their ability to act quickly—often within 10 cycles of the electrical system—allows them to minimize damage to equipment and maintain system stability.

In contrast to circuit breakers, devices like fuses operate by melting a conductor when excess current flows, which, while effective, does not provide the rapid response necessary for modern distribution systems where protection and restoration speed are critical. Transformers do not serve a protective function; they are designed to change voltage levels within the electrical power system. Relays, while they can assist in protection by detecting faults and initiating the operation of circuit breakers, do not interrupt the current themselves; they rely on circuit breakers to perform that function. Therefore, circuit breakers are the primary line of defense in preventing damage from short circuits and other electrical faults in distribution substations, operating efficiently within a brief time frame.