Which device serves as the principal protective device in distribution substations?

The circuit breaker functions as the principal protective device in distribution substations due to its critical role in protecting electrical circuits from overloads and faults. When an overload or fault condition occurs, the circuit breaker automatically interrupts the circuit, thereby preventing damage to the system components, including transformers and other equipment.

Circuit breakers are designed to detect abnormal conditions and react quickly to isolate the fault, which enhances the overall reliability and safety of the power distribution system. In addition to providing protection, they also allow for the restoration of service after a fault has been cleared, by enabling the resetting of the breaker.

In contrast, switches are primarily used to control the flow of electricity but do not offer the same level of protection against faults as circuit breakers. Fuses serve a protective function as well, but they must be replaced once they have blown, making them less efficient for repeated interruptions. Transformers, on the other hand, are crucial for voltage modification within the power distribution system but do not provide any direct protective capabilities. Thus, the circuit breaker stands out as the most essential device for safeguarding distribution substations.