Which of the following is not a common type of circuit breaker used in switchgear?

Hydraulic circuit breakers (HCB) are not typically classified as a common type of circuit breaker used in switchgear. The other options—Air Circuit Breakers (ACB), Oil Circuit Breakers (OCB), and Vacuum Circuit Breakers (VCB)—are well-documented and widely utilized in various applications within switchgear.

Air Circuit Breakers operate using air as the medium to extinguish the arc, making them suitable for high current applications. Oil Circuit Breakers rely on oil not only for insulation but also for arc extinction, and they are usually used in high-voltage systems. Vacuum Circuit Breakers utilize a vacuum to quench the arc when the circuit is interrupted. Each of these types has its own advantages and is selected based on specific application requirements.

Hydraulic circuit breakers, however, do not have a significant presence or standard application in switchgear environments. They are more commonly associated with other types of systems where mechanical force is needed to operate, but they don't fit the typical definitions or mechanisms of circuit breakers found in switchgear design and operational practices. This distinction makes them the correct choice for the question asked.