Which of the following is NOT a principal type of circuit breaker contact?

The principal types of circuit breaker contacts are typically categorized based on their design and mechanism of operation. The wedge, bayonet, and brush contacts are known types that serve specific purposes in creating and interrupting electrical circuits.

Wedge contacts are designed to ensure a secure connection that can handle high current without causing excessive wear or arcing. Bayonet contacts involve a locking mechanism that allows for quick and reliable connection and disconnection, making them convenient for circuit breaker applications. Brush contacts are used in applications where continuous electrical connection is necessary, providing flexibility as they can accommodate different movement and alignment.

Disc contacts, while they may exist in other electrical systems, are not recognized as a principal type of circuit breaker contact. This is mainly because the function of circuit breaker contacts is to efficiently and safely interrupt electrical flow, and shapes like discs do not optimize this critical task as effectively as the other types mentioned. Thus, identifying "disc" as not being a principal type makes sense in the context of circuit breaker technology.