Which type of test is performed on a circuit breaker after installation at the site to verify its readiness and proper functioning before putting it into service?

In a standard Miniature Circuit Breaker (MCB), which component is specifically responsible for providing the time-delayed tripping action during an overload condition?

In a High Rupturing Capacity (HRC) fuse, the space surrounding the fuse element is filled with a powder such as silica sand. What is the primary role of this filling material during the interruption of a fault current?