Braking Capacity

Definition: The maximum current a circuit breaker can safely interrupt under normal operating conditions without damage.

Making Capacity

Definition: The maximum current a circuit breaker can safely close onto (or make) without damage, especially when closing onto a short-circuit.

Short-Time Capacity

Definition: The maximum current a breaker can carry for a short time (e.g., a few seconds) without tripping or damage.

Rated secondary current of C.T. = 5 A

Pickup current = 5 $\times$ 1.25 = 6.25 A

Fault current in relay coil $=\frac{4000\times 5}{400}$ = 50A

∴ Plug-setting multiplier (P.S.M.) $=\frac{50}{6.25}=8$

Corresponding to the plug-setting multiplier of 8

(See Fig.), the time of operation is 3.5 seconds.

∴ Actual relay operating time

= 3.5 $\times$ Time-setting

= 3.5 $\times$ 0.6

= 2.1 seconds