Fuses are often used to protect passive components. Passive components or devices can include things like lighting contactors, relays, time clocks and the like. Passive devices will have a short-circuit current rating (SCCR) instead of an interrupting rating since they are not intended to interrupt current at fault levels. Passive devices cannot defend themselves against a fault condition which is why they require a fuse or circuit breaker ahead of them to provide the required protection in the event that a fault condition occurs in any of the circuits they control.

Each different fuse type has different trip and let through characteristics. The let through value is the maximum amount of current that the fuse can let through to downstream components being protected before the fuse blows during a fault.

The “up-over-down” method is used with a fuse chart to determine the amount of let through current at the load side of a specific fuse when protecting passive components that cannot defend themselves against a fault (see image).

Since a passive device cannot defend itself, it will not interfere when the overcurrent protective device (OCPD) ahead of it senses a fault condition and interrupts the fault. Since no interference occurs, a passive device is a perfect candidate for protection by a fuse that has been selected using the up-over-down method on the fuse chart. The NEC^® does not require specific testing for fuses selected using the up-over-down method when protecting passive components.