Quite often, to achieve compliance with NEC^® 110.10, a fuse is selected to protect passive components when the Available Fault Current (AFC) is higher than the equipment’s short-circuit current rating (SSCR).

Example: A 30-amp time clock with a SCCR of 5,000 amps, installed in a location where the AFC is 40,000 amps.

Since NEC^® Section 110.10 requires the overcurrent device protecting the time clock to be able to protect the time clock before extensive damage occurs in the event of a large fault condition on a circuit controlled by the time clock, something needs to be done in order to comply with NEC^® 110.10 and ensure that the time clock will never see more than 5,000 amps under a fault condition.

Selecting the right fuse and using the up-over-down method on the fuse chart is one option to ensure that the fuse will never let through more than 5,000 amps under a large fault condition occurring in the circuit that the time clock controls.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics. The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit…