Selecting a Fuse Because of its Current Limiting Characteristics.

This chart shows that if a 30-amp RK-1 fuse was installed where the AFC was 40,000 amps, the fuse would let no more than 1,800 amps through before it blows. If the passive equipment being protected had a SCCR of 5k RMS, then this fuse could be used to comply with NEC 110.10.
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This chart shows that if a 30-amp RK-1 fuse was installed where the AFC was 40,000 amps, the fuse would let no more than 1,800 amps through before it blows. If the passive equipment being protected had a SCCR of 5k RMS, then this fuse could be used to comply with NEC 110.10.

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 equipment’s short-circuit current rating (SSCR).

Example (see image): 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).

2017-2023 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…

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Selecting a Fuse Because of its Current Limiting Characteristics.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

Refer to the image showing an example of a fuse chart being used to determine the let current on the load side of a fuse protecting a time clock.

What does the item in the blue circle represent?

A: The size of the fuse protecting the time clock.
B: The length of wire to the time clock.
C: The AFC at the time clock.
D: The final amount of let through.
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