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…
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?
Please register or sign in for electrical continuing education courses. If you were already signed in, your session probably expired, please sign back in. |