RES Technical Corner by Brett Eliasz, PE

Technical Corner

For the article this month we will look at how to achieve appropriate arc energy reduction using fuses.

The general concept here is that the longer it takes a fuse to melt or circuit breaker to open increases the incident energy that is present at that point in the electrical system. The incident energy is what feeds the arc that creates a hazard for electrical workers during an electrical fault.

Article 240.67 of the NEC dictates that a fuse must have a clearing time of .07 seconds or less. This went into effect January 1st of 2020. The circuit breaker section of NEC 240.87 has been around for a few code cycles, where both have the same goal of arc energy reduction.

What is interesting about using fuses to satisfy NEC 240.67 is that one might think that a reduced Bolted Fault Current is beneficial to an owner, which it is, however as you see in the below figure reducing the Bolted Fault Current will in turn decrease the Minimum Arc Current. Reducing too much will limit your choices when planning on using fuses in the electrical system.

To meet the clearing time of .07 seconds the minimum Arc current must be looked at. Most manufacturers publish these tables for the fuses they offer.

For example: A 2000A fuse is desired to provide the appropriate overcurrent protection in a 480V system. The calculated Bolted Fault Current at that point is 60kA using the parameters of the system. The Minimum Arc Current is taken from the figure above. 30kA Using the Littlefuse tables you will need a 2000A Type LDC fuse which will clear the fault within .07s. Using the Mersen fuse tables at the 2000A level you will see that the A4BQ type requires a minimum of 21.9kA of Arc current, the A4BY type requires 24.5kA and the A4BT requires a minimum of 31.1kA so we would choose one of the first two. If we could not find a fuse, then we would have to introduce a Relay based system when a fuse alone cannot guarantee compliance with NEC 240.67.

Reference’s courtesy of 2017 NEC, Littlefuse and Mersen.

Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the Rochester Engineer and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director