Limiting Arc Flash - Current Limiting Fuses

As the dangers of arc flash have become more widely recognized, extensive efforts have been going into developing methods to prevent or limit the power of arc flashes. One approach is the use of current limiting fuses.

The purpose of a current limiting fuse is to reduce the severity of an arc flash. As with most arc flash solutions, a current limiting fuse is effective under some circumstances, but not all.

An article in Electrical Contracting Products describes a current limiting fuse this way:

"In a current limiting fuse, the fusible link's typical design has many places where the cross- sectional area has been reduced in size, and the link is usually encapsulated in quartz sand. When a short circuit fault occurs, the reduced cross sectional areas on the fusible link will heat up quickly and melt, thus opening the fusible link."

So far this sounds like a conventional fuse, except for the quartz sand. But, that is a big difference. When a fuse melts an arc will bridge the gap that is formed. Until the gap becomes too wide for the arc to bridge, the fuse will continue conducting. An arc flash explosion can happen in as little as 1/1000 of a second, so a faster acting fuse was required.

The purpose of the quartz sand is to absorb heat and quench the arc that forms within the fuse. This results in a faster acting fuse and reduces the time need to clear the fault.

Although a current limiting fuse can not prevent an arc flash from occurring, under some circumstances it can limit the severity of an arc flash by cutting off the current flow before the arc flash builds to its maximum intensity. This makes current limiting fuses valuable tools for reducing arc flash injuries.

NEC Standard, Article 110.16
Introduction to Arc Flash Safety
Arc Flash Labels
NFPA 70E Electrical Safety Standards
Magazine Article - NFPA 70E PPE
Current Limiting Fuses
Arc Flash Safety Blog

Arc Flash Tutorial by: Steve Hudgik