Limiting Arc Flash - Current Limiting Fuses

As the dangers of arc flash become more widely recognized, extensive efforts are going into developing methods to prevent or minimize 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. The sand makes a big difference in the fuse's behavior, however. 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. The energy of an arc flash explosion depends largely on the duration of arc, so a faster means of interrupting the current was needed.

In a current limiting fuse, the quartz sand absorbs energy and quenches the arc that forms within the fuse. This results in a faster acting fuse that interrupts the current before it can reach its maximum intensity. In many cases, this will dramatically decrease the severity of an arc flash.

Current-limiting fuses are not a cure-all for arc flash, however. At lower current levels, they may take longer to clear a fault, actually increasing the duration of an arc flash and increasing its intensity. For this reason, careful analysis of an electrical system, along with clear labeling of hazards, continues to be of paramount importance in protecting workers' safety.