Protecting IEC Style Motor Starters

UL Classes RK5, RK1 and J are the most popular. The Class RK5 (Fusetron^TM/Tri-onic^®) is the least expensive and is adequate for most applications. The Class RK1 (Low-Peak^TM/Amp-trap^®) is used where a higher degree of current limitation is required for component protection or system coordination. The RK5 and RK1 are dimensionally interchangeable. Since its introduction in 1983, the Class J time delay fuse has become an increasingly popular choice. The LPJ/AJT provides a somewhat higher degree of current limitation than the RK1. More importantly, the LPJ/AJT is approximately half the physical size of the Class RK5 and RK1 fuses.

The new IEC European style motor starters and contactors are becoming increasingly popular, but they present new problems in protection. These devices represent substantial savings in space and cost but they have a much lower withstand capability than their NEMA counterparts.

In order to achieve the same level of protection for IEC style devices that we expect for NEMA devices, the LPJ/AJT Class J Time Delay fuse is the best choice, sized at 1.25 to 1.50 times motor full load amps. Also, the LPJ/AJT has the advantage of being smaller than RK5 fuses and thereby fits the trim IEC package.