Industrial Power Distribution Protection Solution

Use Class RK fuses (RK1 or RK5) for motor circuits. These fuses feature dual-element time-delay design that handles the high inrush currents during motor starting (typically 500-600% of rated current for 5-20 seconds) without nuisance opening. Class RK1 offers the highest current limitation for better protection of downstream conductors. Class RK5 provides economical protection with good time-delay characteristics.

Select Class RK for all motor circuits. Size at 125-175% of motor full-load current. Consult NEC for specific sizing requirements.

Selective coordination requires that only the fuse closest to the fault operates, leaving the rest of the system intact. Achieve this by selecting devices with adequate time-current separation, typically a 2:1 ratio between upstream and downstream devices at fault levels. For fuse-to-fuse coordination, consult coordination tables in Bussmann documentation. For fuse-to-breaker coordination, verify that fuse clearing time is at least twice the breaker instantaneous trip time.

Use Bussmann coordination tables and software tools to verify selective coordination. Contact our FAE for complex coordination studies.

Breaking capacity must exceed the maximum prospective fault current at the fuse location. Calculate fault current based on transformer kVA rating and impedance, utility source impedance, and conductor impedance to the fault point. Add 25% margin to the calculated value. Most industrial facilities require 100kA minimum, with larger systems requiring 200kA or higher. Bussmann offers fuses with breaking capacities from 10kA to 300kA.

Calculate maximum fault current and select fuse with breaking capacity at least 25% higher. Consult Bussmann catalog or contact FAE for assistance.

Class J fuses can physically replace Class RK fuses only if the fuse block/holder is also replaced with a Class J rated holder. Class J fuses are smaller (more space-efficient) but require Class J mounting dimensions. Voltage rating must also be adequate. Never substitute a higher ampere rating fuse or use a fuse with inadequate voltage or breaking capacity.

Class J can replace Class RK only with compatible holder. Consult electrician for proper replacement per NEC requirements.

Class RK1 and RK5 differ in current limitation capability. RK1 provides superior current limitation, meaning it opens faster and limits let-through current more effectively during faults. This protects downstream conductors better and reduces arc flash hazards. RK5 offers economical motor protection with adequate time-delay characteristics but lower current limitation. For critical applications with expensive conductors or in areas with high incident energy, RK1 is preferred.

Use RK1 for superior protection and arc flash reduction. Use RK5 for economical general motor protection.