Three steps to select the right fuse for control circuit protection

Fuses are the same size and generally look the same, and this identity crisis can leads to mistakes in selecting a replacement fuse, putting workers and equipment at risk.

By Nick Offerman July 1, 2015

Control circuit fuses have a variety of different voltage ratings, ranging from 32 V to 600 V, and interrupting ratings up to 100 kA. As the physical size of these fuses does not vary with voltage or interrupting rating, the most common cause of misapplication is due to improper voltage or interrupting rating selection. When a system fault occurs and an improperly sized fuse tries to interrupt an overcurrent event, there can be adverse consequences. This mistake can lead to compromised system integrity—impacting personnel and equipment safety.

Type, voltage, interrupt rating

Find the right control circuit or supplemental fuse in three simple steps:

  • Select the fuse type. Select from time-delay fuses for inductive loads or fast-acting fuses for resistive loads.
  • Select the voltage rating required. The voltage rating must be equal to or greater than the circuit voltage for the proper application of an overcurrent protective device. The device can have a higher rating, but not a lower one; when an overcurrent protective device is applied beyond its rating, there may be potential for fire and arcing energy, posing a severe fire risk to other components in the panel.
  • Verify that the interrupting rating of the selected fuse is sufficient for the circuit application. The interrupting rating must be equal to or greater than the available short-circuit current. An overcurrent protective device must be able to safely interrupt short-circuit currents to which the equipment can be subjected. If the fault current exceeds a level beyond the capacity of the protective device, the device may rupture, causing additional damage. Therefore, it is important to use a fuse that can sustain the largest potential short-circuit currents. Failure to apply fuses with the appropriate interrupting rating can be a serious safety hazard.

Short-circuit current ratings

Circuit protection calculations can be easier with available online compliance software that helps calculate short-circuit current rating (SCCR) based on the components used in an electrical panel.The software guides users through the process of entering the electrical panel’s components and then calculates an assembly SCCR, providing needed documentation. Such design tools ease compliance with 2008 NEC and UL 508A Supplement SB for assembly SCCR marking requirements for Industrial Control Panels [409.110], Industrial Machinery Electrical Panels [670.3(A)], and HVAC Equipment [440.4(B)].

– Nick Offerman is a product manager at Eaton’s Bussmann Division; edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering,