How to select a fuse holder

Just about every electrical circuit has the potential for unplanned, damaging overcurrent events, and in many of those cases, overcurrent circuit protection is needed to safely manage those events. The most common overcurrent protection is fuses. Many commonly available electronic fuses cannot be dropped into conductive paths due to their shape and size. Typically, a fuse accessory is required to integrate these fuses with a couple of primary functions: integrate fuses safely in electrical circuits, ensure a robust current path (voltage does not creep to other components) and provide a method to replace fuses once they manage overcurrent events as intended.

This document describes how to take advantage of those primary functions and explains some of the features of fuse accessories. It can also provide insight into how to select different types of fuse accessories and illustrate situations that may be encountered when selecting a fuse accessory. Lastly, there are a couple differentiating points between product types, potential to provide electrical safety for people, and why they are important to consider. 

When selecting a fuse accessory to integrate into an electrical circuit, here are a couple items to keep in mind:

Installation Type:

For electronic circuits there are two installation types - printed circuit board applications and wire-in, wire-out applications. The fuseholders available and possible features will differ greatly depending on the application.

Fuses Accepted:

The most common electronic fuses that require accessories are cylindrical; therefore, fuse accessories typically accept different cylindrical fuses based on the diameter of the fuse (most are 1/4” (6.3 mm) and 5 mm fuses), i.e. Eaton's HB fuse holder shown. Some fuse accessories within the Eaton Bussmann Series product line have the ability to accept multiple size fuses.

Accessibility/Replaceability:

During normal operation and after an overcurrent event is properly managed by the opened fuse, this brings two points of consideration: fuse accessibility and how easy it is to replace the fuse to regain operation. Fuse accessories can provide an easy way to change a fuse, as well as provide protection for people against electrical hazards interacting around the equipment in which it is installed. There are a couple different ways of defining this protection, sometimes referred to as Shock Safety, namely in IEC60127-6 or UL4248/IEC60529. It is very important to provide protection against electrical hazards, especially as voltages increase. While this is important, a designer also should consider how easy it is to replace the fuse and who can perform such functions. An end user of this equipment may prefer more difficult access to this fuse for electrical safety or easy access due to limited exposure to untrained persons. For more information on proper fuse replacement, please see Appendix or the Bussmann Series SPD.

Circuit Parameters:

The electrical parameters are critical to evaluate when selecting fuses, and fuse accessories. Voltage ratings are important during both normal operating conditions (nominal current) and also during opening. The maximum voltage rating during operation is generally defined by the standard it is evaluated to. Factors include, but are not limited to, dielectric strength of materials, voltage creepage between conductive surfaces and voltage clearances to conductive surfaces, as well as additional conditions of acceptability.

Practically speaking, creepage and clearance of different circuit voltage levels should be considered in the application to reduce the conditions of acceptability not typically known to users in the field since fuse holders are often mounted in metal enclosures. Eaton does not recommend assuming that published voltage ratings can be field installed as such.

The current rating of a fuse accessory is listed as a maximum current rating and should not be exceeded by the nominal current. One of the primary factors in determining the current rating is defined by heat rise per UL standards or power dissipation according to IEC standards. Fuse holders are evaluated on heat rise and power dissipation with dummy fuses instead of actual fuses (which have higher resistance) at the maximum desired current to standardize testing. Since there are multiple different fuses with different resistances that can be placed inside electronic fuse holders, a standardized dummy fuse is chosen. This results in is a higher heat rise than the defined laboratory conditions. Therefore, a derating, similar to fuse derating, is recommended. Generally this derating factor is in line with fuse derating of 80% but could be more depending on additional environmental factors.

Lastly, ensuring fuse holders can withstand high short circuit currents is important as well. UL4248 recognized fuse holders achieve a 10 kA short circuit current rating by default, so each application should be evaluated individually.

Environmental conditions:

As mentioned in the circuit parameters section, environmental conditions must be considered as well. The most applicable would be the ambient temperature and any airflow, or lack thereof, provided by the environment. The laws of physics dictate that sources of resistance will result in energy losses, commonly in thermal energy or heat. Fuses and fuse accessories (and all current paths) have inherent resistances making them a heat source. The management of that heat can affect fuse performance (outlined in Fuse technology: terminology, specifications and device selection) so this needs to be considered. Better airflow or less restriction can help. This is more important to consider as the fuse and fuse holder sizing is closer toward the nominal current of the circuit. Also, the more enclosed a fuse holder is, the more heat will be trapped.

Other environmental considerations can be outside the electrical circuit itself, such as solid and/or liquid interactions with dust, water drops, equipment washing, complete water immersion, or other  ambient conditions. Fuse holders may be capable of providing additional protection not otherwise provided by an enclosure to ensure these harmful contaminants do not interfere with the circuit path. Eaton has a group of fuse holders specifically evaluated to IEC 60529, which established ingress protection (IP) ratings, in order to allow easy understanding of environmental protection offered within the Bussmann series portfolio. 

Want more information?

Fill out this form if you would like to be contacted by an Eaton Electronics representative.