This guide has been prepared to aid the circuit and system designer in the selection of supplemental fuses. Traditionally, these have been fuses that interrupt the primary equipment power source when an overcurrent condition develops in a device or product. This guide will cover only fuses intended for electronic applications.

Introduction

Fuses that are covered in this Application Guide typically range in physical size up to 10.4 X 40 mm. These fuses fall within the ANCE-248 (Mexico)/CSA C22.2, 248 / UL 248, Supplemental Fuse Standard for North American applications. Current ratings covered by this Standard range from milliamperes to as high as 60 amperes, with voltage ratings up to 600 volts AC or DC. These devices are available in a variety of mounting schemes including plug-in, cartridge, through-hole and surface mount.

Fuses are quite complex despite their simple appearance. Each fuse type and current rating will have unique operating characteristics, maximum operating limits and internal construction. Subtle variations between manufacturers' designs will affect equivalency even between two seemingly identical devices. Further, any given type of fuse from a single manufacturer will have electrical and mechanical parameter variations that result from slight production variations during processing within a given lot and from lot to lot. Safety agency approvals, e.g., ANCE, CSA, and UL, are related to parameters for fuses employed in worst case applications. EIA will set minimum, non-safety related, requirements for fuses that will not serve to diminish minimum requirements set by the safety agencies. These will be comprised of additional requirements not associated with safety-related parameters. The goal of EIA is to standardize qualification performance and characteristics of equivalent fuses, manufactured by multiple sources in an effort to accommodate interchangeability.

Careful investigation by the design engineer must be carried out to determine what type of fuse is best for a given application. It may be beneficial to use both a fuse and some other device to provide more complete circuit or component protection.

Typically, an engineer designs a circuit to meet specified requirements. During the design phase it is important to consider circuit protection needs. The engineer should always assume that some type of circuit protection will be required. The fuse may ultimately be determined to be unnecessary, resulting in cost savings. However, the proper spacing will have been made available to safely accommodate the fuse, if it is required, and the product development can proceed on schedule within the initially budgeted costs.

Design parameter considerations to be addressed

•safety agency approvals;

•open circuit voltage;

•short circuit current potentially available (Interrupting current rating)

•steady state circuit current;

•space limitation;

•worst case current inrush or current spikes (peak current, time duration and multiple event time intervals must be anticipated);

•maximum permitted voltage drop across the fuse at the standard steady state circuit current;

•mounting method (clips, soldered leads, holders, surface mount, etc.);

•plating compatibility with clips, holders and solder;

•environmental issues: temperature, humidity, shock vibration exposure (during production, during storage, during transportation and in field operation);

•I²t limitations;

•processing requirements (wave solder, IR reflow, aqueous detergent cleaning);

•open circuit indication (local and/or remote);

•RoHS compliance;

These are only a few of the preliminary considerations before beginning circuit design.

After a specific fuse is selected, the next step is to determine if the choice was the proper one. Fuses are somewhat unique in the realm of electronic components; if they function properly during stress testing, they are either weakened or made inoperable. Nondestructive test results, as well as tight production control, must be used to predict whether or not a given fuse will continue to perform as desired in a given application. This usually requires significant testing of a given fuse in the final product. Testing in the actual application is essential in many cases even though it is both a time consuming and costly part of the development process. A critical task, often left unfinished when any fuse is finally selected, is adhering to approved parameters. This should be done using a specification that sets limits on critical electrical and mechanical properties.

The Application Guide that follows will help in avoiding the pitfalls associated with erroneous fuse selection and sizing parameters. If problems should arise that have not been covered in this Application Guide, (fuseholder selection), the fuse manufacturers are available to assist in the selection and optimization process.