ECIA EIA-772-A

This guide has been prepared to aid the circuit and systemdesigner in the selection of supplemental fuses. Traditionally,these have been fuses that interrupt the primary equipment powersource when an overcurrent condition develops in a device orproduct. This guide will cover only fuses intended for electronicapplications.

Introduction

Fuses that are covered in this Application Guidetypically range in physical size up to 10.4 X 40 mm. Thesefuses fall within the ANCE-248 (Mexico)/CSA C22.2, 248 / UL 248,Supplemental Fuse Standard for North Americanapplications. Current ratings covered by this Standard rangefrom milliamperes to as high as 60 amperes, with voltage ratings upto 600 volts AC or DC. These devices are available in a variety ofmounting schemes including plug-in, cartridge, through-hole andsurface mount.

Fuses are quite complex despite their simple appearance. Eachfuse type and current rating will have unique operatingcharacteristics, maximum operating limits and internalconstruction. Subtle variations between manufacturers’ designs willaffect equivalency even between two seemingly identical devices.Further, any given type of fuse from a single manufacturer willhave electrical and mechanical parameter variations that resultfrom slight production variations during processing within a givenlot and from lot to lot. Safety agency approvals, e.g., ANCE, CSA,and UL, are related to parameters for fuses employed in worst caseapplications. EIA will set minimum, non-safety related,requirements for fuses that will not serve to diminish minimumrequirements set by the safety agencies. These will be comprised ofadditional requirements not associated with safety-relatedparameters. The goal of EIA is to standardize qualificationperformance and characteristics of equivalent fuses, manufacturedby multiple sources in an effort to accommodateinterchangeability.

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

Typically, an engineer designs a circuit to meet specifiedrequirements. During the design phase it is important to considercircuit protection needs. The engineer should always assume thatsome type of circuit protection will be required. The fuse mayultimately be determined to be unnecessary, resulting in costsavings. However, the proper spacing will have been made availableto safely accommodate the fuse, if it is required, and the productdevelopment can proceed on schedule within the initially budgetedcosts.

Design parameter considerations to beaddressed

• safety agency approvals;

• open circuit voltage;

• short circuit current potentially available (Interruptingcurrent rating)

• steady state circuit current;

• space limitation;

• worst case current inrush or current spikes (peakcurrent, time duration and multiple event time intervals must beanticipated);

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

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

• plating compatibility with clips, holders and solder;

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

• I²t limitations;

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

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

• RoHS compliance;

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

After a specific fuse is selected, the next step is to determineif the choice was the proper one. Fuses are somewhat unique in therealm of electronic components; if they function properly duringstress 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 continueto perform as desired in a given application. This usually requiressignificant testing of a given fuse in the final product. Testingin the actual application is essential in many cases even though itis both a time consuming and costly part of the developmentprocess. A critical task, often left unfinished when any fuse isfinally selected, is adhering to approved parameters. This shouldbe done using a specification that sets limits on criticalelectrical and mechanical properties.

The Application Guide that follows will help inavoiding the pitfalls associated with erroneous fuse selection andsizing parameters. If problems should arise that have not beencovered in this Application Guide, (fuseholderselection), the fuse manufacturers are available to assist in theselection and optimization process.

Edition:

A

Published:

06/01/2008

Number of Pages:

18

File Size:

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