EIA 772-A-2008

$24.70

User’s Application Guide to Fuses

Published By	Publication Date	Number of Pages
ECIA	2008	18

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Category: ECIA

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Description

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.