Author Topic: Fuses and Circuit Protection  (Read 5759 times)

Online bd

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Fuses and Circuit Protection
« on: September 05, 2017, 11:42:10 PM »
Figure 1.  Comparison chart of common low voltage fuse configurations regularly used by the automotive industry.


Glass Tube Fuses:

Glass tube fuses were used in GM trucks during 1973 - 1978 model years in component sizes: AGC 3A, 4A, 10A, 15A, 20A, and 25A.  In addition, dual SFE 4A fuses housed in watertight inline fuse holders were used to protect the factory ammeter and its connecting circuit during the 1967 - 1975 model years until the advent of the factory voltmeter in 1976.  Fuse capacities (ampere ratings) are stamped into the metal end-caps of glass tube fuses.  All AGC style fuses are fast-acting and share the same physical dimensions of 1/4" diameter by 1-1/4" long.  All SFE style fuses are 1/4" in diameter but vary in length from 5/8" to 1-7/16", according to ampere rating.  Only the SFE 20A fuse is directly interchangeable (shares the same physical dimensions) with its AGC 20A counterpart.  The SFE 4A fuses used to protect the factory ammeter circuit are 5/8" long.

Blade Fuses:

Compact, modular blade fuses replaced glass tube fuses beginning in 1979 through the 1987 (91) model years in component sizes: ATO/ATC 3A, 5A, 7.5A, 10A, 15A, 20A, 25A, and 30A.  The bodies of ATO/ATC fuses are translucent plastic and color-coded in compliance with a universal standard for rapid visual identification (Fig. 1, leftmost column).  All ATO/ATC "regular" blade fuses are fast acting and share the same, roughly square, overall physical dimensions of 18.7 mm tall by 19.1 mm wide by 5.1 mm thick, including two 6.7 mm tall by 5.25 mm wide tin electroplated zinc alloy blades needed for plugin circuit connection.  The ATO/ATC fuse series are physically and electrically identical with one minor exception - the ATO (open) style fuse exposes the fuse element to the atmosphere through an open vent window located directly below the element between the blades, whereas the ATC (closed) style fuse completely encases the fuse element within an enclosed chamber.  Neither the ATO nor ATC style fuses are airtight, dust-tight, or watertight, making them completely interchangeable.  Optional ATO "glow" fuses, which possess LED indicators that illuminate when the fuses melt open, adding diagnostic convenience, are available from the aftermarket at a premium price in the more common amperage ratings.

Circuit Breakers:

Discrete, Type 1 automatic reset, dual-post circuit breakers were used in 1977 - 1978 GM light trucks in a 30-amp capacity to protect the newly introduced, cab entry doors power windows option.  Thereafter, the Type 1 auto-reset circuit breakers migrated to the newer design ATO/ATC case size and blade configuration in the continued 30-amp capacity for the protection of the still optional power window and rear window defogger circuits.  Auto-reset circuit breakers rated 35 amperes were used exclusively for rear A/C applications in the "G" series vans.

The headlamp switches used in all 1973 - 1987 (91) GM light-duty trucks contain an inaccessible and independently non-serviceable, Type 1 auto-reset circuit breaker rated for 16 amperes.  The integrated circuit breaker is incorporated strictly to protect the dual and quad headlamp circuits.  Running and instrument lamps are separately powered and protected using replaceable fast-acting fuses.  Servicing the integrated headlamp circuit breaker requires replacement of the headlamp switch as a complete assembly.

Fusible Links:

Fusible links in various gauges were used throughout 1973 - 1987 (91) GM truck production.  A fusible link is an ultra-slow-acting fuse capable of sustaining remarkable surge (in-rush) current for brief periods.  In typical applications, fusible links are used as primary circuit protection for potentially high current draws that exceed 30 amperes.  The ampere capacity of a fusible link is dependant on its effective, or net, cross-sectional area and overall length, as well as the operating environment in which it is used.  Fusible link amperage capacities rarely, if ever, are stated due largely to their imprecise and unrepeatable, hence, unpredictable fusing behavior.  The primary advantages of fusible links are their relatively low cost, minimal space requirements, high surge current tolerance, and exceptional durability.

A fusible link is composed of high strand count, optionally tin electroplated, copper alloy wire jacketed in Hypalon (chlorosulfonated polyethylene) insulation.  The high strand count of the link improves flexibility, whereas tinning (electroplating) of the copper strands impedes the formation of surface oxidation on the wire, which in turn provides stable long-term electrical continuity between the link and its crimped terminations.  Fusible links are capable of operating at sustained temperatures of up to 150 C (302 F) without damage to the wire or insulation.  The Hypalon jacket is designed to safely enshroud the wire link and remain intact without rupturing or igniting in the event that the fusible wire contained within the jacket melts and arcs or flames in response to excess electrical load.  By broadly held convention, fusible links are four AWG (American Wire Gauge) smaller in cross-section (i.e., 4 AWG numerically greater) than the wires they protect and are universally 6" in length regardless of gauge (up to 9" in length for standalone, highly specialized applications), see table.


18 GA
22 GA  (0.3 (mm)2)
16 GA
20 GA  (0.5 (mm)2)
14 GA
18 GA  (0.8 (mm)2)
12 GA
16 GA  (1.0 (mm)2)
10 GA
14 GA  (2.0 (mm)2)
  8 GA
12 GA  (3.0 (mm)2)
  6 GA
10 GA  (5.0 (mm)2)
  4 GA
  8 GA  (8.0 (mm)2)
  2 GA
    6 GA  (13.0 (mm)2)

It is extremely important to maintain strict compliance with the original engineering specifications when servicing a fusible link in order to preserve fusing characteristics and circuit performance.  Altering the gauge or length of a preexisting fusible link during service can severely impede the effectiveness of the link and its ability to safely protect the connected wiring from overcurrent damage.  The result of any such inconsistency can prove catastrophic!  For a step-by-step illustrated guide on how to replace a fusible link, see the related article How to make a Fusible Link by VileZambonie.

Over the course of its history, GM employed fusible links in AWG (metric) wire sizes: 20 AWG (0.5 (mm)2), 18 AWG (0.8 (mm)2), 16 AWG (1.0 (mm)2), 14 AWG (2.0 (mm)2), and 12 AWG (3.0 (mm)2) in the manufacture of its trucks; 16 AWG (1.0 (mm)2) and 14 AWG (2.0 (mm)2) are the most prevalent.

Safety Considerations:

As a matter of electrical safety and fire prevention, NEVER substitute a higher rated fuse, circuit breaker or fusible link (hereinafter, fuse) for the circuit protection installed by the factory.  Wire gauges are matched to the original fuse specification.  Substituting a larger fuse rating than originally specified can easily result in wires overheating, insulation melting and/or igniting in flames.  It is a serious matter!  In addition, under no circumstances should a fusible link be replaced by conventional wire!  The insulation jacketing conventional wire will not tolerate or survive the extreme heat produced by fusing wire, which dramatically increases the potential for fire.  If ever a discrepancy exists between a fuse capacity callout in the wiring manual and the fuse capacity imprinted on the fuse box for a specific fuse and circuit ALWAYS rely on the fuse capacity imprinted on the fuse box.

Historical Trivia:

The "AG" glass tube fuse class originally stood for "All Glass" and was produced in several contrasting series, each with different diameters and lengths, among them "AGA", "AGC", "AGU", "AGW" and "AGX".  AGC is alternately known as 3AG; AGA(1AG); AGU(5AG); AGW(7AG) and AGX(8AG).  As previously stated, all AGC fuses are 1/4" diameter and 1-1/4" long.

The "SFE" glass tube fuse class stands for "Society of Fuse Engineers."  All SFE fuses are 1/4" in diameter but vary in length from 5/8" to 1-7/16", according to ampere rating.

The GM truck divisions discontinued using glass tube fuses beginning with the 1979 production year in favor of the ATO/ATC regular blade fuses.  The last vestiges of glass tube fuses were eliminated from holdout U.S. built vehicles in 1981.

Figure 2.  Commonly encountered schematic symbols representing circuit protection components as illustrated in factory and aftermarket wiring diagrams.

« Last Edit: January 31, 2023, 01:36:24 PM by bd »
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