Fuses, Fusible Links, and Circuit Breakers

Fuses and circuit breakers are designed to protect electrical circuits by opening the circuit if the current flow is excessive. The most common kinds of circuit protection devices are fuses, fusible links, circuit breakers, and positive temperature coefficient (PTC) thermistor protection devices. Fuses and circuit breakers are rated in amps, and their ratings are usually marked on them. Fusible links are typically rated by their wire size.

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FIGURE 36-37
Various fuses.

Fuses are typically used in lighting and accessory circuits where current flow is usually moderate. Usually, a fuse contains a metal strip that is designed to overheat and melt when subjected to a specified excessive level of current flow, breaking the circuit and stopping the excessive current flow from potentially damaging the wiring harness and more valuable components. Fuses come in a variety of configurations, from cylindrical glass cartridge fuses to plastic blade fuses Figure 36-37. They also come in a variety of sizes and amp ratings. Fuses are typically housed in fuse boxes located around the vehicle, typically under the hood and/or dash.

A fusible link is made of a short length (usually 6" [15 cm] or less) of smaller diameter wire that has a lower melting point than standard wire and insulation that is fire resistant. Fusible links are typically placed near the battery to protect the wiring harness between the battery and any fuse boxes. In most cases, they are used to carry higher current flows than fuses, and typically feed power to one or more circuits Figure 36-38. Fusible links are fairly durable, and do not fail very often, unless there is a substantial short circuit in the system or the fusible link wire is abused by excessive flexing or pulling on it. Some newer vehicles use maxi-fuses, which are large blade-type fuses, instead of fusible links.

FIGURE 36-38
A fusible link is typically placed near the battery and carries the current needed to power an individual circuit or a range of circuits.

Circuit breakers are different from fuses and fusible links in two ways. First, they are not destroyed by excess current. And secondly, they can be reset, either automatically or manually. In a circuit breaker, a bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit when current flow becomes excessive. In most types, as the strip cools, it resumes its original shape. The contacts then close, completing the circuit once more. These are called self-resetting circuit breakers. Manual breakers must be reset by hand, which could involve flipping a lever or inserting a small rod to reset the bimetal spring once it cools down.

PTC thermistors are also used as circuit protection devices. They have very low resistance at room temperature, but increase in resistance as the temperature increases. If too much current starts to flow through a PTC, the small voltage drop creates heat in the PTC. The increased heat produces increased resistance, which further increases the voltage drop. This cycle continues quickly until the PTC reaches its maximum resistance. This heightened resistance effectively shuts off most of the current flow to the protected device. PTC s generally reset once power is removed and they are allowed to cool. They are typically integrated into components such as power window motors and door locks.