Protective Devices

Electrical circuits are protected from too much current by fuses, fusible links, and circuit breakers. Such devices will interrupt a circuit to prevent high current from melting conductors and damaging loads. Each of these circuit protection devices is sensitive to current, not voltage, and is rated by current-carrying capacity. They are usually located at, or near, the power source for the circuit being protected. As such, they are usually a good starting point during electrical problem troubleshooting. Remember, though, these devices (blow) or open a circuit because of a problem. Always locate and correct the problem before replacing a fuse or fusible link or resetting a circuit breaker.

• FUSES Fuses are the most common circuit protection device. Fuses have a fusible element, or low­ melting-point metal strip, in a glass tube or plug-in plastic cartridge.
  These fuses are located in a fuse block under the dash or behind a kick panel. Most circuits - other than the headlamp, starter, and ignition systems - receive power through the fuse block. Battery voltage is supplied to a buss bar in the block. One end of each fuse is connected to this bar, the other end to the circuit it protects. Fuse ratings range from 0.5 to 35 amps, but 7.5 ­amps to 20 amps fuses are most common.


• FUSIBLE LINKS
  Some circuits use fusible links, or fuse links, for overload protection. A fusible link is a short length of smaller gauge wire installed in a circuit with larger conductors. High current will melt the link before it melts the circuit wiring. Such fuse links have special insulation that blisters or bubbles when the link melts. A melted link must be replaced with one of the same size after the cause of the overload has been identified and the problem corrected.


• Circuit breakers
  Are used for protecting circuits temporary overloads may occur and where power must be quickly restored. A bimetal strip is used, similar to that in a temperature-sensitive switch. When heated, the two metals expand differently and cause the strip to bend. The "breaker" is normally closed and it opens when the bimetal element bends. Some circuit breakers are self-resetting, others must be manually reset.
  Circuit breakers are used on vehicles to protect circuits for the defogger, heater, air conditioner, power windows, power door locks, and sun roof.

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