Let's begin by looking at why fuses are used and why we might need more than one type. As we saw in the first installment of this series, a circuit is a complete loop from a battery and back, with a device installed somewhere along that loop where electricity is used to do some kind of work or to perform a task. An example would be a simple circuit to light a bulb. We would connect one contact of the bulb to battery +, and the other side to battery -.
It doesn't matter to a light bulb which contact is hooked to which side of the battery. This makes a complete circuit. The bulb lights because it's filament is resistive to electrical flow, so it heats up and glows. Any time electricity is used to do something, heat is generated. A circuit is engineered to carry a certain amount of electricity, with larger wires used in circuits that carry more power. If a circuit is asked to carry more than it is supposed to, excessive heat will be generated and if enough heat is made, the circuit will burn apart at that part of the circuit that has the most resistance.
In a well designed and soundly assembled circuit, the fuse provides that point of resistance, the weak link that will burn apart and open the circuit if the electrical flow exceeds the design limits. Two things can cause a circuit to overload and burn, or blow, the fuse. Sometimes someone adds something to a circuit that demands more power than the circuit can handle. This can result in burned connections, melted wiring, or blown fuses. Sometimes insulation on a wire can rub through, usually on a sharp bodywork edge, the edge of a frame or bracket, or where a wire goes through a hole. This allows the conductor to make contact with ground, and causes a short circuit and often melted insulation. If the short happens between the battery and the fuse, lots of wiring can be burned up if the short makes really good contact. For this reason, manufacturers go to great lengths to make sure the power supply to a fusebox is well protected and routed away from trouble. Sometimes a bigger problem is if the short is not making real good contact or makes contact once in awhile when the wire moves. These intermittent problems can drive mechanics and owners crazy, because they can be very hard to reproduce and find.
Accidents can also cause wiring to short out and these are dangerous because fires can be started. Lightning occasionally strikes a vehicle and the results can be very interesting. Twice I have helped repair trucks that had been damaged by being welded on without the precaution of having the batteries unhooked. In one of those cases, an ECM and a complete engine wiring harness was needed. I also once repaired a wrecker that had been dragged onto a guardrail during a recovery and the main wiring harness under the truck was chopped in two, but not before shorting out the ECM, the injector driver, and the personality module. The cost of repairing these types of problems is usually very impressive.
If you look closely at the first picture, you can see that the blue MINI fuse is blown. The metal conductor inside the fuse is burned in two after the fuse has done its job. Fuses have other features in addition to being able to be visually checked. The picture below shows that they can be checked with a meter or a test light, without having to remove them from the fusebox. If power exists at both contacts, the fuse is good. The fuse is bad if there is only power at one contact, and if there is no power at either, the fuse can still be checked by using a meter to check continuity between the contacts. Note also that in addition to having the amperage rating printed on the fuse, they are also color coded for amperage.
Circuit breakers are the next circuit protection device we will look at. Circuit breakers perform the same function as fuses, but they are re-settable. They operate thermally, with a bi-metallic strip that bends and breaks contact when it gets hot. Auto reset breakers reset automatically when they cool off. Other types have a button that pops out, indicating an overload, and the button can be pushed back in to reset. Some heavy circuit breakers like those on liftgates have a flag that pops out and is reset by pushing it back in. Some can replace glass tube fuses, and some can replace blade fuses. Others have unique blade configurations that must be replaced with the same thing. There are studded types that bolt into trailer nose receptacles or power distribution panels. There are too many different types of breakers to list here, but you can go to http://www.electerm.com/breaker.html to see a good chart of the different types and their features.
Circuit breakers are nice because they let you keep going if a problem is intermittent, and don't need replacement every time they trip. Rarely they do need to be replaced, but repeated overloads can damage the mechanism and arcing can burn the contacts. Typically they will still work but may trip prematurely or may not allow enough voltage through the circuit. Fuses are still used for the circuits a customer would most likely check, because they are easy to check and can be bought almost anywhere. Fuses also protect computer power sources, because a breaker may allow a momentary voltage spike to get through, where a fuse will immediately blow in an overload.
Confusion often exists as to whether something is a breaker, a relay, or a module of some kind. Note that some breakers are fairly large and made of plastic like a relay or module. They may have blade type connections where they plug into a fusebox or electrical panel like a relay, but if it has only two blades, it is a circuit breaker. A relay or module will ALWAYS have more than two blades, and a breaker will NEVER have more than two. The only exception I can think of is a two contact flasher, and those have a unique blade arrangement that is different from any breaker. A breaker will always have an amperage rating printed on it, but they often aren't coded for color like fuses, nor are they all universally interchangeable like standard blade fuses.
I will say just a few words about the third type of circuit protection, the fusible link. A fusible link is a special piece of wire that burns through at a specific load, like a fuse. The wire is covered with an high temperature insulation that won't burn when the link melts, and this insulation makes it impossible to tell visually if the link is bad. A meter or a light must be used to check for continuity through the circuit. Links are rated by the diameter of the conductor, or more precisely the area of the wire's cross section, in millimeters. Unless you know what to look for, they are hard to spot, and are not used for more than one or two circuits. They are used for things like alternator outputs and starter switch power.
Well, that's all for this installment, but stay tuned next for a look at relays, which are one of the most important electrical components on a modern vehicle.
Thanks, Bruce
I will say just a few words about the third type of circuit protection, the fusible link. A fusible link is a special piece of wire that burns through at a specific load, like a fuse. The wire is covered with an high temperature insulation that won't burn when the link melts, and this insulation makes it impossible to tell visually if the link is bad. A meter or a light must be used to check for continuity through the circuit. Links are rated by the diameter of the conductor, or more precisely the area of the wire's cross section, in millimeters. Unless you know what to look for, they are hard to spot, and are not used for more than one or two circuits. They are used for things like alternator outputs and starter switch power.
Well, that's all for this installment, but stay tuned next for a look at relays, which are one of the most important electrical components on a modern vehicle.
Thanks, Bruce
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