Fuse Theory
Fuses, circuit breakers and relays are common devices used in vehicles and multiple electrical devices. The purpose of a fuse is to protect the user from dangerous situations caused by electrical chaos, example, short circuit, equipment failure, excessively high current draw.
Circuit breakers are used as an alternative to fuses. Although they sound advantageous to fuses, there properties of self-restoration should not be abused.
I am not responsible for any damage incurred in your electrical applications after reading this article.
Definition of a Fuse
Non-restorable protection device, once operated, it must be replaced. Fuses can be thought of as a line interrupt device. In service, the power line is live with electricity at the connection end. Out of service, the power line is dead at the connection end.
Fusing can be found in single circuit fuse links, or multi-circuit distribution blocks. High powered audio systems are advised to use distribution block fusing to reduce wire routing, and increase system simplicity.
Definition of a Circuit Breaker
A self-restorable protection device, once operated, it will self-reset, and be automatically placed back in service. Similar to the fuse, except the device will reset itself to the operate mode after a line fault (short circuit, or high current drawn).
Uses of Fuses and Circuit Breakers
Fuses and Circuit Breakers serve as safety devices, protection devices.
--+12V power-----------wire(lead end)--------------[fuse link]--------------more wire(tail end)-----------
Fusing or linking provides a safe method of providing and distributing power to our 12V toys. The basic scheme is as follows shown in the above ascii diagram. 12V power is drawn from the battery, it leads to fuse, the fuse separates the load/tail end of the cable which connects to the 12V toy from the power source. If too much current is drawn the fuse link breaks open, and the 12V power delivered to the device is removed, and hopefully the fault can be properly cleared.
In car audio applications, all power lines (+12V feed) should be properly fused and separated. The ground does not normally require fusing. Fusing the ground wire is at the discretion of installer.
The reason why the ground is not fused is that the car body is at the same potential as the negative battery terminal. For simplicity, we can say in general, same voltage means no sparking from ground wire to the car body. Therefore chafes and tears in cable insulation on a negative cable are not as cumbersome as positive cables. When the positive battery terminal is disconnected, the negative cable becomes mono-pole and cannot conduct electricity.
Fusing should be a thought out process. If you run a large gauge cable to the rear of your vehicle (say 0 gauge), then you should link (fuse the wire) very close to the battery, and at the distribution point (via a distribution block). When installers refer to fusing close to the battery/voltage source, the reasoning is to prevent a cooked wire by mistake. If a short circuit were to occur at the end of a cable located in the trunk, current will flow from the battery to the fuse-link. If the fuse link is in the trunk and the wire is shorted to ground, the entire length of cable will draw a large current, and in most events, will cause a fire. If the fusing was near the battery, the current stops at the fuse link, within inches of the battery. It is very unlikely that a 6 inch lead end of cable will have a short versus a 20 foot length lead end, which is routed through grommets, doors, under seats and through trunks.
After understanding the fundamentals of fusing theory, here is the introduction to the circuit breaker. Much like the kitchen, when you run a toaster, microwave, and oven at once, your breaker trips and you reset it, and all is well. The most common vehicle circuit breakers are self resetting. After a trip, some time later it resets, and restores its protected cable. All fuses can technically be replaced with circuit breakers, but it’s not a good idea. When fuses blow, there is usually a good reason why it blew. Nevertheless, the option is there.
Breakers offer the user of never having to run out of fuses, because the breaker doesn’t use a fuse. Sounds great that it self-restores the broken circuit, and you don’t have to pay for fuses but . . . . . ..
Danger of circuit breakers, dumb devices.
Circuit breakers are dumb devices, and don’t normally come in large sizes (High Current) with great availability. Many install shops like to use circuit breakers because the cost of a fuse link can sometimes rival the cost of a breaker, and convenience, one trip, by a specific event is tolerable, ie: all power windows being opened at once.
The danger of a circuit breaker, say a power wire shorted itself (came loose somewhere), great, the breaker trips, the problem hasn’t been fixed, you didn’t find the loose wire. It resets, and trips, resets, and trips. So what? What if that wire was near a fuel line, and it’s sparking every time the breaker resets? The circuit breaker doesn’t know that, and will continue to trip and reset until you either clear the fault, or cut the link from the battery. Use circuit breakers with caution.
Isolation & Trouble Shooting
Fuses are far better for circuit protection than circuit breakers, because they offer fault occurrence knowledge. As mentioned, if a fuse blows, there is a reason why. And you can isolate circuits using a fuse, much like changing other fuses in your car. This makes for easy trouble shooting.
Every piece of equipment should be fused if possible. Here is the golden rule, If it gets a 12V input, it gets a fuse. It makes no sense to not fuse a device, when a fuse costs pennies, but an amp costs hundreds.
Sometimes fuses don’t come in the proper size, ie: 15A, 20A, 30A etc. Say you need 60Amps of protection, and you can’t find a 60 AMP fuse, do a parallel link of two 30A fuses in parallel. Note, it is better to use a single fuse if possible.
If the amp is fused for 30 amps, give it a 30 amp fuse link. If you have 3 amps, at 30 amps a piece, you should be fusing with 30 amps per amplifier. If using a distribution block, fuse the lead link with a minimum of 90 amps, and each distribution line should be fused at 30 amps. It’s always okay to under fuse by putting in a smaller fuse, however, it will be a hindrance if it keeps blowing. If you over fuse, you risk the chance of heating up cables.
If the head unit was installed using the factory head unit power source, the head unit may not require fusing, as the factory supply is a fused link.
I think you got the jist of the story by now . . .. .
Written by ECTWO; rev0
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