Fuses & Circuit Breakers
Sep 16, 2010
If the positive (live) side of a low voltage DC circuit connects directly to the negative side, a large current will flow. This is known as a short circuit because the current will take the shortest path back to the battery and bypass the rest of the circuit. If the current exceeds the maximum capacity of the cable this will cause the cable to heat up and can result in an electrical fire or damage the wiring. Short circuits can be caused because insulation is damaged by chafing or due to prolonged exposure to sunlight, oil or corrosive chemicals; by a damaged or faulty electrical device; or because a poorly secured connection comes loose. It is also worth noting that although it is becoming less common nowadays, some equipment may have its case or body wired to negative.
In order to protect against the danger of electrical fire all circuits must be protected by introducing either a fuse or a circuit breaker into the circuit. This includes battery cables which are often overlooked.
A fuse or circuit breaker will carry current up to a specified load rating. If the current exceeds the load rating a fuse will ‘blow’ while a circuit breaker will ‘trip’. A fuse includes a short section of wire which melts rapidly when a specific current is reached, thereby creating a physical break in the circuit and preventing any current from flowing. Alternatively, a circuit breaker senses a rise in current either thermally or magnetically and will ‘trip’ if its specified current rating is exceeded, also creating a break in the circuit.
The fuse or circuit breaker fitted must have a lower rating than the maximum current capacity of the wire or cable which it is intended to protect. Where multiple circuits are protected by a common fuse or circuit breaker, this must be rated for the lowest rated wire. Individual devices and components are not protected by fuses and circuit breakers installed within wiring circuits. Some equipment may be fitted with an internal fuse.
Fuses have the benefit of being cheap as are most fuse boxes and fuse holders, some of which can be inserted ‘in-line’ if an extra device is added at some later stage. However ‘blown’ fuses need to be replaced, so spares for each type and rating must always be kept readily at hand. Fuses can corrode, shake loose due to vibration or simply deteriorate with age and eventually fail without warning.
Circuit breakers are generally more expensive than fuses but can be infinitely reset without the need for spares. Circuit breaker switches are available for many situations, thereby halving the number of components needed.
If a fuse blows or a circuit breaker trips, you should allow only one replacement or reset before investigating the cause. Small current surges can occur on circuits without any short circuit; these can be a very occasional nuisance. However, if a blow or trip recurs then the cause must be found.
These handy automotive fuses are economical and readily available in ratings from 1 amp to 40 amps. Blade fuse boxes are also available for marine installations, which can make a rewiring job much easier.
Midi and Mega Fuses
A high current fuse should be installed on both the negative and positive side of battery cables and can also be fitted to protect high current devices. Midi fuses cover a range from 30 amps to 150 amps while Mega fuses cover 100 amps to 300 amps. Midi and Mega fuses must be fitted into an appropriate holder which encloses the fuse and will prevent melted metal from escaping in the event of a short circuit. All these items are listed in our Fuses & Breakers section.