The electrical systems in our vehicles are protected, just like those in our homes, by ultra simple little devices called fuses.

They blow because the circuit they monitor has been asked to carry more than its share of the load due to a faulty connection or component -- or maybe there's just one of those seemingly inexplicable and random power surges that electricity is notorious for. But however it happens, when a fuse blows, you're going to be left without the services of one of your vehicle's systems.

This could be merely annoying, no stereo or sunroof, for example. But blowing some fuses could leave you without stop or signal lights, a heater blower, a radiator fan or windscreen wipers.

Knowing how to check and replace a blown fuse could get you safely back on the road and might just save you a costly trip to a repair shop.

Fuses and other electrical safety and reliability oriented gadgetry, such as circuit breakers and relays, have proliferated in the past couple of decades as more and more gadgets -- everything from power antennas to adjustable pedals -- have been added to vehicles.

My late 1990s minivan has an electrical power distribution centre under the hood and a second fuse block under the driver's side of the instrument panel containing approximately 35 fuses and about 15 relays. The 1968 Morgan sports car that shares the garage space gets by with just two fuses. But then it doesn't have a heater, a radio or even wipers for that matter.

A fuse's job is to sacrifice itself to protect an electrical circuit.

Think of a battery with wires running from the positive and negative terminals with the ends connected to the contact points of a light bulb. The fuse would be located in one of the wires. It allows electrical current to flow up to a certain amperage or load level.

When this amperage is exceeded, heat is produced and the link in the fuse will literally burn up or melt, breaking the circuit and stopping the flow of electricity.

If the fuse wasn't there, the continued flow of current through the wire could damage the component being operated or heat the wire to the point that the insulation melts. The exposed hot wire could then come into contact with something and cause a fire.

Automotive fuses have taken a variety of shapes over the years but the most common were cylindrical glass tubes with metal caps on each end and a wire filament running through the centre. These fitted into metal spring clips at each end.

In recent years, these have been replaced by rectangular plastic types with two protruding terminals that plug into appropriate slots in the fuse box. They come in a variety of sizes, and a range of amperage ratings that commonly includes three, five, 10, 15, 20, 25, 30 and 40 amp sizes.

Some circuits are protected by self-resetting fuses or circuit breakers that are only serviceable by a technician.

As with most things to do with your vehicle, the owner's manual is the place to start if you want to attempt to troubleshoot the failure of a system such as, say, the interior lights, says Brian Holmes, service expert with the Canadian Automobile Association (CAA) of Central Ontario.

It will show you where the fuse boxes are located -- usually, one is under the hood and another under the dash. It will also tell you what each fuse controls, where it is located and its amperage rating. Often the plastic lids of these boxes or panels will have a diagram to help you find the fuse you want to check.

A quick tip: always make sure the lid is properly re-installed, particularly on under-the-hood types, so water can't get in.

Once you locate the suspect fuse, you have to remove it. Most can be extracted by hand, but some auto makers provide plastic tweezers to help, and you might have to resort to a pair of pliers. If you do, be careful not to crush the fuse as you then may not be able to determine if it was at fault or not.

With both old and modern types of fuses, a visual inspection will reveal whether the enclosed link is intact or blown.

If the fuse is blown, toss it and insert a new one. Don't install a lower amperage fuse than the owner's manual calls for as the electrical demand from the system will likely soon blow it again, Holmes advises. And definitely do not install one of higher amperage, he adds, as this could damage the component involved or create a fire hazard. Don't try a fix with a piece of wire either, for the same reasons.

With the new fuse in place, try the system it protected. If it works, pat yourself on the back. If it doesn't work, or functions for only a short time and fails, check the fuse again. If it's blown, you need to get it checked by a technician.

Electrical service can be quick and easy or a drawn-out and expensive process as some faults are very hard to diagnose, particularly if they are intermittent, Holmes says.

He warns that wiring can be damaged by something as simple as throwing a snow shovel into the trunk, but often the culprit is installing aftermarket accessories, such as stereos or fog lights, or hooking up a trailer lighting kit.

He suggests you only attempt these things if you know what you're doing. If not, get professional help, as messing with the electrical system on today's cars can potentially damage very expensive electronic devices, such as computers.

Holmes suggests familiarizing yourself with the fuse box locations, and stashing a couple of spare fuses for the more important systems in your glove box, as good roadside insurance.