Auto questions: how wires, fuses and connectors work

This article is the first step toward understanding the electrical system of your car.

Wires, fuses and connectors are three of the most basic components of an automotive electrical system but though they may seem simple, there is a great deal more to them than meets the eye. In this article we’ll discuss some of the basic characteristics of each and why they’re built the way they are.

Automotive electrical wire consists of an inner core of metal (usually copper) and an outer sheath of plastic. The metal (called the conductor) carries electrical current from one point in your car to another. The plastic (called the insulator) keeps the conductor from touching other metal parts and sending electricity somewhere that you don’t want it. The conductor can be either solid meaning that it is one round filament of metal or stranded meaning there are several filaments twisted together. Why the difference? Stranded wire is more flexible than solid wire and the more strands the higher the flexibility. This is important in cars since the wires will be subjected to constant movement and vibration that could cause a solid wire to break. You will seldom find solid wire on an automobile. The filaments in the conductor can be solid copper or copper coated with solder. Solder is a mixture of tin and some other metals that binds to the copper and makes it easier to connect wires together. Copper has a reddish orange color while solder is silver colored.

Wire comes in sizes ranging from finer than a human hair to diameters measured in inches. Wire size is usually given in terms of wire gauge. If you look at a typical piece of wire say from a house, you will find stamped on its insulation somewhere a series of numbers and letters that say for instance “14 AG”. This stands for 14 American Wire Gauge and indicates the size of the conductor. A higher number indicates a smaller wire. Exactly the opposite of what you’d expect. Typical automotive wire will range from 18 AWG to 26 AWG. The more electricity you expect your wire to carry, the larger wire (lower gauge) you need.

There are many different types of insulation available on wire also. Each type is tailored to a certain set of operating conditions. Some stays flexible at low temperatures while another can resist high temperatures. Automotive wire must endure temperatures from -40 degrees F to 140 degrees F as well as water, mud, salt, oil, gas and tar. As you might expect a good deal of work goes into selecting the right insulation. Information about the insulation is also stamped on the outside of the wire.

Fuses are called electricity’s safety valve. They are designed to interrupt the flow of electricity when some condition occurs that would cause an excessive amount of electricity to flow. Their primary function is to prevent fire but they also protect sensitive electrical equipment. There are three or four different types of fuses that you might see in an automobile but they all operate in the same way. The first type consists of a cylinder of glass with a metal cap at each end. Inside the glass you can see a piece of metal that is connected to the end caps. The metal can range is size from a small wire to a 1/8 in wide strip. This is the fusible link. The next type resembles the first except the glass tube is replaced with a tube made of off-white ceramic. The last type is a small square or rectangle of plastic with two metal tabs protruding from one edge.

Now how does a fuse work? Suppose you have a wire that connects a battery to some electrical device, say a light bulb. Electricity flows from the battery through the conductor of the wire to the bulb. As the electricity flows, it also heats up the wire. The more electricity that flows, the more the wire heats up. Now suppose you cut the wire and attached each end of the wire to an end of a fuse. Now the electricity flows from the battery through the metal inside the fuse (the fusible link) to the bulb. The fusible link is made of a special type of metal that melts at a fairly low temperature and it heats up just like the wire does when electricity flows through it. If something goes wrong and suddenly a lot of electricity starts flowing in the wire, the fusible link heats up to its melting point and melts which interrupts the flow of electricity and prevents damage to our wires and equipment. We said earlier that the fusible links varied in size and the reason should now be apparent. The smaller the link the less electricity it takes to melt it. So we can choose a fuse appropriate to the amount of electricity that should flow in any given circuit. All fuses regardless of their size or shape work on this same principle.

Connectors are one of the most common failure points in electrical circuitry. Because of this and because the automotive environment is so harsh, automotive manufacturers use specialized connectors that they have built specially for them. Sometimes they even build them themselves. Connectors are designed to connect one or more wires together and protect the connection points from water, salt, dirt and chemicals that can cause corrosion or otherwise destroy the connection. A connector consists of two parts; one side has protruding metal pins while the other has metal sockets into which the pins slide. Metal that is exposed to the air corrodes. This corrosion forms a film on the metal and this film is an insulator. If the connector is to work properly, this film must be removed before the connection is made. The sockets are designed to cut through this layer of corrosion as the pins are being inserted thereby exposing clean metal and making a good contact. In some cases the pins and sockets are coated with a thin film of gold since gold is one of the few metals that do not corrode. The design of reliable connectors is an art that has been developed over many years.