For any electrical system to operate, it must make a complete circuit. This simply means that the power flow from the battery must make a complete circle. When an electrical component is operating, power flows from the battery to the component, passes through the component causing it to perform its function (lighting a light bulb), and then returns to the battery through the ground of the circuit. This ground is usually (but not always) the metal part of the vehicle or truck on which the electrical component is mounted.
Perhaps the easiest way to visualize this is to think of connecting a light bulb with two wires attached to it to the battery. If one of the two wires attached to the light bulb were attached to the negative post of the battery and the other were attached to the positive post of the battery, you would have a complete circuit. Current from the battery would flow to the light bulb, causing it to light, and return to the negative post of the battery.
The normal automotive circuit differs from this simple example in two ways. First, instead of having a return wire from the bulb to the battery, the light bulb often returns the current to the battery through the chassis of the vehicle. Since the negative battery cable is attached to the chassis and the chassis is made of electrically conductive metal, the chassis of the vehicle can serve as ground wire to complete the circuit. Secondly, most automotive circuits contain switches to turn components on and off as required.
Every complete circuit must include a component or load which is using the power from the power source. If you were to disconnect the light bulb from the wires and touch the two wires together (don't do this), the power source, in this case the battery, would attempt to deliver all its power instantly to the opposite pole. This tremendous current flow causes extreme heat to build within the system, often melting the insulation on the wiring.
Because grounding a wire from a power source makes a complete circuit, but without a load to use it, this phenomenon is called a short circuit. Common causes are: broken insulation (exposing the metal wire to a metal part of the vehicle), or an internally shorted switch. Water leaking into normally sealed components can also conduct electricity to undesired locations.
Some electrical components which require a large amount of current to operate (blower motor, rear defogger, headlights, etc.) have a relay in their circuit. Since these circuits carry a large amount of current, the thickness of the wire in the circuit (gauge size) is also greater. If this large wire were connected from the component to the control switch on the instrument panel, and then back to the component, a voltage drop would occur in the circuit. To prevent this potential drop in voltage, an electromagnetic switch (relay) is used.
The large wires in the circuit are connected from the battery to one side of the relay, and from the opposite side of the relay to the component. The relay is normally open, preventing current from passing through the circuit. An additional, smaller, wire is connected from the relay to the control switch for the circuit. When the control switch is turned on, it grounds the smaller wire from the relay and completes the circuit. This closes the relay and allows current to flow from the battery to the component. The horn, headlight, and starter circuits are three which use relays.
It is possible for larger surges of current to pass through the electrical system of your vehicle or truck. If this surge of current were to reach an electrical component, it could burn it out. To prevent this, fuses, circuit breakers and fusible links are connected into the current supply wires. These are nothing more than pre-planned weak spots within the system. Too much current WILL damage something; if the something is easy to find and easy to replace, major components and wiring will be protected. Fuses are designed to pass the amount of current (amperes) shown on the fuse. If a current flow develops which exceeds the rating, the fuse element separates (blows), causing an open circuit and stopping the current flow.
A circuit breaker is essentially a self-repairing fuse. The circuit breaker opens the circuit the same way a fuse does. However, when either the short is removed from the circuit or the surge subsides, the circuit breaker resets itself and does not have to be replaced as a fuse does.
A fuse link or main link is a wire that acts as a fuse. It is normally connected between the starter relay and the main wiring harness or the positive battery cable and the main wiring harness under the hood. The fuse link (if installed) protects all the chassis electrical components by cutting off the main power supply if necessary. If the vehicle shows absolutely no electrical response when the key is turned, the main link is suspect. Check it first; it's rare for a battery to get so low that something won't work.