In a series circuit, components are connected like links in a chain. If any link opens, current to all of the components is cut off. In a parallel circuit, all components are connected directly to the voltage supply Figure 36-30. If any connection or component fails in a parallel circuit, current continues to flow normally through the remaining circuits. This is one reason why parallel circuits are used in automotive applications, such as headlight and taillight systems. If one lamp fails, current continues to flow through the other lamps in parallel. In a series circuit, all would go out, which could be disastrous.

Also, since all components connect directly to the battery terminals, the metal of the vehicle’s chassis can become one of the conductors. One terminal of the battery and one terminal of each component can be connected anywhere on the body or chassis to complete the circuit. This is called a common, or ground, connection. It saves a lot of connecting wire.

While electricity always behaves according to the laws of physics, when it operates in a parallel circuit, there are some additional laws you need to know. The parallel circuit laws listed here should be applied when working with parallel circuits:

- The voltage across all branches of a parallel circuit are equal.
- The total current in a parallel circuit equals the sum of the current flowing in each branch of the circuit.
- The total resistance of a parallel circuit decreases as more branches are added. Total parallel circuit resistance will always be less than the lowest branch resistance.

Let’s look at those laws a bit more closely. A feature of a properly working parallel circuit is that the voltage across each branch is the same as the other branches. No matter how many branches are added, or removed, as long as they are in parallel, the voltage across them will be the same as across each of the other branches, including the battery. Another feature of a parallel circuit is that the current flowing in each branch is determined by the resistance of that branch along with the voltage used by that branch.

In a parallel circuit where the resistors in each branch are the same, the current flowing in each branch is also the same. The sum of their individual currents is equal to the total current flowing in the entire parallel circuit. When the resistances are not equal, the current divides in accordance with the resistance of each branch, but the total current flow is still the sum of the currents flowing in each branch Figure 36-31.