Lesson 1/Learning Event 2
In automotive circuits only one insulated wire to the battery is used. One battery terminal, usually
the negative, is connected to the vehicle frame by a ground cable, and the frame serves as one
conductor. When the switch is closed, current flows from the negative terminal of the battery,
through the frame to the bulb, then through an insulated wire, light switch, and to the positive
terminal of the battery.
Now, let's imagine that the insulation wears off exposing a bare spot on the wire between the light
switch and the bulb. Let us further imagine that the exposed wire touches the frame. Battery
current can now flow from the frame to the touching wire, through the switch, and back to the
battery. Before, the high resistance of the bulb prevented a large amount of current from flowing.
With the wire grounded on the frame, current can bypass the bulb, and the circuit now has very
little resistance. As a result, so much current will flow through the wire that it will get hot enough
to melt and can set fire to any nearby flammable objects.
A SHORT CIRCUIT occurs when two exposed wires touch. This will let the current flow in the
circuits for both wires even though the switch is closed in only one circuit. You can realize the
results of this defect if you will imagine that a car has a short circuit between the wires to the tail
and stoplights. When the taillight switch is turned on, battery current from the frame divides and
flows through both lights and then joins to form one path at the short circuit. So both lights are
burning even though the stoplight should not be.
When an OPEN CIRCUIT exists, no current flows. For example, a light circuit is open when its
switch is off or open, when a wire conductor is broken, or when the bulb filament is burned out.
An open circuit occurs in a circuit that is good when you turn off the switch. The other conditions
are defects in a bad circuit.
A HIGH-RESISTANCE CONNECTION is usually the result of corrosion at a battery post, loose
wires at any terminal, or frayed wires. (A frayed wire has some but not all of the strands of wire
broken.) A high-resistance connection reduces the amount of current flowing so the circuit cannot
work as it should. In a light circuit, for instance, the bulb filament would not glow as brightly as it