2-2
18. List the steps to follow before starting work on a circuit breaker and the items to be checked
when maintaining circuit breakers.
CIRCUIT PROTECTION DEVICES
Electricity, like fire, can be either helpful or harmful to those who use it. A fire can keep people
warm and comfortable when it is confined in a campfire or a furnace. It can be dangerous and destructive
if it is on the loose and uncontrolled in the woods or in a building. Electricity can provide people with the
light to read by or, in a blinding flash, destroy their eyesight. It can help save people’s lives, or it can kill
them. While we take advantage of the tremendous benefits electricity can provide, we must be careful to
protect the people and systems that use it.
It is necessary then, that the mighty force of electricity be kept under control at all times. If for some
reason it should get out of control, there must be a method of protecting people and equipment. Devices
have been developed to protect people and electrical circuits from currents and voltages outside their
normal operating ranges. Some examples of these devices are discussed in this chapter.
While you study this chapter, it should be kept in mind that a circuit protection device is used to keep
an undesirably large current, voltage, or power surge out of a given part of an electrical circuit.
INTRODUCTION
An electrical unit is built with great care to ensure that each separate electrical circuit is fully
insulated from all the others. This is done so that the current in a circuit will follow its intended path.
Once the unit is placed into service, however, many things can happen to alter the original circuitry. Some
of the changes can cause serious problems if they are not detected and corrected. While circuit protection
devices cannot correct an abnormal current condition, they can indicate that an abnormal condition exists
and protect personnel and circuits from that condition. In this chapter, you will learn what circuit
conditions require protection devices and the types of protection devices used.
CIRCUIT CONDITIONS REQUIRING PROTECTION DEVICES
As has been mentioned, many things can happen to electrical and electronic circuits after they are in
use. Chapter 1 of this module contains information showing you how to measure circuit characteristics to
help determine the changes that can occur in them. Some of the changes in circuits can cause conditions
that are dangerous to the circuit itself or to people living or working near the circuits. These potentially
dangerous conditions require circuit protection. The conditions that require circuit protection are direct
shorts, excessive current, and excessive heat.
Direct Short
One of the most serious troubles that can occur in a circuit is a DIRECT SHORT. Another term used
to describe this condition is a SHORT CIRCUIT. The two terms mean the same thing and, in this chapter,
the term direct short will be used. This term is used to describe a situation in which some point in the
circuit, where full system voltage is present, comes in direct contact with the ground or return side of the
circuit. This establishes a path for current flow that contains only the very small resistance present in the
wires carrying the current.
According to Ohm’s law, if the resistance in a circuit is extremely small, the current will be
extremely large. Therefore, when a direct short occurs, there will be a very large current through the
wires. Suppose, for instance, that the two leads from a battery to a motor came in contact with each other.
If the leads were bare at the point of contact, there would be a direct short. The motor would stop running
