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Circuit Protection Devices

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
because all the current would be flowing through the short and none through the motor. The battery would
become discharged quickly (perhaps ruined) and there could be the danger of fire or explosion.
The battery cables in our example would be large wires capable of carrying heavy currents. Most
wires used in electrical circuits are smaller and their current carrying capacity is limited. The size of wire
used in any given circuit is determined by space considerations, cost factors, and the amount of current
the wire is expected to carry under normal operating conditions. Any current flow greatly in excess of
normal, such as there would be in the case of a direct short, would cause a rapid generation of heat in the
wire.
If the excessive current flow caused by the direct short is left unchecked, the heat in the wire will
continue to increase until some portion of the circuit burns. Perhaps a portion of the wire will melt and
open the circuit so that nothing is damaged other than the wire involved. The probability exists, however,
that much greater damage will result. The heat in the wire can char and burn the insulation of the wire and
that of other wires bundled with it, which can cause more shorts. If a fuel or oil leak is near any of the hot
wires, a disastrous fire might be started.
Excessive Current
It is possible for the circuit current to increase without a direct short. If a resistor, capacitor, or
inductor changes value, the total circuit impedance will also change in value. If a resistor decreases in
ohmic value, the total circuit resistance decreases. If a capacitor has a dielectric leakage, the capacitive
reactance decreases. If an inductor has a partial short of its winding, inductive reactance decreases. Any
of these conditions will cause an increase in circuit current. Since the circuit wiring and components are
designed to withstand normal circuit current, an increase in current would cause overheating (just as in
the case of a direct short). Therefore, excessive current without a direct short will cause the same
problems as a direct short.
Excessive Heat
As you have read, most of the problems associated with a direct short or excessive current concern
the heat generated by the higher current. The damage to circuit components, the possibility of fire, and the
possibility of hazardous fumes being given off from electrical components are consequences of excessive
heat. It is possible for excessive heat to occur without a direct short or excessive current. If the bearings
on a motor or generator were to fail, the motor or generator would overheat. If the temperature around an
electrical or electronic circuit were to rise (through failure of a cooling system for example), excessive
heat would be a problem. No matter what the cause, if excessive heat is present in a circuit, the possibility
of damage, fire, and hazardous fumes exists.
Q1. Why are circuit protection devices necessary?
Q2. What are the three conditions that require circuit protection?
Q3. What is a direct short?
Q4. What is an excessive current condition?
Q5. What is an excessive heat condition?

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