Module 3—Introduction to Circuit Protection, Control, and Measurement
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AIII-1 to AIII-10
CIRCUIT PROTECTION DEVICES
Upon completion of this chapter you will be able to:
1. State the reasons circuit
protection is needed and three conditions requiring circuit protection.
2. Define a direct short, an
excessive current condition, and an excessive heat condition.
3. State the way in which circuit protection
devices are connected in a circuit.
4. Identify two types of circuit protection devices and label the
schematic symbols for each type.
5. Identify a plug-type and a cartridge-type fuse (open and not open)
6. List the three characteristics by which fuses are rated and state the meaning of
Identify a plug-type and a cartridge-type fuse (open and not open) from illustrations.
List the three categories of time delay rating for fuses and state a use for each type of time-delay rated fuse.
8. List the three categories of time delay rating for fuses and state a use for each type of time-delay rated
fuse. Identify fuses as to voltage, current, and time delay ratings using fuses marked with the old military, new
military, old commercial, and new commercial systems. List the three categories of time delay rating for fuses and
state a use for each type of time-delay rated fuse.
9. Identify a clip-type and a post-type fuse holder
from illustrations and identify the connections used on a post-type fuse holder for power source and load
10. List the methods of checking for an open fuse, the items to check when replacing a fuse,
the safety precautions to be observed when checking and replacing fuses, and the conditions to be checked for when
conducting preventive maintenance on fuses.
11. Select a proper replacement and substitute fuse from a
listing of fuses.
12. List the five main components of a circuit breaker and the three types of circuit
breaker trip elements.
13. Describe the way in which each type of trip element reacts to excessive
14. Define the circuit breaker terms trip-free and nontrip-free and state one example for the use
of each of these types of circuit breakers.
15. List the three time delay ratings of circuit breakers.
16. Define selective tripping, state why it is used, and state the way in which the time delay ratings of circuit
breakers are used to design a selective tripping system.
17. Identify the factors used in selecting
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
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.
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.
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.
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.
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.
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.
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.
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.
Why are circuit protection devices necessary?
Q2. What are the three conditions that require circuit
Q3. What is a direct short?
Q4. What is an excessive current condition?
Q5. What is an
excessive heat condition?
CIRCUIT PROTECTION DEVICES
All of the conditions mentioned
are potentially dangerous and require the use of circuit protection devices. Circuit protection devices are used
to stop current flow or open the circuit. To do this, a circuit protection device must ALWAYS be connected in
series with the circuit it is protecting. If the protection
device is connected in parallel, current will simply flow around the protection device and
continue in the circuit.
A circuit protection device operates by opening and interrupting current to the
circuit. The opening of a protection device shows that something is wrong in the circuit and should be corrected
before the current is restored. When a problem exists and the protection device opens, the device should isolate
the faulty circuit from the other unaffected circuits, and should respond in time to protect unaffected components
in the faulty circuit. The protection device should NOT open during normal circuit operation.
The two types of
circuit protection devices discussed in this chapter are fuses and circuit breakers.
A fuse is the simplest circuit protection device. It derives its name from the Latin word "fusus," meaning "to
melt." Fuses have been used almost from the beginning of the use of electricity. The earliest type of fuse was
simply a bare wire between two connections. The wire was smaller than the conductor it was protecting and,
therefore, would melt before the conductor it was protecting was harmed. Some "copper fuse link" types are still
in use, but most fuses no longer use copper as the fuse element (the part of the fuse that melts). After changing
from copper to other metals, tubes or enclosures were developed to hold the melting metal. The enclosed fuse made
possible the addition of filler material, which helps to contain the arc that occurs when the element melts.
For many low power uses, the finer material is not required. A simple glass tube is used. The use of a glass tube
gives the added advantage of being able to see when a fuse is open. Fuses of this type are commonly found in
automobile lighting circuits.
Figure 2-1 shows several fuses and the symbols used on schematics.
Figure 2-1.—Typical fuses and schematic symbols.
While a fuse protects a circuit, it is destroyed in the
process of opening the circuit. Once the problem that caused the increased current or heat is corrected, a new
fuse must be placed in the circuit. A circuit protection device that can be used more than once solves the
problems of replacement fuses. Such a device is safe, reliable, and tamper proof. It is also resettable, so it can
be reused without replacing any parts. This device is called a CIRCUIT BREAKER because it breaks (opens) the
The first compact, workable circuit breaker was developed in 1923. It took 4 years to design a device
that would interrupt circuits of 5000 amperes at 120 volts ac or dc. In 1928 the first circuit breaker was placed
on the market. A typical circuit breaker and the appropriate schematic symbols are shown in figure 2-2.
Figure 2-2.—Typical circuit breaker and schematic symbols.
Q6. How are circuit protection devices connected to the circuit they are intended to protect and why are
they connected in this way?
Q7. What are the two types of circuit protection devices?
Q8. Label the schematic symbols shown in figure 2-3 below.
Figure 2-3.—Schematic symbols.
Fuses are manufactured in many shapes and sizes. In addition
to the copper fuse link already described, figure 2-1 shows other fuse types. While the variety of fuses may seem
confusing, there are basically only two types of fuses: plug-type fuses and cartridge fuses. Both types of fuses
use either a single wire or a ribbon as the fuse element (the part of the fuse that melts). The condition (good or
bad) of some fuses can be determined by visual inspection. The condition of other fuses can only be determined
with a meter. In the following discussion, visual inspection will be described. The use of meters to check fuses
will be discussed later in this chapter.
The plug-type fuse is
constructed so that it can be screwed into a socket mounted on a control panel or electrical distribution center.
The fuse link is enclosed in an insulated housing of porcelain or glass. The construction is arranged so the fuse
link is visible through a window of mica or glass. Figure 2-4 shows a typical plug-type fuse.
Figure 2-4.—Plug-type fuses
Figure 2-4, view A, sows a good plug-type fuse. Notice the construction and the fuse link. In figure
2-4, view B, the same type of fuse is shown after the fuse link has melted. Notice the window showing the
indication of this open fuse. The indication could be either of the ones shown in figure 2-4, view B.
plug-type fuse is used primarily in low-voltage, low-current circuits. The operating range is usually up to 150
volts and from 0.5 ampere to 30 amperes. This type of fuse is found in older circuit protection devices and is
rapidly being replaced by the circuit breaker.
fuse operates exactly like the plug-type fuse. In the cartridge fuse, the fuse link is enclosed in a tube of
insulating material with metal ferrules at each end (for contact with the fuse holder). Some common insulating
materials are glass, bakelite, or a fiber tube filled with insulating powder.
Figure 2-5 shows a glass-tube fuse. In figure 2-5, view A, notice the fuse link and the metal
ferrules. Figure 2-5, view B, shows a glass-tube fuse that is open. The open fuse link could appear either of the
ways shown in figure 2-5, view B.
Figure 2-5.—Cartridge-tube fuse.
Cartridge fuses are available in a variety of physical sizes and are used in many different circuit
applications. They can be rated at voltages up to 10,000 volts and have current ratings of from 1/500 (.002)
ampere to 800 amperes. Cartridge fuses may also be used to protect against excessive heat and open at temperatures
of from 165° F to 410°F (74°C to 210°C).
Q9. Label the fuses shown in figure 2-6 according to type.
Q10. Identify the open fuses shown in figure 2-6.
Figure 2-6.—Fuse recognition.
You can determine the physical size and type of a fuse by
looking at it, but you must know other things about a fuse to use it properly. Fuses are rated by current,
voltage, and time-delay characteristics to aid in the proper use of the fuse. To select the proper fuse, you must
understand the meaning of each of the fuse ratings.
current rating of a fuse is a value expressed in amperes that represents the current the fuse will allow without
opening. The current rating of a fuse is always indicated on the fuse.
To select the proper fuse, you must
know the normal operating current of the circuit. If you wish to protect the circuit from overloads (excessive
current), select a fuse rated at 125 percent of the normal circuit current. In other words, if a circuit has a
normal current of 10 amperes, a 12.5-ampere fuse will provide overload protection. If you wish to protect against
direct shorts only, select a fuse rated at 150 percent of the normal circuit current. In the case of a circuit
with 10 amperes of current, a 15 ampere fuse will protect against direct shorts, but will not be adequate
protection against excessive current.
The voltage rating of a fuse is NOT an indication of the voltage the
fuse is designed to withstand while carrying current. The voltage rating indicates the ability of the fuse to
quickly extinguish the arc after the fuse element melts and the maximum voltage the open fuse will block. In other
words, once the fuse has opened, any voltage less than the voltage rating of the fuse will not be able to "jump"
the gap of the fuse. Because of the way the voltage rating is used, it is a maximum rms voltage value. You must
always select a fuse with a voltage rating equal to or higher than the voltage in the circuit you wish to protect.
TIME DELAY RATING
There are many kinds of electrical and electronic circuits that
require protection. In some of these circuits, it is important to protect against temporary or transient current
increases. Sometimes the device being protected is very sensitive to current and cannot withstand an increase in
current. In these cases, a fuse must open very quickly if the current increases.
Some other circuits and
devices have a large current for short periods and a normal (smaller) current most of the time. An electric motor,
for instance, will draw a large current when the motor starts, but normal operating current for the motor will be
much smaller. A fuse used to protect a motor would have to allow for this large temporary current, but would open
if the large current were to continue.
Fuses are time delay rated to indicate the relationship between the
current through the fuse and the time it takes for the fuse to open. The three time delay ratings are delay,
standard, and fast.
A delay, or slow-blowing, fuse has a built-in delay that is activated when the
current through the fuse is greater than the current rating of the fuse. This fuse will allow temporary increases
in current (surge) without opening. Some delay fuses have two elements; this allows a very long time delay. If the
over- current condition continues, a delay fuse will open, but it will take longer to open than a standard or a
Delay fuses are used for circuits with high surge or starting currents, such as motors, solenoids,
Standard fuses have no built-in time delay. Also, they are not
designed to be very fast acting. Standard fuses are sometimes used to protect against direct shorts only. They may
be wired in series with a delay fuse to provide faster direct short protection. For example, in a circuit with a
1-ampere delay fuse, a 5-ampere standard fuse may be used in addition to the delay fuse to provide faster
protection against a direct short.
A standard fuse can be used in any circuit where surge currents are not
expected and a very fast opening of the fuse is not needed. A standard fuse opens faster than a delay fuse, but
slower than a fast rated fuse.
Standard fuses can be used for automobiles, lighting circuits, or
electrical power circuits.
Fast fuses are designed to open very quickly when the current through the
fuse exceeds the current
rating of the fuse. Fast fuses are used to protect devices that are very sensitive
to increased current. A fast fuse will open faster than a delay or standard fuse.
Fast fuses can be used to
protect delicate instruments or semiconductor devices.
Figure 2-7 will help you understand the differences
between delay, standard, and fast fuses. Figure
2-7 shows that, if a 1-ampere rated fuse had 2 amperes of
current through it, (200% of the rated value), a fast fuse would open in about .7 second, a standard rated fuse
would open in about 1.5 seconds, and a delay rated fuse would open in about 10 seconds. Notice that in each of the
fuses, the time required to open the fuse decreases as the rated current increases.
Figure 2-7.—Time required for fuse to open.
Q11. In what three ways are fuses rated?
Q12. What does the current rating of a fuse indicate?
Q13. What does the voltage rating of a fuse indicate?
Q14. What are the three time delay ratings
Q15. Give an example of a device you could protect with each type of time delay fuse.
IDENTIFICATION OF FUSES
Fuses have identifications printed on them. The
printing on the fuse will identify the physical size, the type of fuse, and the fuse ratings. There are four
different systems used to identify fuses. The systems are the old military designation, the new military
designation, the old commercial designation, and the new commercial designation. All four systems are presented
here, so you will be able to identify a fuse no matter which designation is printed on the fuse.
You may have
to replace an open fuse that is identified by one system with a good fuse that is identified by another system.
The designation systems are fairly simple to understand and cross-reference once you are familiar with them.
OLD MILITARY DESIGNATION
Figure 2-8 shows a fuse with the old military designation. The tables in the lower part of the figure
show the voltage and current codes used in this system. The upper portion of the figure is the explanation of the
old military designation. The numbers and letters in parentheses are the coding for the fuse shown in figure 2-8.
Figure 2-8.—Old type military fuse designation.
The old military designation always starts with "F," which stands for fuse. Next, the set of numbers
(02) indicates the style. Style means the construction and dimensions (size) of the fuse. Following the style is a
letter that represents the voltage rating of the fuse (G). The voltage code table in figure 2-8 shows each voltage
rating letter and its meaning in volts. In the example shown, the voltage ratings is G,
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics