Module 3—Introduction to Circuit Protection, Control, and Measurement
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 1-21 to 1-30
1-31 to 1-40
, 1-41 to 1-50
1-51 to 1-60
, 1-61 to 1-70
1-71 to 1-73
2-1 to 2-10
, 2-11 to 2-20
1-21 to 2-30
, 2-31 to 2-40
2-41 to 2-42
, 3-1 to 3-10
3-11 to 3-20
, 3-21 to 3-30
33-31 to 3-39
AI-1 to AI-3
,, AII-1 to AII-2
AIII-1 to AIII-10
Figure 2-16.—Circuit breaker components.
The FRAME provides an insulated housing and is used to mount the circuit breaker
2-17). The frame determines the physical size of the circuit breaker and the maximum allowable
voltage and current.
The OPERATING MECHANISM provides a means of opening and closing the
breaker contacts (turning, the circuit ON and OFF). The toggle mechanism shown in figure 2-17 is the quick-make,
quick-break type, which means the contacts snap open or closed quickly, regardless of how fast the handle is
moved. In addition to indicating whether the breaker is ON or OFF, the operating mechanism handle indicates when
the breaker has opened automatically (tripped) by moving to a position between
ON and OFF. To reset the circuit
breaker, the handle must first be moved to the OFF position, and then to the ON position.
Figure 2-17.—Circuit breaker construction.
The ARC EXTINGUISHER confines, divides, and extinguishes the arc drawn between contacts each
time the circuit breaker interrupts current. The arc extinguisher is actually a series of contacts that open
gradually, dividing the arc and making it easier to confine and extinguish. This is shown in figure 2-18. Arc
extinguishers are generally used in circuit breakers that control a large amount of power, such as those found in
power distribution panels. Small power circuit breakers (such as those found in lighting panels) may not have arc
Figure 2-18.—Arc extinguisher action.
TERMINAL CONNECTORS are used to connect the circuit breaker to the power source and the load.
They are electrically connected to the contacts of the circuit breaker and provide the means of connecting the
circuit breaker to the circuit.
The TRIP ELEMENT is the part of the circuit breaker that
senses the overload condition and causes the circuit breaker to trip or break the circuit. This chapter will cover
the thermal, magnetic, and thermal-
magnetic trip units used by most circuit breakers. (Some circuit breakers make use of solid-state
trip units using current transformers and solid-state circuitry.)
THERMAL TRIP ELEMENT
A thermal trip element circuit breaker uses a bimetallic element that is heated by the load
current. The bimetallic element is made from strips of two different metals bonded together. The metals expand at
different rates as they are heated. This causes the bimetallic element to bend as it is heated by the current
going to the load. Figure 2-19 shows how this can be used to trip the circuit breaker.
Figure 2-19.—Thermal trip element action: A. Trip element with normal current; B. Contacts open.
Figure 2-19, view A, shows the trip element with normal current. The bimetallic element is not heated
excessively and does not bend. If the current increases (or the temperature around the circuit breaker increases),
the bimetallic element bends, pushes against the trip bar, and releases the latch. Then, the contacts open, as
shown in figure 2-19, view B.
The amount of time it takes for the bimetallic element to bend and trip the
circuit breaker depends on the amount the element is heated. A large overload will heat the element quickly. A
small overload will require a longer time to trip the circuit breaker.
MAGNETIC TRIP ELEMENT
A magnetic trip element circuit breaker uses an electromagnet in series with the circuit load as in figure
2-20. With normal current, the electromagnet will not have enough attraction to the trip bar to move it, and the
contacts will remain closed as shown in figure 2-20, view A. The strength of the magnetic field of the
electromagnet increases as current through the coil increases. As soon as the current in the circuit becomes large
enough, the trip bar is pulled toward the magnetic element (electromagnet), the contacts are opened, and the
current stops, as shown in figure 2-20, view B.
Figure 2-20.—Magnetic trip element action; Closed contacts
The amount of current needed to trip the circuit breaker depends on the size of the gap between the trip
bar and the magnetic element. On some circuit breakers, this gap (and therefore the trip current) is adjustable.
THERMAL-MAGNETIC TRIP ELEMENT
The thermal trip element circuit
breaker, like a delay fuse, will protect a circuit against a small overload that continues for a long time. The
larger the overload, the faster the circuit breaker will trip.
The thermal element will also protect the
circuit against temperature increases. A magnetic circuit breaker will trip instantly when the preset current is
present. In some applications, both types of protection are desired. Rather than use two separate circuit
breakers, a single trip element combining thermal and magnetic trip elements is used. A thermal-magnetic trip
element is shown in figure 2-21.
Figure 2-21.—Thermal-magnetic element action
In the thermal-magnetic trip element circuit breaker, a magnetic element (electromagnet) is connected in
series with the circuit load, and a bimetallic element is heated by the load current. With normal circuit current,
the bimetallic element does not bend, and the magnetic element does not attract the trip bar, as shown in figure
2-21, view A.
If the temperature or current increases over a sustained period of time, the bimetallic element
will bend, push the trip bar and release the latch. The circuit breaker will trip as shown in figure 2-21, view B.
If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar, release the
latch, and the circuit breaker will trip, as shown in figure 2-21, view C. (This circuit breaker has tripped even
though the thermal element has not had time to react to the increased current.)
Q30. What are the five
main components of a circuit breaker?
Q31. What are the three types of circuit breaker trip elements?
Q32. How does each type of trip element react to an overload?
Circuit breakers are classified as being trip free or non-trip free. A trip-free
circuit breaker is a circuit breaker that will trip (open) even if the operating mechanism (ON-OFF switch) is held
in the ON position. A non-trip-free circuit breaker can be reset and/or held ON even if an overload or excessive
heat condition is present. In other words, a non-trip-free circuit breaker can be bypassed by holding the
operating mechanism ON.
Trip-free circuit breakers are used on circuits that cannot tolerate overloads and on nonemergency circuits.
Examples of these are precision or current sensitive circuits, nonemergency lighting circuits, and nonessential
equipment circuits. Non-trip-free circuit breakers are used for circuits that are essential for operations.
Examples of these circuits are emergency lighting, required control circuits, and essential equipment circuits.
TIME DELAY RATINGS
Circuit breakers, like fuses, are rated by the
amount of time delay. In circuit breakers the ratings are instantaneous, short time delay, and longtime delay. The
delay times of circuit breakers can be used to provide for SELECTIVE TRIPPING.
tripping is used to cause the circuit breaker closest to the faulty circuit to trip. This will remove power from
the faulty circuit without affecting other, non-faulty circuits. Figure 2-22 should help you understand selective
Figure 2-22.—Use of circuit breakers in a power distribution system.
Figure 2-22 shows a power distribution system using circuit breakers for protection. Circuit breaker
1 (CB1) has the entire current for all seven loads through it. CB2 feeds loads 1, 2, 3, and 4 (through CB4, CB5,
CB6, and CB7), and CB3 feeds loads 5, 6, and 7 (through CB8, CB9, and CB10). If all the circuit breakers were
rated with the same time delay, an overload on load 5 could cause CB1, CB3, and CB8 to trip. This would remove
power from all seven loads, even though load 5 was the only circuit with an overload.
Selective tripping would
have CB1 rated as long time delay, CB2 and CB3 rated as short time delay, and CB4 through CB10 rated as
instantaneous. With this arrangement, if load 5 had an overload, only CB8 would trip. CB8 would remove the power
from load 5 before CB1 or CB3 could react to the overload. In this way, only load 5 would be affected and the
other circuits would continue to operate.
PHYSICAL TYPES OF CIRCUIT BREAKERS
All the circuit breakers presented so far in this chapter have been physically large, designed to control large
amounts of power, and used a type of toggle operating mechanism. Not all circuit breakers are of this type. The
circuit breaker in figure 2-23 is physically large and controls large amounts of power; but the operating
mechanism is not a toggle. Except for the difference in the operating mechanism, this circuit breaker is identical
to the circuit breakers already presented.
Figure 2-23.—Circuit breaker with an operating handle.
Circuit breakers used for low power protection, such as 28-volt dc, 30 amperes, can be physically small.
With low power use, arc extinguishers are not required, and so are not used in the construction of these circuit
breakers. Figure 2-24 shows a low power circuit breaker of the push-button or push-pull type. This circuit breaker
has a thermal trip element (the bimetallic disk) and is non-trip-free. The push button is the operating mechanism
of this circuit breaker.
Figure 2-24.—Push-button circuit breaker.
You will find other physical types of circuit breakers as you work with electrical circuits. They are
found in power distribution systems, lighting panels, and even on individual pieces of equipment. Regardless of
the physical size and the amount of power through the circuit breaker, the basic operating principles of circuit
Q33. What is a trip-free circuit breaker?
Q34. What is a non-trip-free circuit
Q35. Where should you use a trip-free circuit breaker?
Q36. Where should you use a
non-trip-free circuit breaker?
The magnetic trip element makes use of a magnetic element (electromagnet).
If current reaches a preset quantity, the magnetic element attracts the trip bar and releases the latch.
thermal-magnetic trip element combines the actions of the bimetallic and magnetic elements in a single trip
element. If either the bimetal element or the magnetic element reacts, the circuit breaker will trip.
Q37. What are the three time delay ratings for circuit breakers?
Q38. What is selective tripping and why
is it used?
Q39. If the power distribution system shown in figure 2-22 uses selective tripping, what is the time delay
rating for each of the circuit breakers shown?
Q40. What factors are used to select a circuit breaker?
Q41. What type of circuit breaker is used on a multimeter?
CIRCUIT BREAKER MAINTENANCE
Circuit breakers require careful inspection and periodic cleaning. Before you attempt to work on circuit
breakers, check the applicable technical manual carefully. When you work on shipboard circuit breakers, the
approval of the electrical or engineering officer must be obtained before starting work. Be certain to remove all
power to the circuit breaker before you work on it. Tag the switch that removes the power to the circuit breaker
to ensure that power is not applied while you are working.
Once approval has been obtained, the incoming power
has been removed, the switch tagged, and you have checked the technical manual, you may begin to check the circuit
breaker. Manually operate the circuit breaker several times to be sure the operating mechanism works smoothly.
Inspect the contacts for
pitting caused by arcing or corrosion. If pitting is present, smooth the contacts with a fine file
or number 00 sandpaper. Be certain the contacts make proper contact when the operating mechanism is ON.
the connections at the terminals to be certain the terminals and wiring are tight and free from corrosion. Check
all mounting hardware for tightness and wear. Check all components for wear. Clean the circuit breaker completely.
When you have finished working on the circuit breaker, restore power and remove the tag from the switch that
applies power to the circuit.
Q42. What steps are to be taken before beginning work on a circuit
breaker? Q43. What items are you to check when working on a circuit breaker?
This chapter has provided the information to enable you to have a basic understanding of circuit protection
devices. The following is a summary of the main points in this chapter.
CIRCUIT PROTECTION DEVICES
are needed to protect personnel and circuits from hazardous conditions. The hazardous conditions can be caused by
a direct short, excessive current, or excessive heat. Circuit protection devices are always connected in series
with the circuit being protected.
A DIRECT SHORT is a condition 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
EXCESSIVE CURRENT describes a condition that is not a direct short but in which
circuit current increases beyond the designed current carrying ability of the circuit.
HEAT describes a condition in which the heat in or around a circuit increases to a higher than normal
CIRCUIT BREAKERS are the two types of circuit protection devices discussed in this chapter.
PLUG-TYPE FUSES are used in low-voltage, low-current circuits. This type fuse is rapidly
being replaced by the circuit breaker.
CARTRIDGE FUSES are available in a wide range of
physical sizes and voltage and current ratings. This type fuse is the most commonly used fuse.
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