Electrical
Circuit Breakers
By: L. W. Brittian, Mechanical-Electrical
Instructor
This is the first in a 7 part series of articles intended
to supplement your knowledge beyond the immediate requirements
of the NEC. The series will cover the types of circuit breakers
that are found in various types of facilities today.
The beginning article lays a foundation (a review for many)
and progresses to introducing molded case, insulated case,
and drawout types with the series ending with the most advanced
of all, microprocessor-based circuit protective devices.
The following topics are covered in this the first part
of the series:
• CIRCUIT BREAKERS
DEFINED
• CIRCUIT BREAKERS
AS SWITCHES
• CURRENT LEVELS TO BE
BROKEN
• OVER-CURRENTS
• CURRENT AND TEMPERATURE
• CIRCUIT BREAKERS
AS HIGH TEMPERATURE LIMIT SWITCHES
• AMPACITIES OF ELECTRICAL
CONDUCTORS
• SHORT CIRCUITS
• SHORTS TO GROUND
• ARCING FAULTS
• BOLTED FAULTS
• SAFETY FIRST, ALWAYS FIRST
• NEC REQUIREMENTS FOR CIRCUIT
BREAKERS
This part of the series begins by defining circuit breakers
then delves into some of the nice to know details about
the relationships of current, temperature, and ampacities
of conductors. The subject of faults and the various types
of faults is then covered. The topic of safety, while next
to last, is highlighted as being of first order importance.
The final topic for this part is a brief listing of some
of the general NEC requirements relating to circuit breakers.
To minimize the length of this paper, only automatic circuit
breaker type overcurrent protective devices are covered.
Restated, fuses, and motor starter type overload relays
are not covered.
Initially those electrical giants, Edison and Tesla; had
only lead wire fuses to protect themselves and their equipment
from overcurrents. Gazing into that fog that is the future,
perhaps we will see these devices become increasingly more
intelligent, and general circuit protective devices taking
on additional task as system monitors.
CIRCUIT BREAKERS DEFINED
The American National Standards Institute (ANSI) defines
a circuit breaker as: “A mechanical switching device,
capable of making, carrying and breaking currents under
normal circuit conditions. Also capable of making and carrying
for a specified time and breaking currents under specified
abnormal circuit conditions, such as those of a short circuit.”
The NEC defines a circuit breaker as “a device designed
to open and close a circuit by non-automatic means, and
to open the circuit automatically on a predetermined overcurrent
without damage to itself when properly applied within it’s
rating.” While the ANSI and the NEC definitions describe
the same family of devices, they do have some differences;
the same is true with the actual circuit breakers themselves.
They are much the same in general terms; however, there
are a number of significant differences between the many
types of electrical circuit breakers installed in various
types of facilities today.
CIRCUIT BREAKERS
AS SWITCHES
Both the ANSI and the NEC definitions acknowledge the potential
for the legitimate use of circuit breakers as switches.
Switches (devices that pass but do not consume electrical
energy) are considered as being control devices; thus one
may also say that a breaker is a control device, or a controller.
A circuit breaker can control and protect an electrical
circuit and people operating the utilization equipment.
An electrical relay is an example of an operating control;
it opens and closes the circuit. Circuit breakers are not
designed as replacements for operating controls such as
relays, contactors, or motor starters.
There is, as you may have intuitively anticipated, an exception.
Some circuit breakers are manufactured for use in a specific
type of application. When a circuit breaker is designed
to also be routinely used as an on-off switch to control
120 or 277volt florescent luminaires they are marked SWD,
for switch duty. This does not mean that a switch duty breaker
can be used to manually control a traffic signal light where
it will be cycled on and off 1,000 or more times per day.
The point is the listing for switch duty (SWD) does not
mean a circuit breaker can be used as a high frequency cycling
operating control, such as a relay that has a life span
rated in tens, if not hundreds of thousands of duty cycles.
While circuit breakers can be legitimately and safely used
as switches, the frequency and duration of such use is limited.
Routinely circuit breakers are manually operated for service-maintenance
and repair type activities. With the preceding enhancing
our understanding, we can say that circuit breakers can
legitimately be used as switches; generally they are not
intended for prolonged repetitive manual breaking and making
type control of electrical energy utilization equipment.
CURRENT LEVELS TO BE
BROKEN
For general consideration, and our immediate purposes,
the amounts of current circuit breakers are required to
open can be divided into the following three broad current
amplitude groups.
The first and lowest is rated load or less. For example:
a 60 amp low voltage molded case thermal-magnetic breaker
must be able to open or close at 48 amps (80% of its rating)
or less.
Next up in current quantity, this same breaker must be
able to open overload level currents. Overloads for our
purposes can be understood by reference to the NEC requirements
for overload protection for motors. Thermal overloads are
commonly sized for some 115% of the motor’s nameplate
full load amps. A motor with a service factor of one, having
a rated load of 10 amps would be overloaded when pulling
11.5 amps or more. Overload currents can for our immediate
purposes be considered to be percentages increases above
rated normal load current.
The third and highest current level grouping is short circuit
currents. Short circuit (fault) currents can be considered
as being fifteen (15) or more times normal rated load currents.
In summation, circuit breakers may be called upon to open
or close a circuit within a range of from no current flow
to as much as fifteen (15) times or more its rated current.
For a 100 amp breaker that could 1,500 amps or more.
As will be covered later, this high value of short circuit
current is routinely exceeded by circuit breakers today.
This should not be considered as implying that circuit breakers
can open unlimited amounts of current. As will be covered
later on, they can not.
OVER-CURRENTS
The National Electrical Code (NEC) defines overcurrent
as “any current in excess of the rated current of
the equipment or the ampacity of a conductor.“
Overcurrent (or excessive current) conditions are caused
by defective conductor insulation, defective equipment,
or an excessive workload burden placed upon the utilization
equipment and its electrical circuit. Fuses and circuit
breakers provide a level of safety against overcurrent conditions
in electrical circuits. We therefore routinely say that
fuses and circuit breakers are overcurrent protective devices
(OCPD). That is, they protect the circuit’s components
from too much current.
CURRENT AND TEMPERATURE
The movement of electrons (electricity) in a conductor produces
a rise in the temperature of the conductor’s material
and its outer layer of electrical insulation. Excessive
temperature rise (caused by an excessive amount of electron
collisions with base material atoms) can result in the melting
of the wires material (assumed to be copper by the NEC).
If it is allowed to rise as high as 1,980 degrees F. For
a point of reference, the NEC limits the operating temperature
of XHHW type conductor insulation to no more than 194 degrees
F. Thus it can be understood that long before the copper
wire will begin to melt, the wires insulation material will
have melted, and perhaps even burned up.
Our first priority, therefore, is the temperature of the
conductor’s electrical insulating materials. Different
types of insulating materials have different maximum design
operating temperatures.
CIRCUIT BREAKERS
AS HIGH TEMPERATURE LIMIT SWITCHES
Electrical energy is transported throughout an electrical
circuit by the conductive path provided by electrically
insulated wires. The material that performs the insulation
function in the circuit has a high temperature limit far
below that of the copper wire. Circuit breakers are routinely
sized to limit thermal energy related damage to the electrical
insulation material and not the copper wire. This being
the case we can say that a circuit breaker limits the temperature
of the connected-protected wire’s insulation materials.
AMPACITIES OF ELECTRICAL
CONDUCTORS
Just how hot an electrically insulated wire can get before
its insulation melts, suffers damage, or has a decrease
in electrical dielectric strength (the ability to perform
as an electrical insulator) are well-known facts. The various
types of materials used as electrical insulation have been
tested and the results listed in what are called ampacity
tables in the NEC in article 310.16.
How long an installed conductor’s electrical insulation
material will last without overload is yet another question.
Research is underway to determine the life of an installed
insulated conductor. No doubt when completed, it will point
to many factors that have a negative impact upon the inservice
life of an insulated conductor. For now we can book a safe
bet that voltage spikes, vibration, environmental factors
such as temperature, dust both electrically and thermally
conductive and non-conductive types, UV light, aggressive
vapors and fluids, and relative humidity will all be proven
to shorten to some degree the life of modern plastic type
electrical insulation materials.
I suspect that many of these same factors also have a negative
impact upon circuit breakers. I do not know of any research
that defines the service life of circuit breakers. Nor am
I aware of any research underway seeking to determine the
service life of circuit breakers. Considering the importance
of the safety provided to people and property that circuit
breakers provide, it is a bit puzzling as to why such research
has not already been undertaken.
For many years various types of materials have been used
as electrical insulators. Today conductors are made using
material for outer jacketing and for filling in the gaps
(indices) between bundled round conductors. These materials
may or may not be considered to be electrical insulators.
Some medium and high voltage cables are made using materials
that are considered to be conductive, or semi-conductive.
Some low voltage instrumentation, control, and signal usage
type cables are made with a layer of conductive material
that acts as a shield to drain off static electrical charges.
SHORT CIRCUITS
A short circuit is an unintended path through which current
can flow. Any time current flows in a path that is not the
normal path, we say that the circuit is shorted. Shorts
are further defined by the nature of the shorted connection.
A direct short is commonly a phase-to-phase short, which
is when two hot (un-grounded) wires make unintended contact
with each other; a phase-to-phase short circuit has thus
been created.
A circuit breaker must be able to respond to a short circuit,
which can present a large current flow in a short period
of time. A short circuit unlike an overload (typically a
percentage increase of rated load current) presents itself
in a very short period of time and will typically be multiples
of the load’s normal operating current.
Breakers are tested to determine their ability to clear
a short circuit without damage to themselves. With a phase-to-phase
short, the breaker will be required to open the circuit
at the circuit’s rated phase-to-phase voltage. This
would be the case independent of whether the system being
grounded or un-grounded is either wye or delta solidly grounded
or un-grounded or resistance (impedance) grounded.
SHORTS TO GROUND
When an insulated hot wire (un-grounded) un-intentionally
makes electrical contact with an electrically conductive-grounded
object, a ground fault is created. The term ground fault
means that there is a defect in the wire’s electrical
insulation; and the faulted wire has shorted to ground.
Many times a phase-to-phase short will develop into a ground
fault, and the other way around. Either a phase-to-phase
short can produce a ground fault, or a ground fault can
produce a phase-to-phase short. The fault can be in one,
two, or three wire’s insulation materials.
Ground fault type circuit breakers (GFCI) will not be covered
in this paper. The short circuit, overload current limiting
nature of these types of breakers, however, will be covered.
It is only the ground fault or residual current feature
of GFCI type breakers that is not covered. A ground fault
can develop a current flow that is limited only by the impedance
of the circuit and the capacity of the energy source supplying
the faulted circuit. Ground faults can occur rapidly and
can be either a low impedance type, developing a significant
amount of electrical energy, or as an arcing type fault
with little total energy consumed. The common breaker is
not designed or calibrated to respond to arcing type shorts
to ground.
Circuit breakers typically will respond to a short to ground
that is of the low impedance type as current levels are
typically multiples of load currents to which the circuit
breaker has been manufactured to sense and then respond
to.
When installed in a grounded system, such as a center grounded
wye system, only about one half of the system’s phase-to-phase
voltage will be broken by the breaker on a ground type fault.
With an un-grounded type system a ground fault on the first
phase-to-ground connection does not result in any current
flow as the system is not referenced to ground. Yet should
a second ground fault develop, the breaker will be required
to break phase-to-phase system rated voltage.
With resistance grounded systems, the impedance of the
supply system’s ground and the circuit’s ground
fault combine to determine the amount of current drawn.
Further, more detailed pursuit of understanding of the various
electrical system grounding (earthing) methods used in America
is beyond the limiting scope of this paper. I suggest that
you read more on grounding, as it is a subject that seems
to always open up disagreements. Mr. Holt’s book titled
Grounding and Bonding, NEC 250, (product # O2NCT2) is just
out and covers this topic much better than I have done.
ARCING FAULTS
When a loose connection (a gap) is made in the faulted
circuit so loose that the current flow is non-continuous,
it is called an arcing or arc fault. This type of circuit
defect is much like a welder using a welding electrode to
produce an electric arc. Arcing type faults are the most
difficult to locate (due to conductor concealment in conduit
or inside of walls and their non-continuous nature) and
can be the cause of fires. This type of defect is the opposite
of a bolted fault; the circuit impedance is higher and the
connection is very irregular (high frequency). The current
flows for only a fraction of a second and then cools down
and may not flow current, or heat up again and produce an
arc across the gap between the two conducting surfaces.
During the AC cycle there are two times that the supply
circuit voltage goes to zero volts; there are two times
when the circuit’s electromotive pressure is zero
and an arc cannot be produced. This zero volts time helps
to increase the faulted circuit’s impedance. This
higher impedance makes it more difficult for the arc to
re-establish itself again. These types of faults produce
heat in a very small area. They can start a fire and not
trip a common thermal-magnetic circuit breaker because their
energy level is so low and they last for such a short time
that they are typically not responded to by a common circuit
breaker.
In response to this unique type of circuit defect, a new
family of circuit protectors called arc fault circuit interrupter
(AFCI) type circuit breakers has been under development
over for the last ten years. Because of the unique components
(microcomputers) of these devices they will not be covered
in this short paper. The common circuit breaker will not
respond to the development of an arcing type fault due to
the low total amount of thermal energy developed by the
arc and the very high frequency of the arc. Perhaps one
can think of an arcing fault as an embryonic electrical
circuit defect, unlike the defect that has fully developed
and matured into an adult electrical fault such as a bolted
fault. Experience shows that on occasion a short circuit
will clear itself before, or during the operation of the
OCPD. That is, it will develop into an open circuit.
BOLTED FAULTS
Occasionally a shorted circuit will evolve that has such
a firm connection (to either a grounded conductive object
or another hot wire) that it is said to be a bolted fault.
We are saying that it was not a loose connection, it was
not wiggling around. A bolted fault offers less impedance
to the flow of current than does an arcing type fault.
A loose connection type of fault may produce enough heat
to melt or plasticize the conductor’s conductive material
and cool enough to then produce a joint so firm and secure
as to be comparable to a welded joint. Then we would call
it a bolted fault. Circuit breakers are typically calibrated
to be capable of responding to a bolted type fault. This
is because a bolted type fault produces sufficient current
flow to cause either the thermal (after some intentional
delay) or the magnetic (non-delay, but only if sufficient
current flow is produced) trip elements to open the circuit.
SAFETY FIRST, ALWAYS
FIRST
The exact nature of electricity-- that it cannot be detected
with the eyes, ears, or the nose, yet if it is touched,
it can kill-- must be remembered at all times. Circuit breakers
are very reliable components of an electrical system; however,
they are man made and are subject to becoming defective.
Proper lock-out-tag-out procedures must be followed when
working on electrical circuits above 50 volts. Personnel
protective equipment must be in serviceable condition and
properly worn. Safety is a requirement, not an option, of
every electrical task, large or small, be it routine or
emergency in nature. Always use the three-step method when
checking for voltage.
Take good care of your electrical test meters, having them
checked at least every three years for insulation strength
and for calibration as listed in the instruction booklet,
or once a year otherwise. While a switch may visually indicate
that the contacts have opened, a meter must be used to confirm
that no voltage remains in the equipment to be worked on.
Many times more than one source of power is provided to
a machine. Some electrical circuits contain motor starting/running
or power factor correction capacitors that may still be
charged even after power has been removed from the circuit.
When working with others do not assume that they know how
to operate your meter, and do not assume that you know how
to operate their meter. Take the time necessary to learn
how to properly operate the test instruments that you will
be required to use. I know that it is temporarily embarrassing
to admit that we do not know something, but being found
dead on the job I believe to be permanently embarrassing.
If you would like to learn more about the use of multi-meters
and such, I suggest that you see if you can get your hands
on a copy of the book titled “Test Equipment,”
published by Delmar, ISBN: 0-8273-4923-8.
NEC REQUIREMENTS FOR CIRCUIT
BREAKERS
The National Electrical code has several requirements for
circuit breakers (overcurrent protective devices). The following
is a listing of some of them. Others can be found in the
various specific articles, such as 430 covering motors.
• Main, feeder, and branch circuit breakers must be
installed in a readily accessible location.
• A working space must be as wide as the equipment,
or at least 30 inches wide and three feet deep, or deep
enough to allow any doors to be opened at a 90 degree angle
in front of the equipment housing a breaker.
• When the operating handle is in the up position
its centerline be not more than 6 ft. 7 inches above the
floor or working platform.
• It must be installed so that it is secure on its
mounting surface.
• When installed the up position must be on and when
the operating handle is moved down this must be the off
position.
• The breaker must be clearly marked as to its off
and on positions.
• The breaker must be clearly marked, such that after
installation the amperage rating is clearly visible. (There
are some exceptions See 240.83)
• The operating handle must be of a trip free design,
so that it cannot be blocked or kept from tripping due to
some type of obstruction keeping the operating handle from
moving to the tripped position.
• When wires are connected to a breaker they must
be properly torqued to the breaker’s termination points.
• The NEC has specific requirements for both AFC and
GFCI type circuit protectors that are mostly applicable
based upon specific locations.
There are specific product type requirements for circuit
breakers to be listed by a nationally recognized testing
lab (NRTL) such as UL, that we will not be covering in this
short paper. That means detailed information relating to
engineering type testing and things that the circuit breaker
manufacture must know about are not covered. This is not
an “everything you ever wanted to know about circuit
breakers” encyclopedic type article.
In the next part of this article the following topics will
be covered.
• FUNCTIONS
• TYPES
• COMPONENTS
• VOLTAGE RATINGS
• AMPERE Ratings
• AMPERE INTERRUPTING CURRENT (AIC)
• TESTING -LISTING OF CIRCUIT BREAKERS
• NOT ALL “BREAKERS” ARE RATED THE SAME
• THE ELECTRICAL ARC
• EFFECTS OF CURRENT FLOW
• THERMAL ENERGY
• THERMAL TRIP ELEMENT
• MAGNETIC TRIP ELEMENTS
• Hydraulic-Magnetic Trip Elements
If you have any questions or comments, please send me an
E-mail.
Remember Work Smarter, Not Harder
L. W. Brittian
Mechanical-Electrical Instructor
lwbrittian@hot1.net
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