06-11-2012, 02:17 PM
ELECTRICAL CIRCUIT BREAKERS
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Electrical Circuit Breakers
This is the first in a 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.
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.
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.
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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.
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.
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-tophase
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.