Electrical Circuit Breakers
This is the first of a series of articles intended to supplement your knowledge beyond the immediate requirements of the NEC. The following is a listing of the topics covered in this article.
This article begins by defining circuit breakers then begins to delve into some of the nice to know details about the relation ships 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 the of first order importance. The final topic for this part is a brief listing of some of the more general requirements relating to circuit breakers. This series of articles will cover the types of circuit breakers that one will find in many types of facilities today. The beginning article begins to lay a foundation (a review for many) and progress to the higher amperage sizes introducing molded case, Insulated case, drawout ending with the most advanced of all the microprocessor based circuit protective devices that are becoming more common. I hope you find this information of value.
At the end of this newsletter, you will find means of providing Mr. Holt with your very valuable comments about this article. Please be sure to put your two cents worth in, as Mike and I think it is worth a lot more than that.
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 its 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 them selves. 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 many 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 (pass, but do not consume electrical power) are considered as being control devices, thus one may also say that a breaker is a control device, or a controller.
To aid in the understanding of a fine point, I offer this comparative example. A gas valve and a steam pressure relief valve are both controllers, one the fuel to the burners that aids in the development steam pressure, and the pressure relief valve that opens on excessive pressure. Both are controllers, one operating (the gas valve) the other safety (the relief valve).
Likewise in a parallel manner; we say that a circuit breaker can control and protect an electrical circuit. An electrical relay is an example of an operating control; it opens and closes the circuit. Circuit breakers are not designed as replacements for relays, operating controls.
There is as you may intuitively have 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 277 volt 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 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 very 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 frequent or repetitive manual breaking and making type control of electrical energy utilization equipment.
Currents To Break
For general consideration and our immediate purposes; the amounts of currents circuit breakers are required to open and close can be divided into the following three broad current amplitude groups.
The first and lowest being rated load or less. For example: a 60 amp low voltage molded case thermo-magnetic breaker must be able to open or close 48 amps (80% of its rating) or less.
Next up in current quantity, this same breaker must be able to open or close Overload currents. Overloads for our purposes can be understood by reference to the NEC requirements overload protection for motors. Thermal overloads are commonly sized for some 115% of the motors nameplate full load amps. A motor with a service factor of one, and 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 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 current ranging from no current flow, to as much as twenty (20) times or more its rated current. For a 100 amp breaker that could 2,000 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 all circuit breakers can open unlimited amounts of current, as will be covered later on, they can not.
Excessive and Over Currents
The National Electrical Code (NEC) defines excessive current as any current in excess of the rated current of the equipment or the ampacity of a conductor.
Overcurrent conditions are caused by defective conductors, equipment, or an excessive workload burden placed upon the utilization equipment. Fuses and circuit breakers provide a level of safety against overcurrent conditions in electrical circuits. We therefore routinely say that fuses and breakers are overcurrent protective devices (OCPD). That is they protect the circuits components from overcurrent. To minimize the length of this paper, only automatic circuit breaker type overcurrent protective devices will be covered.
A circuit breakers primary functions are to provide overcurrent protection, and Isolation from energized circuit components and un-energized circuit components. Breakers must perform these functions when properly applied without fail in all circumstances completely and safely, while protecting the electrical circuit against overcurrent induced damage between normal rated current and the breaking capacity of the breaker called Ampere Interrupting Capacity (AIC). Now that is a big job and an important job.
Modern breakers routinely do their job day in and day out
with very little maintenance. Like all things that are made by man, they do have limits
and they do fail. Hopefully this paper will help you to better understand and appreciate
the task performed by those little black boxes.
Current and Temperature
The movement of electrons (electricity) in a conductor produces a rise in the temperature of the conductor. Excessive temperature rise (caused by an excessive amount of electron collisions with base material atoms) can result in the melting of the wires material (copper), 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 burned up.
Our first priority therefore is the temperature of the conductors electrical insulating materials. Differing types of insulating materials have different maximum design operating temperatures.
Circuit Breakers As High Temperature
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 the possible damage to the 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 wires insulation materials.
Ampacities Of Electrical Conductors
Just how hot an electrically insulated wire can get before its insulation melts, suffers damage or a decrease in electrical dielectric strength (the ability to perform as an electrical insulator) and other physical features 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 conductors 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 span of an insulated conductor. For now we can book a safe bet that voltage spikes, vibration, environmental factors such as temperature, 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 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.
Ampacities Of Electrical Conductors
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 an unintended contact with each other; a phase-to-phase short circuit has 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 not multiples of rated load current) presents its self in a very short period of time and will typically be multiples of the loads normal operating current.
Breakers are tested to determine their ability to clear a short-circuit without damage to them selfs. With a phase-to-phase short, the breaker will be required to open the circuit at the circuits rated phase-to-phase voltage. This would be the case independent of the system being grounded or un-grounded, that is either wye or delta solidly grounded or un-grounded or resistance 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 words ground fault means that there is a defect in the wires 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 wires insulation materials.
Ground fault type circuit (GFCI) breakers 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 in this paper. A ground fault can present 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 of low impedance type consuming a significant amount of electrical energy or as an arcing type fault with little total energy consumed. Some breakers are not designed to respond to arcing type shorts to ground.
Most breakers today will respond to a short to ground that is of the low impedance type as the current levels are normally multiples of normal load currents to which the circuit breaker has been manufactured to sense and respond to.
When installed in a grounded type system, such as a center grounded wye system, only one half of the systems 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 systems ground and the circuits 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.
When a loose connection 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 produces an electric arc. Arcing type faults are the most difficult to locate (due to conductor concealment in conduit or inside enclosed cavities of walls) 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). They may last for only a fraction of a second and then cool down and not flow current, or heat up again and produce an arc between the two conducting surfaces.
During the A-C cycle there are two times that the supply circuit voltage goes to zero volts, there are two times when the circuits electromotive pressure is zero and an arc cannot be produced. This zero volts time helps to increase the faulted circuits impedance. The higher impedance makes it more difficult for the arc to re-establish its self again. These types of faults produce heat in a very localized area, this is how they can start a fire and not trip a common thermal-magnetic circuit breaker, their energy level is so low and they last for such a short time, that they are typically not detected by a circuit breaker or a fuse.
In response to this unique type of circuit defect, an entirely 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 nature (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 liken 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.
Occasionally a short will develop that has a firm solid 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, that it was not wiggling around. A bolted fault offers a lesser impedance to the flow of current than does an arcing type fault.
A loose connection type of fault may produce enough heat as to melt or plasticize the conductors material and having cooled enough to then produce a joint so firm and secure as to be comparable to a welded fault. Then we would call it a bolted fault. Circuit breakers are typically calibrated as to be capable of responding to a bolted type fault as they result in sufficient current flow to cause either the thermal or the magnetic trip elements of open the circuit.
Safety First, Always First
The exact nature of electricity, that is it can not 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. Proper 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. 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.
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 more permanently embarrassing.
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
There are specific product type requirements for 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 informaion relating to engineering type testing and things that the circuit breaker manufacture must know about are not covered.
In the next part of this article we will review additional information covering 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 and some information about magnetic trip elements.
Remember Work Smarter, Not Harder
Copyright © 2002 Mike Holt Enterprises,Inc.