• FUNCTIONS
• TYPES
• COMPONENTS
• VOLTAGE RATINGS
• AMPERE RATINGS
• AMPERE INTERRUPTING CAPACITY (AIC)
• TESTING-LISTING OF CIRCUIT BREAKERS
CIRCUIT BREAKER FUNCTIONS
Later in this article we will cover several more specific features of some specialized types of circuit breakers, but for now let us begin by saying that a circuit breaker’s main functions are:
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Sense the current flowing in the circuit
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Measure the current flowing in the circuit
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Compare the measured current level to its pre-set trip point
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Act within a predetermined time period by opening the circuit as quickly as possible to limit the amount of energy that is allowed to flow after the trip point has been reached.
In a condensing manner, we can say that a circuit breaker functions to provide overcurrent protection, and isolation from energized and un-energized circuit components. As safety devices, these functions must be performed, without failure, without damage to the protected circuit’s components, from no current through the breaker’s rated 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.
CIRCUIT BREAKER TYPES
Medimum and low voltage circuit breakers are commonly separated into the following groups based upon the type of material used to make the frames or cases:
• Molded case, (MCCB) the most common low voltage type
• Insulated case, (ICCB) the intermediate voltage and amperage sizes
• Metal clad, the higher in voltage (medium) and amperage rating
CIRCUIT BREAKER COMPONENTS
The five basic components of a circuit breaker are:
• Frame or case made of metal or some type of electrical insulation
• Electrical contacts
• Arc extinguishing assembly
• Operating mechanism
• Trip unit, containing either a thermal element, a magnetic element or both
CIRCUIT BREAKER VOLTAGE RATINGS
Low voltage (under 600 volts) circuit breakers are commonly rated for, 120 volts, 240 volt, 277 or 480 volts AC. Some breakers are rated for used in DC circuits, while others are rated for use in either AC or DC circuits.
Single pole circuit breakers are rated for a voltage potential between the one hot wire and a grounded surface. Breakers that are intended to be part of a two or three phase circuit are rated for a voltage potential from opposite potential, to opposite potential, or phase-to-phase. You must not use two single pole 240 volt breakers to control a 480 volt circuit, but two single pole breakers rated 277 volts could be used to control a 240 volt circuit.
When improperly applied outside of its rating, a breaker may not be able to extinguish the arc when attempting to clear a fault. Some breakers have what is called a slash (/) rating such as 120/240 or 277/480. Breakers that are slash rated should not be used on un-grounded systems, as they have not been tested for safe operation on these types of systems. For a more detailed coverage of this topic review Mr. Holt’s article “Understanding Circuit Breaker Markings,” in the November 2001 issue of EC&M magazine. Cooper-Bussmann also has an article covering slash rated circuit breakers, if you would like to read still more.
CIRCUIT BREAKER AMPERE RATINGS
Circuit breakers have an ampere rating (typically marked on the end of the operating handle). This is the maximum continuous current that the breaker can carry without exceeding its rating. As a general rule the circuit breaker’s ampere rating should be the same as the conductor’s ampacity. In other words we would not want to put a 60 amp breaker on a 10 amp wire. Breakers are tested in open air, with a temperature of some 40 or 50 degrees C.
When a breaker is placed within an enclosure, cooling airflow is restricted; this reduces the ability of the breaker to carry a current to 80% of its ampere rating. When they are installed in an electrical enclosure, breakers will trip when a current in the amount of their rating is placed upon them continuously. Breakers are designed to be able to safely carry a current in excess of their rating for very very short periods of time to allow some types of electrical equipment (called inductive loads) such as motors to start up.
While not as common, some breakers are rated for 100% continuous loads. These are typically called supplementary protectors (SP) and not circuit breakers.
AMPERE INTERRUPTING CAPACITY (AIC)
Circuit breakers are tested and then rated as to their ability to open the protected circuit with a specific amount of current flowing in the circuit. Circuit breakers typically have AIC ratings of between 5,000 and 200,000 AIC. The amount of fault current available must not exceed the breaker’s ability to safely open the circuit. Not only must the breaker be rated for the applied voltage, and continuous amperage load; it must also have an AIC rating equal to or greater than the available current at the location in the circuit where it will be installed. Breakers that have been installed so that the available fault current exceeds its AIC rating may blow up, just like a bomb would explode were it to attempt to clear a fault current above its rating. When opening a faulted circuit, it is possible for smoke and fire to be exhausted from a breaker. If you would like to see a breaker belch fire and smoke, see if you can locate and view the Cooper-Bussmann fuse company videotape titled “Specification Grade Protection”. The visual impact of this tape will likely enhance your appreciation of the importance of an electrical device’s AIC rating far better than any words of mine.
In your safety classes, you likely have received training in the step to the side routine before manually switching electrical circuits, and this videotape will reinforce the value of this easy safety step. This is also a good reason why sheet metal covers called dead front trim should be re-installed on loadcenters, panelboards, and the like before operating switching devices.
Electrical engineers tell us that the two major factors that govern the amount of fault current that can be delivered in a system are the KVA rating of the transformer and the impedance of the transformer. The presence of connected electric motors in the circuit also adds to the amount of potential fault current. Considering 480 volt systems, combined transformer and motor fault currents can range from 14,400 amps for a 500 KVA transformer with an impedance of 5.0% to some 90,000 amps for a 3500 KVA transformer with 5.75% impedance. Selecting all circuit breakers for higher AIC ratings may be the safety first and cost last method.
An engineering level study of a facility’s electrical system every five years (or before plant remodeling is undertaken) is a good idea. The study should include a review of the AIC of the plant’s breakers and the fault current that the plant’s electrical circuits can deliver to the line terminals of all major circuit breakers (OCPD).
TESTING-LISTING OF CIRCUIT BREAKERS
Molded case low voltage circuit breakers are typically tested to UL standard 489. UL uses the following test goals to determine if a breaker is considered to be safe (incompliance with their safety standard):
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The breaker must interrupt the maximum short circuit current two times.
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The breaker must protect itself and the connected conductor and the equipment it is installed in.
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After having been tested the breaker must be fully functional and pass a thermal calibration trip test at 250% of its rated ampacity; and pass a dielectric withstand test at two times its rated voltage or a minimum of 900 volts.
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The tested breaker must also operate properly and have continuity in all of its poles.
UL-489 listed circuit breakers are tested with a four-foot length of wire, as they must perform during the test as they would when installed in the real world, so wire is connected to make the test a bit more realistic. During the test the conductor’s insulation must not be damaged. The connected wires must not be pulled loose from the breaker-conductor termination lug. The breaker case must not be damaged as a result of cable whip forces (caused by the potentially huge amount of magnetic force developed under short circuit conditions). The connected wire acts to some degree as a heat sink for the breaker. That is, it helps to dissipate heat produced within the breaker. This is because the breaker’s case acts as not only an electrical but a thermal insulation also, in that it tends to retard the rate of heat transfer. This is one reason why breakers have wire size ranges marked on them. Too small a wire attached to the breaker cannot adequately aid in cooling the breaker.
The temperature at the circuit breaker’s terminals must not rise more than 50 degrees C. above the ambient air temperature surrounding the breaker. The UL-489 test standard has been used to test many, many circuit breakers over the years and has proven to be a pretty good standard by which the safety of circuit breakers can be determined.
If you would like information about European standards covering circuit breakers (IEC-947-2), I suggest that you read Cashier technique # 150. Be prepaired for a good bit of concentrating; this document is written at the engineering level. The good folks with Square D can help you locate it on their WEB site.