Sunday, January 25, 2009

Arc Extinguishers

In Module 5, a number of ways or combinations of ways to extinguish an arc was discussed. Low voltage power circuit breakers use some type of Arc Extinguishers (arc chutes or arc chambers) mounted above and around the main contacts to extinguish arcs in air (Figures 21 and 22). This leads to the name low voltage power air circuit breakers.

Arc chutes, in some form, have been used to extinguish arcs for more than a half century. The primary purpose of an arc chute is to extinguish an arc each time a circuit breaker interrupts a current. This is accomplished by confining, dividing and cooling the arc. This accomplished, the arc is not able to sustain itself through current zero.

Not all arc extinguishers are created equal and, therefore, some are more efficient than others. Efficiency is very important because the amount of contact damage caused by arcing is directly related to how fast or efficiently arcs are extinguished. More efficiency leads to longer contact life.

During the arcing process, ionized gases are generated and normally vented, in some fashion, harmlessly away from the circuit breaker, breaker compartment, and any operator who might be in close proximity to the equipment. It is also known that the high pressure created by these gases, if controlled properly, can be put to good use during interruption.
To this end, the molded case low voltage power circuit breaker design, for example, utilizes this gas pressure to help with the interruption process while minimizing gas leakage back into the circuit breaker itself. This improvement is accomplished through the use of seals in the arc chamber and a close fit of pieces and parts. This can only be accomplished with molded frame designs. Obviously, the design and process is a bit more involved than just described. For now, the most important thing to remember is that the original arc extinguisher concept is still used today, but great strides have been taken to improve upon the original concept with significant improvements in overall efficiency.
Operating Mechanism
You learned in Module 5 that some type of a mechanism is provided with all circuit breakers for opening and closing. Low voltage power circuit breakers are no exception. A low voltage power circuit breaker operating mechanism is composed of a number of different parts, assemblies and accessories, all dedicated to ensuring that the circuit breaker opens and closes consistently.
The mechanism is a two-step spring charged stored energy type providing three basic functions:

• A means to charge the closing springs
• A means to close/open the circuit breaker using the stored energy of the closing and opening springs
• A means to perform an Open-Close-Open duty cycle
Two varieties of the mechanism exist:
• Manual
• Electrical (Motor Operated)

The manually operated circuit breaker has its closing springs charged manually through the use of some type of charging handle. The circuit breaker is closed using a manual close button which is a mechanical process. As the circuit breaker closes, a set of smaller opening springs are charged. The circuit breaker is opened using a manual trip (open) button, which is a mechanical process.

Safety interlocks, accessory items and trip units can also cause the circuit breaker to trip through mechanical means. Most manually operated power circuit breakers can be equipped with an optional device to electrically release the spring's stored energy, thus closing the circuit breaker.

Previously, it was not practical or even possible to convert manually operated low voltage power circuit breakers to electrically operated circuit breakers in the field. This is no longer impossible with newer low voltage power circuit breaker designs. Such designs permit manually operated circuit breakers to be converted to electrically operated circuit breakers by field installing UL Listed electrical operators.


An electrically operated circuit breaker can be operated every way a manually operated circuit breaker can be operated. In addition, a small electric motor is normally used to automatically charge the closing springs, and an electrical means to close or trip (open) the circuit breaker is provided.

Integral Trip Unit
For a circuit breaker to do its job, a means must be provided enabling the circuit breaker to perform automatically or in response to other commands. In short, the circuit breaker is a rather dumb device without a brain (intelligence source). This source of intelligence is the trip unit.
As required by ANSI Standards, low voltage power circuit breakers must be provided with an Integrally Mounted Trip Unit. This means that the trip unit must be inside of, or part of, the circuit breaker. Prior to the advent of the first solid state trip unit, electromagnetic type tripping devices, commonly called dual-overcurrent magnetic trips, were used with all low voltage power circuit breakers. In recent times, this type of tripping device on low voltage power circuit breakers has disappeared from the scene. For this reason, only microprocessor-based trip units will be discussed in this module.

A typical microprocessor-based trip unit used with low voltage power circuit breakers offers the following capabilities:
• Programming
• Monitoring
• Diagnostic
• Communication
• Testing


The capabilities of a particular trip unit depends on the trip unit design itself and system requirements. Some trip units can only offer basic features, while others can offer basic features or, if required by the system, additional sophisticated and highly advanced features.
The operating response of a trip unit is graphically represented by time-current characteristic curves. These curves show how and when a particular trip unit will act for given values of time and current. A characteristic curve is represented by a band created by a minimum and maximum value of time or current.

The programmable or adjustable features of a trip unit permit movement of its characteristic curve or parts of the curve . This movement can be done in both a horizontal and vertical direction. Some trip units even allow the shape of the curve to be changed.

Most trip units offer protection combinations of:

• (L) Long delay protection - protection against overloads and short circuits
• (S) Short delay protection - protection against short circuits
• (I) Instantaneous protection - protection against short circuits
• (G) Ground fault protection - protection against ground faults

A trip unit offering all four of these protection at one time is commonly called an LSIG Trip Unit. Other combinations are also available, such as LI, LS, LSI, LIG and LSG.
The long, short and ground functions would have programmable values of current and time. Obviously, instantaneous has no associated time because the trip is instantaneous (Figure 26). Trip units have these different programmable features programmed so they coordinate with one another and with the requirements of the system being protected to provide the closest possible system coordination and protection against all eventualities. This coordination discipline is where you start hearing phrases like curve shaping and close coordination. No attempt will be made in this module to get into the details of this discipline. It is quite specialized and best left to individuals with the proper training.



More advanced trip units are able to monitor and display currents, energy, power, power quality and power factor. They also may be able to diagnose problems and provide advance warnings of potential problems, such as harmonics. Two way communications for remote monitoring and control is also available. This affords the user a cost effective way to monitor and control expansive, multi-location facilities with a wide array of protective equipment and operational machinery.

Trip and no trip tests can usually be performed on the trip unit and circuit breaker utilizing integral testing capabilities or separate test kits. Normally, the tests can be performed with the circuit breaker in service and full protection provided during the testing. This type of testing is secondary testing. Primary testing involves specialty testing equipment and a testing expertise, and is not discussed in this module.

Accessory Items
Accessories used with low voltage power circuit breakers are usually added to the circuit breaker to provide additional features, such as status indication and remote operation. It is possible, however, that some accessories for some circuit breaker designs might be mounted remotely from the circuit breaker. These devices might be totally mechanical, totally electrical or a combination. The intent here is to briefly discuss the function of commonly used accessory items, although all low voltage power circuit breakers do not necessarily offer all of the devices being discussed, nor is this list all inclusive.
• Electrical Operator - This is an assembly of devices including a small spring charging motor that when added to a manually operated circuit breaker converts it to an electrically operated circuit breaker. This allows for remote operation (open/close) of the circuit breaker. The ability to field install this device is more common with newer low voltage power circuit breakers. Power circuit breakers normally use to be either manual or electrical by design, and could not be easily converted.
• Operations Counter - An operations counter is a counting device, usually linked in some fashion to the operating mechanism. It is used to count the open and close operations of the circuit breaker, and serves as a maintenance aid.
• Auxiliary Switch - An auxiliary switch consists of "normally open" (NO) and "normally closed" (NC) contacts (Figure 27). The contacts on some switches are convertible from NO to NC and vice versa. The contacts are frequently referred to as "a" or "b" contacts. The "a" being open when the circuit breaker is open and the "b" closed when the circuit breaker is open. In short, these auxiliary contacts change "state" when the circuit breaker main contacts change "state." An auxiliary switch is normally mounted on the circuit breaker. Contacts from these switches are frequently used for electrical operation of a circuit breaker, remote signaling, and electrical interlocking.


• Undervoltage Release (UVR) - An undervoltage release, normally a circuit breaker mounted electromechanical device, trips the circuit breaker when the voltage falls below a predetermined level.
• Shunt Trip (ST) - A shunt trip is an electromechanical device which is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it provides for remote controlled electrical tripping.
• Spring Release (SR) - The spring release device is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it permits the circuit breaker to be closed electrically from a remote location.
• Bell Alarm (OTS) - The bell alarm, frequently called an overcurrent trip switch (OTS) on a power circuit breaker, is normally circuit breaker mounted. Its function is to provide a signal to indicate that the circuit breaker has tripped open automatically (trip unit command). It will not operate if the circuit breaker is tripped open by other means, such as the use of a manual trip button, an electrical control switch, or the operation of an undervoltage release device.
• Locking Devices - Low voltage power circuit breakers normally have a wide array of mechanical locking devices to prevent unauthorized circuit breaker operation.


• Mechanical Interlocks - These devices provide a way to mechanically interlock two circuit breakers. A typical use for such a device is to prevent one circuit breaker from closing while another circuit breaker is already closed.
• Capacitor Trip Device - A capacitor trip device is normally mounted externally from the circuit breaker. It uses a small storage capacitor for AC control of the circuit breaker to ensure reliable tripping power during fault conditions.
• Lifting Device - Because some low voltage power circuit breakers can be sizable and heavy, a variety of devices is usually available to lift and move the circuit breaker once it is out of its compartment (Figure 29).
 Rail Mounted Lifting Device Being Used to Lift a Magnum DS Power Circuit Breaker


• Truck Operated Cell Switch (TOC) - A TOC switch is usually mounted in the circuit breaker compartment and is activated by movement of a drawout circuit breaker into and out of the "Connected" position. As the circuit breaker moves, the contacts are activated providing a means for remote indication of the circuit breaker's position.

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