As you might imagine by now, there is a wide variety of low voltage power circuit breakers available in the market today. We will not concentrate on what circuit breakers are called. Instead, we will look at characteristics, features and governing standards. Then, no matter who the manufacturer or what a circuit breaker is called, you will be better prepared to discuss the subject.
Low voltage power circuit breakers are considered rugged, long-lived, flexible and, to varying degrees, field-maintainable. Let's briefly look as some of the areas that might set a low voltage power circuit breaker apart from other types of low voltage circuit breakers, such as:
• Method used to make and break circuits
• Ratings
• Construction/Maintainability
• Integral Trip Units
• Operating Mechanisms
• Testing
Method Used to Make or Break Circuits
Because they make and break power circuits in air using Arc Chutes, as opposed to Vacuum, SF6 or oil, they are considered Air Circuit Breakers.
Ratings
Low voltage power circuit breaker interrupting ratings and frame size designations can vary to some degree from one manufacturer to another or from one part of the world to another. One thing that is common to most power circuit breakers is the fact that they are rated for continuous operation at 100% of their current rating in their enclosure. What you see on the nameplate is what you get. There is no derating necessary when enclosed, if they are applied as specified by the manufacturer. This is not the case with all types of low voltage circuit breakers when applied in an enclosure. Low voltage power circuit breakers also have a short time rating in addition to an interrupting rating making them naturally suited for selectivity and coordination with downstream devices. Downstream devices are devices, such as other circuit breakers, that are farther into the electrical system.
You will recall from an earlier discussion, and it is worth mentioning again, that the short time rating is composed of two components - short delay current and short delay time, which are adjustable (programmable). As far as selectivity is concerned, let's say it is the response to a set of circuit or system conditions, usually in terms of current, in a certain time frame. It is really the ability to withstand a certain level of current (kA) for a given time period (cycles) while a downstream device selectively takes care of the problem by interrupting. This is also known as discrimination. The degree of selectivity is usually limited by the sophistication of the trip unit and the physical ability of the circuit breaker to withstand the potentially large thermal and mechanical stresses created by a fault current.
Construction/Maintainability
Low voltage power circuit breakers are essentially an assembly of parts on a metal frame or in an encased housing of insulating material. It is important to know that no set of standards dictates the type of frame construction for low voltage power circuit breakers. That decision is left in the hands of the manufacturer. You could look at it like the frame and body of a car holding all the other parts, like the motor, wheels, bumpers, seats and radio. This type of circuit breaker, to varying degrees, has the ability to be maintained in the field.
In addition, it is available in either a Fixed or Drawout configuration, with drawout being the most commonly used type.
Trip Units
Trip Units today used on low voltage power circuit breakers are almost universally of the solid state, microprocessor-based design. Years ago this same type circuit breaker used only electromechanical type trip units. Because this type of trip unit used with a low voltage power circuit breaker is almost non-existent, it is only mentioned briefly in this module. It is important to note that ANSI Standards require that the trip units on low voltage power circuit breakers be integrally mounted.
Operating Mechanisms
Low voltage power circuit breakers operate through two-step stored-energy spring mechanisms. The springs used to close the circuit breaker contacts, called closing springs, can be manually or electrically charged. The springs used to open the circuit breaker, called opening springs, are usually charged automatically when the breaker is closed.
Because of the increased closing forces required and the closing speed, low voltage power circuit breakers use two-step, stored energy mechanisms. That is, the closing springs are charged and remain charged with the breaker open until a "close" button or some other type of release is activated to close the breaker. As mentioned in Module 5, the low voltage power circuit breaker is required by ANSI Standards to provide an open-close-open duty cycle. This dictates the need for a two-step stored energy mechanism.
In the workplace
Low voltage power circuit breakers are most commonly applied in switchgear assemblies like the one shown here.
Frequently, low voltage power circuit breakers are used to control (and protect against overloads and short-circuits on) fans, pumps and lighting panels.
An assembly such as this one could be used to serve the HVAC needs of a manufacturing facility.
Because they are built to withstand such intense service conditions, low voltage power circuit breakers are ideal for industrial applications such as this. Labels: Circuit Breaker
1 comments:
Thank you for sharing this useful information. keep sharing.
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