Low voltage circuit breaker

There are two type:

1-low voltage dc power circuit breakers

2-low voltage ac power circuit breakers

Large quantities of low voltage AC power circuit breakers are used throughout industry in comparison to very small numbers of DC devices

Circuit Breakers are often classified by certain modifying words, such as low voltage power. Low voltage AC power is considered to be for applications at 1000 volts AC and below

low voltage power circuit breakers are viewed as 600 volt circuit breakers applied at a number of different voltage levels, such as 240 or 480 volts.

requirements that support and further explain the following reasons for applying low voltage power circuit breakers:

Continuity of Service:

the low voltage power circuit breaker has the strength to withstand the stresses of a fault for up to 1/2 second or 30 cycles, instead of opening immediately. This ability to delay opening allows for a circuit breaker nearest the fault to clear the fault. This helps to prevent facility outages or a wide shutdown of facility equipment.

Maintainability :

A low voltage power circuit breaker is designed to be maintained in the field. This extends the useful service life of the circuit breaker. Especially for heavy, repetitive duty applications, maintenance of the circuit breaker is quite an important feature. Low voltage power circuit breakers allow for the inspection and replacement of parts on site.

Safety:

Low voltage power circuit breakers are tested as drawout devices in an enclosure. As such, four distinct circuit breaker positions relative to its enclosure are provided for maximum operator safety.

The four drawout circuit breaker positions allow for the following uniquely different functions:

1-Connected Position  : The circuit breaker is fully connected and functional.

2-Test Position : The circuit breaker's primary connections are disconnected. Secondary connections are not disconnected and testing can be safely performed because the circuit breaker is not energized. This is not possible with a circuit breaker that is permanently mounted.

3-Disconnect Position : Neither the primary nor secondary electrical connections of the circuit breaker are made. This position is often used as a storage position for the circuit breaker within its enclosure.

4-Withdrawn Position : In this position, the circuit breaker has no electrical connections. It is far enough out of its enclosure, usually on some type of integral extension rails, to permit inspection and maintenance without turning the power off to an entire assembly of equipment.

Reliability:

Low voltage power circuit breakers are tested for and must be able to meet high electrical and mechanical endurance ratings. Electrical endurance is the number of operations at rated continuous current and maximum system voltage. Mechanical endurance is the number of operations with no voltage applied.

Principle of operation:

Low voltage power circuit breakers are considered rugged, long-lived, flexible and, to varying degrees, field-maintainable.

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

Trip Units:

 today used on low voltage power circuit breakers are almost universally of the solid statYears ago this same type circuit breaker used only electromechanical type trip units.  it is only mentioned briefly in this module

Basic low voltage power circuit breakers are generally composed of:

• Frame or Chassis
• Primary Contacts
• Arc Extinguishers
• Operating Mechanism
• Integral Trip Unit
• Accessory Items

Primary Contacts:                                            

Primary open/close contacts in a low voltage power circuit breaker provide a means for isolating or connecting a part of a circuit from or with the rest of the system.

Arcing also occurs when opening and closing low voltage power circuit breakers under load, except to a much larger degree. The primary contact design challenge is to ensure that the arcing is dealt with first to protect the surface of the main contacts from arc damage

A primary contact assembly is composed of:

• fixed (stationary) part
• moving part

The fixed and moving main and arcing portions of the assembly can be in any number of configurations, some more efficient than others (Figures 8 and 9).

Figure 8. Contact Assembly Mounted (Front View)

Figure 9. Contact Assembly Mounted (Rear View)

Arc Extinguishers:

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 (Figure 21 ). This leads to the name low voltage power air circuit breakers.

Figure 10. Top Rear View Magnum DS Circuit Breaker Showing
Arc Chambers and One Arc Chute Removed

Operating Mechanism:

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)

1-Manual:

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.

2-Electrical  (motor operated):

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:

the circuit breaker is a rather dumb device without a brain (intelligence source). This source of intelligence is the trip unit

Figure12. Integrally Mounted Trip Units

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.

Accessory Items:

1-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.

2-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.

3-Auxiliary Switch - An auxiliary switch consists of "normally open" (NO) and "normally closed" (NC) contacts (Figure 13)

Figure 13. Graphical Representation of a Four Contact Auxiliary Switch (2A and 2B)

4-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.

5-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.

6-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