101 Basic Series - inw- · PDF file101 Basic Series Learning Module 6: ... Although medium...

Learning Module 6:

Medium Voltage Power Circuit Breakers
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What You Will Learn We’ll step through each of these topics in detail:

Introduction 4

Components 4Frame 5Operating Mechanism 5Trip Unit 5Trip Intelligence 6Arc Extinguisher 7

Arc Extinguishing Technologies 7Vacuum Interrupter Technology 7Spiral Contacts 9SF6 Gas Technology 9Puffer-Type SF6 Gas Technology 10Rotary Arc-Type SF6 Gas Technology 11

Review 1 12

Ratings 13Ratings and Standards Vary 13Maximum Voltage 14Continuous Current 14

Enclosure and Mounting 14Enclosure 14Mounting 15Fixed Mount 15Drawout Mount 16

Standards 18

Helping the Customer 19Circuit Breaker Details 19Configuration and Accessory Details 19

Review 2 20

Glossary 21

Review 1 Answers 24

Review 2 Answers 24


Welcome Welcome to Module 6, which is about Medium Voltage Power Circuit Breakers.

By medium voltage, we mean a voltage range of 1000 volts to 38 kV. (Some con-sider 72.5 kV as the upper end of medium voltage, but we will work with 38 kV for the purpose of this training module.)

Figure 1. Family of Medium Voltage Vacuum Circuit Breakers (ANSI and IEC Types)

Like the other modules in this series, this one presents small, manageable sec-tions of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you’ve just read.

You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

A Note on Font Styles Key points are in bold.

Glossary items are italicized and underlined the first time they appear.

Viewing the Glossary Printed versions have the glossary at the end of the module. You may also browse the Glossary by clicking on the Glossary bookmark in the left-hand margin.


Introduction To understand where the medium voltage power circuit breaker fits into the scheme of things, you need to understand the basics of power distribution in an industrial environment.

An industrial distribution system consists of:

• metering devices to measure power consumption

• main and branch disconnects

• protective devices

• switching devices to start and stop power flow

• conductors

• transformers

Power may be distributed through various Switchgear and Switchboards, trans-formers and Panelboards. The medium voltage power circuit breaker is found in switchgear assembly. A switchgear assembly controls electric power circuits.

Figure 2. Typical Industrial Power Distribution

The Circuit Breaker is the main device – the heart – of the switchgear. It pro-vides centralized control and protection of medium voltage power equipment and circuits. Its operation covers load switching, control and fault protection for gener-ators, motors, transformers, capacitors and all types of feeder circuits.

This type of power equipment is normally found in industrial, commercial and electric utility installations. Typical applications include electric utility sys-tems, industrial distribution systems, commercial buildings, municipal pumping stations, and transportation systems.

Components There are four main parts to a medium voltage power circuit breaker. These are:

· Frame

· Operating Mechanism

· Trip Unit

· Arc Extinguisher


Figure 3. Typical Medium Voltage Vacuum Circuit Breakerwith Front Cover Removed, Exposing Operating Mechanism

Frame A medium voltage power circuit breaker is essentially an assembly of parts on a rugged metal Frame. Depending upon factors such as ratings and interrupting method, they come in a variety of shapes, sizes and configurations.

Operating Mechanism The medium voltage power circuit breaker uses a stored-energy Operating Mech-anism to open the circuit breaker. It has a motor-charged, spring-type, stored-energy closing mechanism. Closing the breaker charges the accelerating springs. Protective relays on the control switch energize a shunt trip coil to release the accelerating springs and open the breaker.

This is a trip-free design – truly mechanically and electrically trip free. Breaker contacts will not touch or close onto a fault, even when a mechanical or electrical “close” command is issued.

The manual controls are usually accessed from the front of the circuit breaker. Although medium voltage power circuit breakers are electrically operated, the closing springs can be charged manually.

For more on operating mechanisms, see Module 5, Fundamentals of Circuit Breakers.

Trip Unit A Trip Unit is typically integral to a circuit breaker. But, the medium voltage power circuit breaker uses externally mounted trip units to provide the oper-ational intelligence. These devices are called Protective Relays.

The protective relays are normally wired to the circuit breaker and Current Trans-formers. They are mounted on a panel or door of the switchgear assembly. They function to detect a defective line or apparatus, as well as dangerous or undesir-able system conditions. The relay energizes the trip coil of the circuit breaker to clear a fault.


Figure 4. Typical Medium Voltage Switchgear Assembly with Protective Relays andOther Devices Shown Mounted on Compartment Doors

Trip Intelligence The protective relay intelligence devices fall into two broad categories. These are:

Electromagnetic–The electromagnetic protective relay has been used widely in the industry for many years, with a high degree of success. It functions to tell the circuit breaker when to operate, based on the specific relay type selected. Many different types of electromagnetic protective relays are available, and each type performs rather specific functions.

Common relay types include: instantaneous overcurrent, time overcurrent and overvoltage. Because these devices are limited in scope, a number of differ-ent relays (each with different protective capabilities) must be used to provide a comprehensive protective package.

Microprocessor-Based–The microprocessor-based protective relay is a multi-function device. It can provide all the features of several electromagnetic relays in one box. It is also easier to mount and wire. Overall, it is a smaller invest-ment than an electromagnetic protective relay solution.

Figure 5. Typical Medium Voltage Switchgear Assembly withMicroprocessor-Based Devices Shown Mounted

In most cases, only one device is required for each three-phase circuit, not one device for each phase. One of the more sophisticated microprocessor-based protective relays is capable of replacing the normal complement of three or four electromagnetic relays, as well as a number of associated meters and switches (Figure 6).


Figure 6. Typical Sophisticated Microprocessor-Based Protective Relay

Because these are individual devices, apart from the medium voltage circuit breaker itself, there will be no further discussion of the tripping intelligence in this module.

Arc Extinguisher The Arc Extinguisher extinguishes the Arc produced when the contacts are pulled apart to interrupt current flow. The higher the voltage, the harder it is to interrupt the flow of current.

There are a number of arc extinguishing technologies in use today. We will look at the most prominent types in the next section.

Arc Extinguishing Technologies

In a low voltage circuit breaker, using air or Arc Chutes is sufficient to extinguish an arc. In the medium voltage range, a different technology needs to be used. The main technology used today is the Vacuum Interrupter.

Vacuum Interrupter Technology

The vacuum interrupter (VI) was briefly discussed in Module 5, Fundamentals of Circuit Breakers.

Figure 7. Schematic for a Typical Vacuum Interrupter

Basically, the vacuum interrupter is a pair of separable contacts (called “pri-mary contacts”) enclosed in a vacuum-tight envelope. The Envelope itself is a ceramic material, with a metal end plate brazed to each end. The metal plates seal the ends and provide support for the parts inside.

Of the two contacts (also called “electrodes”) inside, one is fixed. The other is movable, through a bellows-type connection. Various shields inside the envelope provide different types of protection to interrupter parts.


Figure 8 depicts the important arcing and interruption phenomena within a vac-uum.

Figure 8. Interruption in a Vacuum

When the circuit breaker is closed, the contacts within the interrupter touch, allow-ing current to flow.

When a fault occurs and interruption is required, the contacts are quickly separated and an arc forms. An arc is formed because the voltage tries to keep the current moving.

Figure 9. Enclosing Contacts in a Vacuum

The arc burns in the metal vapor evaporated from hot spots on the contact sur-faces. This metal vapor continuously leaves the contact region and recondenses on the contact surfaces and surrounding metal shield, which protects the ceramic envelope.

At Current Zero, the arc extinguishes, contact vapor production stops, and the original vacuum condition is restored. Current zero is a point in the AC current sine wave where the value is zero.

Figure 10. Current Zero Points in an AC Cycle

The vacuum in the envelope is considered a Dielectric. The Dielectric Strength is the maximum voltage the dielectric can withstand without breaking down. The Transient Recovery Voltage (TRV) is the most severe waveform the interrupter will


have to withstand. This is why the speed of the dielectric recovery and the strength of the dielectric inside the interrupter are critical issues for successful cir-cuit interruption. If the dielectric does not reach sufficient strength fast enough, the arc will re-ignite.

Vacuum interrupters for circuit breaker duty must be capable of interrupting cur-rents of 12 to 50 kA (and up), at voltages up to 38 kV (Figure 11).

Figure 11. Rear View of Circuit Breaker withVacuum Interrupters Installed (One Per Phase)

Spiral Contacts A newer technology is now being used in the vacuum interrupter. It involves using spiral-shaped copper-chrome contacts inside the vacuum tube. They provide a self-induced magnetic effect that moves the arc root around the contact periphery. This very efficient arc control method prevents hot spots, minimizing contact erosion.

Figure 12. Spiral Contacts

SF6 Gas Technology Another arc extinguishing technology is SF6 (sulfur hexafluoride) technology. It is popular outside the United States. It is specifically associated with European man-ufacturers of medium and higher voltage circuit breakers.

SF6 technology was briefly discussed in Module 5, Fundamentals of Circuit Breakers. The main contacts are enclosed in a chamber of SF6 gas, which hap-pens to be a very good dielectric (Figure 13).


Figure 13. Enclosing Contacts in SF6 Gas

In short, this arc interruption technology results the in arc energy being used and absorbed, while the arc is simultaneously cooled.

There are several SF6 interrupter designs, but only two types will be covered here:

• Puffer

• Rotary Arc

Puffer-Type SF6 Gas Technology

The puffer-type of SF6 interrupter is the older of the two SF6 technologies. It is more capable but more complicated than the rotary arc-type.

The interrupter is shown below in the closed position, and in the opening sequence.

Figure 14. Typical SF6 Puffer Interrupter in Closed Position

Figure 15. Typical SF6 Puffer Interrupter During Opening SequenceHigh Current Arc on Left and Near Current Zero on Right


During current interruption, a piston compresses the SF6 gas in a cylinder, all of which is enclosed in an epoxy-type enclosure. After the main current-carrying contacts part, the current transfers to the arcing contacts. Once the arcing con-tacts part, the SF6 gas in the compression chamber blasts the arc through the nozzle. The heat created by the arc breaks the SF6 molecules into fluorine and sulfur. Arc energy is absorbed and the arc is cooled.

As current zero is approached, the heat energy subsides as more SF6 gas enters the system. At current zero, the high-pressure SF6 gas flows through the nozzle and extinguishes the arc.

Compressing the SF6 gas requires significant mechanical energy. A circuit breaker equipped with this type of technology requires a higher-energy operating mechanism than is required by an equivalent vacuum circuit breaker. In addition, this interrupter type has a large number of parts.

Rotary Arc-Type SF6 Gas Technology

Rotary arc SF6 technology is less complicated in design than the puffer type. It has fewer parts and does not require such a high-energy operating mech-anism. However, it is not effective over as wide a range of short circuit currents and voltages as the SF6 puffer or vacuum. Suitable applications are somewhat limited.

Figure 16 shows a typical SF6 rotary arc interrupter.

Figure 16. Cross-Sectional View of Typical SF6 Rotary Arc Interrupter

As the contacts part, the arc transfers from the main contacts to an annular con-tact. This causes the current to switch into the coil behind it. The coil’s magnetic field, produced by the load current itself, causes the arc to rotate rapidly. The arc is cooled by moving through the SF6 gas.

The SF6 gas is normally at rest inside this interrupter. The arc’s movement acts like a mixer, mixing hotter and cooler gas. This helps cool the arc. Contact ero-sion is also reduced due to this rapid arc movement.

As current zero is approached, the dwindling arc must sustain enough speed to be lost in the SF6 environment in order to withstand the transient recovery voltage. (This is mentioned because the magnetic field produced by the load current causes the all important arc rotation. As the current decreases, the magnetic field decreases, and the arc rotation slows.)

At current zero, the arc is cooled and extinguished.


Review 1 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you’ve already read.

1. The ________ ________ is the heart of the switchgear.

2. There are four main parts to a medium voltage power circuit breaker. These are:________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. A microprocessor-based trip intelligence is superior to an electromagnetic-based trip intelligence. In your own words, explain why._______________________________________________________________________________________________________________________________________________________________________________________

4. Vacuum interrupters for circuit breaker duty must be capable of interrupting currents of ___ to ___ kA (and up), at voltages up to ___ kV.

5. SF6 arc extinguishing technology is popular outside the United States. The two mostly commonly used types are __________ and __________ __________.


RatingsRatings and Standards Vary

Medium voltage power circuit breaker ratings vary in different parts of the world. As a matter of fact, “medium voltage” is not uniformly defined around the world. Although the standard we are using in this training module defines medium voltage as 1000 volts to 72.5 kV (a commonly accepted medium voltage range in the United States, as well as other parts of the world), one foreign country consid-ers 1000 volts and above to be “high voltage.”

Medium voltage power circuit breaker ratings charts specify many different types of ratings. Usually, a chart indicates the ratings required by the governing standards where the circuit breaker is applied.

ANSI or IEC is normally the governing standard for medium voltage power circuit breakers. ANSI is associated with U.S. standards, and IEC is associated with international standards. However, neither the standards nor the ratings charts are identical. Charts also vary from manufacturer to manufacturer (Figures 17 and 18). To give an idea of the differences, consider the following example.

Figure 17. Partial Sample of a Medium Voltage ANSI Ratings Chart

Figure 18. Partial Sample of a Medium Voltage IEC Ratings Chart

All of the indicated ratings are important. A number of them are merely calcula-tions. For the sake of this discussion, we will cover only three of them.

These are:

• Maximum Voltage

• Continuous Current

Breaker Type Medium Voltage (kA rms)

Continuous Current (Amps)

Short Circuit Current (kA rms)

270VCP-W750 27 60012002000

16

270VCP-W1000 27 60012002000

22

270VCP-W1250 27 60012002000

25

Breaker Type Voltage Class(kA rms)

Normal Current (Amps)

Short Circuit Current (kA rms)

240VCP-W16 24 63012502000

16

240VCP-W20 24 63012502000

20

240VCP-W25 24 63012502000

25


• Short Circuit Current

These three ratings are common to both ANSI- and IEC-rated circuit breakers. In most instances, a grasp of these three ratings will allow you to assist a customer with a medium voltage power circuit breaker selection.

Maximum Voltage This is the maximum voltage at which the breaker can operate. It is termed Maximum Voltage on ANSI charts and Voltage Class on IEC charts. The operating voltage where the circuit breaker is applied should not exceed the circuit breaker’s rated maximum voltage.

Typical maximum voltage ratings encountered with ANSI applications are: 4.76, 8.25, 15, 27 and 38 kV. Typical voltage classes encountered with IEC applications are: 3.6, 7.2, 12, 17.5 and 24 kV.

Continuous Current This is the amount of current the breaker can carry continuously at 60 cycles without exceeding the temperature rise limit. It is termed Continuous Current on ANSI charts and Normal Current on IEC charts. This maximum rating should always be in excess of the utilization equipment rating to provide for a short-time overload capability.

Typical continuous current currents encountered with ANSI applications are: 600, 1200, 2000 and 3000 amps. Typical normal currents encountered with IEC appli-cations are: 630, 1250 and 2000 amps.

Short Circuit Current This is the level of three-phase short circuit current that the circuit breaker can safely interrupt. The Short Circuit Current is a rating at the circuit breaker’s maximum voltage (ANSI) or voltage class (IEC).

Typical short circuit currents encountered with ANSI applications are: 16, 29, 33, 37 and 63 kA. Typical short circuit currents encountered with IEC applications are: 16, 25, 31.5, and 40 kA.

Enclosure and MountingEnclosure The medium voltage power circuit breaker is always placed in a switchgear

assembly. The switchgear assembly is usually referred to as a Metal-Enclosed Assembly. The phrase “metal-clad” means that compartments within the switch-gear assembly are separated by metal barriers (Figure 19).

Figure 19. Typical Medium Voltage Metal-Clad Assembly Structure withTwo Vacuum Circuit Breakers (Side View)


This is different from a metal enclosed assembly, where the equipment is enclosed, but not necessarily separated by barriers. The metal enclosed assem-bly is typically associated with low voltage equipment.

Figure 20. Typical Vacuum Switchgear Assembly with One Circuit Breaker inUpper Compartment and One Below with Door Closed

Mounting There are two methods for mounting a circuit breaker in the switchgear. These are:

• Fixed Mount

• Drawout Mount

Fixed Mount Fixed Mount circuit breakers are usually found in outdoor applications. When installed outdoors, a special type of housing must be provided to protect the equipment from the elements. This outdoor protection is provided in a number of ways.

Fixed medium voltage power circuit breaker designs exist for a limited range of applications and voltages.


In the WorkplaceThis outdoor substation utilizes fixed medium voltage power circuit breakers to perform a series of capacitor switching functions.

Fixed Medium Voltage Power Circuit BreakerInstalled in Outdoor Enclosure

Each breaker must be housed in a weatherproof enclosure to protect it from the elements.

Drawout Mount The Drawout Mount (or removable) type is – by far – the most common medium voltage power circuit breaker in use. It is used almost exclusively through 38 kV. Prior to the introduction of vacuum designs, medium voltage power circuit break-ers were much larger (Figure 21). For example, one vertical structure could accommodate one Magnetic Air Type Circuit Breaker. The circuit breaker was removable, typically rolled out of the structure on wheels.

Figure 21. Typical Medium Voltage Magnetic Air Circuit Breaker (Front Barrier Removed)

Today, medium voltage power circuit breakers are frequently small enough to be stacked two units high in one vertical compartment.

The circuit breaker has three positions: DISCONNECT, TEST and CONNECT. Normally, the circuit breaker is manually pushed to the TEST position, then mechanically levered between the TEST and CONNECT positions (Figure 22).


The secondary and primary electrical connections are automatically connected or disconnected as the circuit breaker is levered from one position to another.

Figure 22. Medium Voltage Vacuum Circuit BreakerBeing Mechanically Levered Into Its Structure

Most drawout medium voltage power circuit breakers can be completely removed from their compartments onto integral extension rails (Figure 23). This feature makes the task of inspecting the circuit breaker and the compartment much simpler.

Figure 23. Medium Voltage Vacuum Circuit BreakerShown Removed From Compartment on Extension Rails

If the circuit breaker must be lifted from the extension rails onto the floor, integral wheels permit it to be rolled around outside of its structure.


Standards The testing required and the standards that must be met by a medium volt-age power circuit breaker depend on the area of the world where the circuit breaker is applied.

A medium voltage power circuit breaker must meet the requirements of ANSI, IEC, IEEE and NEMA to be considered “world-class” (Figure 24).

Figure 24. Dominant Worldwide Standards

In addition, UL is an important consideration in certain local areas.

Standards and testing to prove compliance are rather stringent. This is because of the voltage and current levels associated with medium voltage equip-ment under abnormal (and even normal) operating conditions.

The testing goes beyond just testing the circuit breaker itself. Because a medium voltage power circuit breaker must function properly with a compatible housing, it is tested separately and in combination with the structural assembly. For this reason, most circuit breaker manufacturers are also switchgear assembly manufacturers.


Helping the Customer

Now you should be ready to assist a customer in matching a product to an appli-cation. When you meet with the customer, conduct a short interview to obtain information in these two areas:

• Circuit Breaker Details

• Configuration and Accessory Details

Circuit Breaker Details Before you can select a specific circuit breaker, get the following information from the customer:

• Which set of standards (ANSI or IEC) apply to the circuit breaker?

• What maximum voltage (ANSI) or voltage class (IEC) is required?

• What continuous current (ANSI) or normal current (IEC) is required?

• What short circuit current is required?

This information alone should be enough to allow you to select an appropriate cir-cuit breaker. However, the more information you have, the better.

Configuration and Accessory Details

It is also important to know how the circuit breaker is to be configured. Ask the customer these three questions:

• Will the circuit breaker be drawout or fix mounted?

• Will the circuit breaker be used indoors or outdoors?

• Will circuit breakers be stacked two to a vertical structure, or mounted one per structure?

Medium voltage circuit breakers do not normally have a large number of acces-sory items. However, you should make yourself aware of anything additional the customer expects.


Review 2 Answer the following questions without referring to the material just presented.

1. For each rating type, identify it as being ANSI, IEC, or both.Normal Current __________Short Circuit Current __________Maximum Voltage __________

2. In your own words, explain the difference between a metal-clad assembly and a metal enclosed assembly._______________________________________________________________________________________________________________________________________________________________________________________

3. There are two methods for mounting a circuit breaker in the switchgear. These are:________________________________________________________________________________________________________________________

4. A medium voltage power circuit breaker must meet the requirements of ______, ______, ______ and ______ in order to be considered “world-class.”

5. There are seven main questions to ask a customer when attempting to match a product to an application. Name three of them.____________________________________________________________________________________________________________________________________________________________________________________


Glossary

ANSI American National Standards Institute. It was organized to simplify and standardize production and construction.

Arc The effect generated when current is forced to be interrupted between two contacts.

Arc Chute A component of the arc extinguisher in a low voltage circuit breaker. It elongates and cools an arc.

Arc Extinguisher A component of a circuit breaker. It actually interrupts the flow of current.

Circuit Breaker A reusable overcurrent protection device. After tripping to break the circuit, it can be reset to protect the circuit again.

Continuous Current The amount of current the breaker can carry continuously at 60 cycles without exceeding the temperature rise limit, according to ANSI charts.

Current Transformer A step-down transformer that steps down supplied current to a current usable by control components such as relays and meters.

Current Zero Also “Zero Point.” A point in the AC current sine wave where the value is zero.

Dielectric The insulating medium between two contacts. Typical dielectrics are air, gas, ceramic or gas and ceramic. A vacuum is the only perfect dielectric.

Dielectric Strength The maximum voltage a dielectric can withstand without breaking down.

Drawout Mount A mounting type for medium voltage power circuit breakers in which the breaker can be completely removed from its compartment onto integral extension rails.

Envelope Main component of a vacuum interrupter. A sealed, evacuated ceramic enclosure, with a metal end plate brazed to each end.

Fixed Mount A mounting type for medium voltage power circuit breakers in which the breakers are fixed in place. Usually found in outdoor applications.

Frame A component of a medium voltage power circuit breaker. Its primary function is to provide a rigid, mechanically strong, insulated housing in which the other components are mounted.

IEC Abbreviation for International Electro-technical Commission. This organization is associated with equipment used internationally.

IEEE Institute of Electrical and Electronic Engineers. A professional organization of scientists and engineers whose purpose is the advancement of engineering.

Magnetic Air Type Circuit Breaker

A type of medium voltage power circuit breaker no longer in common use.


Maximum Voltage The highest voltage at which the breaker can operate, according to ANSI charts. The operating voltage where the circuit breaker is applied should not exceed the circuit breaker’s rated maximum voltage.

Medium Voltage Power Circuit Breaker

A specific type of circuit breaker, used in a switchgear assembly, with a voltage range of 1000 volts to 38 kV.

Metal-Clad Assembly Equipment in the assembly is enclosed, and separated by metal barriers into individual compartments. Typically associated with medium voltage equipment.

Metal-Enclosed Assembly

Equipment in the assembly is enclosed, but not necessarily separated by barriers. Typically associated with low voltage equipment.

NEMA Abbreviation for National Electrical Manufacturers Association. An organization of manufacturers of electrical products.

Normal Current The amount of current the breaker can carry continuously at 60 cycles without exceeding the temperature rise limit, according to IEC charts.

Operating Mechanism

Opens and closes the contacts of a circuit breaker.

Panelboard A wall-mounted electrical power distribution device for use in commercial and industrial applications. It provides circuit control and overcurrent protection for light, heat or power circuits.

Protective Relay The specific trip unit used with a medium voltage power circuit breaker. Functions to detect a defective line or apparatus, as well as dangerous or undesirable system conditions.

SF6 An arc extinguishing technology involving the use of sulfur hexafluoride gas.

Short Circuit Current A rating of the level of three-phase short circuit current that the circuit breaker can safely interrupt. The short circuit current is a rating at the circuit breaker’s maximum voltage (ANSI) or voltage class (IEC).

Switchboard A floor-standing electrical power distribution device for use in commercial and industrial applications. It divides large blocks of electrical current into smaller blocks of current used by electrical equipment.

Switchgear An assembly of switching and interrupting devices, along with control, metering, protective and regulating equipment.

Transient Recovery Voltage

The most severe waveform the vacuum interrupter will have to withstand.

Trip Unit “brain” of a circuit breaker.UL Underwriters Laboratory. An independent laboratory that

tests equipment to determine whether it meets certain safety standards when properly used.


Vacuum Interrupter An arc extinguishing technology. Features a pair of separable contacts enclosed in a vacuum-tight envelope. Because the environment inside the interrupter envelope is a vacuum, an arc cannot be sustained easily.

Voltage Class The highest voltage at which the breaker can operate, according to IEC charts. The operating voltage where the circuit breaker is applied should not exceed the circuit breaker’s rated voltage class.


Review 1 Answers 1. Circuit breaker

2. FrameOperating MechanismTrip UnitArc Extinguisher

3. Answer should basically say: “The microprocessor-based protective relay is a multi-function device. It can provide all the features of several electromagnetic relays in one box. It is also easier to mount and wire. Overall, it is a smaller investment than an electromagnetic protective relay solution.”

4. 12, 50, 38

5. Puffer, Rotary Arc

Review 2 Answers 1. IECBothANSI

2. Answer should basically say: “A metal-clad assembly has internal compart-ments which are separated by metal barriers. A metal enclosed assembly does not necessarily have internal separation barriers”

3. Fixed mount, Drawout mount

4. ANSI, IEC, IEEE and NEMA

5. Any three of the following:Which set of standards (ANSI or IEC) apply to the circuit breaker?What maximum voltage (ANSI) or voltage class (IEC) is required?What continuous current (ANSI) or normal current (IEC) is required?What short circuit current is required?Will the circuit breaker be drawout or fix mounted?Will the circuit breaker be used indoors or outdoors?Will circuit breakers be stacked two to a vertical structure, or mounted one per structure?

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