Mod 2 Circuit Breaker with Video

Module 2 Circuit Breaker Fundamentals Introduction............................................................................. 2 Objectives................................................................................. 2 Standards ................................................................................. 4 Ratings ...................................................................................... 5 The Interrupting Rating ...................................................... 8 MVA versus KA ......................................................................10 Principles of Arc Interruption ..........................................14 Contacts .................................................................................22 Insulation Requirements ..................................................24 Circuit Breaker Controls ....................................................25 Methods of Operation .......................................................26 Auxiliary Switches ...............................................................38 Summary ................................................................................39

INTRODUCTION C ircuit breaker designs vary from relatively simple molded-case breakers used in 120-volt lighting circuits to complex units used in 765,000-volt transmission circuits. There is no single design that is both practical and economical for every application. The means employed for extinction of fault-level currents, and the speed at which the circuit breaker must operate, are two of the major factors considered in breaker design. For this reason, the operating mechanism, the arc-interrupting medium, the insulation system, and the design of the main frame must be taken into account for every intended application. The types of circuit breakers included in this module are categorized in terms of the insulation medium employed for arc extinction, their associated operating mechanisms, and components unique to their respective designs. OBJECTIVES U pon completion of this module and lab practice, the participant will demonstrate, by attain- ing a minimum average grade of 80% (between lab and final exam), that he/she is able to: 1. Explain the functions and ratings of a circuit breaker. 2. Summarize the principles of circuit breaker arc interruption and the materials used for arc interruption. 3. Outline the general construction of circuit breakers. 4. Describe a typical operating mechanism and control circuit for a power breaker. The American National Standards Institute (ANSI) defines a circuit breaker as: “A mechanical switching device, capable of making, carrying and breaking currents under normal circuit con- ditions. Also capable of making and carrying for a specified time and breaking currents under specified abnormal circuit conditions, such as those of a short circuit.” The National Electric Code (NEC) defines a circuit breaker as “a device designed to open and close a circuit by non-automatic means, and to open the circuit automatically on a predetermined over-current without damage to itself when properly applied within its rating.” 2

Circuit Breaker Fundamentals The first circuit breakers looked very much like a knife switch (Figure 1). These switches would be closed and opened by hand. Modern circuit breakers operate like a switch when turning on and off power to equipment, but as the ANSI definition indicates, they also have to provide pro- tection to the systems they are supplying power to. When a fault or short circuit occurs a circuit breaker becomes a protective device and will automatically disconnect and isolate the affected part of the system. The ability to protect the equipment it supplies is what separates a circuit breaker from a simple switch Whether used manually or automatically, the medium voltage circuit breaker must be applied in such a way that it operates without damage to itself. 18-0009 FIGURE 1 Typical 3Ø Knife Switch 3

STANDARDS T he Institute of Electrical and Electronic Engineers (IEEE) C37 and the International Elec- trotechnical Commission (IEC) 62271 electrical standards govern the ratings, performance, features, and testing of circuit breakers and switchgear. The primary goal is to ensure that the circuit breakers serve the intended purpose of safely protecting the electrical distribution system. Without electrical standards, the utility grids would be unreliable, costly to operate, and difficult to connect. Standards for medium voltage circuit breakers and switchgear can be broken down into two general categories. Those categories are: 1. Design, manufacturing and factory testing. 2. Field maintenance and testing. Among the manufacturing standards are: • C37.04 IEEE Standard Rating Structure for AC High Voltage Circuit Breakers • C37.06 IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis—Preferred Ratings and Related Capabilities for Voltages Above 1000V • C37.09 IEEE Standard Test Procedure for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis • C37.54 American National Standard for Indoor Alternating Current High-Voltage Circuit Breakers Applied as Removable Elements in Metal-Enclosed Switchgear—Conformance Test Procedures • IEC 62271-100 High-voltage switchgear and control gear Part 100 High-voltage alternating- current circuit-breakers • IEC 62271-200 High-voltage switchgear and control gear Part 200, AC metal-enclosed switchgear and control gear for rated voltages above 1 kV and up to and including 52 kV Field maintenance and testing of newer medium voltage circuit breakers and switchgear is gov- erned by the manufacturer’s instruction manual when available. In some cases the manufacturer will reference other industry-recognized safety and technical standards within their literature. 4

Circuit Breaker Fundamentals Older equipment manufactured in the early 1970’s and beyond provided little direction on the proper maintenance and testing of this critical equipment. Inspection, cleaning and lubrication were the primary areas addressed. Little or no information was provided regarding testing of the system insulation or the primary current path. In situations such as this, or when the manufac- turer’s instruction manual is not available, industry-recognized standards should be used. Two of these standards are: 1. International Electrical Testing Association (NETA), ANSI/NETA MTS, Standard for Maint- enance Testing Specifications for Electrical Power Equipment and Systems 2. National Fire Protection Association (NFPA), NFPA 70B, Recommended Practice for Electrical Equipment Maintenance Both of these maintenance testing standards are similar in nature and provide guidelines for electrical power system maintenance and testing. ANSI/NETA MTS, as an ANSI standard, will be referenced throughout this course of instruction. RATINGS C ircuit breakers must conform to the particular characteristics of the circuit to which they are applied. The design specifications must conform to the applicable ANSI/IEEE standards and provide the following: 1. Voltage ratings a. Rated Maximum Voltage: The highest rms (root mean square) voltage for which the circuit breaker is designed, and the upper limit for operation. b. Nominal Voltage Rating: The system voltage to which the breaker is applied. For example; a 15kV maximum rated circuit breaker applied on a 13.8kV nominal voltage system. c. Rated Voltage Range Factor K: The ratio of the rated maximum voltage to the lower limit of the range of operating voltage in which the required symmetrical and asym- metrical interrupting capabilities vary in inverse proportion to operating voltage. d. Power Frequency Withstand Voltage: The maximum voltage the circuit breaker insula tion system is designed to withstand at rated frequency for one minute. 5

e. Basic Impulse Level (BIL): The surge voltage (lightning strike) the circuit breaker is designed to withstand. Defined as the voltage that peaks in 1.5 microseconds and falls to one-half that value in 50 microseconds. 2. Current ratings a. Continuous Current Rating: Standard current rating based on the maximum RMS amperes at line frequency that the breaker will carry continuously under usual service conditions without exceeding the heating limits specified in the standardization rules of ANSI/IEEE. b. Rated Short Circuit Current (Required Symmetrical Interrupting Capability): The value of the symmetrical component of the short-circuit current in RMS amperes at the instant of arcing contact separation that the circuit breaker shall be required to interrupt at a specified operating voltage, on the standard operating duty cycle, and with a DC component of less than 20% of the current value of the symmetrical component. c. Asymmetrical Interrupting Current (Figure 2): The value of the total RMS short-circuit current at the instant of arcing contact separation that the circuit breaker is required to interrupt at a specified operating voltage and on the standard operating duty cycle. This is based upon a standard time constant of 45ms (X/R ratio =17 for 60 Hz and 14 for 50 Hz systems) and an assumed relay operating time of .5 cycles. 18-0010 FIGURE 2 Short Circuit Current Components 6

Circuit Breaker Fundamentals d. DC Component: The “%dc” that circuit breakers are certified to interrupt, is based on the contact part time and a standard x/r decrement curve. The combination of the contact part time and the nominal x/r value, results in the maximum value for % dc that the circuit breaker must interrupt. The nominal x/r of 17 coincides well with the typical 60 Hz industrial substation and utilities distribution systems. The %dc is then used to compute the total interrupting current of the circuit breaker at the moment of contact opening. The following equation shows how this total current is computed I Asymmetrical = 1 Symmetrical √1 + 2 (%DC ÷ 100)2 If the nameplate does not provide the %DC component such as Figure 7 does, the DC component can be calculated using the following formula. %DC = 100e(-t/45) Where e is 2.718281828459045 and t is the contact part time (in ms) of the circuit breaker (including ½ cycle relay time). The value of “45” in the denominator of the exponent is the circuit time constant of 45 ms, which is the time constant of decay of the dc component in a circuit with an X/R ratio of 17 on a 60 Hz system. As an example, to find the asymmetrical interrupting capability of a 36kA, 3-cycle rated breaker with a published opening time of 25 milliseconds, a contact part time of 33 milliseconds is used. The contact part time includes half cycle of minimum relay ing time added to the opening time of the breaker. Using the 33 millisecond contact part time, the breaker is capable of interrupting the 36kA symmetrical current with a 48% dc component riding on top of the symmetrical current. When these values are plugged into the formula above, the total rms current is 52.5kA. Since, this breaker is certified as a 3-cycle breaker it is certified to interrupt a total current of 52.5kA from 3 cycles to 2 seconds. If a 5-cycle breaker was certified, the contact part time is 50 msec. The total interrupting current rating would be 39.5kA, whether it clears in 5-cycles or 2 seconds. e. Rated Closing and Latching Current: The breaker’s capability to close and latch at any power frequency making current whose maximum peak is equal to or less than 2.6 times the 60 Hz power frequency rated shortcircuit current, or 2.5 for 50 Hz power frequency times the rated short-circuit current. 3. Operating time in cycles This rating is defined as the time from the energizing of the trip coil with normal voltage until the circuit is interrupted. 7

THE INTERRUPTING RATING C ircuit breakers are designed to interrupt load current and the abnormally high currents present during a short circuit. This latter requirement is the determining factor for many of the principal construction features and has a direct effect on the breaker’s physical size and cost. To understand this, first consider the effects of the magnetic and thermal forces that are present during fault current. When electric current is flowing through a conductor, a magnetic field is produced around that conductor, the intensity of which has a direct relationship to the magnitude of current, Figure 3(A). When two such current-carrying conductors lie adjacent to one another, a force of attraction, Figure 3(B), or repulsion, Figure 3(C), will exist between them because of the interaction of their magnetic fields. If the currents are extraordinarily high, the forces produced may be enormous. FIGURE 3 Magnetic Fields 8

Circuit Breaker Fundamentals The magnitude of current flowing in a circuit under normal conditions is determined by the energy demand of the apparatus connected to the circuit. This amplitude of current flow is referred to as normal or load current. A short circuit essentially disconnects the load and replaces it with zero impedance. In this situation, the circuit current is limited only by the impedance of the circuit conductors and transformers from the generation to the point of fault. The resulting fault current can reach extremely high levels, and its effects can be very destructive, making it imperative to inter- rupt the circuit as rapidly as possible in order to keep damage to a minimum. Ohm’s law clearly demonstrates this problem: I = E Z Where: I = current E = applied voltage Z = circuit resistance Using this formula for a typical load value yields: E = 2,300 volts Z = 10 ohms I = 230 amperes However, under short-circuit conditions the circuit impedance Z might only be .05 ohms giving: E = 2,300 volts Z = .05 ohms I = 46,000 amperes If this magnitude of current were to flow through a breaker, enormous magnetic forces would be set up between the current carrying poles and the temperature of the breaker would get extremely high, perhaps high enough to destroy the breaker. The destructive forces set up by high-level fault currents must be successfully interrupted within a very short time. By examining this example, you see that the magnitude of the short circuit current decreases as the resistance increases. The level of fault current is limited by the impedance of the short circuit current path from the source to the fault and back to the source. Generally speaking, the further away from the generated source, the lower the level of fault current. 9

MVA VERSUS KA T he older circuit breaker rating structure recognized the prevalent medium voltage inter- ruption technology (air magnetic) of the time. It was based on a “constant MVA” inter- rupting capacity (Figure 4) over a defined range of operating voltages. At the maximum design voltage of the air magnetic circuit breaker, the interrupting capacity was limited by the ability of the arc chutes to handle the transient recovery voltage that appears across the circuit breaker contacts following interruption. As the operating voltage was reduced, the in- terrupting capability of the circuit breaker would increase, because the contacts could cope with higher interrupting currents and transient recovery voltage became less of a concern. Finally, a limit would be approached at which the contacts could not absorb further increas- es in heat during interruption. The maximum design voltage was designated as “V,” and the range over which the inter- rupting current capability increased as voltage decreased was defined in terms of voltage range factor “K.”The voltage V/K defined the associated lower limit of voltage. GENERAL ELECTRIC MAGNE-BLAST CIRCUIT BREAKER TYPE AM-13 8-500-5H SER. NO. 0204A2040-004 RATED 1200 15000 INIT 8 RATED 13800 CY 60 MAXIMUM TIME CY VOLTS AMP DESIGN VOLTS RATED 500 INT AMP AT 21000 MAX INT 25000 MOM 40000 INT MVA RATED VOLTS AMP AMP CLOSING 6174582 G1 VOLTS 125 6CLOSING DC VOLT 90-130 COIL AMPS RANGE POTENTIAL 6174582 G1 VOLTS 125 AMP 6 DC VOLT 70-140 TRIP COIL RANGE U/V TRIP VOLTS CURRENT AMP COIL TRIP COIL AC RELAY VOLTS C VOLT WT. 1650 COIL RANGE IMPULSE 95 KV MECH ML-13 DATE 3/67 WITHSTAND TYPE MFG CAUTION! BEFORE INSTALLING OR GEI-88764 OPERATING READ INST. PHILADELPHIA, PA. MADE IN U.S.A. R N.P. 202519 17-0740 FIGURE 4 Nameplate with MVA Interrupting Rating 10

Circuit Breaker Fundamentals In the range of V/K to V, the interrupting current varied so that the product of voltage and interrupting current was a constant value. Simply stated, the interrupting MVA (interrupting current X voltage X 1.732) was constant over this range. For example; a circuit breaker with a maximum design voltage of 15kV and a K factor of 1.3, (Figure 5) the circuit breaker’s interrupting rating was within its tolerance between 15kV and 11.5kV (15kV ÷ 1.3). POWELL Powered by Safety R Power/Vac R Circuit Breaker Houston, Texas, USA www.powellind.com Ratings Nameplate 713.790.1700 Type 731 13.8-500-3 Serial No. 570132-01-016 15RATED MAX kV RATED 1200 AMPS 60 Hz IMPULSE 95 kV INIT 5 CYC CURRENT WITHSTAND TIME VOLTAGE RATED SHORT 18 kA RATED VOLTAGE 1 . 30 CLOSE & LATCH 37 kA CIRCUIT AMPS RANGE FACTOR CAPABILITY AMP CLOSE 0209B8103G009 VOLTS 125VDC CLOSING 6.0 VOLT 100 -140 COIL 125VDC AMPS 5.9 RANGE TRIPPING TCROIIPL -10209B8104G002 VOLTS AMPS VOLT 70 -140 RANGE TRIP -2 VOLTS TRIPPING VROANLTGE COIL AMPS CHARGING 0177C5050G004 VOLTS 125VDC MOTOR WT 470lbs CONNECTION 0209B8267P001 MECH ML-18 DATE 11-12 DIAGRAM TYPE MFG VAC.INTER.TYPE 50E1 REQ: CEP-20796 SO: 570132 D-409 ORDERING PVSD1A1A2A2202D1A2 CAP SWITCHING NO RATING NO. AMPS CAUTION: BEFORE INSTALLING OR OPERATING READ INSTRUCTIONS GEK86132 17-0738 FIGURE 5 Nameplate with Interrupting Rating and 1.30 Voltage Range Factor 11

These relationships are summarized in Figure 6. Maximum symmetrical Interrupting capability = interrupting capability - rated 1 x KxI (rated V/operating V) Rated symmetrical interrupting current I V/K Rated maximum voltage = V 17-0741 FIGURE 6 Interrupting Rating; KA versus MVA 12

Circuit Breaker Fundamentals The “constant MVA” rating structure served the industry well for many years. However, as new interrupting technologies became available, the “constant MVA” relationship became a poor representation of the actual physics of interruption. In particular, one of the desir- able characteristics of a vacuum interrupter is the dielectric withstand capability across the open contacts to recover nearly instantaneously following an interruption. Therefore, the interrupting capability of the vacuum interrupter does not increase significantly as the operating voltage is decreased from rated maximum design voltage. The 1999-2000 revisions to the ANSI standards recognized this by changing the voltage range factor (K) to equal 1.0, which effectively removes it from the rating structure (Figure 7). POWELL Powered by Safety R Power/Vac R Circuit Breaker Houston, Texas, USA www.powellind.com Ratings Nameplate 713.790.1700 Type VB 15 0-25-4 Serial No. 025-PR-0216-0144 15RATED MAX kV RATED 1200 AMPS 60 Hz IMPULSE 95 kV INIT 5 CYC CURRENT WITHSTAND 68 TIME VOLTAGE kA RATED SHORT 25 kA SHORT TIME 25 CLOSE & LATCH CIRCUIT AMPS CURRENT 2s CAPABILITY AMP DC 35 % RATED VOLTAGE 1 . 00 OPERATING 0 - 0.3s - CO - 3m - CO COMP RANGE FACTOR DUTY CYCLE CLOSING CLOSE 02B2A2008G002 VOLTS 125VDC AMPS 6.0 VOLT 100 -140 COIL TRIPPING 10 . 2 RANGE AMPS TRIP -102B2A2009G008 VOLTS 125VDC TRIPPING VOLT 70 -140 COIL AMPS RANGE TRIP -2 VOLTS 125VDC VOLT COIL RANGE CHARGING 0177C2164G001 VOLTS MOTOR CONNECTION 0701P00WR3713 WT 590lbs TMYEPCEH ML-17 MDAFTGE 02-16 DIAGRAM A CLASS C2 CABLE CAPACITANCE CURRENT SWITCHING CHARGING A CLASS C2 BSAWCITKC-THOC-BUARCRKECNATP CISAOPLABTAENDK A CLASS C2 TRANSIENT INRUSH kA TRANSIENT INRUSH kHz CURRENT FREQUENCY VAC.INTER.TYPE 47A REQ: CEP847201 SO: 622529-05 D-451 ORDERING PVMIIFIA2A22020112 MANUAL 01 . 4IB . 66000B NO. CAUTION: BEFORE INSTALLING OR OPERATING READ INSTRUCTIONS 17-0739 FIGURE 7 Nameplate with Interrupting Rating and 1.00 Voltage Range Factor 13

PRINCIPLES OF ARC INTERRUPTION T he principles of arc interruption depend greatly on the medium in which the arc is inter- rupted. The arc behavior is different in open air compared to oil, vacuum or SF6. Regardless of the medium, the circuit breaker must initiate a physical separation in the current- carrying path and provide an insulating medium that is sufficient to prevent the current from continuing to flow. The circuit is usually opened by drawing out an arc between the contacts until the arc can no longer sustain itself and is extinguished. Arc Interruption: Air Circuit Breaker An air circuit breaker employs air as the interrupting insulation medium. Of all the insulating media mentioned, air is the most easily ionized; hence, arcs formed in open air tend to be severe and persistent. Each pole unit of an air circuit breaker consists of a set of main contacts and a set of arcing contacts. The arcing contacts open after the main contacts and the arc is drawn on them, thereby avoiding severe burning and pitting of the main contact surfaces, Figure 8. As the contacts separate, the arc is transferred to arc runners in the throat of the interrupting cham- ber, and a puff of air is used to blow the low energy arc into the arc chute. The current is made to flow through coils that set up a strong magnetic field that forces the arc deep into the interrupting chamber. These coils are typically called blowout coils. The arc chutes are made of insulating material such as porcelain or slate. Their function is to stretch out the arc; cool it; and, at an early current zero, extinguish it. The arc chutes are designed to totally enclose the contact assemblies and provide an enclosed channel in which the arc is dissipated and kept from striking adjacent poles or grounded members. 14

Circuit Breaker Fundamentals FIGURE 8 Breaker Operation 15

Arc Interruption: Air Blast Circuit Breaker This design, Figure 9(a), uses a compressed air mechanism. When compressed air flowing from the reservoir through piping to the pneumatic mechanism is admitted on the upper side of the mechanism piston, the piston moves downward causing the shaft cam to rotate and the operating rod to pull open the moving contact arm. Arrows, Figure 9(b), indicate the flow of compressed air from the reservoir to the mechanism for a breaker opening operation. Rotation of the shaft cam causes various levers to open the blast valve thus allowing com- pressed air to pass from the reservoir up the blast tube and between the separating contacts. The compressed air, after aiding in extinguishing the arc, passes up through the arc chute coolers and out the top of the arc chute. The moving contact arm continues to move away from the contact fingers until it reaches its full open position. The shaft cam allows the blast valve levers to drop off the cam and the blast valve closes after circuit interruption. The closing operation is just the reverse of the opening operation. First, the compressed air, flowing from the reservoir to the pneumatic mechanism, flows in the bottom of the mechanism and underneath the piston thereby causing the piston to move upward instead of downwards as in the opening or tripping operation. This flow of compressed air is controlled by the opening and closing of the magnet valves on the pneumatic mechanism. During a breaker closing operation, the magnet valve on the bottom of the pneumatic mechanism is open, allowing compressed air to flow underneath the mechanism piston while the magnet valve on the top of the mechanism is closed. The upward movement of the piston causes the operating rod to push the moving contact arm up. The moving contact engages the contact fingers, completing the current path from the lower breaker terminal to the upper breaker ter- minal. In closing the breaker, the shaft cam rotates clockwise but the blast valve levers are designed so that the blast valve does not open. Due to the speed of the moving contact arm, air blast on closing is not required. 16

Circuit Breaker Fundamentals 17-0743 FIGURE 9(a) Westinghouse Type CA Air Blast Circuit Breaker 17

Arc chute Arc splitter Cooler Arc tip Arc tip Contact finger Upper terminal Moving Lower contact terminal arm Blast tube Operating rod Blast valve Shaft cam Magnet valve Piston Reservoir Check valve Pneumatic mechanism 17-0742 Magnet valve Incoming air supply Wiring channel FIGURE 9(b) Westinghouse CA Air Blast Breaker Air Flow 18

Circuit Breaker Fundamentals Arc Interruption: Vacuum Circuit Breaker In vacuum circuit breakers, the contacts are drawn apart in a chamber from which air has been evacuated, Figure 10. Since the arc in an air breaker is essentially an electric conductor made up of ionized air particles and contact vapor, an arc is extremely difficult to form in a vacuum. This is the reason that a vacuum makes a very effective arc interruption medium. It is impossible, however, to achieve a perfect vacuum; thus, some arc is always possible. Also, a small amount of the contact material will melt and be thrown into the gap between the contacts. This condition will cause an arc until the gap is great enough to extinguish it. FIGURE 10 Vacuum Interrupter Cutaway 17-0501 19

The contacts inside a vacuum bottle are only spaced 1/4 inch in a 5 kV breaker to 7/8 inch in a 34.5 kV breaker, but due to the vacuum, or lack of air, this space is sufficient to maintain insulation between line and load. The normal vacuum in a bottle is from 10-6 to 10-8 Torr. One Torr is equal to a column of mercury one millimeter high or 1/760 of an atmosphere. A vacuum does not dissipate heat as readily as other insulating media does. Some manufac- turers of the higher amperage vacuum circuit breakers will install heat sinks on the end of the bottle to help dissipate the heat. Although this type of breaker has certain advantages in terms of its size and simplicity, it is sometimes necessary to have heat sinks installed on the bottles. Additional information on vacuum interruption is located in Appendix A. Arc Interruption: Sulphur Hexafluoride (SF6) Circuit Breaker The SF6 circuit breaker, Figure 11, is similar to the vacuum circuit breaker except the vacuum is replaced by an inert, non-toxic, odorless, tasteless gas – Sulphur Hexafluoride. When pres- surized, the dielectric strength of the gas increases, and extinguishes the arc very rapidly. Once the arc is extinguished, the gas quickly recombines. It also has excellent heat-dissipating characteristics, and its dielectric strength is much greater than that of air or oil. FIGURE 11 Upper Stationary Medium Voltage SF6 Circuit Terminal Arcing Contact Nozzle Breaker Pole Unit Stationary Movable Main Contact Arcing Contact 20 Piston Movable Main Contact Operating Lower Lever Terminal Alumina SF6 Pressure Silicate Test Valve 96-0240 SF6 1- 6

Circuit Breaker Fundamentals Arc Interruption: Oil Circuit Breaker Oil circuit breaker contacts are immersed in oil. Current interruption takes place in the oil, which cools the arc developed, and quenches the arc. For medium voltage circuit breakers, all three phases can be placed in one oil tank, Figure 12. The oil tank on medium voltage oil circuit break- ers is normally sealed and the electrical connections between the contacts and external circuits are made through bushings similar to oil transformers. 18-0012 FIGURE 12 GE Medium Voltage Minimum Oil Circuit Breaker 21

CONTACTS Medium voltage circuit breaker contact design varies between manufacturers and between the interrupting mediums. Regardless of manufacturer or interrupting medium, circuit breaker movable contacts operate from an external operating mechanism. These contacts are moved from the closed position when the breaker is opened, thus interrupting the circuit. When in the closed position, the operating mechanism is held in place by some means so that the moving contacts are held tightly closed and properly mated with the stationary contacts. When the circuit breaker has to interrupt a circuit, the trip circuit receives an impulse, permitting the operating mechanism to move the contacts to the open position. On medium voltage circuit breakers there are three principal types of circuit breaker contacts: butt, wedge and bayonet. Butt contacts consist of two solid elements with flat mating surfaces, Figure 13. Silver-plated or silver-alloyed tips reduce contact resistance and reduce heating and pitting. Butt contacts are the primary contact type for vacuum circuit breakers and can be found on some air circuit breakers. 18-0011 FIGURE 13 Typical Single Butt Contact Surface 22

Circuit Breaker Fundamentals The wedge (moving contact) is squeezed into spring jaws, Figure 14, when the breaker is closed, providing excellent contact area. Forcing the wedge between the spring jaws also produces a wiping motion that maintains a clean contact surface. Arcs are normally drawn on the top of the blade, thus protecting the actual contact area from damage. The moving contacts are usually made of hard copper alloy with renewable arcing tips. These tips are very large which gives them a high thermal capacity and thus minimizes burning. The stationary contacts consist of fingers arranged in pairs so that they may surface well on both sides of the moving contact when the breaker is closed. Flat springs on the fingers permit the contacts to align themselves automatically on the wedge so that full current-carrying capacity can always be obtained. One or more pairs of fingers are used, depending upon the current capacity required. As a rule, the blade is large to aid in radiating heat away from the arcing tips. Wedge contacts are the primary contact type on many air circuit breakers for arcing contacts, main contacts, or both. 18-0013 FIGURE 14 Typical Wedge Contacts 23

The bayonet contact, Figure 15, consists of a cylindrical male side and a matching female re- ceptor with spring(s) to keep the two parts locked together. This type of contact is used chiefly in oil circuit breakers and SF6 gas circuit breakers. 18-0014 FIGURE 15 High Voltage SF6 Circuit Breaker Bayonet Contact INSULATION REQUIREMENTS Insulation is a major consideration in the design and construction of circuit breakers and serves four basic purposes: • Arc quenching • Cooling • Normal voltage insulation • Impulse (surge) voltage insulation While some of the insulation components in the breaker serve specific purposes such as the insulation between the circuit breaker frame and the energized parts, they must all work together to make up the circuit breaker insulation system. 24

Circuit Breaker Fundamentals The higher the voltage the insulation requirements become more stringent. Therefore, the in- sulation system is more complex. To simplify the insulation system, it may be broken into three different segments as follows: 1. Breaker frame insulation material (oil, air, SF6, etc.) 2. Bushings 3. Miscellaneous insulation such as wood, phenolic, and thermoplastic wire insulation. CIRCUIT BREAKER CONTROLS R emote close and trip, automatic reclosing, trip-free operation, anti-pump operation, and other such requirements all contribute to the complexity of the breaker control circuit. Fig- ure 16A shows a typical circuit breaker closing circuit, and Figure 16B shows a tripping circuit. AC or DC 52 Y Control Switch FIGURE 16A a Typical Close Circuit X X Y FIGURE 16B Y Typical Trip Circuit CC X 25 96-0243A-1 SSM I Fig 11 A Typical ClosiCnSg SchemSaI tic GT 24, 48,125 R SI 51 50 or 250 V DC 52 b TC 52 a 96-0243B-1 SSM I Fig 11 B Typical Tripping Schematic

The schematic of the closing circuitry is shown in Figure 16A. When the control switch contact is closed, it energizes the X coil through the normally closed Y contact. The X contact energizes the closing coil (CC) which closes the breaker. When the 52/a contact closes, the Y relay is energized through the X contact and the 52/a contact. The Y contacts change state, which deenergizes the X coil and seals in the Y relay through the control switch. As long as the control switch is held in the closed position, the Y relay stays energized and prevents another close. Therefore, there is only one close operation of the circuit breaker, for each turn of the control switch. This feature is called anti-pump. To trip the breaker, the time overcurrent contact (51) or the instantaneous overcurrent contact (50) is closed by system current, Figure 16B. This energizes TC through the 52/a contact. (“a” contacts are closed when the breaker is closed and open when the breaker is open; the op- posite is true for “b”). If the trip is initiated by the time overcurrent contact, the seal-in coil is energized and operates the seal-in contact, thereby sealing in the trip circuit. As soon as the breaker trips, the 52/a contact opens and resets the trip circuit. Circuit breaker tripping is normally performed by operation of the control switch to the trip posi- tion, (CS/T) switch. This is the basis for the operation of circuit breakers from 480 volts to 765 kV with stored energy mechanisms. The big difference is that the larger the breaker, the more components that are involved in the safe operation of the circuit breaker. Circuit breaker control will be covered in more detail in Module 5. METHODS OF OPERATION T he previous description of breaker control operation discussed only the electrical controls for the breaker. Solenoids, springs, pneumatics, magnetics, or hydraulics perform the actual breaker operation. 26

Circuit Breaker Fundamentals Solenoid The solenoid type of mechanism consists of a powerful closing coil, relatively small trip coil, latching device, auxiliary switch, and system of levers, links and toggles for multiplying the short stroke of the closing coil armature, Figure 17. Energizing the solenoid closes the breaker. When the solenoid is energized, the armature is pulled in, pushing the three-link mechanism to latch the breaker. When the breaker has latched closed, the solenoid is deenergized by means of the auxiliary switch. When the trip coil is energized, the breaker is unlatched and the contacts are forced open by gravity and a spring action. Solenoids may be either AC or DC design. The solenoid is not considered to be a “stored energy” device because it will only operate when power is applied to it. The solenoid is very temperature sensitive and has voltage limitations; hence most have been removed from service and replaced with the stored energy mechanism. FIGURE 17 GE MS Solenoid-Operated Mechanism 27

Stored-Energy (Spring-Operated) Mechanisms Stored-energy, spring-operated mechanisms use large springs for the closing and opening operation of the circuit breaker. These springs can be charged manually or electrically with a universal (AC/DC) motor. The stored-energy mechanism performs two functions. It stores closing energy by compress- ing or charging the closing spring, applies the released energy to close the breaker and si- multaneously charges the opening springs, Figure 18. Because the closing spring must have sufficient energy to both close the breaker and charge the opening springs, it will be larger than the opening springs or there will be more than one closing spring. The opening springs will be much smaller than the closing spring. A stored energy, spring-operated mechanism may rest in any one of four positions: • Breaker open, closing spring discharged • Breaker open, closing spring charged • Breaker closed, closing spring discharged • Breaker closed, closing spring charged Regardless of the manufacturer, the spring-operated mechanism performs the same functions. That is, the closing spring is charged by an electric motor. When the spring is fully charged the motor shuts off. When the closing spring pressure is released, the motor activates to recharge the closing spring, and the breaker moves to the closed position and charges the opening spring. When the opening spring pressure is released, the circuit breaker moves to the open position. Closing springs do not hold the contacts closed. Over a period of time, they would weaken, causing the contacts to bounce, vibrate, and burn. On stored energy, spring-operated mecha- nisms, the contacts are held in the closed position by a prop and roller operating mechanism. The prop and roller puts the contact linkage into a mechanical bind, forcing the contacts to stay tightly closed. 28

Circuit Breaker Fundamentals 18-0015 FIGURE 18 GE ML-18 Mechanism Close and Opening Springs 29

In the closed position, Figure 19, the insulated coupling (12) holds the contacts (8) closed due to the alignment of the prop (2), cam main roller (5), secondary latch roller (6), trip latch (11), and the secondary latch (14). The trip latch holds the secondary latch from rotating clockwise. The secondary latch is positioned against the secondary latch roller (yellow), which in turn ex- tends the cam main roller (red) against the prop. The opening spring (not shown in this view) is exerting pressure on the contacts to open. Note that the secondary latch (14) is held against the secondary latch roller (6), which is pushing the main roller (5) and its linkage into a vertical position. The main roller is in turn held against the prop (2), which prevents it from overextending. The centerline of the insulating coupling pin is in a straight line with the main roller through the camshaft. In this position, the contacts (8) are unable to open until the linkage collapses, which cannot happen until the trip latch (11) releases the secondary latch (14). 18-0016 FIGURE 19 Mechanism in Closed Position Opening Springs Charged 30

Circuit Breaker Fundamentals To open (trip) the breaker, Figure 20, the trip latch is rotated clockwise, allowing the secondary latch to rotate counter-clockwise. When it does, the main roller (5) and the secondary latch roller (6) collapse. This allows the opening springs to pull the contacts open. The bell crank lever (7) is used to change motion in one direction into motion in another direction. As the linkage collapses, the bell crank rotates, allowing the contacts (8) to open. 18-0017 FIGURE 20 Mechanism in Open Position 31

In Figure 21 the circuit breaker is open and reset. That is, the close spring is charged in preparation for a close signal. The trip latch (11) and the secondary latch (14) are reset to the same position as when the breaker is in the “closed” position. In order for the mechanism to be in this position, the cam (3) has to be rotated slightly counter-clockwise until the prop (2) is lifted up, allowing the main roller (5) and linkage (green) to slip into the crook of the prop. In so doing, the linkage is slightly extended, and the secondary latch (14) engages the front of the frame (shown in blue), which allows a gap between the trip latch and the secondary latch. The prop and cam reset to their original position. If the breaker close button is pressed, the closing springs will accelerate the contacts closed. The cam and prop will rotate, extending the linkage and forcing the components into the same positions as shown in Figure 19. 18-0018 FIGURE 21 Mechanism in Open Position Closing Spring Charged 32

Circuit Breaker Fundamentals Pneumatically Operated Mechanism Pneumatic operating mechanisms are operated with high pressure air, Figure 22. Air is stored at high pressure in order to operate the circuit breaker. Closing is accomplished by energizing the pilot valve. It allows air to push open the inlet (clos- ing) valve. This allows high pressure to enter the closing piston chamber, which moves the linkage to the closed position where it is held closed by a mechanical prop. When the breaker is closed an auxiliary switch contact (b) deenergizes the pilot valve, which drops out and vents the air from the closing piston chamber allowing it to reset. Pressure switches perform the following functions: 1. Call compressor on and off 2. Provide an alarm if air pressure falls to a predetermined level 3. Prevent breaker closure if the pressure is too low The circuit breaker is opened using spring pressure (not shown). Pilot Valve Exhaust Coil Valve Pilot Valve Inlet Valve Main From Closing Reservoir Piston FIGURE 22 13-0071 Pneumatic Operating Mechanism 33

Hydraulically Operating Mechanism Hydraulic mechanisms, Figure 23, are used to operate some high voltage circuit breakers. However, in some cases, high voltage (72kV class) circuit breakers are used in medium voltage applications due to a higher interrupting rating that other breaker types may not be able to achieve. These mechanisms operate by using pressurized aircraft-quality fluid acting against a piston. There are two energy storage methods used to create the pressure needed: pressurized dry nitrogen in a gas accumulator or a spring (spring pack). The pressurized dry nitrogen energy storage method uses an accumulator. The accumulator is divided into a hydraulic oil volume and nitrogen volume by a piston. The nitrogen volume is pre- charged to a given pressure (pre-charge pressure usually between 1500 psig and 2200 psig). A hydraulic pump element, driven by a charging motor, draws oil from a low pressure reservoir and compresses the oil into a high pressure volume which drives a piston to compress the dry nitrogen in the accumulator. During circuit breaker operation, stored energy is released from the nitrogen accumulator to the hydraulic system via a control valve responding to a signal from the trip or close coil. This energy is transferred to the drive piston of the mechanism causing it to move into either the open or closed position. There is no direct mechanical link between the nitrogen accumulator and drive piston. A heater and thermostat must be used to ensure adequate nitrogen pressure during severe cold temperatures. The spring pack energy storage method uses a stack of discs or bevel washer-like springs. The basic difference between these two hydraulic systems is that the pumped hydraulic fluid acts upon a piston that compresses the springs for stored energy rather than compressing a gas. The same hydraulic fluid is also used to open the circuit breaker. Because the cross sectional area of the drive piston is greater on one end compared to the other, a difference in pressure is realized across the piston even though it’s the same pressurized fluid. When the breaker is called upon to open, the trip coil opens a port to dump the fluid on one side of the piston back to the low pressure reservoir. The remaining high pressure fluid on the other side of the piston opens the circuit breaker. 34

Circuit Breaker Fundamentals FIGURE 23 Hydraulically Operating Mechanism 35

Magnetically Operated Mechanism The newest mechanism operating technology is the magnetic actuator, Figure 24. The actuator is a bi-stable magnet system, in which armature change-of-state is accomplished by the magnetic field of two electrically excited coils. The armature is held magnetically in the limit positions (open or closed) by the fields of two rare-earth permanent magnets. Switching operations are achieved by excitation of one of the two coils until the retaining force of the permanent magnets is exceeded. The armature of the magnetic actuator is linked to an operating shaft connected via insulated push rods to each of the vacuum interrupters. A Capacitor Assist C. Manual Open 18-0019 B. Electronic Controller D. Proximity Sensors FIGURE 24 Magnetic Actuator 36

Circuit Breaker Fundamentals The magnets are controlled via an electronic controller and magnetic proximity sensors, Figure 25. As the armature shaft moves from the open position to the closed position or vice versa, the proximity sensors tell the controller when to turn on or off. The proximity sensors act as the auxiliary contacts for this mechanism. 18-0020 FIGURE 25 Magnetic Actuator Proximity Sensors 37

AUXILIARY SWITCHES A uxiliary switches (Figure 26) are mechanically connected to most mechanisms. They provide the following functions: 1. Indicates circuit breaker position and interrupts close and trip coil currents 2. Performs logic in the DC control circuits 3. Prevents the close coil from energizing if the circuit breaker is already closed 4. Prevents the trip coil from energizing if the circuit breaker is already open 5. Contacts can be set to function as a 52a or a 52b 6. Adjustments can also be made in intermediate steps of 15–22° FIGURE 26 Auxliary Switch 18-0021 38

Circuit Breaker Fundamentals SUMMARY T he design and construction features of circuit breaker equipment must take into account three important factors: • Maximum voltage and continuous current rating • Interrupting rating • Speed of operation A particular design is selected in order to meet all of the requirements and operational charac- teristics for circuit interruption under normal and abnormal conditions. Circuit breakers can be categorized, in terms of the insulation media employed for arc interrup- tion, into four distinct design groups: Open air , Vacuum , Oil , SF6 . The insulation requirements are determined by the maximum design voltage of the breaker. The means employed for arc control, phase-to-phase and phase-to-ground insulation require- ments, and bushing design must be taken into account. High voltage applications require operating mechanisms that extinguish fault-level current in 2 to 3 cycles. This requirement is met by the pneumatic or hydraulic mechanism. It is extremely important that circuit breaker maintenance programs be designed to determine the operational integrity of the operating mechanism, the quality of the insulation system, and the ability of the breaker and its associated auxiliaries to respond to overload and, most im- portantly, fault conditions. January 2018 39

REVIEW QUESTIONS Objective 1 – Explain the functions and ratings of a circuit breaker. 1. A circuit breaker is used to open and close an electrical circuit manually; that is, it is _________________. 2. The circuit breaker must be applied in such a way that it operates ______________. 3. The maximum voltage the circuit breaker insulation system is designed to withstand at rated frequency for one minute is referred to as the ____________________. 4. The current ratings of a circuit breaker are the ____________ rating and the maximum interrupting rating. Objective 2 – Summarize the principles of circuit breaker arc interruption and the materi- als used for arc interruption. 5. A short circuit essentially ____________________ and replaces it with zero impedance. 6. Intensity of the magnetic field around a conductor depends on the _______________ in the conductor. 7. In order to interrupt the flow of current, the circuit breaker must initiate a ____________________________ in the current-carrying element and provide an insulating medium that is sufficient to prevent current from continuing to flow. 8. The circuit is usually opened by ________________ between the contacts until the arc can no longer sustain itself and is extinguished. 9. Of all the insulating media mentioned, air is the most ___________________; hence, arcs formed in open air tend to be severe and persistent. Objective 3 – Outline the general construction of circuit breakers. 10. The four main insulating materials for circuit breakers are air, oil, ______ and ________. 40

Circuit Breaker Fundamentals Objective 4 – Describe a typical operating mechanism and control circuit for a power breaker. 11. As the _______________, the closing spring is compressed. 12. Energizing the solenoid on a circuit breaker’s electrically controlled operating mecha- nism __________________. 41

(This page intentionally left blank) 42