Application of New Technologies in Power Circuit Breakers with Higher Interrupting Capacity and Short Time Ratings

Transcription

1 Application of New Technologies in Power Circuit Breakers with Higher Interrupting Capacity and Short Time Ratings Charles J. Nochumson, P.E. Cutler-Hammer, Inc. 601 Oakmont Lane, Suite 310 Westmont, IL, Abstract - A new Low Voltage Power Circuit Breaker (LVPCB) has been introduced which has higher interrupting capacity and higher short delay current ratings than previously available in LVPCBs. The entire new power circuit breaker including operating mechanism and contacts has been designed and tested to meet or exceed the stringent requirements of ANSI standard C37.16 Low-Voltage Power Circuit Breakers and AC Power Circuit Protectors Preferred ratings, Related Requirements, and Application Recommendations, ANSI C37.50 Low-Voltage AC Power Circuit Breakers Used in Enclosures Test Procedures and ANSI standard C37.17 Trip Devices for AC Low Voltage PCBs. This paper will discuss the evolution of technology in materials and electronics, which has enabled improved performance, as well as provide examples of specific applications in the industry where the new PCB higher interrupting capacity and short time ratings are beneficial to the user. A need to review both short delay time as well as short delay current ratings of equipment will be discussed. Zone selective interlocking functions to reduce damage to protected equipment while still maintaining coordination will also be reviewed. I. A BRIEF DESCRIPTION OF AVAILABLE CLASSES OF LOW VOLTAGE CIRCUIT BREAKERS Low Voltage Power Circuit Breakers (LVPCBs) are rated and tested to ANSI C37 standards. They are mainly used in Low Voltage Metal Enclosed Drawout Switchgear built per ANSI C37 standards. LVPCBs offer field maintainability of main contacts, operating mechanism, and arc chute replacement. LVPCBs are tested in the enclosure and are rated for 100% applications. They offer 2 step stored energyclosing mechanisms, which have typical opening or closing times of 3 to 5 cycles. For electrical operation, they are typically combined with an internal motor to change the breaker closing springs and solenoids to release the spring stored energy for closing and opening of the breaker. They are typically applied in systems with selective coordination, synchronizing schemes and automatic transfer scheme applications. Molded Case Circuit Breakers (MCCBs) are tested according to UL489. They are typically applied in Low Voltage Switchboards, Motor Control Centers and Panelboards. MCCB current carrying parts, mechanism and Originally presented at the 1999 TAPPI Conference in March Copyright TAPPI trip devices are completely contained within a molded case, and are not designed to be field maintainable. MCCBs as standard are tested in open air and are de-rated to 80% when placed in an enclosure. Optional MCCBs are available which have been UL tested for applications at 100% in the enclosure. Molded case circuit breakers have over-center toggle operating mechanisms. For electrical operation, they are typically combined with external motor operating mechanisms, which move the breaker handle. The external motor operator is not fast enough for synchronizing or many automatic transfer scheme applications. MCCBs are also available in current-limiting type. Insulated Case Circuit Breakers (ICCBs) are also rated and tested in according to UL489. They are typically applied as mains in Low Voltage Switchboards, Motor Control Centers and some transfer switches. Similar to the MCCB, the ICCB is generally contained in a sealed molded case, and is not designed to be fully field maintainable. ICCBs are found as either 80% rated devices or 100% rated devices when mounted in the proper enclosure. ICCBs utilize 2 step stored energy mechanisms, similar to LVPCBs. ICCBs are normally not fast enough to qualify as current-limiting type. II. THE HISTORIC EVOLUTION OF LOW VOLTAGE POWER CIRCUIT BREAKER TECHNOLOGY The first low voltage power circuit breaker designs go back to the year 1929, when Dr. Slepian of Westinghouse invented the DE-ion arc chute. During a fault, this arc chute device enabled the LVPCB to effectively protect downstream equipment by splitting the resulting arc into several segments, which were then individually extinguished. Since 1930, power circuit breaker technology has advanced in several areas as shown in the timeline, Fig 1. A. Interrupting Ratings Interrupting capacity is defined as the maximum short circuit current the breaker can safely interrupt. Early designs of power circuit breakers were designed with interrupting capacity of 15/25kA. The latest designs of LVPCBs include interrupting ratings of up to 100kA at 480 volts, without the use of integral current-limiting fuses. Many users have elected to move away from the use of LVPCBs with current limiting fuses for some of the following reasons: Presented at the 1999 IEEE IAS Pulp and Paper Industry Conference in Seattle, WA: IEEE Personal use of this material is permitted.

2 ANSI/UL Low Voltage Power Circuit Breaker Timeline Year Introduced Interrupting Rating Mechanism Type Trip Sensor Type 15/25 ka 25 ka 30 ka 50/65 ka 100kA Solenoid Solenoid Stored Energy Stored Energy Stored Energy Spring Spring Spring Electro- Electro- Solid State Microprocessor Microprocessor mechanical mechanical Peak Sensing RMS Sensing RMS Sensing Figure 1: Timeline of Low Voltage Power Circuit Breaker Technology Developments including interrupting capacity improvements, available mechanisms and trip sensors from 1930 through When current limiting fuses are utilized as part of feeder circuit breakers, medium to high fault currents will cause the limiter to open, thus losing coordination with downstream protective devices. 2. Increased floor space of the low voltage metal-enclosed switchgear is required when fuses are incorporated into the assembly. 3. Increased downtime and cost associated with the replacement of blown current-limiters. Increases in ratings have come primarily from improved materials in breaker frame construction. Earlier designs used an open type, metal frame. Today s designs use engineered thermoset composite resins in a molded frame. The rigidity of the frame is a critical design point during fault current interruption, since much of the energy required to interrupt the fault is absorbed in the frame. Increased interrupting ratings mean higher spring energies in the operating mechanism. Steel frame breakers deflect during fault interruption and in some cases must absorb up to 40% of the spring energy. Thus, open steel frame breakers require larger springs and frames, which reduce overall mechanism life. Newer epoxy resin design frames have superior characteristics in overall frame deflection and include a molded arc chamber that provides support to the current path and protection from phase to phase arcing during interruption. The improved structural rigidity of the frame allows higher interrupting and short time ratings in a smaller overall package. Typical 3200 amp breaker frames and power contact designs in both steel and molded frame breakers are shown in Fig. 2. Figure 2: Shown at left is a comparison in size of a new epoxy resin frame LVPCB versus a traditional open steel frame construction. Both breakers are rated for 3200A at 600 volts. Shown at right is a newer finger-type design power contact assembly versus the traditional wedge-type design.

3 B. Operating Mechanism Early designs of operating mechanisms included solenoid operators. In 1967, higher closing speed requirements resulted in development of a two-step stored energy mechanism for Power circuit breakers. This mechanism allowed for the capability of fast reclosure (Open, Close- Open duty cycle) via the addition of a charging spring. This also facilitated easy remote operation, fast reclosing (5 cycles or less), and assured that energy was always on hand to open/close the breaker. The latest designs continue to use the two-step stored energy operating mechanism. C. Trip Unit Sensing Early design trip units in power circuit breakers were electromechanical type. Solid state sensing trip units were introduced in the late 1960 s, which replaced the initial designs. Early versions of solid state trip units used analog circuitry and were able to effectively sense and trip on peak currents. In some cases, this resulted in nuisance tripping conditions. In the 1980 s, digital solid state controls became available that were able to effectively integrate the current waveform to calculate a true rms current value. This effectively eliminated early nuisance tripping problems. Microprocessor based rms trip units were introduced in the early 1990 s and advanced versions of these are used in the latest design LVPCBs. Today, Low Voltage Power Circuit Breakers (LVPCBs) and metal-enclosed switchgear are designed and tested to ANSI, NEMA, and UL standards [1]. II. APPLICATIONS OF POWER CIRCUIT BREAKERS ANSI offers the following definition for Rated Short-Circuit Current for LVPCBs: Rated Short-Circuit Current for Unfused Circuit Breakers: The rated short-circuit current of an unfused circuit breaker is the designated limit of available (prospective) current at which it shall be required to perform its short-circuit current duty cycle (O-15s-CO) at rated maximum voltage under the prescribed test conditions. This current is expressed as the rms symmetrical value of current measured from the available current wave envelope at a time ½ cycle after short circuit initiation. Unfused circuit breakers shall be capable of performing the short-circuit current duty cycle with all degrees of current asymmetry produced by three-phase or single-phase circuits having a short-circuit power factor of 15% or greater (X/R ratio of 6.6 or less). [2] A. Definitions: Short Delay Current And Short Delay Time Ratings Short delay current ratings and short delay time settings are critical factors in the overall function of a LVPCB when used in a selectively coordinated system. For purposes of this paper, in conjunction with trip unit settings, the following definitions will be utilized: Short delay current rating of a LVPCB: The maximum current for which the circuit breaker can remain closed for a short delay time. The maximum short delay time of a LVPCB: The time that a Power Circuit Breaker must keep its contacts closed at a maximum short delay rated current (typically ½ second or 30 cycles). {The ANSI test standard for LVPCBs actually requires a short delay fault test of 30 cycle duration, a 15 second zero current interval and then another 30 cycle short delay fault test.} There are significant differences in short time delay ratings for various circuit breaker types. Typically, the maximum short delay time rating for Molded Case Circuit Breakers (MCCB) equipped with electronic trips is 18 cycles and the maximum short delay time rating for Insulated Case Circuit Breakers (ICCB) is 30 cycles. It s important to note that although a ICCB may have a 30 cycle short delay time, the current magnitude of that 30-cycle delay is at significantly less current magnitude than it s rated interrupting capacity. Most MCCBs and ICCBs available on the market are equipped with a fixed instantaneous override circuit (when adjustable instantaneous is not furnished). These breakers frequently have interrupting ratings, which are higher than the breaker's short delay current rating. When the fault current value exceeds the short delay current rating of the breaker, the fixed instantaneous override circuit immediately trips the breaker. This means that system coordination can only be achieved up to the short delay current rating of the breaker. For fault currents above the short delay rating, the breaker opens and coordination is lost. However, the breaker can still interrupt currents up to its published interrupting capacity. The industry test circuit standards used for testing low voltage circuit breakers include power factor test ranges and X/R test ranges shown in Table 1. When circuit breakers are used in actual installations where the X (reactance) and R (resistance) represent the total X and R from the faulted point in the circuit back to the utility generating source, a de-rating factor should be applied to the circuit breaker interrupting rating if the calculated X/R ratio exceeds those listed in the

4 test standards. Typical recommended de-rating factors are shown in Table 2. The LVPCB derating factors shown in Table 2, are essentially the reciprocals of the multiplying factor based on ANSI C37.13, Table 3. This ANSI multiplying factor is applied to the calculated symmetrical short circuit current, before comparing it to the LVPCB interrupting rating. The ratio of X to R determines the maximum degree of asymmetry possible in the faulted circuit. In other words, the X to R ratio is the degree to which the first half cycle peak current exceeds the steady state rms value of the fault. The actual amount of asymmetry will depend on when the fault occurs in the voltage wave. The test circuit X/R ratio determines the first half cycle Peak Multiplication Factor. The Peak Multiplication Factor times the calculated or test rms symmetrical current equals the first half cycle peak current. See Fig. 3 for the relationship of X/R ratio to Peak Multiplication Factor. Figure 3: Relation of X/R Ratio to Multiplication Factor

5 B. The Need to Specify the X/R Ratio The new low voltage power circuit breaker's RMS sensing trip unit measures various points on the current wave within each cycle and then calculates the rms value of the wave. This calculation is used to support long delay and short delay tripping decisions. However, the majority of trip units still utilize peak sensing measurement for those circuits associated with instantaneous trip adjustment or over-ride circuits. For example, if an insulated case circuit breaker has a published short delay current rating of 50,000 amperes rms symmetrical and an interrupting rating of 100,000 rms symmetrical amperes, it must be determined from the manufacturer whether the 50,000 ampere short delay current rating and associated over-ride circuit is based on a symmetrical wave (X/R = 0) or some other X/R ratio, such as 6.6, which is utilized with LVPCBs. In this example, assuming the 50,000 ampere short delay current rating is based on a perfect symmetrical wave, then the first half cycle Peak Multiplying Factor would be 1.41, or the breaker would begin to open instantaneously for peak current values over 1.41 x 50,000 A= 70,500 peak amperes. If this circuit breaker were applied in an actual circuit location that had an available fault current of 35,000 amperes rms symmetrical and an X/R ratio of 6.6, the Peak Multiplying Factor would be 2.3 (see Fig. 3). This 35,000 rms symmetrical available fault current would have a first half cycle peak of 35,000 X 2.3 or 80,500 peak amperes. Since the available peak amperes of 80,500 exceeds the breaker instantaneous override peak setting of 70,500 amperes, the instantaneous override circuit would cause the breaker to open. This would not allow the short delay circuit to continue timing out. As a result of the breaker opening, coordination with downstream breakers would be lost. In this case of a hypothetical 50kA short delay current rated breaker, the breaker would trip on a 35kA rms fault without any short time delay instead of waiting for a load side breaker to trip (Since the X/R ratio it was rated for was 0 and not the 6.6 required for the example circuit). Thus, it is important to specify not only the breaker short delay current rating required, but also that it must be based on a test circuit with an X/R ratio of 6.6. In the following discussion, all currents given are rms symmetrical values, unless otherwise noted. C. A Discussion of System Coordination A number of variables can impact overall system coordination. When a fault occurs at point "A" (refer to Fig. 4), the amount of fault current flowing from the source (without motor contribution) through the main breaker M1, feeder breaker F1, 200-Ampere breaker in DP-1 and the 50- ampere breaker in LP-1 would be of the same magnitude. The fault current magnitude would be determined by the system available fault current, the motor contribution, and the total impedance between the faulted point and the utility and motor sources, and the impedance of the arc itself.

6 Figure 4: Typical Power Distribution System To achieve perfect coordination, only the 50-Ampere, 3- pole branch MCCB in Panel LP-1 would open, while the 200-ampere feeder MCCB in Panel DP-1, the feeder breaker F1, and main breaker M1 in the main service equipment would remain closed. This would remove only the faulted circuit from service, while allowing the remainder of the circuits in the facility to remain energized. Typically, a molded case circuit breaker with a frame size under 1200-amperes has a maximum short delay current rating of approximately 10 to 13 times its frame size. This being the case, the 200-ampere breaker in DP-1 would have a short delay current rating of about 2000 to 2600 amperes. For short circuit currents above 2000 to 2600 amperes, the MCCBs adjustable instantaneous circuit would open, which would open the breaker contacts, removing the circuit from service. The same would be true on an electronic breaker with an instantaneous override circuit, which would open the breaker, or with a thermal magnetic breaker, where the magnetic coil would open the breaker and take the circuit offline. In the case of extremely high faults, the fault current itself is instrumental in opening the breaker contacts. At a fault current level below approximately 2,000 amperes, the molded case feeder breaker in Panel DP-1 would coordinate with the 50-ampere molded case circuit breaker in panel LP-1. For a fault current level above 2000 amperes, both the 200-ampere MCCB in panel DP-1 and the 50-ampere MCCB in panel LP-1 would open and all circuit loads in Panel LP-1 would be de-energized. Similarly, if the 800-ampere feeder breaker in the main service equipment were an MCCB, it would have a short delay current rating of approximately 8000 amperes. If the fault current at point "A" was above approximately 8000 amperes, the MCCB feeder F1, the 200-ampere feeder MCCB in DP-1 and the 50-ampere MCCB breaker in LP-1 would all probably open and de-energize the loads connected to Panel DP-1. If the fault current at point "A" was above the short delay current rating of the 2500-ampere main breaker M1 in the main service equipment, and breaker M1 were of the MCCB design, the entire facility would be without power if the breaker opened. D. The LVPCB Difference Using LVPCBs in the system shown in Fig. 4 can make a substantial difference in overall system performance. If the main M1 and the feeder F1 in the main service equipment were LVPCBs with a short delay current rating and interrupting rating without instantaneous trips of 65,000 amperes, and the available fault current at their point of application was less than 65,000 amperes, the LVPCBs would continue to stay closed up to their short delay time setting. Feeder breakers such as those at F1, F2, F3, F4, and F5 typically would have a short delay time setting of 6 cycles to allow downstream instantaneous devices to operate, or inrush currents to flow. The main breaker M1 would have a short delay time setting of 12 cycles to allow the main service equipment feeder breakers -- F1 through F5 -- to open first if a fault existed on their load side. This would create a selectively coordinated system. For example, if the fault current at point "A" were approximately 55,000 amperes, the 50-ampere breaker in LP- 1 and the 200-ampere breaker in DP-1 would open. However, the LVPCB type feeder breaker F1 and the LVPCB type main breaker M1 would remain closed. As a result, only the loads associated with LP-1 would be deenergized. If the feeder circuit breakers F1, F2, and F3, or the main circuit breaker M1, were specified as LVPCBs and equipped with instantaneous trips, they would still have the same interrupting capacity. However, their short delay current rating would be reduced to the maximum available setting of the adjustable instantaneous pick-up -- typically 12 times the rating plug. Thus, the 800-ampere feeder circuit breakers F1, F2, and F3, would trip instantaneously at 9,600 amperes, and the main circuit breaker M1 would trip instantaneously at 30,000 amperes. Their available short delay current rating would essentially be reduced to these current values. E. Guidelines for Specifying LVPCBs To avoid unnecessary trips and unplanned instances of system de-energizing, the following general guidelines should be used when specifying LVPCBs: Specify main breakers and tie breakers with long delay and short delay without adjustable instantaneous; Select feeder breakers with long delay and short delay without adjustable instantaneous where coordination with down stream (load side) devices is required. Select feeder breakers with long delay, short delay and instantaneous for those feeder breakers feeding an INDIVIDUAL transformer or an INDIVIDUAL motor. For example, where F4 is feeding an individual transformer, the instantaneous current pick-up would be set slightly above (approximately 10%) the calculated maximum fault current on the secondary of the 225 kva transformer (without secondary motor contribution), multiplied by the transformer voltage ratio. For a 225 kva transformer with 5% impedance, the secondary maximum fault current with unlimited primary available current would be {(624 secondary FLA) / 0.05} equal to 12,480 amperes. The instantaneous pick-up setting

7 would then be {[12,480 x (208V / 480V)] x 1.10}, or 6000-ampere instantaneous setting passing through F4 would have to be either a fault in the transformer primary winding or a fault on the primary side of the transformer. As a result, coordination with secondary devices would not be required. Short delay would still be furnished for this breaker to give good low-level short circuit protection. It would also allow for transformer inrush current of approximately 12 x primary FLA for 0.1 seconds. In the case of an individual motor, such as feeder F5, the instantaneous current pick up would be set slightly above (approximately 10%) motor inrush 1/2 cycle current (typically 15 times full load amperes). New electronic trip units are available as standard with Long Delay, Short Delay and field selectable Instantaneous (instantaneous protection or no instantaneous protection). This allows for more flexibility in applying the breakers for various applications in the electrical distribution system, and approximately 6000 amperes. Values of current above this more flexibility in utilizing spare breakers for different applications. F. The Time/Current Relationship The relationship between time and current for the feeder breaker F1 circuit of Fig. 4 is shown in Fig. 5. It uses PCBs in the main service equipment, and MCCBs in DP-1 and LP- 1. At 2000 amperes, coordination is lost between the 200- ampere MCCB and the 50-ampere MCCB. Coordination is achieved between the 800-ampere PCB in the main service equipment and the 200-ampere feeder breaker in DP-1. Coordination is also achieved between the 2500-ampere main PCB and the 800 ampere feeder PCBs. To achieve complete coordination for all devices, the breakers in Panel DP-1 would have to be outfitted with ICCB or PCB units with short delay current ratings equal to the available fault current at their particular locations. Figure 5: System Coordination Curves

8 G. Achieving Coordination When doing a coordination study on a given feeder breaker, it's critical to select the largest overcurrent device on the load side grouped equipment (panelboard, motor control center, or busway bus plug) it supplies. Similarly, when selecting settings of main breakers, it is important to select the largest load side feeder overcurrent device or the load side feeder overcurrent device with the highest settings to achieve full coordination. Many of today's Molded Case Circuit Breakers or Insulated Case Circuit Breakers -- both of which are tested per UL standard 489 for Molded Case Circuit Breakers -- may have interrupting capacities equal to those listed in Table A, Columns 5 and 8. However, when their short delay current ratings are compared to those shown in Column 7, they are much lower. This significantly reduces selectivity or continuity of service. Table A indicates the three phase short circuit ratings in symmetrical rms amperes for breakers with and without instantaneous trips, and for breakers without current limiting fuses. Column 9 gives the short delay current ratings for the new LVPCBs. The short delay current rating is the maximum current at which the new LVPCBs can keep its contacts closed for up to 30 cycles. If the fault current exceeds the indicated current values, the breaker will override the selected short time setting and open the contacts through an instantaneous override circuit. The breaker can interrupt fault current maximums up to the ratings indicated by Column 8. The instantaneous override circuit for the new LVPCB is based on a system X/R ratio of 6.6. AIC Rating - Symmetrical RMS Amperes AIC Rating - Symmetrical RMS Amperes Short Delay With Instantaneous Trips Without Instantaneous Trips Current Rating AC Breaker ANSI Typical New PCB ANSI Typical New PCB New PCB Voltage Frame C37, PCB Maximum C37, PCB Maximum Maximum Amperes Table 1 Maximum Table 1 Maximum (3) (4) ,000 65, ,000 22,000 65, ,000 85, ,600 65,000 65, ,000 42,000 65, ,000 85, ,000 65,000 65, ,000 50,000 65, ,000 85, ,200 85,000 85, ,000 85,000 65, ,000 85, , , , ,000 85,000 85, , , ,000 (1) (2) 130, ,000 (2) 85, , , ,000 65, ,000 22,000 65, ,000 85, ,600 50,000 65, ,000 42,000 65, ,000 85, ,000 50,000 65, ,000 50,000 65, ,000 85, ,200 65,000 85, ,000 85,000 65, ,000 85, ,000 85,000 85, ,000 85,000 85, , , ,000 (1) (2) 85, ,000 (2) 85, , , ,000 50, ,000 22,000 50, ,000 85, ,600 42,000 50, ,000 42,000 50, ,000 85, ,000 42,000 50, ,000 42,000 50, ,000 85, ,200 65,000 65, ,000 65,000 65, ,000 85, ,000 85,000 85, ,000 85,000 85, , , ,000 (1) (2) 85, ,000 (2) 85, , ,000 Notes ( ) : 1. Not an ANSI listed frame rating. Conventional PCB's may utilize fan cooling to achieve the indicated frame continuous rating. 2. No ANSI short circuit rating available. 3. Breakers can interrupt currents up to the indicated values in Column 8 without adjustable instantaneous circuit, but will only maintain contacts closed (short time rating) up to the values in Column 9 at which time a non-adjustable instantaneous override will initiate interruption. 4. Short Delay Current Ratings for the new PCB - current value that the breaker will remain closed (withstand) for 30 cycles or 0.5 seconds. Table A: 3 Phase Short Circuit Ratings of ANSI Rated LVPCBs, Typical LVPCBs Available Today and the New LVPCB with Increased Ratings

9 III. EXAMPLES REQUIRING HIGHER THAN NORMAL SHORT DELAY CURRENT RATINGS A. Example #1 Double Ended Switchgear with 2 Mains & Tie with Closed Transition Transfer or Tie Breaker Normally Closed Fig. 6 shows a typical Double-Ended switchgear line-up with each incoming service fed by a 2000/2666 kva (AA/FA) Dry Type transformer. Condition # 1 -- Tie Breaker Normally Open With Closed Transition Re-Transfer This system is normally operated with tie breaker "T" open, and a closed transition retransfer is used. Both 480-volt sources must be suitable for parallel operation, and properly protected. The available fault current through each main breaker "M1" and "M2" is 39,100 amperes symmetrical rms. Using ANSI standards for lumping motor contribution at the main bus for a 480 volt system, 4 x transformer secondary full load amperes yields 4 x 2406 or approximately 9600 amperes. This additional motor contribution would be added at Bus 1 and Bus 2 for a total available fault current at each bus of 48,700 amperes. If voltage should be lost to Line 1, main breaker M1 automatically opens. Since the tie breaker T is open, the loads on Bus 1 experience a complete voltage outage. When the tie breaker closes, voltage is returned to Bus 1, and motor circuits on the bus would typically have to be restarted in sequential order. When voltage is returned to Line 1, many industrial users do not want to lose the loads to Bus 1 again during an open transition retransfer, which first opens the tie breaker and then closes the main M1 breaker. A closed transition retransfer is achieved by first closing the main M1 breaker and then, a few cycles to a few seconds later, opening the tie breaker. During the time of retransfer when both mains and the tie breaker are closed, if a fault were to occur on the load side of one of the feeder breakers -- such as location "A" near its load side terminals -- then feeder breaker F2 would be required to interrupt approximately {(2 x 39,100) + (2 x 9600)} amperes, or 97,400 amperes. NEC requires, "Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals Figure 6: Double Ended Switchgear with 2 Mains & Tie with Closed Transition Transfer or Tie Breaker Normally Closed

10 of the equipment. In order to avoid an NEC violation, feeder breakers would need an interrupting capacity of 97,400 or nominally 100,000 A.I.C. If a full short circuit study was performed, the motor contribution at point "A" would typically be somewhat less than the 2 x 9600 amperes utilized in the above calculation. This reduction would be due to the cable impedance between the motors and point "A", and the fact that the motor contribution from MCC-2 does not pass through breaker F2. If a fault occurred at point "B" on the load side of a combination starter in MCC-2 during the closed transition transfer, breaker F2 would require a short time rating equal to the available fault current at point "B" to ensure a coordinated system. The fault current flowing through breaker F2 during a fault at point "B" would be the available fault current at point "A" reduced by the cable and combination starter impedance from "A" to "B". For increased coordination, a high short delay current rating of 85,000 amperes rms symmetrical would be desired. At a minimum, all feeder breakers and equipment downstream from the main switchgear should have an interrupting capacity rating for the higher available fault current when the sources are paralleled with the tie closed. Since the mains do not experience fault currents higher than the current provided by one source, they could have interrupting capacity and short time ratings equal only to the one source feeding it. The tie breaker would experience only the fault current from one source and motor contribution from one side, and could have a rating equal to the highest combination of one source, plus motor contribution on that bus. It is also recommended that reverse current protection be provided at the main breakers. Condition # 2 -- Tie Breaker Normally Closed - Paralleling Sources The same situation as described in Condition 1 when a closed transition retransfer occurs, is applicable to parallel source operation. Many industrial systems are operated with both sources paralleled due to the critical nature of processes where even a few cycles of lost power to the load could have significant financial consequences. Many systems supplying welding equipment are operated with two or more sources in parallel in order to generate adequate voltage and current during the welding process. Referring to Fig. 6, reverse current and reverse power relaying, as well as a synchronism check must be included with the main LVPCBs M1 and M2. It would be recommended under this continuous parallel operation to consider replacing M1 and M2 with Network Protectors equipped with forward phase and ground fault time overcurrent protection in addition to network relaying. Same Bus Parallel Operation with Utility With the upcoming advent of electric utility de-regulation, a growing number of industrial and commercial customers have begun to consider local generation for either peak shaving, selling power back to the utility or reducing overall power consumption from the utility. Another advantage of co-generation is a reliable back-up power source in the event of a utility outage. The availability of an inexpensive source of power for the generator, such as process gas or steam, is a significant factor in justifying these systems. On-site generation is common place in the pulp & paper, petrochemical and other major industries. The simple system shown in Fig. 7 consists of two 1000 kw generators paralleled on the same bus. They are then paralleled with the utility. Each generator has an approximate full load current of 1500 amperes, and can supply approximately 15,000 amperes of fault current, based on a Z equal to 10%. The utility or customer supply has a 2500 kva transformer, which will let through approximately 50,000 amperes fault current. If the Generator Switchgear and the Main Switchgear are relatively close together and G1, G2, GT1, GT2, and M1 breakers are closed, approximately 80,000 amperes, plus motor contribution of approximately 12,000 amperes -- or a total of 92,000 amperes -- would be available at both the generator bus and the main switchgear during parallel operation. This would require all LVPCBs except LB1 (which would be operated only when GT1 was open) to have an interrupting capacity of 100,000. The highest possible short time rating would be needed for feeder breakers F1, F2, and F3 for coordination purposes. This would provide a greater degree of coordination, as well as selective tripping with over-current devices in the motor control centers. Non-Parallel Operation With Utility Fig. 8 illustrates how multiple generation can be used to serve the emergency and critical loads of a facility. The Tie breaker GT is normally operated closed, and the generators each are paralleled to common bus L/R. Although not required, Tie breaker GT is provided to enhance reliability. If a bus fault occurred on bus L or R, the generation system would not go completely off-line, as would happen if a solid bus without a Tie breaker were used instead. The Tie breaker allows for maintenance on either bus L or bus R, while still keeping two generators available for supplying the emergency and critical loads. B. Example #2 Multiple Generators Paralleled on the

11 Figure 7: Multiple Generators paralleled on the Same Bus From the generator bus, the emergency and critical loads are supplied through a feeder breaker (GF1, GF2, GF3, or GF4) to a distribution panel, such as EDP2. Current is then routed through a transfer switch -- in this case, ATS1. Since the emergency loads are typically lighting, and the critical loads are usually far from the emergency switchgear fed by smaller conductors, the motor contribution for this example is assumed to be insignificant. Each generator is capable of supplying 24,000 amperes of fault current. A total of 96,000 amperes of fault current for the four generators would be available at the Bus L/R. Generator LVPCBs G1, G2, G3, and G4 would only be required to have interrupting capacities of 3 x 24,000 amperes, or 72,000 amperes. For example, for a fault at point "A" in the conductors between Generator G2 and Generator Breaker G2, generator breaker G2 would only have to interrupt the fault current from Generators G1, G3, and G4. Generator G2 fault current would not pass through generator breaker G2. For a load side main Bus L fault at point "B", generator breaker G2 would only have to interrupt the fault current from G2. The tie breaker GT would have to be capable of interrupting 2 x 24,000 amperes. Feeder breakers GF1, GF2, GF3, and GF4 would need to be capable of interrupting 4 x 24,000 amperes, or 96,000 amperes. For example, if a fault occurred at point "C", all four generators would contribute to the fault current. The highest possible short delay current rating for the LVPCB feeder breakers would also be needed to allow the MCCB breakers in downstream EDP panels to open first if a fault occurred on the load side of an ATS (for example at point "D"). This would keep the entire EDP panel from losing service. A better design to support coordination and continuity of service to the critical loads would involve eliminating the EDP panels equipped with MCCB breakers. Instead, each transfer switch would be fed directly from additional LVPCBs. However, this improved design would be more costly, and would require automatic transfer switches able to withstand (momentary) time ratings equal to the short delay time setting on the power circuit breakers [Refer to the discussion for example 3]. Cautions to Keep in Mind In many cases, the X/R ratio of generators is very high, and the system X/R ratio can exceed the tested X/R ratio equal to 6.6 for LVPCBs. This would require de-rating of the breaker interrupting and short delay current capability. For smaller generators, or systems where the symmetrical rms interrupting capacity of the breaker appears to be adequate for the calculated symmetrical rms current, derating may also be required. A breaker with higher interrupting capacity and a higher short delay current rating may be needed so the

12 Figure 8: Power Distribution System with Multiple Generators breaker's interrupting capacity will still exceed the system's available symmetrical rms current when the de-rating appropriate X/R factor is applied. In large generating systems, it is critical that the X/R ratio is checked when using load side molded case circuit breakers. Also, spot network transformers may have a higher X/R ratio than conventional transformers, and may require de-rating of conventional equipment and selection of higher than normal device interrupting and short delay current ratings. C. Example #3 Large KVA Transformers or Transformers with Low Impedance Since the installed cost for low voltage substations is significantly lower in terms of $ s/kva for larger KVA rated substations, there is a trend toward higher ratings including 3000KVA, 3750KVA and beyond. The example shown in Fig. 9 utilizes an outdoor 3750 kva liquid filled transformer with a double secondary throat connection. The two secondary bus runs feed a 3000 ampere main breaker, which in turn feeds 800 ampere feeder breakers. With this configuration, several users have elected to rely on Insulated Case Circuit Breakers (ICCBs) to obtain an interrupting capacity that was high enough to support this application. Since this system has 78,466 amperes available from the transformer [3750 kva / (0.480 x x Z)], plus approximately 15,000 amperes of motor contribution, it required devices with 100,000 ampere interrupting capacity. The use of fused LVPCBs would result in likely loss of selectivity for high fault conditions and significantly higher costs along with additional floor space. Thus, ICCBs with higher 100KA interrupting ratings, has been the historic best solution. However, this technology only supports short delay current rating of 25,000 amperes for the feeder breakers and 35,000 amperes for the main breakers. If a fault greater than 25,000 amperes occurred on the load side of any of the combination starters within a motor control center, total coordination for that MCC would be lost. In the event of a 50,000-ampere fault at point "A" on a branch circuit of MCC1, the entire MCC1 would be out of service because of breaker F1 opening. In addition, all MCCs

13 connected to switchgear bus 1 would be lost due to the opening of main breaker M1. If a fault were to occur, Figure 9: Power Distribution System with 3750KVA Power Transformer a current magnitude greater than 35,000 amperes would be more than likely based on the high available fault current of the source. With the advent of new LVPCB technology with short delay current ratings of 85,000 amperes and interrupting ratings of 100,000 amperes, significantly better coordination can be achieved without sacrificing the floor space required by fused LVPCBs. IV. CONSIDERATION OF SHORT DELAY TIME SETTINGS AS THEY AFFECT LOAD SIDE EQUIPMENT PROTECTION Fig. 10 shows a facility fed by a basic single line, with normal loads served by LVPCB Feeders F1, F2, and F3. The facility has its critical computer or process loads served by LVPCB feeder F4, through ATS 1, LVPCB feeder CUPS, through the Uninterruptable Power Supply "UPS", and critical feeders CF-1, CF-2, and CF-3. In addition, there is a maintenance bypass LVPCB that's designated CBP. Critical panels CP-1, CP-2 and CP-3 serve the critical loads and utilize molded case circuit breakers. Circuit Breakers CF-1, CF-2, and CF-3 were selected as LVPCBs to provide coordination with the molded case breakers in the critical panels. Typically with modern microprocessor trip units and breaker operating mechanisms, a 6-cycle short time delay difference is sufficient to achieve coordination between these types of devices. Since coordination between breakers is desired at all levels, bypass breaker CBP and CUPS would have their short delay time set at 12 cycles. Feeder F4 would require an 18-cycle short delay time setting and the main M1 would require a 24- cycle short delay time setting. If the scheme incorporated two incoming lines with normal mains M1, M2 and a tie breaker, the tie breaker short delay time would be set at 24 cycles and the main breakers short delay time set at either 24 or 30 cycles. For a fault at point "A" of current magnitude at least ten times the MCCB frame, the MCCB breaker in CP-1 would open, while all upstream LVPCBs would stay closed. For a fault at point "B" in the main bus of panel CP-1, or in the conductors at Point "C", feeder breaker CF-1 would time for 6 cycles and then open to clear the fault. For a fault at point "D" in the bypass feeder, breaker CBP would time for 12 cycles and then clear the fault. If a fault occurred at point "E", Main Switchgear LVPCB short delay would time for 18 cycles and then clear the fault. This would mean the fault current would be passing through the main switchgear bus and ATS1 for the 18-cycle period of time. In this application, an Automatic Transfer Switch "ATS1" would have to "withstand" the fault current passing through it for a minimum of 18 cycles. Most conventional contactor type automatic transfer switches have a "Momentary" or "Withstand" time rating of only 3-cycles. Transfer switches outfitted with Insulated Case Breakers or PCBs are available with up to 30-cycle withstand time ratings. The transfer switch momentary current rating must be greater than the available fault current at its point of application. Per ANSI standards, low voltage switchgear bus is designed and braced to carry rated short circuit current for 30 cycles. On the other hand, the switchboard bus, the motor

14 Figure 10: Ratings of Load Equipment for Short Time Delay Service control center bus, and the panelboard bus have only 3 cycle momentary ratings. While busway is only required to be rated for a 3-cycle momentary rating, some types are available with a 6-cycle rating. If a fault were to occur at point "A" at the critical panel, at point "F" at MCC 1, or at point "G" on the bus plug feeder, the appropriate molded case circuit breaker would open within one cycle. As a result, the critical panel or MCC or busway 3-cycle momentary rating would be sufficient. V. ZONE SELECTIVE INTERLOCKING Today's modern microprocessor trip units are usually available with zone selective interlocking. If zone selective interlocking is specified to be wired between trip units within a switchgear assembly -- or from one switchgear assembly to another switchgear assembly -- this feature can be used. Zone selective interlocking allows the short delay time settings to be set in 6-cycle steps for coordination purposes. However, it will override these settings when the fault condition is located directly on the load side of each protective device. Zone selective interlocking makes the full short delay capability of the selected breaker available for coordination with load side protective devices, while it provides fast opening of a breaker when a fault is located directly on its load side to minimize damage. If zone selective interlocking were specified for the power circuit breakers located in the main switchgear in Figure 10, the time settings previously discussed would still be used. A fault occurring on the main switchgear bus at point "H", is on the load side of main breaker M1, and the line side feeders, F1, F2, F3 and F4. Due to the location of the fault, only the trip unit in main breaker M1 would sense high fault current. Since breakers F1, F2, F3, and F4 did not sense a high fault current they would not send a restraining signal to main breaker M1. This would indicate that the fault was not on the load side of any of the feeder breakers. Instead of timing out to the M1 breaker preset value of 24 cycles, main breaker M1 would open in approximately 3 to 5 cycles, minimizing damage. If the fault were located at point "E", then breaker F4 would send a restraining signal to main breaker M1 indicating the fault was on the load side of the F4 feeder breaker. Main breaker M1 would then continue to time out to its preset 24-cycle value. When the F4 feeder breaker

15 opened in 18 cycles to clear the fault, the fault current would no longer be present at M1 main breaker and M1 breaker trip unit would stop timing. Similarly, consider the results if zone selective interlocking were wired between the Main Switchgear and the UPS Switchgear breaker trip units. If a fault occurred at point E, breakers CUPS and CBP would not send a restraining signal to breaker F4. Without a restraint signal, breaker F4 would open in 3 to 5 cycles in lieu of its preset timing value of 18 cycles, and would minimize damage. On the other hand, if a fault was at point D, breaker CBP would send a signal back to breaker F4 allowing it to proceed and time out to its 18 cycle short delay setting. Meanwhile breaker CBP would open after 12 cycles and cleared the fault. Obviously, if zone selective interlocking were wired between the UPS switchgear breakers and the critical switchgear breakers, for the same fault at point D, breaker CBP would open in 3 to 5 cycles. Problems can arise when Insulated Case Breakers or LVPCBs are used as both mains and feeder breakers in switchboards (in lieu of ANSI metal-enclosed switchgear), and can subject the switchboard bus to momentary time requirements past its 3-cycle rating. This being the case, the momentary current rating of load side equipment and conductors, along with the momentary time rating must be considered. VI. SUMMARY Adequate short delay current and short delay time ratings are needed to facilitate coordinated distribution systems during fault conditions. This ensures that only the circuit breaker closest to the fault opens. An MCCB and a LVPCB with identical published short delay current ratings will respond to the same fault current in a different manner, based on their individual X/R ratio design test capabilities. The specifier should understand the circuit X/R ratio for his system, and it s effect on the published short time rating of the circuit breaker protective devices. When the X/R ratio of the application exceeds breaker test standards, a de-rating factor must be used to calculate the effective short delay current and interrupting rating of the breaker. There is a need to review both current and time ratings of load equipment during design. The availability of a new LVPCB with higher than previously available short delay current and interrupting capacity can in many applications enable the systems designer to engineer a fully coordinated system, resulting in better protection while improving system performance. VII. REFERENCES [1] ANSI C37.13 IEEE Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures ANSI C37.16 Low Voltage Power Circuit Breakers and AC Power Circuit Protectors Preferred Ratings, Related Requirements, and Application Recommendations ANSI C37.17 Trip Devices for AC and General Purpose DC Low-Voltage Power Circuit Breakers ANSI C IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit-Breaker Switchgear ANSI C37.50 Low-Voltage AC Power Circuit Breakers Used in Enclosures Test Procedures ANSI C37.51 Metal-Enclosed Low-Voltage AC Power-Circuit-Breaker Switchgear Assemblies Conformance Test Procedures NEMA SG3 Low-Voltage Power Circuit Breakers NEMA SG5 Power Switchgear Assemblies UL1066 Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures UL1558 Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear [2] ANSI C , Paragraph [3] UL489 and UL1066 Test Standards for low voltage power circuit breakers