ANSI MV TechTopics Catalog PDF

www.usa.siemens.com/techtopics TechTopics catalog The released collection from No. 01 to No. 99 Answers for infrastructure and cities. Medium-voltage TechTopics is a series of papers that discuss issues of interest to users or specifiers of medium-voltage electric...

www.usa.siemens.com/techtopics TechTopics catalog The released collection from No. 01 to No. 99 Answers for infrastructure and cities. Medium-voltage TechTopics is a series of papers that discuss issues of interest to users or specifiers of medium-voltage electrical equipment, including: 5  - 15 kV arc-resistant, metal-clad switchgear (GM-SG-AR and NXAIR P) 5  - 38 kV non-arc-resistant, metal-clad switchgear (GM, GM-SG and GM38) 5  - 38 kV arc-resistant, gas-insulated switchgear (8DA10 and 8DB10) 5  - 27.6 kV metal-enclosed, load-interrupter switchgear (SIMOSEC) 2 .3 - 15 kV motor controllers (Series 81000™, SIMOVAC and SIMOVAC-AR) 1  5.5 - 38 kV outdoor vacuum circuit breakers (SDV6 and SDV7) 5  - 38 kV retrofit and replacement circuit breakers. Topics discussed in medium-voltage TechTopics will include application, selection and discussion of the applicable standards for these products. Medium-voltage TechTopics issues will be added to address recurring questions. TechTopics No. 12, 40, 49, and 51 are reserved. TechTopics issue Pages No. 01 - Surge limiter application recommendations for metal-clad switchgear up to 15 kV 5-7 No. 02 - Loss of vacuum 8-9 No. 03 - Vacuum vs. SF6 10-11 No. 04 - kA rated circuit breakers and switchgear 12-15 No. 05 - Reclosing applications - minimum reclosing time 16-17 No. 06 - Three-cycle versus five-cycle interrupting time 18-19 No. 07 - Current transformers - Use of 600 V CT’s in metal-clad switchgear 20-21 No. 08 - Heat generation estimation for type GM switchgear (up to 15 kV) 22-23 No. 09 - Heat generation estimation for type GM38 switchgear (up to 38 kV) 24-25 No. 10 - Heat generation estimation for Series 81000™ controllers 26 No. 11 - Fast bus transfer times for type GMI circuit breakers 27-29 No. 13 - Use of latched contactors to switch transformers 30-31 No. 14 - X-radiation emissions by vacuum interrupters 32-33 No. 15 - Expected life of electrical equipment 34-35 No. 16 - Bus joint fundamentals 36-37 No. 17 - Main bus continuous current ratings 38-39 No. 18 - Bus joint and primary disconnect plating 40-41 No. 19 - Bus joint current density 42 No. 20 - Power factor correction capacitor - sizing for motors 43-46 No. 21 - “Bus bracing” in metal-clad switchgear 47-48 No. 22 - “Bus bracing” in metal-enclosed switchgear 49-50 No. 23 - Circuit breaker ratings - type GMI circuit breakers 51-54 No. 24 - Checking integrity of vacuum interrupters 55-56 No. 25 - Shunt reactor switching applications 57 No. 26 - Ground bus ratings 58-59 No. 27 - Standards for medium-voltage metal-clad switchgear 60-61 No. 28 - Standards for outdoor high-voltage circuit breakers 62-64 No. 29 - Derating factors for reclosing service 65-66 2 TechTopics issue Pages No. 30 - Altitude correction factors 67-68 No. 31 - Solar radiation correction factors 69-70 No. 32 - Capacitor switching applications 71-72 No. 33 - Clearance requirements in switchgear and control equipment 73-74 No. 34 - Three-cycle vs. five-cycle interrupting time - type 3AK1 circuit breakers 75-76 No. 35 - Transient recovery voltage 77-79 No. 36 - Early “b” contacts 80 No. 37 - Low current switching capabilities 81-82 No. 38 - Harmonic filter applications 83-84 No. 39 - Heat generation estimation for type NXAIR P switchgear (up to 15 kV) 85-86 No. 41 - Circuit breakers or switches - application considerations 87-88 No. 42 - Circuit breakers or vacuum contactors - application considerations 89-90 No. 43 - Interposing relay requirements 91-93 No. 44 - Anatomy of a short-circuit 94-96 No. 45 - Accuracy of current transformers (CTs) used in medium-voltage control equipment 97 No. 46 - Selection of current transformer (CT) ratio in medium-voltage control 98-99 No. 47 - 7.2 kV equipment basic insulation levels (BIL) 100-101 No. 48 - Fan-cooling control circuit for forced-air cooled circuit breakers 102-103 No. 50 - Ground sensor current transformer cable routing 104-105 No. 52 - Insulation of switchgear terminations 106-107 No. 53 - Use of SF6 gas in medium-voltage switchgear 108-110 No. 54 - Interrupter switch technology comparison - type SIMOSEC SF6 switch - conventional air switches 111-112 No. 55 - Capacitor trip devices 113-114 No. 56 - Switchgear outdoor enclosure type - Why isn’t it NEMA 3? 115-116 No. 57 - Arc flash hazard labels 117-119 No. 58 - What is the difference between E-rated and R-rated current-limiting fuses? 120-122 No. 59 - Control power sources for switchgear 123-124 No. 60 - Use of cable for connections in medium-voltage switchgear 125-126 No. 61 - Circuit breaker “standard duty cycle” 127-128 No. 62 - A bit of history on circuit breaker standards 129-130 No. 63 - Recovery of SF6 gas from type SIMOSEC switches at end-of-life condition 131-132 No. 64 - NFPA 70E - Changes in 2012 edition 133-134 No. 65 - Arc-furnace switching applications 135-137 No. 66 - Clearances 138-139 No. 67 - %dc component 140-141 No. 68 - Heat generation estimation for historic switchgear type D (to 4.76 kV) and type F (to 15 kV) with 142-144 air magnetic circuit breakers No. 69 - Fast bus transfer times for type GMSG circuit breakers 145-146 No. 70 - Arc-resistant switchgear accessibility types 147-149 No. 71 - Generator circuit breakers 150-152 No. 72 - Generator circuit breaker applications - delayed current zeroes 153-155 No. 73 - Generator circuit breaker applications - transient recovery voltage 156-157 3 TechTopics issue Pages No. 74 - Heat generation estimation for type GM-SG or GM-SG-AR switchgear (up to 15 kV) 158-159 No. 75 - Ferroresonance in ungrounded systems with voltage transformers connected line-to-ground 160-161 No. 76 - Heat generation estimation for SIMOSEC load-interrupter switchgear 162-163 No. 77 - Residual voltage on load side of an open circuit breaker 164-166 No. 78 - Personal protective equipment (PPE) required with metal-clad switchgear 167-169 No. 79 - Working space required around electrical equipment 170-172 No. 80 - Special tests - type SDV7 distribution circuit breaker gearbox lubrication 173-175 No. 81 - Arc-flash incident energy mitigation 176-177 No. 82 - Continuous current capability in ambient temperatures other than 40 °C 178-180 No. 83 - Arc-resistant construction for outdoor distribution circuit breakers 181-183 No. 84 - Space heater - sizing and application principles 184-186 No. 85 - Temperature limitations for user’s power cables 187-188 No. 86 - Use of unshielded cables for connections in medium-voltage switchgear and motor controllers 189-190 No. 87 - Ground and test devices 191-197 No. 88 - Application of maintenance grounds in switchgear 198-200 No. 89 - Venting of exhaust gases from arc-resistant equipment 201-203 No. 90 - Temperature ratings for external cables 204-206 No. 91 - Current transformer relaying accuracies - IEEE comparted to IEC 207-210 No. 92 - Heat generation estimation for SIMOVAC non-arc-resistant and SIMOVAC-AR arc-resistant 211-213 medium-voltage controllers No. 93 - Capacitor switching performance classes 214-215 No. 94 - Circuit breaker interlocking and operating requirements 216-218 No. 95 - Tie circuit breakers and out-of-phase applications 219-221 No. 96 - Phase sequence versus phase arrangement 222-223 No. 97 - Ratings for retrofitted switchgear 224-226 No. 98 - Ground protection in metal-clad switchgear - ground sensor current transformers vs. residual 227-230 connection of current transformers No. 99 - Special tests - type SDV7 distribution circuit breaker gearbox lubrication 231-233 4 TechTopics No. 01 Surge limiter application recommendations for metal-clad switchgear up to 15 kV www.usa.siemens.com/techtopics While current chopping is not a significant problem with 5. The reignition produces a high-frequency transient Siemens vacuum interrupters, there are certain switching current, as the source-side and load-side voltages are situations where special consideration should be given brought back together. If the magnitude of the high- because of the possibility of multiple reignitions. Even frequency component is large enough, a current zero will though they are rare, if multiple reignitions do occur, excess be produced that does not coincide with a natural current voltages are possible on downstream (primarily inductive) zero. equipment. Multiple reignitions can occur on almost any 6. The vacuum circuit breaker can interrupt the current at a type of interrupting technology, including air magnetic, oil, high-frequency current zero. air blast, vacuum and SF6. This process, once begun, repeats until the contact gap Multiple reignitions can occur if the following sequence of becomes sufficiently large that the dielectric strength of the events occur together: gap exceeds the voltage imposed across the gap. At this 1. A motor is switched OFF during starting, or while stalled point, further reignitions will not occur. (locked rotor current flowing) 2. The interrupter contacts part just before (less than 1 ms) a natural current zero 3. The circuit breaker interrupts at the natural current zero, but before the contact gap is large enough to withstand the recovery voltage following interruption 4. The circuit has a particular combination of load-side and source-side capacitances and inductances that results in a high-frequency transient recovery voltage (TRV) across the interrupter contacts. If the “right” circuit parameters exist, the rate-of-rise of the TRV can exceed the rate-of-rise of dielectric strength in the contact gap. If this occurs, the arc will reignite. Answers for infrastructure.  Rated voltage kV 3.6 6 7.5 11 15 MCOV of ZnO element kV 2.9 3.6 5.4 9.0 11.0 Surge limiter data Series gap sparkover voltage 1.2 μs x 50 μs wave kV 8 10 15 25 31 0.5 kA switching surge discharge voltage 8 μs x 20 μs wave kV 8 10 15 25 31 1.5 kA switching surge discharge voltage 8 μs x 20 μs wave kV 8.3 12.4 16.5 28.9 33.0 12.0 2.4 6.9 12.47 Solidly grounded wye system applications kV 8.32 ---- 4.16 7.2 13.2 13.8 12.0 6.9 System 4.16 4.8 12.47 Delta system applications kV 2.4 7.2 applications 4.8 6.9 13.2 8.32 13.8 12.0 6.9 4.16 4.8 12.47 Low resistance or high resistance grounded wye system applications kV 2.4 7.2 4.8 6.9 13.2 8.32 13.8 Surge limiter part 18-665-161-xxx -001 -002 -003 -004 -005 Figure 1: Surge limiter selection according to system type If multiple reignitions do occur, the downstream voltage can Surge limiters should also be applied when any load has less escalate and damage the protected equipment. Figure 3 of than full insulation integrity, such as open dry-type the paper entitled “Surge Limiters for Vacuum Circuit transformers, which frequently have less that full BIL Breakers” by S. H. Telander, et. al., shows the escalation of capability. voltage that accompanies multiple reignition, and Figure 4 of Zinc oxide surge arresters, with their higher energy this paper depicts the zone of conditions where multiple absorption capability, can also be used. They are more reignition might occur. As discussed in the paper, the region expensive than surge limiters and if they are not needed to of concern involves rotating machines, because of their large protect against lightning or switching surges, are not inductance and relatively weak insulation structure. necessary. To protect against the overvoltages caused by multiple Surge limiters and surge arresters differ in several reignitions, we recommend that surge limiters be applied on fundamental respects. The type 3EF surge limiter can absorb those circuits with conditions that fall within the zone. the trapped energy associated with a vacuum interruption, Essentially, this means that surge limiters should be applied whereas a surge arrester has a greater energy absorption for all applications in which a circuit breaker is used to supply capacity to deal with system phenomena, including lightning a motor that has a locked rotor current of 600 A or less. This strikes and switching surges from all sources. The 3EF surge recommendation applies to full-voltage starting applications. limiter has a lower (i.e., better) protective voltage level than It also applies to reduced-voltage starting applications, as the an equivalent surge arrester. circuit breaker can interrupt the locked rotor current if the driven load jams or stalls. In capsule form, our recommendations are as follows: Selection of surge limiters for use in equipment up to 15 kV can be made using the application chart in Figure 1. 1. For transformers of full BIL rating, no additional protection is needed. If applied in accordance with these recommendations, Siemens vacuum interrupters will impose dielectric stress on 2. For transformers of reduced BIL rating, add some form of load equipment that are not significantly different from the protection (either surge limiters at the switchgear, surge stresses associated with traditional air magnetic or oil circuit capacitors at the transformer, or surge arresters at the breakers. transformer). 3. For motors with locked rotor current under 600 A, add some form of protection (either surge limiters at the switchgear, surge capacitors at the motor or surge arresters at the motor). 4. For motors with locked rotor current over 600 A, no additional protection is needed. The information provided in this document contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners. Siemens Industry, Inc. 7000 Siemens Road Wendell, NC 27591 Subject to change without prior notice. Order No.: E50001-F710-A154-X-4A00 All rights reserved. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 02 Loss of vacuum If a vacuum interrupter should lose vacuum, several operating As predicted, the “flat” interrupter did not successfully clear the situations should be considered: affected phase, and the “flat” interrupter was destroyed. The laboratory backup breaker cleared the fault. a. With contacts open Following the test, the circuit breaker was removed from the b. When closing switchgear cell. It was very sooty, but mechanically intact. The c. When closed and operating normally soot was cleaned from the circuit breaker and the switchgear cell, the faulty interrupter was replaced, and the circuit breaker d. When opening and interrupting normal current was re-inserted in the cell. Further short circuit interruption e. When opening and interrupting a fault. tests were conducted the same day on the circuit breaker. Cases a, b and c are relatively straightforward. Generally, the Field experience in the years since that test was conducted system sees no impact from loss of vacuum in such a situation. supports the information gained in the laboratory experiment. Cases d and e, however, require further discussion. One of our customers, a large chemical operation, encountered separate failures (one with an air magnetic circuit breaker and Suppose there is a feeder circuit breaker with a vacuum one with a vacuum circuit breaker) on a particular circuit interrupter on phase 3 that has lost vacuum. If the load being configuration. Two different installations, in different served by the failed interrupter is a delta-connected countries, were involved. They shared a common circuit (ungrounded) load, a switching operation would not result in a configuration and failure mode. The circuit configuration, a tie failure. Essentially, nothing would happen. The two good circuit in which the sources on each side of the circuit breaker phases (phase 1 and phase 2, in this example) would be able were not in synchronism, imposed approximately double rated to clear the circuit, and current in the failed interrupter voltage across the contact gap, which caused the circuit (phase 3) would cease. breaker to fail. Since these failures resulted from application in The alternative case of a grounded load is a different situation. violation of the guidelines of the ANSI/IEEE standards, and In this case, interruption in the two good phases (phase 1 and greatly in excess of the design ratings of the circuit breakers, phase 2) would not cause current to stop flowing in phase 3, they are not indicative of a design problem with the and the arc would continue to exist in phase 3. With nothing to equipment. stop it, this current would continue until some backup However, the damage that resulted from the failures is of protection operated. The result, of course, would be interest. In the case of the air magnetic circuit breaker, the unit destruction of the interrupter. housing the failed circuit breaker was destroyed, and the Since the predominant usage of circuit breakers in the 5-15 kV adjacent switchgear units on either side were damaged range is on grounded circuits, we investigated the impact of a extensively, requiring significant rebuilding. The air failed interrupter some years ago in the test lab. We magnetic circuit breaker was a total loss. In the case of the intentionally caused an interrupter to lose vacuum by opening vacuum circuit breaker, the failure was considerably less the tube to the atmosphere. We then subjected the circuit violent. breaker to a full short circuit interruption. Answers for infrastructure. The vacuum interrupters were replaced, and the arc by- In fact, the MTTF (mean time to failure) of Siemens power products (soot) cleaned from both the circuit breaker and vacuum interrupters has now reached 57,000 years (as of the compartment. The unit was put back into service. 2010). Our test experience in the laboratory, where we routinely Questions raised by customers regarding loss of vacuum were explore the limits of interrupter performance, also supports legitimate concerns in the 1960s, when the use of vacuum these results. interrupters for power applications was in its infancy. At that time, vacuum interrupters suffered from frequent leaks, and More recently, several tests were performed in our high-power surges were a problem. There was only one firm that offered test laboratory to compare the results of attempted vacuum circuit breakers then, and reports suggest that they interruptions with “leaky” vacuum interrupters. A small hole had many problems. (approximately 1/8” diameter) was drilled in the interrupter housing, to simulate a vacuum interrupter that had lost We entered the vacuum circuit breaker market in 1974, using vacuum. The results of these tests were very interesting: Allis-Chalmers’ technology and copper-bismuth contact materials. In the early 1980s, after becoming part of the 1. One pole of a vacuum circuit breaker was subjected to an worldwide Siemens organization, we were able to convert our attempted interruption of 1,310 A (rated continuous current vacuum designs to use Siemens vacuum interrupters, which = 1,250 A). The current was allowed to flow in the “failed” had been introduced in Europe in the mid-1970s. Thus, when interrupter for 2.06 seconds, at which point the laboratory we adopted the Siemens vacuum interrupters in the U.S., they breaker interrupted. No parts of the “failed” circuit breaker or already had a very well established field performance record. the interrupter flew off, nor did the circuit breaker explode. The paint on the exterior of the interrupter arcing chamber The principle conceptual differences in the modern Siemens peeled off. The reaminder of the circuit breaker was vacuum interrupters from the early 1960s designs lies in undamaged. contact material and process control. Surge phenomena are 2. A second pole of the same vacuum circuit breaker was more difficult to deal with when copper-bismuth contacts are subjected to an attempted interruption of 25 kA (rated used than with today’s chrome-copper contacts. Similarly, leaks interrupting current = 25 kA), for an arc duration of 0.60 were harder to control with vacuum interrupters built largely seconds, with the laboratory breaker interrupting the by hand than with today’s units. Today, great attention is paid current at the time. The arc burned a hole in the side of the to process control and elimination of the human factor arc chamber. The circuit breaker did not explode, nor did (variability) in manufacture. parts of the circuit breaker fly off. Glowing particles were The result is that the Siemens vacuum interrupters today can ejected from the hole in the arcing chamber. None of the be expected to have a long service life and to impose dielectric mechanical components or other interrupters were stress on load equipment that is not significantly different from damaged. Essentially, all damage was confined to the failed the stresses associated with traditional air magnetic or oil interrupter. circuit breakers. Our experience suggests rather strongly that the effects of a vacuum interrupter failure on the equipment are very minor, compared to the impact of failures with alternative interruption technologies. But the real question is not what the results of a The information provided in this document contains merely general failure might be, but rather, what is the likelihood of a failure? descriptions or characteristics of performance which in case of actual The failure rate of Siemens vacuum interrupters is so low that use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the loss of vacuum is no longer a significant concern. In the early respective characteristics shall only exist if expressly agreed in the terms 1960s with early vacuum interrupters, it was a big problem. of contract. A vacuum interrupter is constructed with all connections All product designations may be trademarks or product names of between dissimilar materials made by brazing or welding. No Siemens AG or supplier companies whose use by third parties for their organic materials are used. In the early years, many hand- own purposes could violate the rights of the owners. production techniques were used, especially when borosilicate glass was used for the insulating envelope, as it could not Siemens Industry, Inc. tolerate high temperatures. Today, machine welding and batch 7000 Siemens Road induction furnace brazing are employed with extremely tight Wendell, NC 27591 process control. The only moving part inside the interrupter is Subject to change without prior notice. the copper contact, which is connected to the interrupter end Order No.: E50001-F710-A155-X-4A00 plate with a welded stainless steel bellows. Since the bellows is All rights reserved. welded to both the contact and the interrupter end plate, the © 2012 Siemens Industry, Inc. failure rate of this moving connection is extremely low. This accounts for the extremely high reliability of Siemens vacuum For more information, contact: +1 (800) 347-6659 interrupters today. www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 03 Vacuum vs. SF6 The technical literature over the past several decades includes For comparison, consider the problems of an air magnetic a number of papers that discuss the merits and demerits of circuit breaker. No air magnetic circuit breaker includes a vacuum and SF6 interrupting technologies. Of course, most of means to indicate that the arc chutes are not installed, or are the papers are biased in favor of one of the competing installed incorrectly. The result of incorrectly installed arc technologies, although several appear to strive for an unbiased chutes is usually complete circuit breaker destruction and or neutral point of view. Perhaps the principle arguments used considerable damage to one or more switchgear cubicles. Yet, by advocates of SF6 are: air magnetic circuit breakers were used for decades and no one clamored for a “loss of arc chute integrity” indicator!  SF6 interrupter can be equipped with a low-pressure alarm A switch to give indication that gas pressure inside the The proponents of SF6 circuit breakers continually raise the interrupter has been lost specter of catastrophic failure of circuit breakers on loss of vacuum. To promote their SF6 technology, they feature their Interruption in SF6 is “soft” (no current chopping). low-pressure alarm switch option. The only reason they need We believe that these issues are not significant relative to this option is because leaks in SF6 are a virtual certainty! Siemens vacuum interrupters, and that the facts strongly favor The reason that leaks are so much more likely with SF6 than the use of vacuum as an interrupting medium. vacuum relates to their construction. A vacuum interrupter is Leak detection constructed with all connections between dissimilar materials made by brazing or welding. No organic materials are used. In SF6 proponents argue that a vacuum circuit breaker needs a the early years, many hand production techniques were used, “loss of vacuum detector” to be equivalent to an SF6 circuit especially when borosilicate glass was used for the insulating breaker with a low-pressure alarm switch. To our knowledge, envelope, as it could not tolerate high temperatures. Today, no supplier has a practical loss of vacuum detector on a machine welding and batch induction furnace brazing are vacuum circuit breaker. Over the years, various efforts have employed with extremely tight process control. The only been made to develop one, but they always stumble on one moving part inside the vacuum interrupter is the copper seemingly immutable law of the universe. It appears that any contact, which is connected to the interrupter end plate with a means to provide an indication (or remote alarm) on loss of welded stainless steel bellows. Since the bellows is welded to vacuum requires changes to the interrupter construction that the contact stem and the interrupter end plate, the failure rate radically increase the likelihood that a leak will occur. of this moving connection is extremely low. The basic design Further, the failure rate of today’s interrupters is so low that (welding/brazing) and the sophisticated process controls loss of vacuum is no longer a significant concern. In the early account for the extremely high reliability of Siemens vacuum 1960s, it was a big problem. interrupters. As of 2010, the mean time to failure (MTTF) for Siemens power vacuum interrupters had exceeded 57,000 interrupter years, illustrating that loss of vacuum is an extremely remote occurrence. Answers for infrastructure. In contrast, an SF6 interrupter requires a sliding or rotating seal The type 3EF surge limiter can absorb the trapped energy at the point where the mechanism penetrates the wall of the associated with a vacuum interruption, whereas a surge interrupter chamber. These seals can age and leak, so that a arrester has a greater energy absorption capacity to deal with means to alarm on low pressure becomes mandatory. system phenomena, including lightning strikes and switching surges from all sources. The 3EF surge limiter has a lower (i.e., The typical SF6 circuit breaker has twice the number of parts in better) protective voltage level than an equivalent surge the high voltage circuit as the equivalent vacuum circuit arrester. breaker. The typical number of moving parts in the high voltage circuit is also twice as many. More significant, the Further information on the subject of voltage surges, which number of inaccessible moving parts (under SF6 or vacuum) is can occur with vacuum interrupters, and their impact on about 10 times as great! This last point is very significant, as equipment selection is contained in “Surge Limiters for Vacuum moving parts obviously have a higher failure rate than non- Circuit Breakers,” by S. H. Telander, et. al., which appeared in moving parts. The greater number of moving parts in the SF6 the July/August 1988 issue of the IEEE Transactions on Industry circuit breaker dictates that it must be less reliable than a Applications. In capsule form, our recommendations for vacuum circuit breaker. In some cases, an SF6 interrupter can vacuum circuit breaker application (vacuum contactor be repaired, but only at the factory, which means that in most application recommendations differ) are as follows: cases, the method of repair will in fact be replacement. 1. For transformers of full BIL rating, no additional protection is The supporters of SF6 technology frequently claim that a needed. leaking SF6 interrupter will still interrupt rated fault current one 2. For transformers of reduced BIL rating, add some form of time. While this may sometimes be true, it is seldom true for all protection (either surge limiters at the switchgear, surge ratings, especially the high interrupting ratings common in capacitors at the transformer or surge arresters at the metal-clad switchgear and outdoor substation type circuit transformer). breakers. The dielectric strength and interrupting capacity of SF6 circuit breakers depends on the existence of adequate gas 3. For motors with locked rotor current under 600 A, add surge pressure. If the SF6 pressure drops to atmospheric level, there is protection (surge limiters at the switchgear, surge capacitors no way of telling whether the arc chamber contains SF6, air or at the motor or surge arresters at the motor). a mixture. As a result, there is no way to predict with 4. For motors with locked rotor current over 600 A, no confidence the performance of the interrupter under such additional protection is needed. conditions. ‘Soft’ interruption The other argument used extensively by advocates of SF6 technology is that vacuum circuit breakers chop current prior to a natural current zero, and gas breakers do not. This is not completely true, as chopping can occur with any type of interrupting medium, including oil, air magnetic, vacuum, SF6 and air blast circuit breakers. Further, current chopping is a statistical phenomenon, and also depends on the characteristics of the circuit being switched and the amount of The information provided in this document contains merely general descriptions or characteristics of performance which in case of actual current interrupted. There are numerous papers that discuss use do not always apply as described or which may change as a result of application of vacuum circuit breakers in detail. While current further development of the products. An obligation to provide the chopping was a major concern in the 1960s, when the respective characteristics shall only exist if expressly agreed in the terms common vacuum technology employed copper-bismuth of contract. contacts, it is no longer an issue with today’s chrome-copper contacts. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their The common areas of concern with application of vacuum own purposes could violate the rights of the owners. circuit breakers are transformers and motors. Siemens application recommendations are contained on page 26 of the Siemens Industry, Inc. selection and application guide for type GM-SG switchgear. We 7000 Siemens Road Wendell, NC 27591 recommend that all transformers have full BIL rating. Any transformers that have reduced BIL (i.e., open dry-type Subject to change without prior notice. designs) require surge protection. This can be provided by the Order No.: E50001-F710-A156-X-4A00 Siemens type 3EF zinc oxide surge limiter, or by a conventional All rights reserved. zinc oxide surge arrester. Limiters and arresters differ in several © 2012 Siemens Industry, Inc. fundamental respects. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 04 kA rated circuit breakers and switchgear The rating structure for circuit breakers used in metal-clad The 1964 rating structure was based on a “constant MVA” switchgear underwent major revision with the completion of interrupting capacity over a defined range of operating the new ANSI/IEEE standards in 1999 and 2000. The voltages. At the maximum design voltage of the air magnetic applicable standards (old and new) are detailed in Table 1. circuit breaker, the interrupting capacity was limited by the ability of the arc chutes to handle the transient recovery C37.06-2000 is a minor editorial revision of the 1997 voltage that appears across the circuit breaker contacts edition. The 1997 edition was published in anticipation of following interruption. As the operating voltage was the changes in the ratings that were finally published in the reduced, the interrupting capability of the circuit breaker 1999 versions of C37.04, C37.09 and C37.010. Together, would increase, as the contacts could cope with higher these revisions comprise the first major structural change to interrupting currents and transient recovery voltage became the circuit breaker rating standards since the change from less of a concern. Finally, a limit would be approached at the total (asymmetrical) current basis of rating to the which the contacts could not absorb further increases in heat symmetrical current basis of rating in 1964. C37.06 was during interruption. revised in 2009 with major organizational and technical changes. The rating structure introduced in 1964 (and modified in 1979 and again in 1987) recognized the prevalent medium- voltage interruption technology (air magnetic) of the time. Previous New Standard Title version version C37.04 1979 1999 Rating structure for AC high-voltage circuit breakers 1997 1979 C37.06 2000 AC high-voltage circuit breakers rated on a symmetrical current basis - preferred ratings and related capabilities 1987 2009 C37.09 1979 1999 Test procedure for AC high-voltage circuit breakers rated on a symmetrical current basis C37.010 1979 1999 Application guide for AC high-voltage circuit breakers rated on a symmetrical current basis Table 1: The applicable standards Answers for infrastructure. The maximum design voltage was designated as “V,” and The new rating structure continues the movement towards the range over which the interrupting current capability harmonization of ANSI/IEEE requirements with those of IEC, a increased as voltage decreased was defined in terms of process that has been pursued since 1951. voltage range factor “K.” The voltage V/K defined the What does this change imply for users of existing equipment associated lower limit of voltage. In the range of V/K to V, rated to the 1987 (or earlier) ratings? Probably very little. the interrupting current varied so that the product of voltage There are hundreds of thousands of circuit breakers installed and interrupting current was a constant value. Stated more that are rated to the old standards, and it is expected that simply, the interrupting MVA (interrupting current X voltage new circuit breakers and switchgear will be available with X 1.732) was constant over this range. These relationships the old “constant MVA” ratings for many years. New or are summarized in Figure 1. replacement circuit breakers with “constant MVA” ratings The “constant MVA” rating structure served the industry, must continue to be designed, rated and tested to the old both users and manufacturers, for many years. However, as standards, as the new standards do not define the full rating new interrupting technologies became available, the structure or test requirements for the “constant MVA” circuit “constant MVA” relationship became a poor representation of breakers. the actual physics of interruption. In particular, one of the desirable characteristics of a vacuum interrupter is the Gradually, however, the new “constant kA” circuit breakers dielectric withstand capability across the open contacts and switchgear are becoming more widely used. The use recovers nearly instantaneously following an interruption. of the “constant kA” ratings simplifies the application of The practical effect of this is the interrupting capability of circuit breakers and switchgear, and also more accurately the interrupter does not increase significantly as the represents the true physics of modern vacuum interruption operating voltage is decreased from rated maximum design technology voltage. Relating this fact to the “constant MVA” rating structure, we see the voltage range factor of a vacuum interrupter is essentially equal to 1.0. Figure 1: Relation of interrupting capability, close and latch capability, rated maximum design voltage and rated symmetrical current "constant This is one of the principle reasons that restructuring of the MVA basis" circuit breaker ratings was undertaken by working groups within IEEE and NEMA over the decade of the 1990s. Tables 2 and 3 briefly summarize the ratings in the 1987 and 2000 versions of C37.06. Close and latch capability = It should be pointed out that the “historic MVA class” rms = 1.6 x K x I included in the 1964 and 1979 versions of ANSI C37.06 peak = 2.7 x K x I (but deleted in the 1987 version) were intended only as convenient labels, not as an arithmetically accurate calculation of the interrupting MVA for a given rating. For example, the calculated MVA interrupting capacity for the Maximum symmetrical Interrupting capability = 350 “MVA class” is 338 MVA rather than 350 MVA. interrupting capability = rated I x KxI (rated V/operating V) TThe table of the new “constant kA” ratings has been kept in the same format as the table for “constant MVA” ratings to facilitate easy comparison. The “MVA class” is no longer relevant. The voltage range factor (K) is also eliminated from the new rating structure, but is shown as K = 1.00 in the new table for comparison. The close and latch ratings have been changed from 2.7 to 2.6 times the maximum symmetrical interrupting capacity (peak amperes) and from 1.6 to 1.55 times the maximum Rated symmetrical interrupting symmetrical interrupting capacity (rms amperes), to correct current I mathematical errors in earlier standards. V/K Rated maximum voltage = V  Dielectric kV Close and latch kA Historic Maximum Rated Maximum Range Continuous "MVA" class kV kA kA factor current Impulse rms Peak 60 Hz BIL 1.6 KI 2.7 KI 1,200 250 4.76 29 36 1.24 19 60 58 97 2,000 1,200 350 4.76 41 49 1.19 2,000 19 60 78 132 3,000 1,200 500 8.25 33 41 1.25 2,000 36 95 66 111 3,000 1,200 500 15.0 18 23 1.30 36 95 37 62 2,000 1,200 750 15.0 28 36 1.30 2,000 36 95 58 97 3,000 1,200 1000 15.0 37 48 1.30 2,000 36 95 77 130 3,000 1,200 1500 38.0 21 35 1.65 2,000 80 150 56 95 3,000 Table 2: ANSI C37.06-1987 (and 1964 and 1979) circuit breaker ratings ("constant MVA" rating basis)  Dielectric kV Close and latch kA Historic Maximum Rated Maximum Range Continuous "MVA" class kV kA kA factor current Impulse rms Peak 60 Hz BIL 1.55 KI 2.6 KI 40 40 1,200 62 104 Not 4.76 50 50 1.00 2,000 19 60 78 130 applicable 63 63 3,000 98 164 40 40 1,200 62 104 Not 8.25 50 50 1.00 2,000 36 95 78 130 applicable 63 63 3,000 98 164 Not 1,200 15.0 25 25 1.00 36 95 39 65 applicable 2,000 40 40 1,200 62 104 Not 15.0 50 50 1.00 2,000 36 95 78 130 applicable 63 63 3,000 98 164 1,200 Not 31.5 31.5 49 82 38.0 1.00 2,000 80 150 applicable 40 40 62 104 3,000 Table 3: ANSI/IEEE C37.06-2009 (also 1997 and 2000) circuit breaker ratings ("constant kA" rating basis) The information provided in this document contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners. Siemens Industry, Inc. 7000 Siemens Road Wendell, NC 27591 Subject to change without prior notice. Order No.: E50001-F710-A157-X-4A00 All rights reserved. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 05 Reclosing applications - minimum reclosing time Siemens circuit breakers used in metal-clad switchgear are Adjusted for the typical closing time of the circuit breaker, this suitable for use in reclosing applications when applied in means there must be at least 0.26 seconds between the accordance with the latest revision of ANSI/IEEE C37.04- closing of the tripping contact (for instance, the overcurrent 1999. In this standard, clause 5.5.1 defines minimum relay trip contact) and the initiation of the reclose command to reclosing time as follows: the circuit breaker. “5.5.1 Minimum reclosing time...The minimum reclosing The ANSI/IEEE application guide for high- voltage circuit time of a circuit breaker is 0.3 seconds. This is the shortest breakers, ANSI/IEEE C37.010-1999, discusses the need for dead permissible time in which the circuit breaker is required to time during reclosing operations in clause 5.9 as follows: reclose with rated control voltage and rated pressure. It may “5.9 Reclosing time… Before a circuit can be successfully be necessary to add an external time delay to meet specific re-energized, there must be sufficient dead time in the application requirements....” circuit breaker for the arc path at the fault to become The purpose of this time delay is twofold. First, a minimum deionized. On a radial line where the load includes a large time delay is needed to be reasonably sure that the arc at the motor component, arcing may be sustained after the breaker fault location (out on the distribution lines) will not re-ignite at the source is opened. Synchronous motors and static and thus create a second fault. This time is commonly referred capacitors included in the load will tend to prolong the to as “deionization time.” Second, the time delay allows time period of arcing….” needed for the circuit breaker’s mechanical linkages and “...A dead time on the circuit of at least 135 ms is normally latches to achieve a stable reset position before a closing required to clear the fault’s ionized gases at 115 kV to operation is initiated. 138 kV for circuit breakers without resistors across the To apply a medium-voltage circuit breaker in a reclosing interrupters. The required dead time is greater for higher application in accordance with ANSI/IEEE C37.04-1999, the voltages or when selective pole tripping is used to clear only user must ensure at least 0.30 seconds (300 ms) of time delay the faulted phases. is incorporated between the initiation of tripping of the circuit breaker and the completion of closing on the reclose operation. Answers for infrastructure.  Dead times on the order of several seconds may be required The need for a minimum time delay on instantaneous reclosing to allow secondary arcs to extinguish. (Secondary arcs result was less clear in earlier versions of the standards. Because of from capacitive coupling between the normal and faulted this, it is reasonable to assume some initial reclose operations phases.)” have failed in practice because the air at the point of the fault had not deionized, and the fault re-ignited when the circuit In accordance with ANSI/IEEE C37.04, it is the user’s breaker reclosed. responsibility to ensure that the required 0.30 second time delay is incorporated in the external control scheme. This All Siemens vacuum circuit breakers (including types GMSG, time delay is not incorporated as an integral element of the 38-3AH3, SDV6 and GMI) are suitable for reclosing operations circuit breaker mechanical design or electrical control when the reclosing system allows at least six cycles (100 ms) circuitry. between “b” switch closing and the initiation of the close command. It is recognized some users will wish to reclose with a time delay of less than 0.3 seconds (300 ms). As the time delay is decreased from 300 ms, the possibility increases that a reclose operation will fail due to excess ionization at the point of the fault. Sources vary in the estimates of the amount of time that must be allowed between interruption of the fault and the The information provided in this document contains merely general subsequent re-energization. Most of the sources indicate there descriptions or characteristics of performance which in case of actual must be at least six cycles between arc interruption (on use do not always apply as described or which may change as a result of opening) and contact make on the subsequent closing further development of the products. An obligation to provide the operation. respective characteristics shall only exist if expressly agreed in the terms of contract. When consideration is given to extremes of circuit breaker arcing time and closing time, we conclude that the reclosing All product designations may be trademarks or product names of relay should be set to issue a reclose command no sooner than Siemens AG or supplier companies whose use by third parties for their six cycles (100 ms) after the “b” switch makes during an own purposes could violate the rights of the owners. opening operation. Siemens Industry, Inc. If the reclosing relay does not monitor the “b” switch to 7000 Siemens Road determine that the circuit breaker is open before it issues the Wendell, NC 27591 close command, then the reclosing relay should be set to issue Subject to change without prior notice. the reclose command no sooner than ten cycles (167 ms) after Order No.: E50001-F710-A158-X-4A00 the opening signal is issued. All rights reserved. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 06 Three-cycle versus five-cycle interrupting time This issue of TechTopics discusses the rated interrupting For Siemens type GMI, SDV and GMSG circuit breakers, the time for circuit breakers used in metal-clad switchgear. average arcing time is approximately 9 ms, which is Historically, ANSI/IEEE C37.04 (1979 or earlier) characterized representative of a very large number of interrupting tests in circuit breakers with interrupting time classes, such as three the short-circuit test laboratory. The longest arcing time cycle, five cycle and eight cycle. These classes always were observed during testing is typically 17 ms, which occurs on rather gross approximations, because they made no tests with maximum offset asymmetrical current interruptions. allowance for production variations, and also because the The latter are tests specifically intended to explore the outer rated interrupting time could be exceeded by up to limits of interrupting performance. 50 percent under certain conditions. Further, a circuit In accordance with ANSI/IEEE C37.09-1999, tests must explore breaker that was just slightly in excess of one rating class both the shortest possible arcing time and the longest possible would fall into the next higher (longer) class, giving the arcing time. impression of a radical change in performance that does not necessarily reflect reality. Thus, there is a need to establish The longest possible arcing time results when contact part some facts pertinent to discussion of three-cycle versus occurs just prior to a current zero that precedes a minor loop of five-cycle circuit breakers. current. Because the first current zero occurs in a fraction of a millisecond after contact part, and the second current zero ANSI/IEEE standards no longer establish three-cycle and five- occurs only a short time later (perhaps 1 ms to 2 ms), the cycle classes, nor do they give assumed values for “contact-part interruption does not take place until the current zero that time” associated with a particular interrupting time. Instead, ends the major loop of current. rated interrupting time is now stated in terms of absolute time in milliseconds. As stated, the purpose of these tests is to expose the circuit breaker to the worst conceivable set of circumstances, so as to ANSI/IEEE C37.04-1999, clause 5.6 defines “rated interrupting establish that under this most extreme condition the circuit time” as “the maximum permissible interval between the breaker successfully interrupts. These worst-case conditions energizing of the trip circuit at rated control voltage and rated rarely occur in actual installations. operating pressure for mechanical operation, and the interruption of the current in the main circuit in all poles.” This definition makes it clear that the rating must consider the ‘worst-case’ conditions for all variables. Thus, it must consider the longest arc duration under the most onerous conditions. Of equal importance, it must consider the longest opening time (including worst case production variations). Answers for infrastructure. Type GMI circuit breakers Description Average time Range of values For Siemens type GMI circuit breakers, the relevant data is detailed in Table 1. Opening time 33 ms 25 ms to 41 ms If ANSI/IEEE C37.04 based rated interrupting time on nominal opening time and average arcing duration, our type GMI circuit breakers would be rated 42 ms (2.5 cycles). However, C37.04 states the rating has to be based on the worst-case conditions, Arcing duration 9 ms 2 ms to 17 ms which means that the type GMI circuit breakers are rated 58 ms (3.5 cycles). Actual interrupting times on production circuit breakers could range from 42 ms (25 ms opening time + 17 ms Interrupting time 42 ms 27 ms to 58 ms arcing duration) to 58 ms (41 ms opening time + 17 ms arcing duration), using the worst case arcing duration. What does this mean to a user with respect to application of Table 1: Type GMI circuit breakers (also type GMSG (five-cycle) circuit the circuit breakers? Basically, nothing. The reason for this breakers) relates to the way circuit breakers are tested in the short-circuit test laboratory. When circuit breakers are tested for short- circuit performance, they are tested to a philosophy that is completely reversed from the manner in which they are rated. Description Average time Range of values For ratings, the circuit breakers are rated in accordance with the worst-case (longest) times. For testing, actual test Opening time 29 ms 25 ms to 33 ms parameters are set up based on the worst-case short-circuit conditions, which means the shortest possible times. What does this mean for the GMI circuit breaker? Using the Arcing duration 9 ms 2 ms to 17 ms data in Table 1, the circuit breaker is tested as though it is the fastest circuit breaker, for example, with the shortest opening time. Therefore, the short-circuit conditions are set up in the laboratory to expose the circuit breaker to the conditions that Interrupting time 38 ms 27 ms to 50 ms would occur if (for the GMI circuit breaker) it had an opening time of 25 ms. Therefore, the circuit breaker is tested as though it was a historic three-cycle circuit breaker (1.5-cycle Table 2: Type GMSG (three-cycle) circuit breakers contact part time in terms of C37.04-1979, clause 5.10.2.2). The result is that the type GMI circuit breaker has the interrupting capability of a three-cycle circuit breaker, even though we must rate it as a five-cycle circuit breaker. If you consider only the nominal value for operating time The information provided in this document contains merely general (33 ms opening) and worst-case arcing time (17 ms arcing), descriptions or characteristics of performance which in case of actual the circuit breaker is a three-cycle circuit breaker. However, use do not always apply as described or which may change as a result of under the limits of production tolerances, it varies between further development of the products. An obligation to provide the 2.5 cycles and 3.5 cycles. Since ANSI/IEEE uses the “maximum respective characteristics shall only exist if expressly agreed in the terms of contract. permissible interval,” it’s a five-cycle circuit breaker (since a 3.5-cycle class doesn’t exist). All product designations may be trademarks or product names of Type GMSG circuit breakers Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners. For type GMSG circuit breakers, two rated interrupting times are offered: 83 ms (five cycles) and 50 ms (three cycles). The Siemens Industry, Inc. data for type GMSG (five-cycle) circuit breakers is the same as 7000 Siemens Road Wendell, NC 27591 shown in table 1. For type GMSG (three-cycle) circuit breakers, the data is as shown in table 2. Subject to change without prior notice. Order No.: E50001-F710-A159-X-4A00 All rights reserved. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics TechTopics No. 07 Current transformers (CTs) - Use of 600 V CTs in metal-clad switchgear www.usa.siemens.com/techtopics For many decades, it has been common practice to use 600 V To demonstrate the dielectric capabilities, C37.20.2 requires window (‘donut’ or toroidal) current transformers (CTs) in that the switchgear be tested using a normal circuit breaker metal-clad switchgear rated through 38 kV. Conceptually, use compartment equipped with the maximum complement of of 600 V CTs in switchgear is exactly analogous to use of 600 V current transformers installed. Typically, our designs are similar bushing CTs in outdoor high-voltage circuit breakers or in large for 1,200 A, 2,000 A and 3,000 A current ratings. Accordingly, power transformers. the arrangement tested is the configuration with the highest continuous current circuit breaker and compartment. This Occasionally, we are asked to explain why such a practice is arrangement is appropriate because it has the largest physical justified. The simple answer is the voltage rating of the current conductors used in the design, which, in turn, dictates that the transformer (by itself) is irrelevant. The important question is dielectric stress is more severe than for lower continuous whether the complete system, of which the CT is a part, meets current ratings having smaller conductors. the performance requirements for the system. The dielectric capability of the complete system is established by the Our types GM-SG (up to 15 kV) and type GM38 (up to 38 kV) combination of air surrounding the circuit breaker primary switchgear designs have successfully passed all required design runback conductor, the insulating tube incorporated in the tests, demonstrating that the integrated system meets the primary disconnect assembly, the air between the CT and the ratings required by the standards. primary disconnect assembly tube, plus the insulation of the CT. For metal-clad switchgear constructed to ANSI/IEEE C37.20.2, the complete system must meet the dielectric test requirements (power frequency withstand as well as lightning impulse withstand) for the completely assembled switchgear. Answers for infrastructure. Why are 600 V window CTs commonly used in metal-clad switchgear? Window-type CTs are less costly than molded high-voltage CTs, resulting in a more economical product for users. Window-type CTs do not incorporate the primary conductor as an integral component of the CT itself, thus isolating the CT from major dielectric as well as thermal stresses. The primary conductor bracing for short-circuit forces is provided by the switchgear structure and the circuit breaker primary runback conductor, instead of by the CT, which allows higher short-circuit levels. Since the CTs are not located directly in the high-voltage bus structure, they are more accessible for maintenance, inspection, changeout (for example, if a ratio change is needed) or for addition of CTs after initial installation. The information provided in this document contains merely general Window CTs are readily available in multi-ratio descriptions or characteristics of performance which in case of actual configuration. use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the Lead-times for window CTs are generally a fraction of respective characteristics shall only exist if expressly agreed in the terms those for high-voltage CTs. of contract. Use of window CTs allows the CTs to be installed in the All product designations may be trademarks or product names of circuit breaker compartment, around the primary Siemens AG or supplier companies whose use by third parties for their disconnect assembly, which saves space in both the bus own purposes could violate the rights of the owners. compartment and in the cable compartment. Siemens Industry, Inc. 7000 Siemens Road Wendell, NC 27591 Subject to change without prior notice. Order No.: E50001-F710-A160-X-4A00 All rights reserved. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 08 Heat generation estimation for type GM switchgear (up to 15 kV) We are often asked to provide estimated heat generation data The amount of heat generated is related to the square of the for our equipment. This issue of TechTopics provides current, so a circuit breaker operating at one-half rated current information that allows calculation of approximate heat will have heat generation only one-quarter of that at full-rated generated by the type GM switchgear under assumed loading continuous current. Because the effect of the square conditions. relationship is very significant, it is overly conservative to estimate heat generation based on the assumption that all The heat generation data given in the table is based on full- sections and all circuit breakers each carry their rated rated continuous current. Actual heat generation calculations continuous current at all times. Air conditioning systems sized must take into account the true loading of the equipment. based upon such estimates will be much larger than the real operating conditions will require. Table 1: Approximate full-load heat generation (in watts (W)) for type GM switchgear (up to 15 kV) Continuous current - circuit breaker (rows 1-3) or main bus (row 4) Rated current 1,200 A 2,000 A 3,000 A 4,000 A Category Actual current 1,200 A 2,000 A 3,000 A 4,000 A Circuit breaker cell with circuit breaker 475 W 871 W 1,396 W 2,480 W Vertical section with main bus 154 W 187 W 220 W 390 W Space heaters per vertical section 400 W 400 W 400 W 400 W Voltage transformer (VT) rollout 50 W 50 W 50 W 50 W 4% of CPT kVA 4% of CPT kVA 4% of CPT kVA 4% of CPT kVA Control power transformer (CPT) (drawout or stationary) rating rating rating rating Microprocessor type 50 W 50 W 50 W 50 W Relaying and instrumentation Electromechanical non-complex 100 W 100 W 100 W 100 W per circuit breaker cell Electromechanical complex 200 W to 300 W 200 W to 300 W 200 W to 300 W 200 W to 300 W Answers for infrastructure. Notes on assumptions: Table 2: Calculations 1. Space heaters, when provided, are not normally Heat controlled by a thermostat. Hence, their load is Category generation represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is 2,000 A circuit breaker at 1,400 A = 871 x (1,400/2,000)2 = 427 W not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of 1,200 A circuit breaker at 250 A = 475 x (250/1,200)2 = 21 W approximately 110. Therefore, in an air- conditioned room, the heaters would be energized A 1,200 A circuit breaker at 600 A = 475 x (600/1,200)2 = 119 W continuously. 2. Heat generated by current transformers is ignored 1,200 A circuit breaker at 550 A = 475 x (550/1,200)2 = 100 W as it is usually insignificant and varies according to the CT ratio as well as the loading. Total heat generation for circuit breaker cells 667 W 3. The CPT heat generation estimate is very conservative and assumes the CPT is operated at Vertical sections with 2,000 A bus at 1,400 A = B 275 W full-rated capacity. If normal loading is at less than 3 x 187 x (1,400/2,000)2 = full rating, heat generation may be adjusted by the square of the percent loading. C Space heaters for three vertical sections = 3 x 400 = 1,200 W 4. Relaying and instrumentation heat generation estimates are very approximate and are normally D VT rollout = 1 x 50 = 50 W estimated on the basis of the number of circuit breaker cells. Extensive relaying and E CPT = 1 x 4% x 10 kVA = 400 W instrumentation may warrant additional conservatism in the estimation of associated heat generation. F Relaying and instrumentation = 4 x 50 = 200 W 5. Conversion factor: watts x 3.415179 = BTU/hour. Total estimated heat generation under assumed loading conditions 2,792 W To estimate the heat generated under actual loading conditions, determine the component heat generation for each of the components indicated in Table 1. Estimated heat generation for circuit breakers should be adjusted for actual loading based on the ratio of the squares of the actual current and the rated current. To be precise, this adjustment should also be made for the actual current loading of the main bus for each individual The information provided in this document contains merely general vertical section, but this is frequently ignored in the interest of descriptions or characteristics of performance which in case of actual simplification. Instead, the main circuit breaker loading is use do not always apply as described or which may change as a result of usually assumed to be equal to the main bus loading in all further development of the products. An obligation to provide the vertical sections. respective characteristics shall only exist if expressly agreed in the terms of contract. Example: Assume a lineup with three vertical sections, one 2,000 A main circuit breaker (loaded to 1,400 A), three All product designations may be trademarks or product names of 1,200 A feeder circuit breakers (loading 250 A, 600 A and Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners. 550 A), 2,000 A main bus and space heaters. The lineup includes one VT rollout, one 10 kVA CPT and microprocessor Siemens Industry, Inc. relaying, and instrumentation. The calculations would be as 7000 Siemens Road described in Table 2. Wendell, NC 27591 If true loading were not considered (for example, all Subject to change without prior notice. calculations performed on the basis of full-rated current), the Order No.: E50001-F710-A161-X-4A00 calculations would yield a heat generation of 4,707 W or about All rights reserved. 170 percent of the “real” heat generation. © 2012 Siemens Industry, Inc. For more information, contact: +1 (800) 347-6659 www.usa.siemens.com/techtopics www.usa.siemens.com/techtopics TechTopics No. 09 Heat generation estimation for type GM38 switchgear (up to 38 kV) We are often asked to provide estimated heat generation data The amount of heat generated is related to the square of the for our equipment. This issue of TechTopics provides current, so a circuit breaker operating at one-half rated current information that allows calculation of approximate heat will have heat generation only one-quarter of that at full-rated generated by the type GM38 switchgear under assumed continuous current. Because the effect of the square loading conditions. relationship is very significant, it is overly conservative to estimate heat generation based on the assumption that all The heat generation data given in the table is based on full- sections and all circuit breakers each carry their rated rated continuous current. Actual heat generation calculations continuous current at all times. Air conditioning systems sized must take into account the true loading of the equipment. based upon such estimates will be much larger than the real operating conditions will require. Table 1: Approximate full