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Breakers v. fuses[1]
- 1. Breakers and Fuses BREAKERS AND FUSES AS CIRCUIT PROTECTIVE DEVICES
- 3. Breakers and Fuses Fuse Replacements Plug Fuse Penny Cartridge Fuse Copper Bar
- 4. Breakers and Fuses Design Specification Switch must be: SAFE RELIABLE TAMPERPROOF RESETTABLE
- 5. Breakers and Fuses Arc Extinguisher Side Plate Steel Plates
- 6. Breakers and Fuses Overall, Circuit Breakers Provide Better Protection than Fuses for Low Voltage Circuit Protection …and here’s why.
- 7. Breakers and Fuses The Issues … Technical Short circuit capacity. Motor circuit protectors. Single-phase protection. High/low level fault protection. Equipment protection. Coordination. Downtime. Current limiting fuse characteristics. - Fuse let-through chart. - Up-Over-Down Method. Series ratings.
- 8. Breakers and Fuses Short Circuit Calculations Unlimited Fault Current 2,500 kVA, Z=5.75% 12,470V∆ Calculated Fault 52,296A 480Y/277V Isc = kVA VxZ = 2,500 x 1,000 x 100 480 x 3 x 5.75 = 52,296A
- 9. Breakers and Fuses Secondary Short Circuit Capacity of Typical Power Transformers 208 Volts, 3-Phase 240 Volts, 3-Phase 480 Volts, 3-Phase Transformer Rating 3-Phase kVA and Maximum Short Circuit kVA Available Short Circuit Current rms Symmetrical Amps Impedance Percent from Primary System Combined Transformer and Motor 300 5% Unlimited 18400 17300 8600 500 5% Unlimited 30600 28900 14400 750 5.75% Unlimited 40400 38600 19300 1000 5.75% Unlimited 53900 51400 25700 1500 5.75% Unlimited 80800 77200 38600 2000 5.75% Unlimited 51400 2500 5.75% Unlimited 64300
- 10. Breakers and Fuses Effects of Impedance Fault Current in Thousands of Amperes (Sym.) 1500 kVA Transformer/5.75% Impedance/480 Volts UTILITY KVA 500,000 60 4 - 750 MCM 2 - 500 MCM 250 MCM #1/0 AWG #4 AWG 50 40 30 20 10 0 0 2 5 10 20 50 100 200 500 1000 2000 Distance in Feet from Transformer to Breaker Location 5000
- 11. Breakers and Fuses Short Circuit Calculations Unlimited Fault Current 12,470V∆ 2,500 kVA, Z = 5.75% Calculated Fault 64,328 Note: Obtain specific impedance values for each system. Do not assume the values shown here will be typical. 12 Feet, 3,200A Copper Feeder Busway 480Y/277V 3,200A 52,296 3,200A Bus 200A 60 Feet (3) 1-Conductor #4/0 Copper THW Insulation Steel Conduit Bus Plug 200A 150A 10,968 29,560 Calculated at 100% Motor Contributions Main Control Panel 100 Feet (3) 1-Conductor #2 Copper THW Insulation Steel Conduit
- 12. Breakers and Fuses Line-to-Line-to-Line Fault Bolted Fault Arcing Fault Systems must be designed for worst case conditions. However, the majority of faults will be arcing type.
- 13. Breakers and Fuses Frequency of Faults UTILITY Least Likelihood of Fault Greatest Likelihood of Fault M M
- 14. Breakers and Fuses Frequency of Faults TYPE OF FAULTS INCIDENCE % FAULT MAGNITUDE Three-phase bolted ? Approaches fault available Single-phase bolted 5% 30-60% of fault available Line-to-line arcing 15% Low to medium (less than 30%) Line-to-ground arcing 80% Very low to low (less than 10%)
- 15. Breakers and Fuses Control Panel
- 16. Breakers and Fuses Motor Circuit Protectors (HMCP) Typical Motor Capability Curve Locked Rotor Time Time Starting Curve Current
- 17. Breakers and Fuses Single-Phase Protection Single-phasing on three-phase loads cannot occur with circuit breakers as all three poles open on a single-phase fault. Single-phasing on motor loads when a single fuse blows can cause heating and eventual damage to motor windings. To eliminate single-phasing when using fuses is both costly and inefficient. Low level arcing faults can continue to be fed through the load.
- 18. Breakers and Fuses Single-Phasing Condition Protective Device Starter A B C “Normal Condition” 10A 10A Motor CLEARING TIME FOR A LOW LEVEL FAULT A B C 10A x 180% 18A Fault 0 10A x 180% 18A Motor Phase 10A A B C Dual Element Fuse A 0 Fault Motor Circuit Protector (MCP) 0 0 Motor Phase A B C Clears after multiple cycles. B C Clears in less than one cycle.
- 19. Breakers and Fuses High/Low Level Fault Protection Circuit breakers will clear high level (short circuit) faults safely and effectively. Circuit breakers in general will clear low level (overload) faults more effectively than fuses (e.g., a motor circuit protector will clear a low current motor fault (X6) in a shorter time than a fuse). Plus prevent single-phasing. Circuit breakers can include ground fault protection. They quickly interrupt dangerous ground faults before they escalate.
- 20. Breakers and Fuses Equipment Protection IEC 947-4-1 defines two levels of coordination for the motor starter under short circuit conditions. TYPE 1 COORDINATION Under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts. TYPE 2 COORDINATION Under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognized, in which case the manufacturer shall indicate the measures to be taken in regards to equipment maintenance.
- 21. Breakers and Fuses Equipment Protection FACTS Type 1 coordinated motor branch circuits are capable of clearing low level faults without damage. Type 1 may not prevent damage to the motor starter components in high level faults. Type 2 does not permit damage to the starter (as noted). Type 1 or Type 2 does not cover protection of the motor. Breakers or fuses may be utilized in Type 1 or Type 2. Type 2 prohibits replacement of parts (except fuses). Most faults in electrical systems are low level. Fuses can nuisance trip on startup under Type 2.
- 22. Breakers and Fuses Equipment Protection Reduction in downtime is critical to manufacturing facilities. CHOICES Type 1 Must replace heaters. May need to replace starter. Type 2 with fuses Must replace fuses. Type 2 with breaker No replacement required. Type 2 intended to keep production running. Repair or replacement is recommended.
- 23. Breakers and Fuses Why do we have molded case, insulated case and metal enclosed breakers? Molded Case Circuit Breakers Westinghouse Series C Molded Case Circuit Breakers 70A - 2,500A Insulated Case Circuit Breakers Westinghouse SPB Systems Pow-R Circuit Breakers 200A - 5,000A Metal Enclosed AC Power Circuit Breakers Westinghouse Types DSII/DSLII Low Voltage AC Power Circuit Breakers 100A - 5,000A
- 24. Breakers and Fuses Time-Current Curve Coordination Study 4.16 kV MOTOR A B C D E 1,000 E 100 Time in Seconds 250 MVA Phase Protection 10 A Ansi 3-Phase Through Fault Protection Curve (More Than 10 In Lifetime) D 250A 1,000 kVA 5.75% 19,600A C B 4,160 V ∆ 480Y/277 V 1,600A 24,400A B 1 Transformer Inrush .1 1,000A 20,000A A 175A Ground Protection .01 100 HP- .5 1 10 100 1,000 10,000 Scale X 100 = Current in Amperes at 480 Volts M 124A FLC = Available fault current including motor contribution.
- 25. Breakers and Fuses Electronic Trip Unit Zone Selective Interlocking - Short Time Delay - Ground Fault Time Delay - Instantaneous Trip Regardless of Short Time Delay - Minimize Damage Ground Fault Setting: 1,200A Pickup 0.5 Sec. Time Delay Zone 1 Breaker 1 Ground Fault Setting: 600A Pickup 0.3 Sec. Time Delay Zone 2 Breaker 2 Fault 1 Zone 3 Fault 2 LOAD Zone Interlock Wiring Ground Fault Setting: 300A Pickup No Time Delay
- 26. Breakers and Fuses Coordination Fuses coordinate well between each other as the 2:1 ratio of the current rating is maintained. Circuit breakers with thermal magnetic trip units will also coordinate well under the same conditions. Circuit breakers with adjustable electronic trip units will coordinate below the 2:1 ratio. Trip unit settings need to be determined by carrying out a coordination study. Fuses cannot do zone selective interlocking.
- 27. Breakers and Fuses Coordination Circuit breaker flexibility. - Adjustable pickup settings give improved current coordination. - Adjustable time delay setting gives improved time coordination. - Zone selective interlocking. • Improving time coordination. • Reducing damage to equipment. • Reducing stress on upstream devices.
- 28. Breakers and Fuses The Issues …Technical …The Advantages of Using Breakers Downtime Resettable and reusable devices. Better coordination. Closer equipment protection. Early warning (alarms). No time lost in searching for replacement fuses. Plus ground fault option.
- 29. Breakers and Fuses Downtime Main with Ground Fault Feeders without Ground Fault Control Panel M
- 30. Breakers and Fuses Current Limiting Available Short Circuit Current I2t = (IRMS)2t Ip Peak Let-Through Current (Ip) rms Let-Through Current (Calculated) IRMS tmelt tarc t Total Clearing Time
- 31. Breakers and Fuses Hundred Thousands 10 B Ip = 2.3 x IRMS SYM 5 Current Limiting Threshold 100 kA Available Fault Current 600A Fuse 600A 65 kA 600A 35 kA 600A 30 kA 600A Ten Thousands 10 5 LetThrough Current 10 Thousands Maximum Instantaneous Peak Let-Through Amperes Fuse Let-Through Chart 45 kA 40 kA 32 kA 5 30 kA 1 1 5 Thousands 10 5 Ten Thousands 10 5 10 Hundred Thousands Available Current in rms Symmetrical Amperes Fuses Tested at 15% Power Factor X/R Ratio = 6.6
- 32. Breakers and Fuses LetThrough Fault Current Hundred Thousands B Ip = 2.3 x IRMS SYM 5 Fuses Tested at 15% Power Factor X/R Ratio = 6.6 10 Ten Thousands 200A Fuse 10 200A Fuse 5 Current Limiting Threshold 10 Thousands 100 kA Available Fault Current Maximum Instantaneous Peak Let-Through Amperes Up-Over-Down Method 5 1 1 10 5 Thousands 5 Ten Thousands 10 5 10 Hundred Thousands 17 kA 100 kA Available Current in rms Symmetrical Amperes This method assumes that there is no downstream fast acting interrupting device.
- 33. Breakers and Fuses Breaker Contacts Standard Contact Current Limiting Contact (Reverse Loop)
- 34. Breakers and Fuses Current Limiting Fuse Characteristics Stand alone current limiting fuse. Fuse Action Fuse is current limiting and clears the fault in the first 1/4 cycle. FAST SLOW Fuse Action FAST Current limiting fuse with modern fast acting circuit breakers. Breaker begins to open. Breaker tries to clear the fault. Dynamic impedance is introduced. SLOW The current limiting fuse then becomes a slow acting device.
- 35. Breakers and Fuses The Circuit Breaker Sees the Fault Before the Fuse Fuse Action FAST SLOW …Therefore, the Up-Over-Down Method DOES NOT WORK because it’s only a theoretical calculation. NEC requires that fuse/breaker series combinations must be tested per UL test procedures. One test is better than a million calculations.
- 36. Breakers and Fuses Fuse manufacturers have acknowledged that the Up-Over-Down Method DOES NOT WORK with today’s modern high interrupting circuit breakers.
- 37. Breakers and Fuses Three Types of Systems 1. Selectively Coordinated System 2. Fully Rated System 3. Series Rated System
- 38. Breakers and Fuses 1. Selectively Coordinated System This system allows or selects the breaker closest to the overcurrent source to open, thus most closely isolating the problem. CONTINUITY OF SERVICE High continuity. PROTECTION All breakers fully rated. COST Most costly of all three systems.
- 39. Breakers and Fuses 2. Fully Rated System In this system, all of the breakers must be fully rated for the system’s available fault current. This allows for quick selection of equipment, but allows for less continuity of service in general. CONTINUITY OF SERVICE Lower than selectively coordinated system. Usually higher than series connected system. PROTECTION All breakers fully rated. COST Lower than selectively coordinated system. Usually higher than series rated system.
- 40. Breakers and Fuses 3. Series Rated System This is a system of series connected breakers which have been tested in combination and shown to effectively protect the system. Downstream breakers are not fully rated for the system’s available fault current but the upstream breaker, which is tested in combination, protects the downstream breaker by operating before damage occurs. CONTINUITY OF SERVICE Continuity of service may suffer. PROTECTION Downstream breakers not fully rated. COST Usually the least costly system.
- 41. Breakers and Fuses Series Ratings Fuses Circuit Breakers 100 kA 100 kA 100 kA 100 kA 100 kA 100 kA 100 kA 100 kA 100 kA 65 kA 14 kA 14 kA 14 kA 14 kA 14 kA 14 kA 14 kA 14 kA FULLY RATED SERIES RATING Series ratings can not be done with fusesthere’s no advantage. Circuit breakers are tested as a component and tested in an assembly.
- 42. Breakers and Fuses The Issues … Practical Safety. Monitoring and communications. Testability. Accessorization. Size. Economics. Cost.
- 43. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Safety Breakers are dead front devices. Fused switches have exposed live parts. Terminal shields and end covers available. Fuses can be easily replaced with devices that are improper and have different characteristics. Handle mechanism allows resettability of breaker without needing access.
- 44. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Monitoring and Many functions can be integral Communications to electronic trip units. - Earth leakage/ground fault. - Monitoring functions. - Metering. - Energy. - Power factor. - Communications.
- 45. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Testability Electronic trip units are field testable. Fuses are not field testable. Push to test in the field. Verify each unit off the assembly line.
- 46. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Accessorization Accessories are difficult to apply to fuse switches and are very costly. Both internal and external circuit breaker accessories are easy to install and are cost effective. Internal. - Shunt trip. - Auxiliary contacts. - Undervoltage release. - Bell alarms. External. - Motor operators. - Interlocks. - Handle mechanisms. - Cylinder locks.
- 47. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Size Smaller devices allow for more room in an assembly. Assemblies with fusible devices are larger. Space saving in control panel.
- 48. Breakers and Fuses Practical Issues Space and Cost Comparisons PRL 4F Fusible PRL 4B Breaker Wall Space 1200A FDP VERT. 1200A ND 73.5” 400A KD 400A KD 400A KD 400A KD 400A FDPW 400A FDPW 400A FDPW 90” 400A FDPW Thru-Feed Lugs 36” 72” $1,410 Additional for Fuses SAVINGS… 59% Wall Space Equipment Cost Savings Installation Time Cost of Fuses and Labor for Replacement Downtime PLUS Spares Floor Space 11.3” 2.83 Sq. Ft. 18” 9 Sq. Ft. 69% Floor Space
- 49. Breakers and Fuses The Issues …Practical …The Advantages of Using Breakers Economics No spares required. Fuses need to be replaced. Breakers are more electrically efficient. (Fuses have a higher wattage dissipation.) Cost Less downtime. Less contractor labor to handle and install. $ $ saved in floor and wall space. Saves panel space.
- 50. Breakers and Fuses Why Use a Breaker in a Distribution System: Prevents single-phasing. Motor protection. Coordination. Zone interlocking. Resettable. Dead front, no exposed parts. Space savings. Prevents downtime. Accessorization. Testable.
- 51. Breakers and Fuses Why Use a Breaker in a Control Panel: Prevents single-phasing. Motor protection. Ground faults. Resettable. Dead front, no exposed parts. Space savings. Prevents downtime. Accessorization. Testable.
Notes de l'éditeur
- Cutler-Hammer/Westinghouse has been a world leader in circuit protective devices since inventing the circuit breaker over seventy years ago.
- The need for molded case circuit breakers was created in 1918 when numerous applications for electrical motors resulted in a demand for a device that would ensure safe operation and, at the same time, protect electrical circuits. During this period, individual motors were used for the first time in industrial plants to operate machine tools, and in private homes to operate appliances. Plant electricians were constantly changing fuses blown during motor startups because of the lack of properly designed fuses for motor circuit protection.
- Inspectors were also concerned about fire hazards. Fuses were being replaced by fuses with too high of an ampere rating. Plug fuses were being bridged by pennies and cartridge fuses were being replaced with copper bars. Inspection authorities became involved and attempted to find a solution to the problem.
- Meetings with switch manufacturers were initiated in an effort to find a solution. Switch manufacturers were asked to develop a switching device that would interrupt a circuit under prolonged overload conditions. The device would have to be safe, reliableand tamperproof. It should also be resettable so as to be reusable after an interruption without replacing any parts.
- During this period of research and development, Westinghouse produced the DE-ION arc extinguisher for use in large oil circuit breakers. Although too large in its initial form to be practical for small circuit breakers, the arc extinguisher was eventually modified into a usable size. The first compact, workable circuit breaker was developed in 1923 when the modified arc extinguisher was coupled with a thermal tripping mechanism. It was not until four years later (1927), however, that Westinghouse research engineers found the ideal combination of materials and design that permitted circuit breakers to interrupt fault currents of 5,000 amps at 120 volts AC or DC. One year later (1928), Westinghouse placed the first circuit breaker on the market. Its acceptance was instantaneous.
- Circuit breakers are usually selected by knowing the systems voltage, total load, and available fault current. Fuse manufacturers typically talk of 200,000 and 100,000 amp faults. Are these real world values? This 2,500 kVA transformers would typically be the largest size in most plants. The fault available is only 52,296 AIC, assuming unlimited utility fault is available. Typically faults available are 500 MVA which would further reduce this value to 48,128. Salesperson’s Warning Note: If double ended substation, tie breaker closed and mains in parallel. 105,000 may exist for a duration.
- This chart shows secondary short circuit capacity of typical power transformers assuming unlimited utility fault available, 100% motor contribution and no wire impedance. Where greater than 100,000 AIC exists, Cutler Hammer manufactures circuit breakers up to 200,000 AIC for these unusual circumstances (because 2,500 kVA can deliver 64,300). But in reality, the fuse manufacturers talk about 200 K that does not exist very often. Plus you have to consider the switch or equipment you are putting the fuse in, even if rated for 200,000. Point out why we have 65 K breakers.
- Here we are showing the available fault current of a typical transformer found in an industrial plant. This 1,500 kVA transformer can deliver 38,600 amps of fault current. The size of the conductor and the distance away from the transformer adds impedance to the system to further reduce the available fault current. Example: A 100 feet of (2) 500 MCM cables causes the available fault current to drop below 31,000 amps. Also note 100 feet of #4 wire causes the fault current to drop below 10,000 amps. This is what you may see at a control panel.
- This is a one line showing the affects of impedance of the fault available through the power distribution system. Points A) Is the main bus fault available off the transfomer secondary. B) Is feeder fault with 100% motor contribution (motors act like generators contributing to the fault current during the fault). C) Cable fault with 60 feet downstream of a feeder. D) Typical fault available in a 100 amp control panel. Note: If the typical 1,500 kVA transformer was used in place of the 2,500 kVA, worst case, the currents would be as follows: A) 31,400 B) 38,600 C) 17,750 D) 6,600
- It is important to realize that the short circuit available currents are calculated assuming a worst case three-phase bolted faultas shown in the left hand corner of the overhead. All short circuit protective devices should be selected such that they can safely interrupt this fault level. Rarely if ever do actual faults approach this level. Most faults are arcing faults, caused by an accidental contact of tools with live conductors or by insulation failure inside a wire or motor. The magnitude of an arcing fault is significantly reduced due to added impedance introduced by the arcing fault. Although the magnitude of the arcing fault can be much lower, these faults can be the most dangerous due to the energy released at the point of the arc and the concern of what path the current flows.
- Where do most faults occur with a distribution system? Most faults occur at the end of the distribution system where it is easier and more likely to get accidental contact with live parts. Other areas in the system typically are much more isolated from personal contact.
- This is the percent of incidence. Magnitude shows percentage of available fault you could expect to see. A lot of the line-to-line arcing faults start out as line-to-ground and when they are not caught quickly, they can cascade into a more severe fault.
- Of these common fault incidences, the contact of a tool inside a control cabinet with exposed live parts such as older design starters, fuses and bus bars.
- Another likely area of faults occurring is in motors. All industry, in one way or another, depends upon electrical motors to perform critical functions. Most faults on a motor circuit are generally caused by an insulation breakdown within the motor windings. The initial fault current is low in relationship to the system capacity with these types of faults. This low level fault causes an arcing condition that, if not arrested could cascade like a chain reaction. As the fault cascades, it broadens the physical area of the fault, shorting out more and more of the motor windings. Prior to 1969, fusible devices and thermal magnetic breakers were the most common form of motor branch circuit protection. Both of these devices provided somewhat acceptable results. However, evidence reported by trade journals indicated that the level of protection was not totally effective. In 1969, available data indicated that approximately 19%* of all fires recorded in the United States were the result of motor failures. * Source IAEI News
- The HMCP Motor Circuit Protector was developed to meet customer needs for improved motor short circuit protection. To achieve the required performance, the HMCP was designed with three closely calibrated current sensors and an adjustable trip mechanism into the high performance Series C Circuit Breaker. The current sensors allowed the HMCP to clear faults in excess of its trip rating in one cycle or less with a high degree of repeatability. The adjustable trip mechanism provided the flexibility to place the trip setting just above the motor inrush, clearing low level faults quickly while ignoring motor starting currents. By utilizing the basic Series C design, the HMCP also offered high interrupting ratings in combination with motor starters, dead front construction, resettability after a trip, a wide variety of accessories and a reliable disconnect in a compact size. Since the introduction of the MCP (motor circuit protector) by Westinghouse in 1970, and its increased use as a motor branch circuit protective device, the incidence of fires, as a result of motor failures, has declined dramatically. In 1983, the incidence of fires of electrical origin in the United States and Canada combined, as a result of motor failures, was down to approximately 3%.* * Source IAEI News
- Another advantage these HMCPs provide is protection from single-phasing after a fault. Most single-phase conditions are caused by a blown fuse resulting from a single-phase fault.
- Explain example. Fault happens on load side of disconnect. One fuse blows. The motor single-phases and overloads causing the current to increase (180%) in the remaining phase. It now takes some time for the remaining fuses to blow. Typically overload relays do not react to this condition. An HMCP trips all phases at the same time eliminating the single-phase condition.
- In review: Breakers will clear high level faults effectively and safely. Breakers in general clear low level more effectively and these are what most commonly happen in an electrical system.
- 1)Those contacts in the starter can weld and just need to bepried apart with a screwdriver. It may have deteriorated the life of that starter and needs replacement at the nextavailable opportunity. 2)You may not get your motor running with the fuse protectionyou would need. 3)Your heaters took a hard hit, it could change their resistance value.
- The answer is coordination. The differentiating factor of these breakers is a term called withstand. Withstand is the ability for the breaker to delay its tripping under a fault condition. Molded Case Circuit Breakers are designed for use at the end of the distribution system. They need to trip rapidly therefore they have a lower withstand capability. (They usually have a withstand of 18 cycles at 10 times their rating.) Power Breakers are intended for use upstream in a distribution system where they must wait for a downstream device to interrupt the fault. This keeps from taking the whole plant down. Metal enclosed can have withstands at 60 cycles for their interrupting rating. Insulated Case can have withstands of 30 cycles. Note: The equipment upstream is braced and the bus bars are designed to be able to handle this fault for this period of time.
- In the coordinated system, we want the device curves on the left to trip first. This makes sure that the closest device to the fault handles it. Note: Oftentimes you need the ability to adjust the curves around transformers inrush points or have the ability to coordinate with a fuse.
- Explain how Zone Selective Interlocking works (see SA-11581C, page 8). Coordinates short time and ground fault - advantage instantaneous trip without time delay on short time or ground fault. 1)Coordination reduces stress on upstream devices andequipment. 2)Set up system as though there was no zone selectiveinterlocking. 3)Positive coordination without time delays. 4)No interlock on Instantaneous trip. We can do this even with molded case circuit breakers.
- Review.
- Ground fault at motor can take main down and all other feeders. (Depends on level of fault and the main.) Ground fault on the panel will take only that machine down. Circuit breakers can do internal or external ground fault sensing. They will catch the ground fault before it cascades into a fault of more magnitude. (Helps with equipment and personal safety.) In Europe, they call this earth leakage protection. It is very common in their panels and making it into the USA. Fuse switches are not recommended for ground fault. For a fused switch to be used as ground fault device, it must be rated to handle the available fault levels while opening. Most switches except bolted pressure switches are not rated for this application.
- Current limiting is defined by UL as a device that limits the total I2t let-through less than the I2t of a 1/2 cycle, when operating inside its current limiting range.
- Fuse manufacturers talk a lot about current limiting and peaklet-through. Fuses work very well at high level faults when there is a lot of energy with the ability to melt the links. As the fault goes down, they become less current limiting as shown by the overhead. Note: Current limiting breakers are also available and most of our breakers limit the current. Salesperson’s Notes: Short circuit devices are tested with power factors that you typically do not recognize as being standard in a plant. The reason is the fault acts in an assymetrical configuration as opposed to the one a plant talks about in normal operation.
- Many peope still subscribe to the Up-Over-Down Method that was once promoted by fuse manufacturers. They assumed a 100 kA fuse protected a 17 kA breaker by using the let-through curve of the fuse. All assumptions they made were on theory, not testing. Lab conditions do not always exist in the field.
- Why new circuit breakers open so much faster than old designs. The contacts are designed utilizing a pivoted reverse loop and causes the current to flow in opposite directions in parallel paths, creating a blow-apart effect. This natural blow-apart action combined with the pivoted contact design dramatically increases the overall blow-apart action which quickly draws a high impedance arc and effectively limits the let-through current. Because of this current limiting action, a fault never reaches its peak value of current. Salesperson’s Note: Right hand rule.
- The Up-Over-Down Method does not account for the dynamic impedance introduced by today’s circuit breakers.
- NEC-240-83 requires that all series rated combinations be tested and labeled. It is because of the requirements of tested combination, that we do not publish let-through current curves. We feel all combinations should be tested.
- When people use the Up-Over-Down Method, what they are trying to achieve is what we call a series rated system. In addition to a series rated system, there are selectively coordinated and fully rated systems. Let’s discuss the difference.
- Most expensive - used for continuity/critical loads. Uses higher withstand rated breakers (power breakers) in order to coordinate trip curves for system - allows downstream breaker to clear fault first. All breakers and fuses are rated for the available fault current.
- Typically using molded case circuit breakers that are all rated for the full available fault. Coordination can be lost above the withstand of the upstream breaker. (Roughly 10 times the rating of the source,i.e., 400 amp breaker 4,000 amp withstand for 18 cycles.)
- Less continuity since upstream breaker may trip before downstream breaker can clear downstream fault. If we rate anything series rated, we have tested it in these conditions.
- This circuit board was tested as a series.
- We reviewed the technical issues, let’s discuss the practical reasons.
- There is no evidence to show fuses are safer than breakers as the fuse manufacturers lead you to believe. More people are injured by accidental contact with live parts as commonly found in fused equipment. Breakers are designed with dead front covers and terminal shields are available to keep you completely isolated from live parts. Handle mechanisms allow you to reset without opening the operator door. With fuses, you must go in and replace the fuses, exposing yourself.
- Earth leakage is driven by the European market typically30 milliamps. With deregulation, you need to understand where you are using energy.
- Breaker performance can be verified through nondestructive testing. Every breaker is tested off the assembly line. All fuse testing is destructive (except resistance tests). You never really know where it will actually trip. Salesperson’s Note: NEMA AB-4 1991 Field Testing of Breakers.
- Self explanatory.
- Fuse manufacturers do 70% of their business in replacement fuses. Fuse manufacturers would love to keep selling you replacement fuses. Over the life of the equipment, it will cost you a lot in replacement fuses. In startup and testing at an OEM, it will cost more in replacement of fuses instead of resetting the breaker.