Selective Coordination

Transcription

1 Introduction and Requirements 5 NEC Code: Definitions (B) Healthcare Healthcare Elevators Emergency Systems Legally Required Standby Systems What Is Selective Coordination? Today, more than ever, one of the most important parts of any installation - whether it is an office building, an industrial plant, a theater, a high-rise apartment or a hospital - is the electrical distribution system. Nothing will stop all activity, paralyze production, inconvenience and disconcert people and possibly cause a panic more effectively than a major power failure. Isolation of a faulted circuit from the remainder of the installation is mandatory in today s modern electrical systems. Power blackouts cannot be tolerated. While it's very important, it is not enough to select protective devices based solely on their ability to carry the system load current and interrupt the maximum fault current at their respective levels. A properly engineered system will allow ONLY the protective device nearest the fault to open, leaving the remainder of the system undisturbed and preserving continuity of service. We may then define selective coordination as THE ACT OF ISOLATING A FAULTED CIRCUIT FROM THE REMAINDER OF THE ELECTRICAL SYSTEM, THEREBY ELIMINATING UNNECESSARY POWER OUTAGES. THE FAULTED CIRCUIT IS ISOLATED BY THE SELECTIVE OPERATION OF ONLY THAT OVERCURRENT PROTECTIVE DEVICE CLOSEST TO THE OVERCURRENT CONDITION. In Article of the 5 NEC this definition had been added: Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings. The two one line diagrams in the figure below demonstrate the concept of selective coordination. The system represented by the one line diagram to the left is a system without selective coordination. A fault on the load side of one overcurrent protective device unnecessarily opens other upstream overcurrent protective device(s). The result is unnecessary power loss to loads that should not be affected by the fault. This is commonly known as a "cascading effect" or lack of coordination. The system represented by the one line diagram to the right is a system with selective coordination. For the full range of overload or fault currents possible, only the nearest upstream overcurrent protective device opens. All the other upstream overcurrent protective devices do not open. Therefore, only the circuit with the fault is removed and the remainder of the power system is unaffected. The other loads in the system continue uninterrupted. Selective Coordination: Avoids Blackouts Without Selective Coordination OPENS NOT AFFECTED UNNECESSARY POWER LOSS Fault With Selective Coordination OPENS NOT AFFECTED Fault Selective coordination can be a critical aspect for electrical systems. Quite often in the design or equipment selection phase, it is ignored or overlooked. And when it is evaluated many people misinterpret the information thinking that selective coordination has been achieved when in fact, it has not. The following sections explain how to evaluate systems as to whether the overcurrent protective devices provide selective coordination for the full range of overcurrents. 5 NEC Requirements Application of Other Articles. The essential electrical system shall meet the requirements of Article 700, except as amended by Article Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. Popular Methods of Performing A Selective Coordination Study Currently three methods are most often used to perform a coordination study: 1. Overlays of time-current curves, which utilize a light table and manufacturers published data, then hand plot on log-log paper. 2. Computer programs that utilize a PC and allow the designer to select time-current curves published by manufacturers and transfer to a plotter or printer, following proper selections. Simply plotting the curves does NOT prove selective coordination. The curves must be analyzed in relation to the various available fault currents at various points in the system. Also, where the plot may appear to show coordination above 0.01 second, the characteristics of the overcurrent devices below 0.01 second may not be coordinated. 3. For fuse systems, V or less, merely use the published selectivity ratios on page 91 for Cooper Bussmann fuses. The ratios even apply for less than 0.01 second. This text will apply to all three methods. Overloads and Low Level Fault Currents In the sections ahead, information is presented as an aid to understanding time-current characteristic curves of fuses and circuit breakers. There are simple methods that will be presented to analyze coordination for most systems V or less. It should be noted that the study of time-current curves indicates performance during overload and low level fault conditions. The performance of overcurrent devices that operate under medium to high level fault conditions are not reflected on standard curves. Other engineering methods may be necessary. Coordination Analysis The next several pages cover coordination from various perspectives. The major areas include: Fuse curves Fuse selective coordination analysis Circuit breaker curves Circuit breaker coordination analysis Elevator circuits Emergency circuits Ground fault protection coordination The section on ground fault protection is included because GFP systems can cause coordination issues. The ground fault protection section discusses GFP requirements and then discusses coordination considerations. 5 Cooper Bussmann 89

2 Fuses Fuse Curves The figure to the right illustrates the time-current characteristic curves for two sizes of time-delay, dual-element fuses in series, as depicted in the one-line diagram. The horizontal axis of the graph represents the RMS symmetrical current in amps. The vertical axis represents the time, in seconds. A A For example: Assume an available fault current level of 0A RMS symmetrical on the load side of the A fuse. To determine the time it would take this fault current to open the two fuses, first find 0A on the horizontal axis (Point A), follow the dotted line vertically to the intersection of the total clear curve of the A time-delay dual-element fuse (Point B) and the minimum melt curve of the A time-delay dual-element fuse (Point C). Then, horizontally from both intersection points, follow the dotted lines to Points D and E. At 1.75 seconds, Point D represents the maximum time the A timedelay dual-element fuse will take to open the 0A fault. At 90 seconds, Point E represents the minimum time at which the A time-delay dual-element fuse could open this available fault current. Thus, coordination operation is assured at this current level. The two fuse curves can be examined by the same procedure at various current levels along the horizontal axis (for example, see Points F and G at the 0A fault level). It can be determined that the two fuses are coordinated, for the overcurrents corresponding to the fuse curves on the graph. The A time-delay dual-element fuse will open before the A timedelay dual-element fuse can melt. However, it is necessary to assess coordination for the full range of overloads and fault currents that are possible. TIME IN SECONDS Point E Point G Point D Point C Point B Figure 3a. A A Available Fault Current Level 0A For analyzing fuse selective coordination for higher level fault currents see the next page, Medium to High Level Fault Currents Fuse Coordination. When using the published Fuse Selectivity Ratios, drawing time current curves is not necessary for any level of overcurrent Point F Minimum Melt Total Clearing Point A 0A CURRENT IN AMPERES ,000 20, Cooper Bussmann

3 Fuses: Selectivity Ratio Guide Medium to High Level Fault Currents Fuse Coordination The illustrations on the next page shows the principles of selective coordination when fuses are properly applied. The available short-circuit current will reach a peak value of I p during the first half cycle unless a protective device limits the peak fault current to a value less than I p. A currentlimiting fuse will reduce the available peak current to less than I p, namely I' p, and will clear the fault in approximately one-half cycle or less. Note that t c is the total clearing time of the fuse, tm the melting time and ta the arcing time of the fuse. Where high values of fault current are available, the sub-cycle region becomes the most critical region for selective operation of current-limiting fuses. The area under the current curves is indicative of the energy let-through. If no protective device were present, or if mechanical type overcurrent devices with opening times of one-half cycle or longer were present, the full available short circuit energy would be delivered to the system. The amount of energy delivered is directly proportionate to the square of the current. So we can see how important it is to have fuses which can limit the current being delivered to the system to a value less than the available current. The amount of energy being produced in the circuit while the fuse is clearing is called the total clearing energy and is equal to the melting energy plus the arcing energy. Selectivity between two fuses operating under short circuit conditions exists when the total clearing energy of the load side fuse is less than the melting energy of the line side fuse. An engineering tool has been developed to aid in the proper selection of Cooper Bussmann fuses for selective coordination. This Selectivity Ratio Guide (SRG) is shown below. *Selectivity Ratio Guide for Blackout Prevention (Line-Side to Load-Side) Circuit Load-Side Fuse Line-Side Fuse Current Rating 601-0A 601-0A 0-A 601-0A 0-A 0-1A 0-A 0-60A Type Time- Time- Dual-Element Fast- Fast- Fast- Fast- Time- Delay Delay Time-Delay Acting Acting Acting Acting Delay Trade Name Limitron Fusetron Limitron Limitron T-Tron Limitron SC Class (L) (L) (RK1) (J) (RK5) (L) (RK1) (T) (J) (G) Cooper Bussmann KRP-C_SP KLU LPN-RK_SP LPJ-SP FRN-R KTU KTN-R JJN JKS SC Symbol LPS-RK_SP TCF FRS-R KTS-R JJS 601 to Time- KRP-C_SP 2:1 2.5:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A 0A Delay (L) 601 to Time- Limitron KLU 2:1 2:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A 0A Delay (L) LPN-RK_SP 2:1 2:1 8:1 3:1 3:1 3:1 4:1 0 Dual- (RK1) LPS-RK_SP to Ele- (J) LPJ-SP 2:1 2:1 8:1 3:1 3:1 3:1 4:1 A ment Fusetron FRN-R 1.5:1 1.5:1 2:1 1.5:1 1.5:1 1.5:1 1.5:1 (RK5) FRS-R 601 to Limitron KTU 2:1 2.5:1 2:1 2:1 6:1 2:1 2:1 2:1 2:1 N/A 0A (L) 0 to Fast- Limitron KTN-R 3:1 3:1 8:1 3:1 3:1 3:1 4:1 A Acting (RK1) KTS-R 0 to T-Tron JJN 3:1 3:1 8:1 3:1 3:1 3:1 4:1 1A (T) JJS 0 to Limitron JKS 2:1 2:1 8:1 3:1 3:1 3:1 4:1 A (J) 0 to Time- SC SC 3:1 3:1 4:1 2:1 2:1 2:1 2:1 60A Delay (G) *Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Cooper Bussmann. General Notes: Ratios given in this Table apply only to Cooper Bussmann fuses. When fuses are within the same case size, consult Cooper Bussmann. TCF (CUBEFuse) is 1 to A Class J performance; dimensions and construction are unique, finger-safe IP-20 design. 5 Cooper Bussmann 91

4 Fuses For the next example, the Selectivity Ratio Guide suggests that the minimum ratio between line side and load side fuse should be at least 2:1. The one-line shows fuses KRP-C-0SP feeding a LPS-RK-SP. The ratio of amp ratings is 5:1 (0:) which indicates coordination between these fuses. Continuing further into the system the LPS-RK-SP feeds a LPJ-60SP. This ratio of amp ratings is 3.33:1 (:60), which also indicates a selectively coordinated system. I p Available Short-Circuit Current 480/277V I 1 p KRP-C-0SP Amp Fuse Let Through Energy Line Side Load Side Time-Delay Fuse KRP-C-0SP t m t c t a LPS-RK-SP Dual-Element Fuse LPS-RK-SP Amp Fuse Let Through Energy t m Line Side Load Side t c LPJ-60SP Dual-Element Fuse Fault LPJ-60SP Amp Fuse Let Through Energy* t c Requirements for selectivity Total clearing energy of load side fuse is less than melting energy of line side fuse. *Area under the curves is not actual energy but is indicative of let-through energy. Actual let-through energy is I 2 rt Cooper Bussmann

5 Fuses Example Fuse Selective Coordination Review the one-line diagram of the fusible system. All the fuses are Fuses. The Selectivity Ratio Guide provides the minimum amp ratio that must be observed between a line-side fuse and a load-side fuse in order to achieve selective coordination between the two fuses. If the entire electrical system maintains at least these minimum fuse amp rating ratios throughout the system, the entire electrical system will be selectively coordinated for all levels of overcurrent. Notice, time current curves do not even need to be plotted. One-Line For Fuse System Coordination Analysis KRP-C-1SP Fuse Check the LPJ-SP fuse coordination with the KRP-C-1SP fuse. Use the same steps as in the previous paragraph. The amp rating ratio of the two fuses in the system is 1:, which yields an amp rating ratio of 3:1. The Selectivity Ratio Guide shows that the amp rating ratio must be maintained at 2:1 or more to achieve selective coordination. Since the fuses used have a 3:1 ratio, and all that is needed is to maintain a 2:1 ratio, these two fuses are selectively coordinated. See the following diagram. Selective Coordination Only Faulted Circuit Cleared KRP-C1SP Fuses LPJ-SP Fuses LPJ-SP Fuses Only These Fuses Open LPJ-SP Fuses Any Fault Level! Opens Not Affected LPJ-SP Fuses Any Fault Level! Check the LPJ SP fuse coordination with the LPJ-SP fuse. The amp rating ratio of these fuses is : which equals 4:1 ratio. Checking the Selectivity Ratio Guide, line-side LPJ (vertical left column) to load-side LPJ (horizontal), yields a ratio of 2:1. This indicates selective coordination for these two sets of fuses. This means for any overcurrent on the load-side of the LPJ-SP fuse, only the LPJ-SP fuses open. The LPJ-SP fuses remain in operation as well as the remainder of the system. Summary Fuse Coordination Selective coordination is an important system criteria that is often overlooked or improperly analyzed. With modern current-limiting fuses, all that the design engineer, contractor, plan reviewer, electrical inspector or user needs to do is adhere to these ratios. It is not necessary to plot the time current curves. Just maintain at least the minimum amp rating ratios provided in the Cooper Bussmann Selectivity Ratio Guide and the system will be selectively coordinated. This simple method is easy and quick. 5 Cooper Bussmann 93

6 Circuit Breakers Circuit Breaker Curves The following curve illustrates a typical thermal magnetic molded case circuit breaker curve with an overload region and an instantaneous trip region (two instantaneous trip settings are shown). Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which the breaker interrupts the circuit. When using molded case circuit breakers of this type, there are four basic curve considerations that must be understood. These are: 1. Overload Region 2. Instantaneous Region 3. Unlatching Time 4. Interrupting Rating 1. Overload Region: The opening of a molded case circuit breaker in the overload region is generally accomplished by a thermal element, while a magnetic coil is generally used on power breakers. Electronic sensing breakers will utilize CTs. As can be seen, the overload region has a wide tolerance band, which means the breaker should open within that area for a particular overload current. 2. Instantaneous Region: The instantaneous trip (I.T.) setting indicates the multiple of the full load rating at which the circuit breaker will open as quickly as possible. The instantaneous region is represented in the following curve and is shown to be adjustable from 5x to 10x the breaker rating. When the breaker coil senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed. The unlatching time is represented by the curve labeled average unlatching time for instantaneous tripping. After unlatching, the overcurrent is not halted until the breaker contacts are mechanically separated and the arc is extinguished. Consequently, the final overcurrent termination can vary over a wide range of time, as is indicated by the wide band between the unlatching time curve and the maximum interrupting time curve. The instantaneous trip setting for larger molded case and power breakers can usually be adjusted by an external dial. Two instantaneous trip settings for a A breaker are shown. The instantaneous trip region, drawn with the solid line, represents an I.T. = 5x, or five times A = 0A. At this setting, the circuit breaker will trip instantaneously on currents of approximately 0A or more. The ± 25% band represents the area in which it is uncertain whether the overload trip or the instantaneous trip will operate to clear the overcurrent. The dashed portion represents the same A breaker with an I.T. = 10x, or 10 times A = 0A. At this setting the overload trip will operate up to approximately 0 amps (±10%). Overcurrents greater than 0A (±10%) would be cleared by the instantaneous trip. The I.T. of a circuit breaker is typically set at its lowest setting when shipped from the factory. 3. Unlatching Times: As explained above, the unlatching time indicates the point at which the breaker senses an overcurrent in the instantaneous region and releases the latch holding the contacts. However, the fault current continues to flow through the breaker and the circuit to the point of fault until the contacts can physically separate and extinguish the arc. Once the unlatching mechanism has sensed an overcurrent and unlatched, the circuit breaker will open. The final interruption of the current represented on the breaker curve in the instantaneous region occurs after unlatching, but within the maximum interruption time. The relatively long time between unlatching and the actual interruption of the overcurrent in the instantaneous region is the primary reason that molded case breakers are very difficult to coordinate. This is an inherent problem since the breaking of current is accomplished by mechanical means. 4. Interrupting Rating: The interrupting rating of a circuit breaker is a critical factor concerning protection and safety. The interrupting rating of a circuit breaker is the maximum fault current the breaker has been tested to interrupt in accordance with testing laboratory standards. Fault currents in excess of the interrupting rating can result in destruction of the breaker and equipment and possible injury to personnel. In other words, when the fault level exceeds the circuit breaker interrupting rating, the circuit breaker is no longer a protective device. In the example graph below, the interrupting rating at 480 volts is 30,000 amps. The interrupting ratings on circuit breakers vary according to breaker type and voltage level. The marked interrupting on a circuit breaker is a three-pole rating and NOT a single-pole rating (refer to pages 29 to 34 for more information). When drawing circuit breaker time-current curves, determine the proper interrupting rating from the manufacturer s literature and represent this interrupting rating on the drawing by a vertical line at the right end of the curve. 0 TIME IN SECONDS Minimum Unlatching Time Overload Region Adjustable Instantaneous Trip Set at 5 Times I.T. = 5X (± 25% Band) Average Unlatching Times Ampere Circuit Breaker Breaker Tripping Magnetically Current in Time in RMS Amps Seconds 5, , Maximum Interrrupting Time 15, , , Average Unlatching Times for Instantaneous Tripping Instantanous Region Interrupting Rating RMS Sym. Amps 240V 42, V 30,000 V 22,000 Adjustable Magnetic Instantaneous Trip Set at 10 Times I.T. = 10X (± 10% Band) Maximum Interrupting Time Interrupting Rating at 480 Volt ,000 CURRENT IN AMPERES 20,000 30,000 40,000 60,000 80,000, Cooper Bussmann

7 Circuit Breakers Medium to High Level Fault Currents Circuit Breakers The following curve illustrates a A circuit breaker ahead of a 90A breaker. Any fault above 1500A on the load side of the 90A breaker will open both breakers. The 90A breaker will generally unlatch before the A breaker. However, before the 90A breaker can separate its contacts and clear the fault current, the A breaker has unlatched and also will open. Assume a 0A short circuit exists on the load side of the 90A circuit breaker. The sequence of events would be as follows: A 90A 0A 1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to start the actual opening process. 2. The A breaker will unlatch (Point B) and it, too, would begin the opening process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible. 3. At Point C, the 90A breaker will have completely interrupted the fault current. 4. At Point D, the A breaker also will have completely opened the circuit. Consequently, this is a non-selective system, causing a complete blackout to the other loads protected by the A breaker. As printed by one circuit breaker manufacturer, One should not overlook the fact that when a high fault current occurs on a circuit having several circuit breakers in series, the instantaneous trip on all breakers may operate. Therefore, in cases where several breakers are in series, the larger upstream breaker may start to unlatch before the smaller downstream breaker has cleared the fault. This means that for faults in this range, a main breaker may open when it would be desirable for only the feeder breaker to open. This is typically referred to in the industry as a "cascading effect." Typically circuit breaker manufacturers do not publish the unlatching times or unlatching curves for their products. TIME IN SECONDS Amp Circuit Breaker Amp Circuit Breaker I.T. = 5X D C B A ,500A CURRENT IN AMPERES 0 4,000A ,000 20,000 30,000 40,000 60,000 80,000,000 14,000A 30,000A I.R. I.R. 5 Cooper Bussmann 95

8 Circuit Breakers Simple Method To Check Circuit Breaker Coordination The previous discussion and curve illustrated two molded case circuit breakers (90A and A) with the unlatching characteristics for both shown on one curve. This illustrated that two circuit breakers with instantaneous trips can not be selectively coordinated for fault currents above a certain level. That level is the fault current at which the upstream circuit breaker operates in its instantaneous trip region. When a fault above that level occurs, the lower circuit breaker (90A in this case) unlatches. However, before it clears the circuit, the upstream circuit breaker(s) (A in this case) also unlatches. Once a circuit breaker unlatches, it will open, thereby disconnecting the circuit from the power source. In the case shown, the curves show the A circuit breaker needlessly opens for a fault on the load side of the 90A circuit breaker. For any fault current greater than where the two circuit breaker curves intersect (in this case 1500A) the upstream circuit breaker does not coordinate with the down stream circuit breaker. However, in most cases, manufacturers do not publish unlatching times or unlatching curves for their circuit breakers. Therefore, even the most detailed coordination study from a software program will NOT show whether or not a circuit breaker system is "selectively coordinated." So then, how can coordination of circuit breakers be assessed when all the circuit breakers used in a system have instantaneous trip settings? There is a very simple method that does not even require drawing the circuit breaker time current curves. This method can be used to analyze software program coordination plots in most cases. The following paragraphs present this method. Circuit Breaker Coordination Simplified Method With Time Current Curve With the simplified method, there is no need to have the unlatching times or draw the unlatching curves. The following curve illustrates the time current characteristics for the 1A circuit breaker, the A circuit breaker and A circuit breaker. The instantaneous trip settings for each of these three molded case circuit breakers are provided on the one-line diagram. The A circuit breaker has a non-adjustable instantaneous trip setting and the curve is as depicted. The A circuit breaker has an instantaneous trip set at 10 times its amp rating (10X) which is 10 times A or 0A. The 1A circuit breaker has an instantaneous trip set at six times its amp rating (6X) which is six times 1A rating or 7A.. Remember from a previous section 2. Instantaneous Region that there is a tolerance associated with the instantaneous trip region for adjustable instantaneous trip settings; the curve shown for the A and 1A circuit breakers are drawn with the tolerances included. 1. This simple method will be shown with the time current curves (but without the unlatching time curves included). 2. However, normally it is not even necessary to draw the time current curves in order to evaluate coordination of circuit breakers having instantaneous trip units. So another section provides this simplified method without needing a time current curve. Below is the one line diagram that will be used for learning these simple methods. Review the one-line diagram below that has three molded case circuit breakers in series: from the main 1A to the A branch circuit with the A feeder in between. The other circuit breakers on the one-line diagram supply other circuits and loads. The fault current path from the power source is depicted by the red arrows/lines superseded on the one-line diagram. One-Line For Circuit Breaker System CoordinationAnalysis 1A MCCB 6 X = 7,A A MCCB 10 X = 4,000A A MCCB It Non-Adjustable Fault > 7,A When the curves of two circuit breakers cross over in their instantaneous trip region, then the drawing indicates that the two circuit breakers do not coordinate for fault currents greater than this cross over point. For instance, interpreting the coordination curves for the A circuit breaker and the A circuit breaker: their curves intersect in the instantaneous region starting at approximately 3A. That means for a fault current greater than 3A on the load side of the A circuit breaker, the A circuit breaker will open as well as the A circuit breaker. This demonstrates a lack of coordination and results in a "cascading effect" that will cause a partial blackout. This curve also shows that for any fault greater than approximately 6500 amps on the load side of the A circuit breaker, the A and 1A circuit breakers will open as well as the A circuit breaker. The reason: for a fault of greater than 6500A, all three of these circuit breakers are in their instantaneous trip region. Both the A and 1A circuit breakers can 96 5 Cooper Bussmann

9 Circuit Breakers unlatch before the A circuit breaker clears the fault current. If this is not understood, re-read the previous section Circuit Breaker Coordination - Medium to High Level Fault Currents. How does this affect the electrical system? Look at the one-line diagram below. For any fault current greater than approximately 6500A on the load side of the A circuit breaker, the 1A and A circuit breakers open as well as the A circuit breaker. The yellow shading indicates that all three circuit breakers opened - A branch circuit, A feeder and the 1A main. In addition, all the loads fed by the other circuit breakers, denoted by the hash shading, are blacked out unnecessarily. This is due to the lack of coordination between the A, A and 1A circuit breakers. Circuit Breaker Coordination Simplified Method Without Time Current Curve It is not even necessary to draw the curves to assess circuit breaker coordination. All that is necessary is to use some simple multiplication. Multiply the instantaneous trip setting times the circuit breaker amp rating (the instantaneous trip setting is usually adjustable but can vary depending upon frame size and circuit breaker type - some have adjustable settings of four to 10 times the amp rating - check specifications of specific circuit breaker). The product of these two is the approximate point at which a circuit breaker enters its instantaneous trip region. (As explained in a previous section 2. Instantaneous Region, there is a tolerance associated with where the instantaneous trip initially picks up. A vertical band depicts the instantaneous trip pickup tolerance. For this easy method, we will ignore the tolerance band; therefore the results differ somewhat from the time current curve example just given.) For instance, the A circuit breaker in this example has its instantaneous trip (IT) set at 10 times its amp rating (10X). Therefore for fault currents above 10 x A or 0A, the A circuit breaker will unlatch in its instantaneous trip region, thereby opening. The same could be determined for the 1A circuit breaker, which has its instantaneous trip set at 6X its amp rating. Therefore, for fault currents above 7A, the 1A circuit breaker unlatches in its instantaneous trip region, thereby opening. The coordination analysis of the circuit breakers merely requires knowing what the numbers mean: 1. Any fault on the loadside of the A circuit breaker greater than 0A will open the A circuit breaker as well as the A circuit breaker. Reason: the A circuit breaker with an instantaneous trip set at 10 times opens instantaneously for any fault current greater than 0A. 2. Any fault on the loadside of the A circuit breaker greater than 7A will open the 1A circuit breaker as well as the A and A circuit breakers. Reason: the 1A circuit breaker with an instantaneous trip set at six times opens instantaneously for any fault current greater than 7A. 3. Any fault on the loadside of the A circuit breaker greater than 7A will open the 1A circuit breaker as well as the A circuit breaker. Reason: the 1A circuit breaker with an instantaneous trip set at six times opens instantaneously for any fault current greater than 7A. So it becomes apparent, to evaluate coordination of circuit breakers with instantaneous trips, the time current curves do not have to be drawn. All that is necessary is to use simple multiplication of the instantaneous trip settings times the circuit breaker amp ratings,and evaluate this in conjunction with the available fault current. Note: Circuit breakers that provide the use of a short time delay do not always assure coordination. The reason is that molded case circuit breakers and insulated case circuit breakers that have a short-time delay will also have an instantaneous trip setting that overrides the short-time delay at some fault level. Molded case circuit breakers with short time delay settings will have an instantaneous trip that overrides the short time delay, typically at a maximum of 10 times the amp rating. These instantaneous overrides are necessary to protect the circuit breakers for higher faults. The same simple procedure for evaluating circuit breakers with instantaneous trips can be used for this type circuit breaker, also. Merely read the manufacturer s literature to determine this instantaneous trip override setting. However, be certain to establish if the instantaneous trip pickup is given in symmetrical amps or asymmetrical amps. Some manufacturers specify the instantaneous override in asymmetrical amps which for practical evaluation purposes moves the instantaneous trip pickup setting to the left (picks up at lower symmetrical fault currents than perceived). See the next two pages for a brief discussion and curves of short-time delay settings and instantaneous overrides. 5 Cooper Bussmann 97

10 Circuit Breakers Short-Time-Delay and Instantaneous Override Some circuit breakers are equipped with short-time delay settings for the sole purpose of improving system coordination. Review the three curves on this page and the next page. Circuit breaker short-time-delay (STD) mechanisms allow an intentional delay to be installed on low voltage power circuit breakers. Short-time-delays allow the fault current to flow for several cycles, which subjects the electrical equipment to unnecessarily high mechanical and thermal stress. Most equipment ratings, such as short circuit ratings for bus duct and switchboard bus, do not apply when short-time-delay settings are employed. The use of short-time-delay settings on circuit breakers requires the system equipment to be reinforced to withstand the available fault current for the duration of the short-time-delay. Ignoring equipment ratings in relation to the protective device opening time and let-through characteristics can be disastrous. Following is a time-current curve plot for two low voltage power circuit breaker with short-time delay and a 20A MCCB. The A CB has a STD set at 6 cycles and the A CB has a STD set at 24 cycles. This type of separation of the curves should allow for selective coordination, assuming that the breakers have been serviced and maintained per the manufacturer's requirements. This is an approach to achieve selective coordination that can diminish electrical safety and component protection. An insulated case circuit breaker (ICCB) may also be equipped with shorttime-delay. However, ICCBs will have a built-in override mechanism. This is called the instantaneous override function, and will override the STD for medium to high level faults. This override may kick in for faults as low as 12 times (12x) the breaker s amp rating. (See curve in left column on next page.) This can result in non-selective tripping of the breaker and load side breakers where overlaps occur. This can be seen in the example. (See curve in right column on next page.) As the overlap suggests, for any fault condition greater than 21,000A, both devices will open, causing a blackout. Zone-Selective Interlocking Zone-Selective Interlocking (ZSI), or zone restraint, has been available since the early 1990s. ZSI is designed to limit thermal stress caused by shortcircuits on a distribution system. ZSI will enhance the coordination of the upstream and downstream molded case circuit breakers for all values of available short-circuit current up to the instantaneous override of the upstream circuit breaker. Caution: Use of Circuit Breaker Short-Time Delay Settings May Negate Protection and Increase Arc-Flash Hazard The longer an overcurrent is permitted to flow the greater the potential for component damage. The primary function of an overcurrent protective device is to provide protection to circuit components and equipment. A short-time delay (STD) setting on a circuit breaker can negate the function of protecting the circuit components. A low voltage power circuit breaker with a short-time delay and without instantaneous trip, permits a fault to flow for the length of time of the STD setting, which might be 6, 12, 18, 24 or 30 cycles. This typically is done to achieve fault coordination with downstream circuit breakers. However, there is an adverse consequence associated with using circuit breaker short-time delay settings. If a fault occurs on the circuit protected by a short time delay setting, a tremendous amount of damaging fault energy can be released while the system waits for the circuit breaker short-time delay to time out. In addition, circuit breakers with short-time delay settings can drastically increase the arc-flash hazard for a worker. The longer an overcurrent protective device takes to open, the greater the flash hazard due to arcing faults. Research has shown that the arc-flash hazard can increase with the magnitude of the current and the time duration the current is permitted to flow. System designers and users should understand that using circuit breakers with short-time delay settings will greatly increase the arc-flash energy if an arcing fault incident occurs. If an incident occurs when a worker is at or near the arc-flash, the worker may be subjected to considerably more arc-flash energy than if an instantaneous trip circuit breaker or better yet a currentlimiting circuit breaker or current-limiting fuses were protecting the circuit. The requirements for doing flash hazard analysis for worker safety are found in NFPA 70E Electrical Safety Requirements for Employee Workplaces." As an example, compare the photos resulting from investigative testing of arcing faults. Further information is provided in Electrical Safety & Arc-Flash Protection in this bulletin. A couple of comparison photos are shown on the next page. These tests and others are detailed in Staged Tests Increase Awareness of Arc-Fault Hazards in Electrical Equipment, IEEE Petroleum and Chemical Industry Conference Record, September, 1997, pp This paper can be found on the Cooper Bussmann web site at One finding of this IEEE paper is that current-limiting overcurrent protective devices reduce damage and arc-fault energy (provided the fault current is within the current-limiting range). Low Voltage Power Circuit Breaker with Short-Time-Delay 98 5 Cooper Bussmann

11 Circuit Breakers Insulated Case Circuit Breaker Instantaneous Override Instantaneous Override Opens at 21,000 Amps 0 0 0A A ICCB A CB 0A ICCB TIME IN SECONDS TIME IN SECONDS Instantaneous Override = 12X BLACKOUT! ,000 CURRENT IN AMPERES 20,000 30,000 40,000 60,000 80,000, ,000 CURRENT IN AMPERES 20,000 30,000 40,000 21,000 AMPS 60,000 80,000,000 Test 4 shows sequential photos of a circuit protected by a circuit breaker with a short-time delay: interrupted at 6 cycles, so this incident lasted 1 10 of a second. The arcing fault was initiated on a three phase, 480V system with 22,A short circuit available. Current-limiting fuses or current-limiting circuit breakers can reduce the risks associated with arc-flash hazards by limiting the magnitude of the fault currents (provided the fault current is within the current-limiting range) and reducing the time duration of the fault. Test 3 photos, to the right, are from tests with the same test setup as shown in Test 4 above, except that KRP-C- 601SP current-limiting fuses protect the circuit and clear the arcing fault in less than 1 2 cycle. The arc-flash was greatly reduced because these fuses were in their current-limiting range. Also, the thermal and mechanical stresses on the circuit components that conducted the fault current were Test 3 Same test circuit as the prior photos, to the left, except the circuit is protected by KRP-C-601SP Cooper Bussmann Current-Limiting Fuses. In this case these fuses limited the current and the fuses cleared in less than a 1 2 cycle. greatly reduced. Recent arc-flash research has shown that arc-flash energy is linearly proportional to the time duration of the fault (given the fault currents are the same). Ignoring the fact that the KRP-C-601SP fuses in Test 3 limited the current let-through, the arc-flash energy released in Test 3 was approximately 1 12 that of Test 4 just due to the faster operation of the KRP- C-601SP fuses (less than 1 2 cycle clearing in Test 3 vs. 6 cycles clearing in Test 4). The actual arc-flash energy was reduced even more in Test 3 because of the current-limiting effect of the KRP-C-601SP fuses. 5 Cooper Bussmann 99

12 Essential Electrical Systems In Healthcare Facilities, Emergency Systems and Legally Required Standby Systems In recent history, there has been an increasing demand for reliability of electrical power for buildings spanning the entire spectrum from single-family residences to places of assembly to industrial facilities. Many building systems have gone to the extent of installing emergency back-up or standby systems to ensure maximum possible reliability and continuity of their electrical system. There are some types of buildings that require an emergency system or legally required standby system such as hotels, theatres, sports arenas, healthcare facilities and many more similar institutions. These life safety electrical systems are used to ensure electrical power in the case of power loss due to an outside source (utility loss) or an inside source (overcurrent condition). Very important loads must remain energized as long as possible during times of normal power outage and emergencies that may be caused by equipment failures, nature or man. However, simply installing a back-up system is not the entire solution, the electrical system must be designed to maximize power to these important loads and minimize outages even under physical catastrophes. It is very important to analyze the characteristics of the overcurrent protective devices (OCPDs) of these life safety systems to ensure they will perform as desired. The NEC has special requirements for these life safety electrical systems. These include several requirements that are based upon providing a system with reliable operation, reducing the probability of faults, and minimizing the effects of an outage to the smallest portion of the system as possible. Below are a few of these sections: maintenance and testing requirements 700.9(B) emergency circuits separated from normal supply circuits 700.9(C) wiring specifically located to minimize system hazards failure of one component must not result in a condition where a means of egress will be in total darkness. The objective of these requirements is to ensure system uptime with the goal of safety of human life during emergencies or for essential health care functions. The 5 NEC has adapted to this demand and has modified a few articles to address the issue of selective coordination and to ensure by design and installation that the intended life safety electrical systems will stay in service for the longest possible period of time. The installation of a back-up system could easily be negated by the use of overcurrent protective devices that are not selectively coordinated. Selective coordination helps ensure by design and installation that some life safety electrical systems will stay in service for the longest possible period of time. There has been a requirement for selective coordination in the NEC for many years. NEC has had a selective coordination requirement for multiple elevators in a building. This requirement is crucial for a few specific reasons such as not stranding passengers for normal operation and for emergency egress as well as keeping elevators in use for emergency firefighting operations. The 5 NEC selective coordination requirements have been expanded and a definition Coordination (Selective) has been added to Article : New Article Definition Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings. NEC (Emergency Systems) and (Legally Required Standby Systems) have added selective coordination of overcurrent protective devices to help ensure life safety. Examples of circuits that require selective coordination would be emergency and egress lighting for the safe evacuation from a building and to assist in crowd and panic control. Also, in many cases, some elevators and ventilation equipment may be classified as an emergency or legally required standby system by the Authority Having Jurisdiction (AHJ) or the locally adopted building code. Also, requires the essential electrical systems of healthcare facilities have overcurrent protective devices that are selectively coordinated. The New NEC Requirements Application of Other Articles. The essential electrical system shall meet the requirements of Article 700, except as amended by Article Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. System Requirement As stated in the NEC requirements above, selective coordination is not just between the branch circuit and feeder. It is the entire circuit path from the main overcurrent protective device to the branch circuit device. See figure below. 5 Cooper Bussmann

13 Essential Electrical Systems In Healthcare Facilities, Emergency Systems and Legally Required Standby Systems A given emergency branch circuit load has a circuit path up through to the main on the normal source and another circuit path up through to the main on the emergency source. The requirement for selective coordination means that all the overcurrent protective devices must be selectively coordinated with each other, from each emergency branch circuit up through to the main of the normal power supply and up through to the main on the emergency power supply. See following example: 3. A coordination study should fully investigate, interpret, and present the level of coordination achieved for the full system and for the determined amount of available short-circuit currents. This requires a person qualified for this task. Too often, coordination studies are performed where the deliverable is just time current curve plots with no interpretation or discussion on the level of coordination achieved (or sacrificed). Also, the studies should be performed in the design phase so that if the study results prompt a change, the change can be without major ramifications. Fusible Systems Cooper Bussmann makes it easy to design fusible systems that are selectively coordinated. For the modern current-limiting fuses, selectivity ratios are published (see Fuse Selectivity Ratio Guide section). It is not necessary to plot time current curves or do a short-circuit current analysis; all that is necessary is to make sure the fuse types and ampere rating ratios for the mains, feeders and branch circuit meet or exceed the selectivity ratio. These selectivity ratios are for all levels of overcurrent up to the interrupting ratings of the respective fuses. The ratios are valid even for fuse opening times less than 0.01 seconds. This means with current-limiting fuses, it is not necessary to do any analysis for less than 0.01 seconds when the fuse types and ampere rating ratios adhere to the selectivity ratios. The industry standard for publishing fuse time current curves is to plot the times from 0.01 seconds and longer. The following example illustrates all that is necessary to achieve selective coordination with a fusible system. How Can Selective Coordination Be Achieved? Selective coordination of the overcurrent protective devices for these life safety systems for both the normal power source path and the alternate power source path can be achieved with either modern current-limiting fuse technology or modern circuit breaker technology. However, not all fuse types or circuit breaker types can be used in any and all situations to ensure selective coordination. The ability to achieve selective coordination depends on the specific circuit parameters (short-circuit currents available at various points in the system) and the fuse or circuit breaker opening characteristics and possible setting options. Proper overcurrent protective device choice and selective coordination analysis is absolutely necessary. However, if the proper choices and analysis are not made so that selective coordination is ensured, the system may be built with deficiencies that may negatively impact life safety. Other Important Considerations 1. Either a system has overcurrent protective devices that are selectively coordinated or the system does not. There is no middle ground. When terms such as enhancing coordination, optimizing coordination, coordination "to the best degree possible," or similar terms are used, it generally means the overcurrent protective devices are not selectively coordinated over the full range of short-circuit currents that are available in the application. 2. Selective coordination is for the full range of overcurrents available at all mains, feeders and branch circuits. It is not acceptable to arbitrarily consider selective coordination is applicable only above certain times such as 0.01, 0.1, or 1.0 seconds. The coverage of selective coordination in the sections on Selective Coordination for Fuses and Circuit Breakers of this SPD publication explains how to assess fuses and circuit breakers for the full range of overcurrents irrespective of the times. Also, these preceding sections provide some easy, quick methods to assess selective coordination for both fuse and circuit breaker systems. 5 Cooper Bussmann 101

14 Essential Electrical Systems In Healthcare Facilities, Emergency Systems and Legally Required Standby Systems Frequently Asked Questions: Fuse System Q. How about when two different current-limiting fuses are plotted on a time current curve and there is a space between the lower ampere fuse total clear curve and the larger ampere fuse minimum melt curve does this ensure selective coordination (if the selectivity ratio is not known)? A. In this case, without knowing the selectivity ratio and having only time current curves, selective coordination can only be ensured for the level of overcurrent up to the point where the larger fuse curve crosses 0.01 seconds. For times where the fuse opening time is less than 0.01 seconds, selectivity ratios are necessary. It is not necessary to plot fuse time current curves; just adhere to the selectivity ratios. Q. What about lighting branch circuits? There has not been a commercially available fusible lighting panelboards, so how can selective coordination be achieved for the 20A panelboard circuits? A. In order to achieve a selectively coordinated fusible system, Cooper Bussmann has the Coordination Module, which is a fusible branch circuit panelboard. So now it is easy to achieve selective coordination from main to branch circuit with an all fusible system, including branch circuit lighting panelboards, by adhering to the simple selectivity ratios. Information on Coordination Module on data sheet Selectivity Ratio Guide For Coordination Module Load-Side Fuse LP-CC FNQ-R KTK-R Line-Side Fuse LPJ_SP 2:1 LPN-RK_SP 2:1 LPS-RK_SP 2:1 FRN-R 2:1 FRS-R 2:1 Ratios only apply to Cooper Bussmann Fuses. When fuses are in the same case size, consult Cooper Bussmann Cooper Bussmann

15 Essential Electrical Systems In Healthcare Facilities, Emergency Systems and Legally Required Standby Systems Circuit Breaker Systems As stated earlier, if a designer wants to use circuit breakers, it may be possible to achieve a selectively coordinated system. There are a host of circuit breaker alternatives in the form of circuit breaker types, optional features, and possible settings. See Circuit Breaker Coordination section in this publication for more in-depth discussion on some of the options and tradeoffs. The designer must understand the characteristics, setting options and override particulars for each circuit breaker considered and in some cases, factor in the specific circuit parameters such as the available short-circuit currents at each point in the system. If circuit breakers are considered, typically the following become important: 1. A short-circuit current study with a coordination study (interpreted properly) is necessary to determine if selective coordination is achieved for each circuit and for the entire circuit path. It is necessary to have the available short-circuit currents and circuit breakers characteristics and settings. Depending on the circumstances, the designer may have to use different type circuit breakers or options in order to achieve selective coordination. 2. Typically the designer will have to utilize circuit breakers with short-time delays and possibly larger frame sizes. This may increase the cost and equipment size. 3. Tradeoffs in system protection: These decisions that may improve the level of coordination may negatively impact the level of component protection. 4. Arc flash hazard level: These decisions that may improve the level of coordination may negatively impact the arc flash hazard level. 5. In many cases, the circuit breaker solution may only be selectively coordinated for the exact designed system (for specific circuit breaker settings and available short circuit currents of a specific system). Most systems get upgraded or change at some point in time and this can significantly increase the available short-circuit currents. This may negate the selective coordination for some circuit breaker systems that may have originally been selectively coordinated. 6. Testing and maintenance: periodic exercising and testing are important for circuit breakers to ensure they operate as intended. If circuit breakers are found not to operate as specified, maintenance or replacement of these circuit breakers is necessary. For instance, if a feeder circuit breaker fails to operate or operates slower than as specified, it may cause the main circuit breaker to open due to a feeder fault. The result would be a much larger portion of these critical system loads being without power. System with Mixture of Fuses and Circuit Breakers Fuses and circuit breakers operate in totally different ways. Fuses are thermal devices that have encased fusible elements that operate under overcurrent conditions by melting or vaporizing at some points of the element, arcing, and clearing the circuit. All circuit breakers have three operating functions to clear a circuit: (1) overcurrent sensing, (2) unlatching, and (3) parting the contacts, arcing, and clearing. There are numerous technologies used for circuit breaker overcurrent sensing. For downstream fuse and upstream circuit breaker, it is not a simple matter to determine if a fuse and circuit breaker will be selectively coordinated. Even if the plot of the time current curves for a downstream fuse and an upstream circuit breaker show that the curves do not cross, selective coordination may not be possible. If a fuse is upstream and a circuit breaker is downstream, at some point the fuse time current characteristic crosses the circuit breaker time current characteristic. For short-circuit currents at that point and higher, the upstream fuse is not coordinated with the down stream circuit breaker. New Requirement Compliance Achieving overcurrent protective device selective coordination for a system requires the proper engineering, specification and installation of the required devices, and in addition, knowledgeable plan review and inspection to ensure compliance. The designer, contractor, and plan review/inspector each have their role in compliance to selective coordination in order to ensure safety to human life. Role of Designers For these vital systems, the designer must select, specify, and document overcurrent protective devices that achieve selective coordination for the full range of possible overcurrents and for faults at all possible points in the system (faults can occur in the branch circuits, sub-feeders, and feeders). The designer should provide the plan reviewer a stamped analysis verifying that selective coordination is achieved as designed for the required electrical systems. Documentation of the basis should be included. Also, this information is necessary so that the contractor quotes and installs the overcurrent protective devices as designed. Role of Plan Review/Inspectors The plan review process is a critical phase. If a non-compliant design does not get caught and corrected in the plan review, it could get red tagged in the inspection phase, which may require a very expensive gear change out. During the plan review process, the engineer must provide the stamped substantiation in the form of documentation that his design achieves selective coordination for these vital circuits. The plan reviewer should not have to prove or disprove that selective coordination is achieved. The engineer s documentation should be clear enough to demonstrate that the design work included the proper selective coordination analysis and that the plans and specifications clearly articulate the required details on type of overcurrent protective devices, ampere ratings, and settings (if circuit breakers). The engineer s documentation should clearly state that the plans specify overcurrent protective devices that achieve selective coordination for these vital systems. During the inspection process, prior to energizing the system, the field inspection should include verifying that the overcurrent protective devices have been installed as specified. If circuit breakers are used, the settings should be verified as per plan. Role of Contractors Contractors must install the system as designed to ensure selective coordination for the system. If a fusible system, install the fuse types and ampere ratings as called for in the design. If a circuit breaker system, install the circuit breaker types with the specified settings. If the contractor opts to suggest a value engineering option for these vital systems, it is critical that the engineer evaluates and approves of any changes as well as the plan reviewer. 5 Cooper Bussmann 103

16 Elevator Circuit Elevator Circuits and Required Shunt Trip Disconnect A Simple Solution. When sprinklers are installed in elevator hoistways, machine rooms, or machinery spaces, ANSI/ASME A17.1 requires that the power be removed to the affected elevator upon or prior to the activation of these sprinklers. This is an elevator code requirement that affects the electrical installation. The electrical installation allows this requirement to be implemented at the disconnecting means for the elevator in NEC (B). This requirement is most commonly accomplished through the use of a shunt trip disconnect and its own control power. To make this situation even more complicated, interface with the fire alarm system along with the monitoring of components required by NFPA 72 must be accomplished in order to activate the shunt trip action when appropriate and as well as making sure that the system is functional during normal operation. This requires the use of interposing relays that must be supplied in an additional enclosure. Other requirements that have to be met include selective coordination for multiple elevators (620.62) and hydraulic elevators with battery lowering [620.91(C)]. There is a simple solution available for engineering consultants, contractors, and inspectors to help comply with all of these requirements in one enclosure called the Cooper Bussmann Power Module. Elevator Selective Coordination Requirement In the 5 NEC, states: Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices. A design engineer must specify and the contractor must install main, feeder, sub-feeder, and branch circuit protective devices that are selectively coordinated for all values of overloads and short circuits. To better understand how to assess if the overcurrent protective devices in an electrical system are selectively coordinated refer to the Selective Coordination Section of this booklet. Below is a brief coordination assessment of an elevator system in a circuit breaker system (example 1) and in a fuse system (Example 2). Power Module Elevator Disconnect All-in-One Solution for Three Disciplines NEC Selective Coordination Hydraulic Elevators Traction Elevators NFPA 72 Fire Safety Interface Component Monitoring ANSI/ASME A17.1 Shunt Trip Requirement The Power Module contains a shunt trip fusible switch together with the components necessary to comply with the fire alarm system requirements and shunt trip control power all in one package. For engineering consultants this means a simplified specification. For contractors this means a simplified installation because all that has to be done is connecting the appropriate wires. For inspectors this becomes simplified because everything is in one place with the same wiring every time. The fusible portion of the switch utilizes LPJ-(amp)SP fuses that protect the elevator branch circuit from the damaging effects of short-circuit currents as well as helping to provide an easy method of selective coordination when supplied with an upstream fuse with at least a 2:1 amp rating ratio. More information about the Cooper Bussmann Power Module can be found at Using the one-line diagram above, a coordination study must be done to see that the system complies with the selective coordination requirement if EL-1, EL-2, and EL-3 are elevator motors. Go to the Selective Coordination section for a more indepth discussion on how to analyze systems to determine if selective coordination can be achieved Cooper Bussmann