Selective Coordination 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: • 700.4 maintenance and testing requirements • 700.9(B) emergency circuits separated from normal supply circuits • 700.9(C) wiring specifically located to minimize system hazards • 700.16 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 2005 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® 620.62 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 2005 NEC® selective coordination requirements have been expanded and a definition Coordination (Selective) has been added to Article 100:
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New Article 100 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® 700.27 (Emergency Systems) and 701.18 (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, 527.26 requires the essential electrical systems of healthcare facilities have overcurrent protective devices that are selectively coordinated.
The New NEC® Requirements 517.26 Application of Other Articles. The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517. 700.27 Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. 701.18 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.
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Selective Coordination 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.
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Selective Coordination 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 www.cooperbussmann.com data sheet 3115.
Selectivity Ratio Guide For Coordination Module
Line-Side Fuse
Load-Side Fuse LP-CC FNQ-R KTK-R LPJ_SP
2:1
LPN-RK_SP LPS-RK_SP FRN-R FRS-R
2:1 2:1 2:1 2:1
Ratios only apply to Cooper Bussmann Fuses. When fuses are in the same case size, consult Cooper Bussmann.
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Selective Coordination Essential Electrical Systems In Healthcare Facilities, Emergency Systems and Legally Required Standby Systems Circuit Breaker Systems
New Requirement Compliance
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:
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.
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.
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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.
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Ground Fault Protection Coordination Considerations
B. Two Step Ground Fault Relaying
FDR GFR
40
1200A CB
30 20
200A CB
10 8 6 4 3 2 1 .8 .6 .4 .3 .2
BLACKOUT AREA
.1 .08 .06 .04 .03 .02
000,001 000,08 000,06
000,04 000,03
000,02
000,01 000,8 000,6
000,4
000,3
.01
000,2
Feeder G.F. Relay 100A 6 Cycle Delay
MAIN GFR
000,1 008
200A CB
100 80 60
006
Circuit Breaker Opens
200
004
Circuit Breaker Opens
Main G.F. Relay 1200A 18 Cycle Delay
400 300
003
1200A CB
600
002
RESULT: BLACKOUT
1,000 800
001 008
Two step ground fault relaying includes ground fault relays on the main service and feeders. In many instances, this procedure can provide a higher degree of ground fault coordination to prevent unnecessary service blackouts. Yet it is mistakenly believed by many that two step ground fault relays assure ground fault coordination. For complete selective coordination of all ground faults, the conventional overcurrent protective devices must be selectively coordinated as well as the ground fault relays. The fact is that even with this two step relay provision, ground fault coordination is not assured on many systems designed with mechanical overcurrent protective devices which incorporate instantaneous unlatching mechanisms.
The system above illustrates the typical problem concerning this point. The main ground fault relay is set at 1200A, 18 cycle delay and the feeder ground fault relay is set at 100A, six cycle delay. These ground fault relay settings could mistakenly be interpreted to mean that feeder ground faults would be cleared by only the feeder ground fault relay opening the feeder disconnect. But the analysis must also include the phase overcurrent device characteristics since these devices also respond to current.
TIME IN SECONDS
This fact is commonly overlooked when applying ground fault relays. Generally, the short-time-delay on the ground fault relay is thought to provide coordination for higher magnitude feeder ground faults. However, as shown by this example the main circuit breaker operates to cause an unnecessary blackout. Note: Circuit breakers with short-time-delay trip settings were not considered in this section. The reason is that a short-time-delay on a circuit breaker defeats the original purpose of protection. Short-circuit currents and high magnitude ground fault currents, when intentionally permitted to flow for several cycles, dramatically increase the burn time and damage to the system as well as increasing the arc-flash hazards to personnel. Electrical systems are not designed to withstand, for long periods, the torturous forces that fault currents produce. Circuit breaker short-time-delay trip settings with typical delays of 6, 18, 24, or 30 cycles can greatly exceed the short circuit withstandability of system components. According to industry standards, the duration for equipment short-circuit current testing is three cycles for switchboard bus (UL891) and three cycles for busway (BU1-1999). The shortcircuit current withstandability for insulated conductors decreases as the overcurrent device operating time increases (reference Insulated Cable Engineers Association Publication P-32-382, “Short-Circuit Characteristics of Cable”). Short-circuit currents and high magnitude ground fault currents must be interrupted as rapidly as possible (preferably with current-limiting devices) to minimize equipment damage. Whenever insulated case and molded case circuit breakers have a short-timedelay feature they also have an instantaneous override This requires the sensing mechanism to override the short-time-delay feature for high ground fault or line-line faults. The result is a lack of coordination with the feeder breakers for any fault current above the instantaneous override setting. Selective coordination is therefore very difficult to achieve.
CURRENT IN AMPS
The two step ground fault relays give a false sense of security. The graph above illustrates that the ground fault relays are coordinated, but overcurrent devices are not coordinated for feeder or branch circuit ground faults above 11,000 amps. This is indicated as the BLACKOUT AREA on the curve. In this case the main overcurrent device and the feeder overcurrent device both open on a feeder circuit fault. Thus the entire system is blacked out; even though two step ground fault relays are provided.
WARNING! For Health Care Facilities: Section 517.17 requires the main and feeders to be 100% selectively coordinated for all magnitudes of ground fault current including low, medium, and high ground fault currents.
Ground Fault 11,000A or Greater
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Ground Fault Protection Coordination Considerations In many cases two step relays do provide a higher degree of ground fault coordination. When properly selected, the main fuse can be selectively coordinated with the feeder fuses. Thus on all feeder ground faults or short circuits the feeder fuse will always open before the main fuse. When selectively coordinated main and feeder fuses are combined with selectively coordinated main and feeder ground fault relays, ground fault coordination between the main and feeder is predictable.
1,000 800 600 400 300 200 LPS-RK
100 80 60
FDR GFR
40
Does Not Open
LPS-RK200SP
Feeder G.F. Relay 100A 6 Cycle Delay
KRP-C1200SP
30
TIME IN SECONDS
KRP-C1200SP
Main G.F. Relay 1200A 18 Cycle Delay
200SP
MAIN GFR
20 10 8 6 4 3 2 1
Only Feeder Disrupted
.8
Any Level Ground Fault Current
.6 .4 .3
The above figure illustrates a selectively coordinated main and feeder for all levels of ground faults, overloads and short circuits. Any fault on the feeder will not disrupt the main service. This system offers full selective coordination for all levels of ground faults or short circuits. 1. The feeder ground fault relay is set at a lower time band than the main ground fault relay, therefore the relays are coordinated.
.06 .04 .03 .02
000,001 000,08 000,06
000,04 000,03
000,02
000,01 000,8 000,6
000,4
000,3
000,2
000,1 008
006
004
003
.01
002
Conclusion: This system is completely selective for all levels of ground faults and short circuits. This system meets the intent of NEC® 517.17 for 100% selectivity.
.1 .08
001 08
2. The feeder fuses are selectively coordinated with the main fuses for all ground faults, short circuits, or overloads on the load side of the feeder. The feeder fuses would clear the fault before the main fuses open.
.2
CURRENT IN AMPS
Complete Ground Fault Selective Coordination Is Necessary To Prevent Blackouts! To assure complete selective coordination for all ground faults, it is essential that the conventional overcurrent protective devices be selectively coordinated as well as the ground fault relays’ requirement. The intent of 517.17 is to achieve “100 percent selectivity” for all magnitudes of ground fault current.
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Selective Coordination Now Required
Selective Coordination Now Required For Emergency Systems, Legally Required Standby Systems, and Essential Electrical Systems in Health Care Facilities
BLACKOUT
Background Selective coordination is now required for increased system reliability, which is vital for these critical systems. Selective coordination can be defined as isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open. The following was added to NEC® 2005 Article 100 Definitions: 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 one-line diagrams in Figure 1 and Figure 2 demonstrate the concept of selective coordination. Selective coordination is an important new NEC® 2005 requirement that is consistent with the critical need to keep these loads powered even with the loss of normal power. Article 700, Emergency Systems, and Article 701, Legally Required Standby Systems have several requirements that are based upon providing a system with reliable operation, reduction in the probability of faults and minimizing the effects of an outage to the smallest portion of the system as possible. Article 517, Health Care Facilities, requires essential electrical systems to meet the requirements of Article 700 except as amended in Article 517. 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. Selective
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coordination of overcurrent devices fits well with the other requirements such as: • 700.4 maintenance and testing requirements • 700.9(B) emergency circuits separated from normal supply circuits • 700.9(C) wiring specifically located to minimize system hazards • 700.16 failure of one component must not result in a condition where a means of egress will be in total darkness
Ensuring Compliance Achieving the proper overcurrent protective device selective coordination requires proper engineering, specification and installation of the required devices. During the plan review process, it is the design engineer’s responsibility to provide documentation that verifies the overcurrent devices are selectively coordinated for the full range of overcurrents that can occur in the system. And the site inspection should verify the overcurrent protective devices are installed as specified to achieve selective coordination. It is possible for both fusible and circuit breaker systems to be selectively coordinated with proper analysis and selection. Selective coordination is easy with Bussmann® fuses by using the published fuse selective coordination ratios; a full short-circuit and coordination study is not necessary to verify selective coordination. Selective coordination with circuit breakers depends on their characteristics and settings as well as the circuit parameters for the specific application. It
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Selective Coordination Now Required is generally difficult to achieve selective coordination with common circuit breakers that incorporate instantaneous trip settings. Typically circuit breakers with short-time delay settings or zone selective interlock features may be necessary, which can add to the cost and may create other system issues. If using zone selective interlocking options, molded case and insulated case circuit breakers still have an instantaneous trip that overrides the zone selective tripping feature. This is necessary to protect the circuit breaker from severe damage. Consequently blackouts can occur even with this zone selective interlocking feature. If circuit breakers are to be considered, a full short-circuit current and coordination study must be done with proper analysis and interpretation. See simple fuse and circuit breaker examples are on page 8.
New Requirements 2005 NEC® 700.27 Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. 701.18. Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. 517.26 Application of Other Articles. The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.
Example (See Figures 1 & 2) If overcurrent protective devices in the emergency system are not selectively coordinated, a fault at X1 on the branch circuit may unnecessarily open the sub-feeder; or even worse the feeder or possibly even the main. In this case, emergency circuits are unnecessarily blacked out. With selective coordination as a requirement for emergency, legally required standby, and essential electrical systems, when a fault occurs at X1 only the nearest upstream fuse or circuit breaker supplying just that circuit would open. Other emergency loads would remain powered.
Notes: 1. Article 517 has no amendment to the selective coordination requirement, therefore selective coordination is required. 2. Selective coordination is required for both the normal supply path and the emergency system path.
Figure 1 - Normal Source Power to Emergency Circuits
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Selective Coordination Now Required
Figure 2 - Emergency Service Power to Emergency Circuits
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Selective Coordination Now Required The Cooper Bussmann SPD Selecting Protective Devices publication (download from www.bussmann.com) has an in-depth discussion on selective coordination analysis with the published fuse selectivity ratios, some simple
evaluation rules for coordination of instantaneous trip circuit breakers, and illustration of short-time delay circuit breakers. Go to www.bussmann.com for Bussmann® Fuse Selectivity Ratios Interactive Guide under Application Info/Software.
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®
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Lineside KRP-C-800SP to Loadside LPJ-100SP 800/100=8:1 Table shows only 2:1 needed Therefore Selective Coordination achieved Lineside LPJ-1000SP to Loadside LPS-RK-20SP 100/20=5:1 Table shows only 2:1 needed Therefore Selective Coordination achieved
1. If circuit breakers are not maintained, extended clearing times or nuisance operation may compromise coordination. 2. If using zone selective interlocking option, molded case and insulated case circuit breakers still have an instantaneous trip that overrides the zone selective tripping feature. Blackouts still can occur since selective coordination can not be achieved.
Other Information Emergency systems are considered in places of assembly where artificial illumination is required and for areas where panic control is needed such as hotels, theaters, sports arenas, health care facilities, and similar institutions. Emergency systems also provide power to functions for ventilation, fire detection and alarm systems, elevators, fire pumps, public safety communications, or industrial processes where interruption could cause severe human safety hazards. Legally required standby systems are intended to supply power to selected loads in the event of failure of the 8
normal source. Legally required standby systems typically serve loads in heating and refrigeration, communication systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes where interruption could cause severe human safety hazards. Essential electrical systems in healthcare facilities are portions of the electrical system designed to ensure continuity of lighting and power to designated areas/functions during normal source power disruptions or disruptions within the internal wiring system. Essential electrical systems can include the critical branch, life safety branch, and equipment systems which are essential for life safety and orderly cessation of procedures during normal power disruptions. ©2005 Cooper Bussmann
