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NEC Article Review
CHAPTER 7 - SPECIAL CONDITIONS
Chapter 7, which covers special conditions, is the third of four chapters that deal with special topics. Chapters 5, 6 and 8 cover special occupancies, special equipment and communications systems, respectively. Remember, the first four Chapters of the NEC are sequential and form a foundation for each of the subsequent four Chapters.
What exactly is a "special condition?" It is a situation that does not fall under the category of Special Occupancies or Special Equipment, but creates a need for additional measures to ensure the "safeguarding of people and property" mission of the NEC put forth in Article 90.
The NEC groups these logically, as you might expect. Here are the general groupings:
Article 700. Emergency Systems
In some cases, an emergency system simply provides a lighted exit path upon loss of main power or in the case of fire. Its purpose isn't to continue normal operations, but rather to provide lighting and controls essential for human life.
These systems are legally required, often as a condition of operating permit for a given facility based on its use. The Authority Having Jurisdiction makes the determination as to whether an emergency system is necessary for a given facility and what it must entail.
A movie cineplex might be required to have lighted exit paths from restrooms, offices, individual theater rooms, and other areas where people are. But there is no threat to human life if a projector stops running or the popcorn machine stops popping.
In certain circumstances, the AHJ may require an emergency system to operate actual process equipment. But normally, those processes use mechanical means to effect an emergency shutdown. For example, consider a black liquor recovery boiler. If it goes out of control and explodes, it leaves a huge crater where a facility and the people in it once stood. Consequently, a black liquor recovery boiler facility must always be able to run a pre-determined shutdown and venting operation to protect the human lives that would surely be lost if an explosion occurred. These systems would seem to be ideal candidates for Article 700, but the shutdown and venting operations do not typically require electricity for implementation. Upon loss of power, spring-loaded or gas-loaded valves dump the process steam and operate vents. Similary, in nuclear powered generating stations, the control rods are driven fully into the core to shut it down-without electricity.
This background information will help you understand that not all emergency actions to save human life will fall under Article 700. The general goal is keep the emergency operation as reliable as possible. One way to do that is to use inherently safe actuation devices, such as valves that go to a predetermined position upon loss of power. Another is to limit what needs to be an emergency load in the first place so the emergency system powers only what is needed to save human life.
In an emergency, it's difficult to administratively control loads. Thus, the emergency system must be able to supply all emergency loads simultaneously. When the emergency power source also supplies power for load shedding or other non-emergency loads, the emergency loads take priority over the other loads, and those other loads may be dropped to support the emergency loads.
As you study Article 700, keep in mind that emergency systems are essentially lifelines for people. The entire Article is based on keeping those lifelines from breaking.
Article 701. Legally Required Standby Systems
In the hierarchy of electrical systems, the emergency systems of Article 700 get first priority. Taking the number two spot are legally required standby systems, which fall under Article 701.
The main difference between the two is that the emergency systems of Article 700 take priority over the legally required standby systems of Article 701. But there are other differences. For example, legally required systems must supply standby power in 60 seconds or less after a power loss, instead of the 10 seconds or less required of emergency systems.
Article 701 systems don't serve the purpose of directly protecting human life. They serve the purpose of running specific loads that, if shut down, would create hazards or impede rescue operations. Thus, hospital communications systems fall under Article 701-evacuation instructions announced over the public address system are part of a rescue operation.
Article 700 basically applies to systems or equipment required to protect people who are in an emergency and trying to get out, while Article 701 basically applies to systems or equipment needed to aid the people responding to the emergency. For example, Article 700 lighting provides an exit path. But, Article 701 lighting might provide illumination of the fire hydrants and switchgear areas.
Article 702. Optional Standby Systems
Taking third priority after emergency and then legally required systems, optional standby systems protect public or private facilities or property where life safety does not depend on the performance of the system. These systems don't affect rescue operations.
Suppose a glass plant loses power. Once glass hardens in the equipment-which it will do when process heat is lost-the plant is going to suffer a great deal of downtime and expense before it can resume operations. An optional standby system can prevent this loss.
You'll see these systems in facilities where loss of power could cause economic loss or business interruptions. Data centers can lose millions of dollars from a single minute of lost power. A chemical or pharmaceutical plant could lose an entire batch from a single momentary power glitch. In many cases, the lost revenue cannot be recouped.
When the power went out in Chicago in August a few years ago, restaurants lost millions of dollars in food inventory due to a loss of refrigeration. But many firms were using optional standby systems and didn't incur such huge losses. In an extended outage, where the logistics of fuel delivery becomes a problem, optional standby systems would have to wait in line behind legally required standby systems, which would have to wait in line behind emergency systems.
Article 720. Circuits and Equipment
Operating at Less Than 50 Volts
Article 725. Class 1, Class 2, And Class 3 Remote Control, Signaling And Power-Limited Circuits
Circuits that fall under Article 725 are remote-control, signaling, and power-limited circuits that are not an integral part of a device or appliance. Article 725 includes circuits for burglar alarms, access control, sound, nurse call, intercoms, some computer networks, some lighting dimmer controls and some low-voltage industrial controls.
Let's take a quick look at the types of circuits:
We'll cover these definitions in more depth later and explain the differences between Class 1, Class 2 and Class 3.
Article 725 provides some cost-savings for installations of circuits that qualify for Article 725 requirements. As with all other Articles in Chapters 5 through 8, the wiring methods required by Chapters 1 through 4 apply. But because of the inherently lower danger of fire risk with qualifying circuits, Article 725 specifies conditions where these methods are not required. For example, you do not need to put a Class 2 splice in a box or enclosure. At the same time, some additional requirements apply to ensure safety. While there's not a net compromise on safety, there is a net cost-savings.
The purpose of Article 725 is to allow for the fact that these circuits "are characterized by usage and power limitations that differentiate them from electric light and power circuits." Article 725 provides alternative requirements for minimum wire sizes, derating factors, overcurrent protection, insulation requirements, wiring methods and materials.
Article 725 consists of three parts. Part I provides general information, Part II pertains to Class 1 cabling, and Part II pertains to Class 2 and 3 cabling. The key to understanding and applying each of the three parts is knowing the voltage levels of the cabling involved and the purposes of that cabling. Article 725 allows you to save time and money when working with particular types of circuits. Thus, it makes sense to become familiar with Article 725 if you work with low-power circuits.
Article 760. Fire Alarm Systems
Interest in security and fire alarm systems rose sharply after September 11 and remains high. This means much more work with this kind of equipment. The work includes new installations, retrofits, upgrades, testing and maintenance.
Article 760 provides the requirements for the installation of wiring and equipment for fire alarm systems, including all circuits the fire alarm system controls and powers. Fire alarm systems include fire detection and alarm notification, voice communications, guard's tour, sprinkler waterflow and sprinkler supervisory systems. NFPA 72-National Fire Alarm Code provides other fire alarm system requirements.
As you study this material, pay close attention to the illustrations. These highlight important requirements many people have difficulty understanding from the Article 760 text alone, plus they show you some of the more common Article 760 violations.
Article 770. Optical Fiber Cables And Raceways
Fiber-optic cable has many advantages over competing technologies. Foremost is sheer information capacity. With today's light multiplexing, a single fiber can carry millions of telephone conversations. Fiber-optic systems are also highly scalable within the same infrastructure. And they enjoy electromagnetic noise immunity. You can see why fiber-optic cable installations are growing in popularity.
Article 770 provides the requirements for installing optical fiber cables and the optical raceways that contain and support optical fiber cables. It also contains the requirements for composite cables, often called "hybrid," that combine optical fibers with current-carrying metallic conductors.
While we normally think of Article 300 when we think of wiring methods, you need to use only Article 770 methods for fiber-optic cables except where Article 770 makes specific references to Article 300. The first such reference, in 770.3, is to 300.21. This addresses requirements for stopping the spread of combustion. Another Chapter 3 reference is to 300.22, which applies when you are installing optical fiber cables and optical fiber raceways in ducts, plenums or other air-handling spaces.
Article 770 does not refer to 300.15, so you don't have to use boxes for splices or termination of optical fiber cable. The FPN in 770.50 states that splice cases and terminal boxes are typically used as enclosures for splicing or terminating and splicing optical fiber cables. While this is a good practice, it's not required-no FPN is an enforceable Code requirement [90.5(C)].
Article 90 states that the NEC is not a design guide or installation manual. Article 770 doesn't tell you how to ensure your system will meet, and test out to, the performance requirements you need or your contract specifies. For example, it doesn't mention bend radii. It doesn't tell you how to install and test cable safely, either, but that doesn't mean you should look into a cable even if you can't see any light coming through it. But light used in fiber-optic circuits usually isn't visible and it can damage your eye. To do a brilliant job of installing fiber optic systems, follow Article 770 along with industry practices and other standards.
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