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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's a situation that doesn't 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 Power Systems
In a few cases, an emergency system simply provides poer for exit lighting or the illumination of exit signs upon loss of main power or in the case of fire. Its purpose isn't to provide power for normal business operations, but rather to provide lighting and controls essential for human life.
These systems are legally required, often as a condition of an 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.
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 nonemergency 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 Power Systems
In the hierarchy of electrical systems, Article 700 Emergency Systems 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 do not serve the purpose of directly protecting human life. They supply 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 illuminate 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 doesn't depend on the performance of the system. These systems are not required for 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 suffer 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.
Mike Holt's Comment: If you desire more information about any of the above changes, be sure to order my Changes book and/or library (Video/DVD).
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