Understanding the purpose of emergency systems helps you meet their special requirements.
Essentially, emergency systems are lifelines for people, and Art. 700 is all about keeping those lifelines from breaking. The Article's main goal is to keep the emergency operation as reliable as possible. One way to do that is to use inherently safe actuation devices, such as valves that “fail safe” to a predetermined position upon loss of power. Another is to limit what qualifies as an “emergency load,” so the emergency system powers only what is needed to save human life (Fig. 1 below).
Emergency systems are, by definition, legally required. A facility may need to meet certain emergency system requirements as a condition of obtaining a permit to operate. The authority having jurisdiction (AHJ) determines whether or not an emergency system is necessary for a given facility. The AHJ also determines specific emergency system requirements, based on intended use and operations.
Sometimes, an emergency system simply provides power for exit lighting (or the illumination of exit signs) when there's a fire or loss of power. The purpose of an emergency system isn't to provide power for normal business operations — it's to provide lighting and controls essential for human life. For specific locations that require emergency lighting, see NFPA 101, Life Safety Code, or the locally adopted building code.
Emergency power systems may also provide power to maintain fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and similar functions.
In an emergency, it's difficult to control loads administratively. Thus, the emergency system must be able to supply all emergency loads simultaneously. When the emergency power source also supplies standby power or other nonemergency loads, the emergency loads take priority over the other loads. Consequently, the system may drop other loads to support the emergency loads.
Equipment approval. The AHJ must approve all equipment used for the emergency system [700.3]. This means the AHJ can reject an installation of Listed equipment and can approve the use of non-Listed equipment. But due to litigation risk, approval of unlisted equipment is becoming increasingly difficult to obtain.
Equipment testing. Emergency power system testing consists of acceptance testing and operational testing [700.4]. To ensure the emergency power system meets or exceeds the installation specification, the AHJ must conduct (or witness) an acceptance test of the emergency system upon completion — and periodically afterward [700.4(B)].
Periodic testing helps ensure the systems are in proper operating condition. Running the emergency power system under load is a generally accepted method of testing. In fact, you must provide a means for testing all emergency lighting and power systems during maximum load.
But the AHJ also presents maintenance requirements. For example, the AHJ must require periodic maintenance of batteries. The AHJ might not spell out the specific maintenance steps, but may refer to manufacturers' maintenance recommendations. Don't think in terms of what maintenance you can “get by with not doing.” Think in terms of what maintenance will prevent a failure of the emergency system. It is unwise to expect a court to split fine hairs in language when the intent is as clear as it is here.
You must keep records of all required testing [700.4(A) and (B)] and maintenance [700.4(C)]. The NEC doesn't specify a record retention period. Today, records are normally electronic, and storage is cheap. So, few facilities have any compelling reason to delete records — regardless of age.
Power sources. An emergency system power source must have adequate capacity to safely carry all emergency loads expected to operate simultaneously [700.5]. If an alternate power source has adequate capacity, you can use it to supply emergency loads [Art. 700], legally required standby loads [Art. 701], and optional standby system loads [Art. 702]. But if the alternate power source lacks adequate capacity to carry the entire load, it must have automatic selective load pickup and load shedding to ensure adequate power — in this order of priority:
A temporary alternate source of power must be available whenever the emergency generator is out of service for more than a few hours [700.5(B)]. Transfer equipment must be automatic, identified for emergency use, and approved by the AHJ [700.6]. Transfer equipment must supply only emergency loads (Fig. 2 below). You must use multiple transfer switches where a single generator supplies emergency loads and other loads.
To warn emergency response personnel, a sign must indicate the type and location of on-site emergency power sources [700.8(A)]. This sign must be placed at the service-entrance equipment.
Upon loss of normal power, emergency power must be available within 10 seconds [700.12]. The emergency power source must be one of the following:
Storage battery. Storage batteries must be of suitable rating and capacity to supply and maintain the total load for 90 minutes. Voltage applied to the load can't fall below 87.5% of normal.
Generator set. A generator acceptable to the AHJ and sized per 700.5 must automatically start the prime mover when the normal service fails. Where internal combustion engines are prime movers, an on-site fuel supply must provide not less than 2 hours of full-demand operation of the system. If an outdoor-housed generator has a readily accessible disconnect within sight (and within 50 feet) of the structure, an additional disconnect isn't required at the structure for the generator feeder conductors that serve or pass through the structure.
Uninterruptible power supply (UPS). UPSs must comply with the applicable requirements for storage batteries and generator sets.
Separate service. You can use an additional service as the emergency power source, if it's installed per Art. 230 and is acceptable to the AHJ. Tapping ahead of the normal service equipment doesn't qualify as the required emergency source of power (Fig. 3 above). To minimize the possibility of simultaneous interruption of emergency supply, the additional service must be:
Served by a separate service drop or lateral.
Electrically and physically separated from all other service conductors.
Unit equipment. Individual unit equipment (battery packs) must have:
Rechargeable battery and charging means.
Provisions for one or more lamps mounted on the equipment, or terminals for remote lamps (or both).
Relaying device arranged to energize the lamps automatically upon failure of the supply to the unit equipment.
Emergency lighting battery pack equipment (ELBE) must be permanently fixed in place. You can use flexible cord-and-plug connection (twist lock not required) for ELBE designed for this purpose, if the cord doesn't exceed 3 feet.
The branch-circuit wiring that supplies ELBE must be the same branch-circuit wiring that supplies the normal lighting in the area. The reason for this is to maintain a charge on the batteries.
However, you must connect the ELBE ahead of any local switches. Otherwise, the emergency lighting can be rendered useless by simply turning off the local lighting.
The branch circuit that feeds the ELBE must be clearly identified at the distribution panel [700.12(F)].
Design and install emergency lighting systems so the failure of any individual lighting element will not leave any space requiring emergency illumination [700.16] in total darkness. This means a single remote head is never sufficient — you need at least two lighting heads for any area.
Circuits. Emergency circuits must supply no loads other than those required for emergency use [700.15].
Permanently mark all boxes and enclosures (including transfer switches, generators, and power panels) for emergency circuits as components of an emergency system [700.9] (Fig. 4 right). A Code violation in this area can impede fire and rescue operations.
Keep all wiring from emergency sources to emergency loads entirely independent of all other wiring and equipment. Why? To ensure a fault on the normal wiring circuits will not affect the performance of emergency wiring or equipment. Art. 700 does permit some exceptions:
Wiring in transfer equipment enclosures.
Luminaires supplied from two sources of power.
A common junction box attached to luminaires supplied from two sources of power.
You can install two or more emergency circuits in the same raceway, cable, box, or cabinet. Design and locate emergency wiring circuits so as to minimize the hazards that might cause failure due to flooding, fire, icing, vandalism, and other adverse conditions.
Overcurrent protection devices for emergency power systems must be selectively coordinated with all supply-side overcurrent protective devices [700.27]. See the definition of “Selective Coordination” in Art. 100. Designing a selectively coordinated system should be left to the system designer. The branch-circuit overcurrent protection devices for emergency circuits must be accessible to authorized persons only [700.25].
In the alternate power source for emergency systems, you don't have to provide ground-fault protection of equipment. But you must provide ground-fault indication [700.7(D)] and [700.26].
By thinking of emergency power systems as lifelines for people, you can quickly cut through the fog of misperception that commonly accompanies the design, installation, and maintenance of these systems. In contrast, a standard backup power system is a lifeline for processes and equipment. Keeping this difference in mind will help you conform to Art. 700 in an efficient and effective manner.