Emergency and standby systems

When normal power fails, some loads must keep running to protect life: exit signs and egress lighting so people can leave safely, fire alarm systems, fire pumps, smoke control fans and elevators for firefighters. Others must keep running to protect property, processes or business: refrigeration, data centers, industrial processes and communications. The code distinguishes these categories and sets stricter rules for the life-safety systems, covering how quickly power must be restored, how wiring is separated, how circuits are protected and how systems are tested. This lesson covers the categories of systems, sources of emergency power, transfer equipment, wiring and selective coordination, sizing generators and batteries, and testing and maintenance.

Categories of systems

In the US, the NEC distinguishes three categories: emergency systems (Article 700), legally required by codes for life safety, such as egress lighting, exit signs, fire alarm and fire pumps; legally required standby systems (Article 701), required by codes for other public needs such as smoke control, sewage handling and communications; and optional standby systems (Article 702), chosen by the owner to protect property or business, such as data centers and refrigeration. Article 708 adds critical operations power systems for facilities designated vital to public safety or national security. NFPA 110 governs generator installations for emergency and legally required standby systems, and NFPA 101 (Life Safety Code) and the building code determine where they are required.

Sources of emergency power

Emergency power can come from several sources, alone or combined:

• Generator sets (diesel, natural gas or propane), sized for the connected emergency loads, starting automatically on loss of normal power. Diesel sets are the usual choice for emergency service because they start quickly and store their own fuel; natural gas units depend on the gas utility, which some codes accept only with conditions.
• Storage batteries: central battery systems or unit equipment (self-contained battery packs with lamps or remote heads) that provide immediate lighting for a rated time.
• Uninterruptible power supplies (UPS) for loads that cannot tolerate even a brief interruption, such as fire alarm control panels (which have their own batteries) and data processing equipment.
• A separate service from the utility, in some cases, where permitted by the authority.

NFPA 110 classifies emergency power supply systems by Type (the maximum seconds to restore power: Type 10 for emergency systems), Class (the minimum hours of operation without refueling, such as Class 2 for 2 hours) and Level (Level 1 where failure could cause loss of life or serious injury, Level 2 otherwise).

Emergency lighting

Emergency lighting must illuminate exits, corridors, stairs and paths of egress so occupants can find their way out when normal lighting fails.

NFPA 101 (7.9) requires emergency illumination along means of egress of an initial average of at least 1 footcandle (10.8 lux), with a minimum of 0.1 footcandle at any point, for at least 90 minutes, and maximum-to-minimum ratios not exceeding 40:1. Exit signs must be illuminated and supplied by emergency power. Unit equipment is listed to UL 924.

Transfer switches

Automatic transfer switches (ATS) monitor normal power and, when it fails, signal the generator to start and transfer the load once the generator's voltage and frequency are acceptable. When normal power returns and stabilizes, they transfer back after a time delay and let the generator run unloaded to cool down. Transfer switches are designed and tested to withstand the available fault current and to switch motor and transformer loads (DCE-340 covers open, closed and delayed transition types). Emergency loads are supplied through transfer switches that serve only emergency loads, so that faults or overloads in other loads cannot compromise them. Large generators may also use load shedding: if the generator becomes overloaded, lower-priority loads are dropped to protect the emergency loads.

Transfer switches for emergency systems must be listed for emergency use (UL 1008), must supply only emergency loads (700.5), and may not be used as service equipment except where permitted.

Wiring and separation

Emergency wiring must survive the events that cause the emergency and must not be affected by failures of other systems:

• Separation: emergency circuits run in their own raceways, cables, boxes and enclosures, not mixed with normal or optional standby wiring, except in transfer switches, exit and emergency luminaires supplied from two sources, and specified common junction boxes.
• Identification: emergency panels, boxes and receptacles are marked (often red) so they are recognized.
• Fire resistance: in high-rise and large assembly buildings, feeders for emergency systems may need fire-rated cable or protection by fire-rated construction for a specified time (typically 1 or 2 hours), so a fire does not disable fire pumps, elevators and smoke control.

700.10 requires emergency wiring to be kept entirely independent of other wiring, and in buildings such as high-rises and large assembly occupancies, feeder circuits must be protected by fire-resistive construction, listed circuit integrity cables or 2-hour fire-rated systems, or installed in spaces fully protected by sprinklers (700.10(D)). Similar requirements apply to fire pump circuits (695.6) and fire alarm circuits.

Selective coordination

If a fault occurs on one emergency branch circuit, only the overcurrent device nearest the fault should open, leaving all other emergency loads energized. If an upstream feeder breaker opened instead, an entire floor's egress lighting or the whole emergency system could go dark during an emergency. Selective coordination means choosing overcurrent devices so that, for every possible fault current, the downstream device opens before any upstream device. It is checked with time-current curves and manufacturers' coordination tables, and often requires fuses with suitable ratios or breakers with adjustable short-time settings and no instantaneous trip upstream.

700.32 requires emergency system overcurrent devices to be selectively coordinated with all supply-side overcurrent devices, for the full range of fault currents, as selected by a licensed professional engineer or similar qualified person. 701.32 requires the same for legally required standby systems, and 708.54 for critical operations power systems.

Sizing generators

Generators must supply the running load and also handle the inrush of large motors starting. When a large motor starts on a generator, its starting current (several times running current, at low power factor) causes a voltage dip and frequency drop that can drop out contactors, extinguish discharge lamps or trip drives. Sizing therefore considers:

• The total running load in kW and kVA, with future growth.
• Step loads: the largest block applied at once, often limited by sequencing transfer switches or motor starts.
• Motor starting kVA and the acceptable voltage dip (often 15 to 20% for emergency loads); reduced-voltage starters and drives reduce starting kVA.
• Nonlinear loads (UPS, drives), which can cause voltage distortion and may require oversizing.

Generator manufacturers provide sizing software and data on transient response for these checks.

Testing and maintenance

Emergency systems must work on the rare day they are needed, so codes require regular testing and records:

NFPA 110 requires weekly inspection and monthly exercise of emergency generators for at least 30 minutes under a load of at least 30% of the nameplate rating (or other conditions), with annual load bank testing where monthly loads are low, and written records. NFPA 101 requires monthly 30-second tests and annual 90-minute tests of battery emergency lighting. 700.3 requires the authority to witness acceptance testing, with periodic testing records kept.

Diesel engines run lightly loaded for long periods suffer wet stacking (unburned fuel accumulating in the exhaust), which is why minimum test loads or periodic load bank tests are required. Fuel quality must also be maintained, since diesel degrades and accumulates water and microbial growth over months.

Lesson 10 covers fire alarm systems, which share many interfaces with emergency power, and Lesson 12 the testing and documentation that close out a project.