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 centres, 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 Canada, the National Building Code (Subsection 3.2.7) determines which buildings need emergency lighting and emergency power, and for how long, and CSA C282 (Emergency electrical power supply for buildings) sets requirements for generator systems that supply emergency loads. Section 46 of the CEC covers the installation of emergency systems, unit equipment and exit signs, and Section 32 the fire alarm systems and fire pumps they supply. Loads that are not required by the building code (for example data centre loads or process equipment) are optional standby loads, wired under the general rules and often supplied by the same generator through separate transfer switches.

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.

CSA C282 requires emergency generators to be capable of supplying the emergency load within 10 seconds of a power failure, to have enough fuel for at least 2 hours of operation at full load (more where the building code requires a longer duration), and to have automatic starting, specified alarms and annunciation.

Emergency lighting

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

The NBC requires emergency lighting to provide an average illuminance of not less than 10 lx at floor or tread level along means of egress, with a minimum duration that depends on the building (30 minutes for most buildings, longer for high buildings and certain occupancies). Exit signs must be continuously illuminated and supplied by emergency power. Unit equipment is certified to CSA C22.2 No. 141.

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 are certified to CSA C22.2 No. 178.1 and installed under Section 46 and CSA C282, with the emergency loads on their own transfer equipment.

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.

Section 46 and Section 32 of the CEC, together with NBC Article 3.2.7.10 and related requirements, require emergency conductors to be separated from other wiring and, for certain systems in buildings such as high buildings, to be protected against fire for specified durations, for example by installation in a fire-rated enclosure, by burial in concrete, or by use of cables certified for circuit integrity.

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.

The CEC requires overcurrent devices in emergency systems to be coordinated in accordance with the Code's rules for these systems, and designers commonly apply full selective coordination for emergency and fire pump circuits as good practice.

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:

CSA C282 requires weekly inspection and monthly operation of emergency generators under load, with annual testing and maintenance and written records. Unit equipment is tested monthly for a short period and annually for its rated duration under CSA C22.2 No. 141 and the fire code. The provincial fire code assigns responsibility for testing to the building owner.

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.