Quatrième année Lesson 6 of 12 · about 13 min
Emergency and standby power
What you’ll learn
- The difference between emergency, legally required standby and optional standby power.
- How an automatic transfer switch hands the load to a generator, and the transition types.
- How UPS types differ, and how to estimate battery runtime and generator kW.
- Why emergency circuits are kept apart, and why backfeeding kills line workers.
Before this: Grounding and bonding as a system · Three-phase power and power factor
When the utility fails, some loads must keep going: the lights that show people the way out, the fire alarm, the fire pump. Others the owner would just like to keep going: the servers, the freezers, the process. Emergency and standby systems are the generators, transfer switches, batteries and UPSs that do that — and the rules that keep them working at the one moment everything else is failing.
Three kinds of backup power
| Kind | Why it exists | Typical loads |
|---|---|---|
| Emergency | Required by code or the authority for life safety | Egress lighting, exit signs, fire alarm, fire pumps, some elevators |
| Legally required standby (NEC term) | Required, but not directly for evacuation | Smoke control, sewage pumps, some communications |
| Optional standby | The owner’s choice, to protect property or business | Servers, refrigeration, a whole house |
In Canada the rules sit in CSA C22.1’s emergency systems section, CSA C282 (emergency generators in buildings) and the building and fire codes; in the US in NEC Articles 700, 701 and 702 with NFPA 110. Those documents — not the installer — decide which loads go on which system, how fast they must be restored and how they are tested.
The transfer switch
The ATS watches the normal supply. When it fails (or sags too far for too long), the ATS signals the generator to start, waits until the generator’s voltage and frequency are steady, and moves the load over. When the utility returns and stays good for a set time, it moves the load back and lets the generator run unloaded to cool down. Its interlocks make sure the two sources are never tied together.
- Open transition — break before make; the load sees a brief outage.
- Delayed transition — pauses in the middle so motors and transformers lose their residual voltage before the other source hits them.
- Closed transition — briefly parallels the sources for a no-break transfer, with the generator synchronised.
- Bypass-isolation — lets the ATS be serviced while the load stays fed.
UPS and unit equipment
A generator takes seconds to start. For loads that cannot see even that gap, a UPS carries them on batteries. A standby UPS switches to its inverter when the supply fails; a line-interactive one also corrects sags and swells; an online double-conversion UPS always runs the load from its inverter, so there is no switch-over at all. Unit equipment — the battery packs with two lamp heads you see in stairwells — is emergency lighting in one box, with its own charger and battery.
Worked: generator kW and UPS runtime (practice numbers)
- A generator is plated 100 kVA at 0.8 PF. Its kW rating = 100 × 0.8 = 80 kW.
- At 600 V three-phase its full-load current is 100,000 ÷ (1.732 × 600) = 96.2 A; at 480 V, 100,000 ÷ (1.732 × 480) = 120.3 A.
- A UPS has two 12 V, 100 Ah batteries: 2 × 12 × 100 = 2,400 Wh stored.
- Load 800 W; inverter about 90% efficient. Runtime ≈ 2,400 × 0.9 ÷ 800 = 2.7 h in theory.
- Real runtime is shorter — batteries deliver less at high discharge rates and as they age. Use the maker’s runtime chart for the number you design with.
Backfeed kills. A generator connected to a building without transfer equipment that isolates it from the utility can feed back through the service and the pole transformer, putting thousands of volts on lines a crew believes are dead. Never “suicide-cord” a generator into a receptacle, and never leave an interlock defeated.
Sur le chantier
Tips
- A building with a generator has two sources. Lock out both — and the UPS output — before you work on emergency circuits.
- Label emergency boxes and raceways the way your code requires; the next electrician needs to know.
- Check the generator’s phase rotation matches the utility before the first transfer, or every three-phase motor on the emergency side runs backwards.
- Generators are exercised and tested under load on a schedule — ask to see the log.
Common mistakes
- Pulling a normal circuit through an emergency raceway or box “because it was handy”.
- Getting the neutral switching and the neutral-to-ground bond wrong at the transfer switch.
- Assuming a UPS’s output is dead because the input breaker is off.
- Sizing a generator on kVA alone and forgetting motor starting.
- Trusting the charge light on unit equipment instead of doing the test.
Check yourself
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1.What is the main job of an automatic transfer switch (ATS)?
The ATS senses the loss of normal power, signals the generator to start, and transfers the load once the generator is ready. It is built (mechanically and electrically interlocked) so the utility and the generator can never be tied together, except in closed-transition designs made and approved for it.
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2.Why are emergency (life-safety) circuits kept separate from normal wiring?
Emergency systems exist for the moment everything else is failing. Separation (their own raceways, boxes and panels, as your code book requires) keeps a problem on the normal side from reaching them.
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3.Why must a portable generator never be plugged into a house through a receptacle ("backfeeding") without transfer equipment?
With the main not isolated, the generator feeds back out the service drop and the pole transformer works in reverse, putting thousands of volts on lines the crews believe are dead. Transfer equipment makes that connection impossible.
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4.An online, double-conversion UPS has no transfer time when the utility fails. Why?
In double conversion, power is always rectified to DC and inverted back to AC. The battery sits on that DC link, so when the input disappears the inverter keeps running from the battery without any switching. Standby and line-interactive types do switch, briefly.
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5.A generator is rated 100 kVA at 0.8 power factor. What is its kW rating?
kW = kVA × PF = 100 × 0.8 = 80 kW. The engine is sized for the kW; the alternator for the kVA.
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6.A UPS has 2,400 Wh of battery and an inverter about 90% efficient. Roughly how long could it carry an 800 W load, in theory?
2,400 × 0.9 ÷ 800 = 2.7 hours. In practice less: batteries give less than their rating at high discharge rates, age, and are not run completely flat. Use the maker’s runtime chart for a real figure.
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7.How do you check that battery-powered emergency lighting (unit equipment) actually works?
A charge light shows the charger works, not that the battery will carry the lamps. The test is to drop the supply (with the test switch or the breaker) and time it. How often and for how long is in your code and fire regulations.
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8.What is the difference between an emergency system and an optional standby system?
Emergency power (egress lighting, exit signs, fire alarm, fire pumps) is legally required and gets the strictest rules. Optional standby — keeping a server room or a freezer running — is a business choice with lighter rules. (The NEC also has a middle category, legally required standby.)
Questions fréquentes
What is the difference between open and closed transition?
An open-transition switch breaks from one source before it makes to the other, so the load sees a short outage — usually a fraction of a second. A closed-transition switch briefly parallels the two sources so the load sees no break at all; that needs the generator synchronised to the utility and usually the utility’s agreement. A delayed-transition switch deliberately pauses in the middle so motors and transformers can lose their stored magnetism first.
Which rules cover emergency power?
In Canada, the emergency systems section of CSA C22.1, CSA C282 for emergency generators in buildings, and the building and fire codes that decide what must be on emergency power. In the US, NEC Article 700 (emergency), 701 (legally required standby) and 702 (optional standby), with NFPA 110 for generator performance. Which loads need what is set by those documents and the authority having jurisdiction — not by the electrician on the day.
Why does the neutral sometimes switch with the phases?
Whether the transfer switch switches the neutral (a 4-pole switch) or leaves it solid (3-pole) depends on how the generator’s neutral is grounded — whether the generator is a separately derived system. It affects ground-fault protection and where the neutral-to-ground bond may be. It is a design decision you install to, and a common place for mistakes.
Voir aussi
- Grounding and bonding as a system — separately derived systems
- Fire alarm basics — a load that must stay on
- Three-phase power and power factor — kW vs kVA on a generator
- Calculateur de puissance triphasée
- Lockout / tagout walk-through — two sources means two lockouts
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