When utility power fails, an automatic transfer switch notices, waits briefly to make sure the outage is real, starts the generator, and moves the load across once the generator is ready. When the utility comes back, it waits again to be sure the supply is stable, moves the load back and lets the generator cool down. Each of those steps has a setting, and the settings decide how long your loads are dark.
The transfer switch
The automatic transfer switch, or ATS, sits between two sources and the load. It watches the voltage and frequency of both sources, decides which one should supply the load, and sends the start signal to the generator. Buildings usually have more than one: emergency loads such as exit lighting and fire alarm on their own switch, and standby loads such as heating, refrigeration or IT equipment on others. Our guide to emergency power vs standby power explains the difference.
The sequence, step by step
The timeline below follows a typical outage from start to finish. Actual delays are set during commissioning to suit the loads, the generator and the code requirements for emergency loads.
- Utility fails. The ATS sees voltage or frequency on the normal source drop outside its limits on any phase.
- Start delay. A short delay rides through momentary dips and automatic reclosing on the utility line, so the generator does not start for every flicker.
- Generator starts. The ATS signals the generator to start. The engine cranks, reaches speed and the alternator builds voltage.
- Transfer to generator. When the generator's voltage and frequency are within limits, the ATS transfers. Emergency loads transfer first; standby loads may follow after further delays so the generator picks up load in steps.
- Running on generator. The building runs on generator power for as long as the outage lasts and fuel allows.
- Utility returns. The ATS sees normal voltage and frequency again and starts a retransfer delay, often several minutes, to make sure the utility is stable before putting the load back on it.
- Retransfer. The ATS transfers back to the utility. This causes another short interruption unless the switch uses closed transition.
- Cool-down and stop. The generator runs unloaded for a cool-down period, then stops and returns to standby, ready for the next outage.
For emergency loads, the building code and CSA C282 set the maximum time from loss of utility to power being restored. The start delay, the generator's start-up time and the transfer delay together must fit within it, which is one reason emergency generators need to be kept warm and their batteries healthy.
Open, delayed and closed transition
How the switch moves between sources also matters.
- Open transition is the most common. It is simple and never connects the two sources together, but the load sees a short break on every transfer, including the return to utility.
- Delayed transition adds a deliberate pause with the load disconnected. Large motors and transformers can generate their own voltage for a moment after disconnection; reconnecting them to a source out of step can cause damaging current and torque. The pause lets that decay.
- Closed transition briefly parallels the generator and utility so retransfer and testing cause no interruption. Because the generator is momentarily connected to the grid, it needs the utility's approval and protection that meets the utility's requirements, which vary by utility and province.
A UPS can cover the gaps for sensitive equipment; see UPS systems for buildings. Where several generators are paralleled, the sequence adds synchronizing and load priority steps, covered in generator paralleling.
Buildings with several transfer switches
Larger buildings often have several transfer switches fed from one generator. If they all transfer at the same instant, the generator sees the whole building's load in one step, including the inrush of motors and transformers, and its voltage and frequency can dip far enough to trip it. Designers avoid this by staggering the transfers: emergency loads first, then standby loads in groups a few seconds apart, with large motors such as chillers and fire pumps given their own timing.
Elevators need particular attention. When several elevators share a generator, the design usually lets only a set number run on generator power at once, and the elevator controls must be told when the building is on generator. Elevators that return energy to the supply when braking can also push a lightly loaded generator off its settings, which is handled in the elevator and generator design.
Testing the sequence
Generators are typically exercised on a regular schedule, and emergency generators are tested on the schedule CSA C282 and the fire code set. A test that only runs the engine without load proves little about the transfer sequence. A useful test transfers real load through the ATS. Record at each test:
- Time from the simulated outage to the generator start, and to the load being transferred.
- Generator voltage, frequency and load once transferred.
- Any alarms, warnings or abnormal noise, smoke or leaks.
- Time from the simulated return of utility to retransfer, and the cool-down period.
- Fuel level, battery and charger condition, and block heater operation.
- Which ATS units transferred, and confirmation that each one returned to normal.
Our guide to standby generator maintenance covers the rest of the routine.
What transfer tests commonly turn up
| Symptom | Common causes |
|---|---|
| Generator slow to start or fails to start | Weak starting batteries, failed charger, block heater not working, fuel problems |
| ATS does not transfer although the generator runs | Generator voltage or frequency outside the ATS limits, ATS controller fault, switch left in manual or test mode |
| Generator trips when load transfers | Too much load in one step, large motors starting together, settings that do not match the generator |
| Load stays on generator after utility returns | Retransfer delay set very long, ATS waiting for a phase that is still low, controller fault |
| Equipment trips or resets on retransfer | Open transition break, motors reconnected out of step, sensitive loads without UPS |
Each of these is easier to fix when it is found on a planned test rather than during a real outage, which is the main argument for testing under load.
Tell people before you test. A transfer test interrupts power to the loads on that ATS unless it uses closed transition. Warn occupants, IT and anyone running sensitive equipment, and make sure elevators, fire alarm and building automation respond as expected. Test switching is done by qualified people following the manufacturer's procedure.
How ehZee can help
We design transfer schemes, select transition types and write sequences of operation for new and existing generator systems, as part of our emergency and standby power design service. Drawings are sealed by a licensed professional engineer where required.
Common questions
Why do our lights go out twice during an outage?
Once when the utility fails and once when the ATS transfers back to utility after the outage. Open transition switches break the load briefly on retransfer. A closed transition switch avoids the second interruption, subject to utility approval.
Can we shorten the delays?
Some can be adjusted, within the generator's capability and the code limits. Shortening the start delay too far makes the generator start on every flicker; shortening the retransfer delay risks returning to an unstable utility. Changes should be made deliberately and recorded.
What is a bypass-isolation transfer switch?
A transfer switch with a built-in bypass that lets the switch mechanism be isolated and withdrawn for maintenance while the load stays connected to a source. It is common where loads cannot tolerate an outage for ATS service.