When the Mains Fails: Generators, ATS and UPS Explained | Cobler
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When the Mains Fails: Generators, ATS and UPS
When the TNB supply fails, some loads never stop, some stop for a few seconds and some wait for the mains. How the essential switchboard, standby generator, automatic transfer switch and UPS work together, how long backup lasts, and who does what during a power cut.
Tan Kok XinBuilding Electrical Fundamentals
Part 8 of 11 in Cobler's Building Electrical Fundamentals course. New here?See the course page.
Part 7 dealt with voltage that rises too high for a moment. This part deals with the opposite problem: the supply disappearing. When the TNB supply is lost altogether, what keeps a building's essential loads running?
Picture a weekday afternoon in the office tower when the TNB (Tenaga Nasional Berhad) supply is lost because of a fault on the grid. The office lights go out, the air conditioning stops and most lifts come to a halt. The emergency lights in the corridors and stairs come on straight away. In the server room, nothing changes at all. A short time later a diesel engine starts in the plant room, and the fire lift, the security systems and the other essential services have power again. The office floors and the chillers stay off until TNB's supply returns.
So a single power cut produces three different results: some loads never stop, some stop for a short time, and some stay off. This part explains why. Part 1 introduced the pieces briefly; here they get their full explanation. It covers the essential switchboard (EMSB), the standby generator, the automatic transfer switch (ATS), the uninterruptible power supply (UPS), how long each backup can last, and what happens when the mains comes back. It also sets out what an operator watches and records, and what is left to the competent person.
Something in this article you want to dig into — or a situation in your own building it doesn't quite cover? Send us your question. We don't run public comments; the team replies to you directly by email.
A building splits its loads into normal and essential
A standby generator cannot usually carry the whole building, so the design decides in advance which loads it must keep running.
The office tower shows why. At its maximum demand the building draws about 1,739 kVA, and the chiller plant alone uses about 35% of the building's electricity. Its standby generator is rated 1,000 kVA, well below what the whole building needs. (kVA measures the total load a generator or transformer must carry; kW, kVA and kVAR explains it.)
So the loads are divided between two kinds of board:
Normal boards feed loads that can wait for the mains to return: office lighting and sockets, most air conditioning, the chiller plant and most lifts.
The essential switchboard (EMSB) feeds loads that must keep working: in the office tower, the fire systems, the fire lift, stairwell pressurisation fans, emergency lighting, security and the server room.
Fire-safety loads are on the essential board because they must work during a fire, even if the fire has also cut the mains. Security and the server room are there because the building's owner has decided they must keep running. The list for a real building comes from its approved fire-safety design and the owner's requirements, and it is shown on the single line diagram.
The automatic transfer switch moves the essential board to the generator
The automatic transfer switch (ATS) watches the mains supply to the essential board. When the mains fails, it starts the generator and moves the essential board from the mains to the generator.
The sequence runs like this:
The mains is lost, or its voltage moves outside set limits.
The ATS waits for a short, deliberately set time. This confirms that the supply has really failed and is not just a brief dip that will recover by itself.
The ATS signals the generator to start.
The diesel engine starts and speeds up. The generator's voltage and frequency must settle at their correct values before it can take load.
The ATS disconnects the essential board from the mains and connects it to the generator.
In a common design, the ATS is interlocked so that it can never connect the mains and the generator to the essential board at the same time. This keeps the generator from feeding power back into TNB's network.
Steps 2 to 5 take time, so the essential loads lose power for a short gap, usually measured in seconds. The exact time depends on the design and the settings. This is the "stop for a short time" group from the opening.
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The ATS selects the mains or the generator for the essential board, and the UPS keeps the server room running through the changeover.
Adding a generator also changes the building's electrical paths, including the path fault current takes. Part 6 made the same point about earthing: sites with generators or multiple supplies need a design that accounts for each operating arrangement.
A UPS covers the loads that cannot stop even for seconds
Some loads cannot tolerate even a few seconds without power. Servers and network equipment would shut down and could lose data. These loads are fed through an uninterruptible power supply (UPS).
A UPS has three main parts:
A rectifier, which turns the incoming alternating current (AC) into direct current (DC) to charge a battery.
A battery, which stores energy.
An inverter, which turns DC back into AC for the load.
In the common online type of UPS, the load always runs from the inverter. When the mains fails, the battery takes over feeding the inverter, so the load sees no break at all. The office tower has a 60 kVA UPS for the server room. That is why the server room did not notice the power cut in the opening.
In the office tower, the server room is also on the essential board, so the UPS is fed from the generator once the ATS has transferred. The UPS battery only has to cover the gap until the generator takes over, not the whole outage.
Emergency lighting has a similar need: people must be able to see the way out the moment the lights fail. Emergency light fittings commonly have their own batteries, or are fed from a central battery, so they light up straight away and do not wait for the generator.
Every backup source has a limit, called its autonomy
Autonomy is how long a backup source can keep supplying its load.
For the generator, autonomy is set by the fuel. The time it can run is the usable fuel in its tanks divided by the rate at which it burns fuel at its actual load. The more load it carries, the faster it uses fuel. A long outage therefore depends on how much fuel is stored and how quickly more can be delivered. The fuel level is one of the most useful readings an operator can take during an outage.
For the UPS, autonomy is set by the battery and the load. UPS batteries are usually sized to cover minutes, not hours: enough to bridge the gap until the generator runs, and in many designs enough for an orderly shutdown of the servers if the generator does not start.
Worth knowing: Battery capacity falls as batteries age, so a UPS's autonomy on the day of a real outage can be shorter than when it was new. Regular testing by the UPS service provider shows whether the batteries still meet the design.
How long the fire-safety loads must be supported is set by the building's approved fire-safety design, not by the operator on the day.
Fire-safety loads come first
The fire-safety loads are the main reason the essential board exists, so they have priority over every other backed-up load.
During a fire, the fire lift carries firefighters, the stairwell pressurisation fans keep smoke out of the escape stairs, and the fire alarm and fire-fighting systems must keep working. Their supplies are designed to stay available when the normal supply is lost, and their cables are commonly routed and protected so that they keep working in a fire.
Two rules follow for the operating team:
Never switch off or isolate a fire-safety supply to save fuel or for convenience.
Any planned work that affects these supplies is arranged by the competent person, together with the building's fire-safety procedures.
When the mains returns, loads come back in order
When TNB's supply returns, the building goes back to normal in steps, not all at once.
The ATS waits until the mains has been steady for a set time. This avoids switching back onto a supply that fails again a moment later.
The ATS moves the essential board back to the mains. In a common design this causes a second short break for the essential loads. The UPS covers the server room again, and the emergency light batteries start recharging.
The generator keeps running without load for a cool-down period, then stops and returns to automatic standby, ready for the next failure.
The normal boards are supplied again, and large loads are restarted in a planned order. Large motors, such as those on chillers and pumps, can draw a high current while they start, several times their running current if they are started directly (How Electric Motors Work explains motors). Starting them one after another instead of all together keeps these starting currents from adding up.
The restart order for a real building, including when to bring back the chillers and the lifts, is set in its written operating procedure.
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During an outage, each group of loads behaves differently: UPS loads never stop, essential loads have short gaps at each transfer, and normal loads wait for the mains.
Who does what during a power cut
During an outage, the operator's job is to observe, record and report. Switching, resetting and repairs are for the authorised competent person, such as the building's chargeman, and for the generator and UPS service providers.
The operator observes and records
The competent person or service provider decides and acts
The time the mains was lost and the time it returned
Any manual operation of the ATS or switchboards
Which boards and areas lost power, and which stayed on
Resetting tripped breakers and restoring supplies
The ATS position indicator and the generator's running lights and alarms
Investigating why the generator or ATS did not act as designed
The generator fuel level, and any fuel delivery
Generator and UPS maintenance, test runs and battery tests
Whether the UPS shows that it is running on battery, and any remaining-time display
Changes to settings, delays or the list of essential loads
Anything that did not come back after the mains returned
The restart of large plant, under the site's procedure
Do not bypass the ATS, operate a switchboard or reset protection unless you are trained and authorised for that task under the site's procedure. If people are trapped in a lift, follow the building's emergency procedure and call the lift service provider.
The outage record is useful long after the event. It shows whether the transfer worked as designed, how long the essential loads were off, and how much fuel was used. The final part of this course shows how to turn records like these into a clear fault report.
(Optional later reading: some buildings also run their generators to reduce their demand charges. That is an economic question, separate from keeping the building safe, and Genset Peak Shaving in Malaysia discusses it. You do not need it for the rest of this course.)
What comes next
Many loads, such as pumps and fans, are started and stopped by control panels, and after a power cut those panels decide what restarts. The next part, Inside an Electrical Control Panel, opens one up in the office tower's chiller plant room and asks: what is inside an electrical control panel, and how does a small control circuit switch a large pump motor on and off?
Check your understanding
Why is the chiller plant in the office tower not on the standby generator? The generator is rated 1,000 kVA, while the whole building draws about 1,739 kVA at maximum demand, and the chiller plant uses about 35% of the building's electricity. The generator is reserved for the essential loads, such as fire systems, the fire lift, security and the server room. The chillers wait for the mains to return.
The building has a standby generator. Why does the server room also need a UPS? The generator takes time to start, and the ATS waits before transferring, so the essential board is without power for a short gap, usually seconds. Servers cannot tolerate that break. The UPS supplies them from its battery with no break until the generator takes over, and again during the second short break when the ATS transfers back to the mains.
Recap: When the mains fails, a building's loads fall into three groups. Loads on a UPS never stop, because a battery and inverter carry them through. Loads on the essential board stop for a short gap while the ATS confirms the failure, starts the generator and transfers to it. Loads on normal boards wait for the mains. Each backup has a limited autonomy: fuel for the generator, battery capacity for the UPS. Fire-safety loads have priority. When the mains returns, loads come back in steps, and large motors are restarted in a planned order. The operator observes, records and reports; the competent person switches, resets and repairs.
Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.
Tan Kok XinBuilding Electrical Fundamentals
