The short answer
UPS for Control Panels
A UPS in a control panel carries the loads that must survive a power interruption, typically the controller, the network and radio, the level transmitter, and the HMI, for long enough to ride through a generator transfer or to report the outage and shut down cleanly. Panel UPS units are either AC units feeding the control transformer side or DC units that back up the 24 V bus directly with a battery module, and the DC type suits most panels better. The UPS is sized for its load and its runtime, its battery is replaced on a schedule, and its status is alarmed, because a UPS with a dead battery is discovered by the outage it was bought for.
Key points
- A UPS carries the loads that must report or ride through. Pumps, heaters, and drives are never on it.
- A 24 V DC UPS module on the control bus is simpler and more reliable than an AC unit for most panels.
- Size for runtime: long enough to cover the generator transfer, or to report the outage and shut down.
- The battery is a consumable. Replace on a schedule and alarm its condition.
- Alarm on-battery, low battery, battery fault, and UPS fault to the controller and SCADA.
What the UPS is for
When utility power fails at a site, two things need to keep working: the controller, so that it knows what happened and manages the restart, and the telemetry, so that SCADA learns of the outage and the operator is notified. At a site with a generator, the UPS carries those loads through the ten to twenty seconds between the utility failing and the transfer switch closing on the generator. At a site without one, the UPS carries them long enough to report the outage, the high level that follows, and the pump status, and then to shut down cleanly before the battery is exhausted. That is the whole job, and it is a small load for a short time, which is why panel UPS units are small.
What goes on it
| On the UPS | Not on the UPS | Why |
|---|---|---|
| The controller and its I/O | Pumps, starters, drives | The controller must see the event; the loads that consume power are what the generator is for |
| The network switch, the radio, the cellular modem | Panel heaters and fans | Reporting is the purpose; heating is not |
| The level transmitter and the critical instruments | Lighting and receptacles | A controller with no level cannot report a high level |
| The HMI, where an operator may be present | Chemical feed pumps | A feed pump on a UPS runs with no process flow |
| The alarm dialer | Anything with an inrush the UPS cannot start | The dialer is the second reporting path |
AC or DC
- AC UPS
- A conventional unit with a battery, an inverter, and an AC output, feeding the control transformer or the panel receptacle circuit. It backs up everything downstream including AC loads, needs space and ventilation, and its inverter is another device to fail. Common in SCADA server rooms and where AC loads must ride through.
- DC UPS module
- A module on the 24 V DC bus between the power supply and the loads, with a battery module beside it. When the supply output disappears, the module switches the bus to the battery without interruption. It backs up only the 24 V loads, which in most panels are exactly the loads that matter, and it has no inverter. The common choice for a control panel.
- Battery-backed power supply
- A power supply with an integral charger and battery connection, doing the same job as the DC module in one device.
- Solar and battery systems
- At sites with no utility power, the battery is the primary source and the panel is designed as a DC system from the start; the sizing is for days, not minutes.
Sizing
- 1
List the backed-up load
Every device that stays on the UPS side, at its continuous current: controller, I/O, switch, radio, transmitter, HMI, dialer. Typically a few amps at 24 V.
- 2
Decide the runtime
With a generator: the transfer time plus margin, a few minutes. Without: long enough to report the outage and the high level, run the radio through the operator response, and shut down; commonly 30 minutes to a few hours. The control narrative says.
- 3
Choose the battery
From the load, the runtime, and the manufacturer runtime curves, with derating for temperature and for the battery age at end of life, which is when it must still meet the runtime. A battery sized exactly for the runtime when new meets half of it at replacement time.
- 4
Check the inrush
The UPS must carry the peak current of the loads at switchover; a radio transmitting or an HMI backlight starting draws more than its average.
- 5
Plan the shutdown
The controller reads the UPS status and the battery level, records the state, sends the final alarms, and where the platform supports it, closes files or parks outputs before the battery dies. A controller that dies mid-write to nonvolatile memory can lose its program.
The battery
Sealed lead-acid batteries in a panel last three to five years in a temperate enclosure and far less in a hot one; every 10 °C above 25 °C roughly halves the life. Lithium battery modules last longer and tolerate temperature better at a higher price. Whatever the chemistry, the battery is replaced on a schedule before it fails, its date is written on it and in the maintenance system, and the UPS runs a periodic self-test that the controller alarms if it fails. A UPS that has been in a panel for eight years with its original battery is not a UPS.
Monitoring
- On battery: the utility or the supply has failed and the UPS is carrying the load. Alarmed to SCADA as the outage indication.
- Low battery: the runtime is nearly gone. The controller executes its shutdown sequence.
- Battery fault or replace battery: the self-test failed or the battery is at end of life. A maintenance alarm.
- UPS fault or bypass: the UPS is not protecting the load. A maintenance alarm with priority.
- Battery voltage and temperature where the module provides them, trended.
The signals reach the controller as relay contacts or over a network interface; the DC modules typically offer both. They are on the I/O list, on the alarm list, and tested at commissioning by pulling the supply input and watching the sequence through to shutdown.
Frequently asked questions
- Does a lift station need a UPS if it has a generator?
- For the controller and the radio through the transfer, yes, unless the program is written for a clean restart and the utility accepts losing visibility of the event. A small DC module with a few minutes of battery is inexpensive and lets the controller manage the staggered restart, as the generator operation page describes.
- How long a runtime is enough without a generator?
- Long enough to report and to keep reporting until someone can respond, plus a clean shutdown. Thirty minutes covers the alarms; two hours covers a crew arriving with a portable generator and lets SCADA see the level the whole time. Longer runtimes are batteries and money; the narrative decides.
- Why did the UPS fail during the outage it was installed for?
- The battery was dead, and nobody knew because the self-test was not alarmed or the alarm was ignored. Batteries in panels fail quietly. The fix is the replacement schedule and the alarm to SCADA, tested by pulling the supply.
- Can the SCADA server run on the same UPS as the panel?
- A server is an AC load with a different runtime need and its own shutdown software; it gets its own AC UPS in the server room, sized for its load and for the time it takes to shut down or to run until the generator carries it. Panel DC modules and server UPS units are different products for different jobs.
Related topics
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Lift Station Generator OperationStandby power at a lift station: what the transfer switch does, how the controls ride through the transfer, staggered pump restarts, what SCADA should see, load testing, and the portable generator connection.
- Control Power SuppliesSizing the 24 VDC supplies that run a controller and its field devices, separating loads that should not share a supply, and keeping the controller alive through a power event.
- Alarm Notification and CalloutGetting the alarm to the person on call when no one is watching the screen: notification paths, escalation, acknowledgment from the field, which alarms qualify, and the failure modes that leave a station in high level with nobody paged.
- Retentive MemoryWhat survives a power cycle in a controller and what does not, how retention is backed up by battery, capacitor, or nonvolatile storage, what should be retentive, and how retained data gets lost.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
Direct contact
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