What is a UPS?
A UPS (Uninterruptible Power Supply) is a backup power system that keeps connected equipment running without interruption when the mains supply fails or moves outside tolerance, by switching instantly to energy from batteries or another storage medium. A UPS also filters out surges, sags and distortion. In OT and IT environments the UPS is the first line of defence for availability: it rides through short disturbances, gives a standby generator time to start and allows a controlled shutdown when the outage lasts longer.
UPS systems protect data centres, SCADA environments, PLC installations, DCS systems, telecom equipment, hospitals and other critical infrastructure. They are a standard element of Business Continuity and High Availability in industrial automation, and complement rather than replace your backup strategy for data.
🧠 How does a UPS work?
A UPS continuously monitors input voltage and frequency. As soon as the voltage drops out, rises or falls too far, or the frequency drifts, the internal energy storage takes over the supply. The main components are:
| Component | Function |
|---|---|
| Rectifier / charger | Converts alternating current (AC) to direct current (DC) and charges the battery |
| Battery or energy storage | Supplies energy while the mains is unavailable |
| Inverter | Converts DC back into a stable AC output voltage |
| Static bypass | Transfers the load straight to the mains on a UPS fault or overload |
| Maintenance bypass | Allows work on the UPS without dropping the load |
| Controller and network card | Monitoring, alarming, logging and communication with management systems |
How these components are combined determines the topology and therefore the quality of protection.
🔋 What types of UPS are there?
The standard IEC 62040-3 (adopted in Europe as EN IEC 62040-3) classifies UPS systems by how much their output depends on the mains. The classification maps onto the three common topologies:
| Topology | IEC 62040-3 class | Operation | Transfer time | Typical use |
|---|---|---|---|---|
| Offline (standby) | VFD - voltage and frequency dependent on the mains | Load runs directly on the mains, the inverter only starts on failure | A few milliseconds | Workstations, small offices, non-critical systems |
| Line-interactive | VI - voltage regulated, frequency follows the mains | Automatic voltage regulation (AVR) corrects sags and overvoltage without drawing on the battery | A few milliseconds | Network equipment, small server rooms, industrial edge systems |
| Online (double conversion) | VFI - voltage and frequency independent of the mains | Load is fed continuously through rectifier and inverter | None (0 ms) | SCADA servers, DCS, data centres, process automation |
An offline UPS has a simple architecture and low cost, but offers only basic protection. An online UPS provides full power conditioning and is therefore the standard for critical OT loads. Its drawback is lower efficiency: double conversion typically loses a few percent of the power as heat. Many online units therefore offer an eco mode that raises efficiency at the cost of a short transfer time.
In control cabinets you will also often find a DC UPS: a 24V DC module with a battery or supercapacitors that sits directly behind the industrial power supply and bridges PLCs, I/O and industrial switches without an inverter.
🏭 Which OT systems does a UPS protect?
In industrial automation even a voltage dip lasting a fraction of a second can restart a PLC, reboot a network switch, disrupt time synchronisation between systems or ruin a process batch.
| System | Why it needs a UPS |
|---|---|
| PLC and I/O | Avoid a production stop and an unknown process state after restart |
| SCADA and HMI | Operators keep visibility and control during the disturbance |
| DCS | Process safety and controlled shutdown |
| SIS | The safety function must stay available on mains loss or move to a safe state |
| Historian | Prevent data loss and corrupted databases |
| Industrial Switch and Firewall | Network and security functions keep working |
UPS systems are usually combined with a standby generator, dual power feeds per device and redundancy in the UPS setup itself. In commercial buildings and data centres a BMS or DCIM system supervises the UPS as part of the building services.
🔄 Which redundant UPS configurations are common?
| Configuration | Description | Effect of losing one UPS |
|---|---|---|
| N | Exactly enough capacity for the load | Load goes to mains via bypass or drops |
| N+1 | One extra module or unit above the required capacity | No impact, and maintenance is possible without risking an outage |
| 2N | Two fully separate power paths (A and B feed) | No impact, even when an entire path fails |
| 2N+1 | Two separate paths, each with an extra module | No impact, even during maintenance on one path |
More redundancy reduces single points of failure, but quickly doubles the investment. A 2N setup only makes sense when the connected equipment has two power supplies. OT environments therefore often choose N+1 for central systems and a decentralised DC UPS per control cabinet.
🔥 What energy storage does a UPS use?
| Storage | Properties | Typical life |
|---|---|---|
| VRLA (sealed lead-acid) | Cheap and proven, sensitive to heat | Design life of 5 or 10 years; often replaced earlier in practice |
| Lithium-ion | High energy density, lighter and more compact, built-in battery management system | Typically 10–15 years |
| NiCd | Robust at extreme temperatures, popular in substations and heavy industry | Long, but more expensive and environmentally harmful |
| Flywheel | Kinetic storage, no chemistry | Bridges seconds to a few tens of seconds, enough for a generator start |
| Supercapacitors | Very high cycle count, fast discharge | Bridges seconds; suited to DC UPS units in control cabinets |
Battery ageing is one of the leading causes of UPS failure. A battery is considered at end of life when its capacity falls below 80% of the original value, and service life roughly halves for every 10 °C above the recommended 20–25 °C. A UPS in a hot switch room will therefore need new batteries much sooner than the datasheet promises. For larger storage alongside or behind the UPS, see battery energy storage.
⚙️ How do you size a UPS in practice?
Good sizing starts with what the UPS must achieve: ride through the disturbance, let the generator start, or carry out a clean shutdown.
- Inventory the load - use your asset inventory to determine the actual power draw in watts per system, not the nameplate rating of the power supply.
- Calculate in both kVA and kW - modern UPS units have an output power factor of 0.9 to 1.0; also allow for inrush currents from motors, transformers and power supplies.
- Keep headroom - size the UPS so that it normally runs below about 80% load, with room for growth.
- Define the required autonomy - see the table below.
- Arrange the shutdown - connect servers and virtualisation platforms to shutdown software so they power down automatically before the battery is exhausted.
- Test periodically - run battery and transfer tests and replace batteries preventively, as set out in your maintenance plan.
Allow for the limitations too: a UPS produces heat that must be cooled, a large installation takes up considerable space, and both the investment and the maintenance are substantial. A UPS also only ever supplies power temporarily; a prolonged outage requires a generator.
| Scenario | Typical autonomy |
|---|---|
| Bridging until the generator takes the load (usually 10–15 seconds) | 5–10 minutes, with margin for a failed start |
| Controlled shutdown of servers and SCADA | 10–15 minutes |
| Running on without a generator | 30 minutes to several hours |
In the Netherlands the installation must comply with NEN 1010, and work on the UPS falls under NEN 3140: a UPS keeps its output live even when the mains feed is switched off, which is a real hazard during maintenance.
The large-scale blackout on the Iberian Peninsula on 28 April 2025 showed why this matters: Spain and Portugal were without power for hours, and organisations without properly sized backup power stopped immediately. Grids such as the Dutch one are very reliable, but short dips caused by short circuits or switching operations still occur, and those are exactly what disrupts sensitive process equipment.
📡 How do you monitor a UPS?
A networked UPS reports battery status, load, input voltage, temperature, remaining runtime and alarms. Most vendors support SNMP (the standard UPS-MIB was defined in RFC 1628 in 1994), and in industry also Modbus or Modbus TCP. Newer systems offer OPC UA or MQTT.
That data ends up in SCADA, a BMS, DCIM or a historian. An operator then sees not only that the mains has failed, but also how many minutes remain. Trends in battery temperature and internal resistance also enable predictive maintenance through monitoring.
🔐 How do you secure a network-connected UPS?
A UPS with a network card is a computer with a web interface that can switch off power to your critical systems. That makes it an attractive target for attackers and a full part of your cybersecurity scope.
- Known attacks - on 29 March 2022 CISA and the US Department of Energy warned that attackers were breaking into internet-connected UPS devices using factory default passwords.
- Known vulnerabilities - in March 2022 Armis published “TLStorm”: three vulnerabilities in APC Smart-UPS devices (CVE-2022-22805, CVE-2022-22806 and CVE-2022-0715) that allowed remote, unauthenticated code execution, partly through unsigned firmware updates. Schneider Electric released patches.
- Impact - an attacker can switch off the output, force shutdowns, suppress alarms or, through manipulated firmware, even cause physical damage. Ransomware on the server running the shutdown software or the management console can also take away visibility of the UPS and the controlled shutdown.
Key security measures:
| Measure | Implementation |
|---|---|
| Never directly on the internet | Inventory every UPS and check that the management interface is not publicly reachable |
| Network Segmentation | Place management cards in a separate management segment of the OT Network, behind a firewall |
| Strong authentication | Change default passwords, enable lockout and use MFA where possible |
| Secure protocols | Use HTTPS and SNMPv3 and disable Telnet, HTTP and SNMPv1/v2c |
| Patch Management | Include UPS firmware in the regular patch cycle |
| Zero Trust | Allow only explicitly required connections, for example from the shutdown server |
IEC 62443 provides the framework for securing the UPS as part of a zone. For manufacturers, a network-connected UPS is a product with digital elements under the Cyber Resilience Act: reporting of actively exploited vulnerabilities applies from 11 September 2026, the full requirements from 11 December 2027. For organisations covered by NIS2 (in the Netherlands the Cyberbeveiligingswet, in force since 15 August 2026), power supply and its protection are part of the duty of care for business continuity and compliance.
❓ Frequently asked questions
What is the difference between a UPS and a standby generator?
A UPS takes over the power supply within milliseconds or with no interruption at all, but can only sustain it for minutes to a few hours. A standby generator can supply power for hours or days, but usually needs 10 to 15 seconds to start. In critical environments the UPS therefore bridges exactly the time until the generator takes over the load.
How long do UPS batteries last?
Lead-acid (VRLA) batteries in a UPS usually have a design life of 5 or 10 years, but are often replaced earlier in practice. At ambient temperatures well above 20–25 °C, their service life roughly halves for every 10 °C. Lithium-ion batteries in a UPS typically last 10 to 15 years.
Which type of UPS suits SCADA and PLCs?
For SCADA servers, a DCS and other critical OT systems, an online UPS (double conversion, class VFI under IEC 62040-3) is the usual choice because it has no transfer time and fully conditions the power. For PLCs and I/O in a control cabinet, a 24V DC UPS is often used. An offline UPS is usually insufficient for these applications.
Can a UPS be hacked?
Yes, a UPS with a network card can be hacked when its management interface is reachable and poorly secured. In 2022 CISA warned of attacks using default passwords, and the TLStorm vulnerabilities in APC Smart-UPS devices became public. Segmentation, strong passwords and firmware updates greatly reduce this risk.
What do N+1 and 2N mean for a UPS?
N+1 means a UPS installation has one more module or unit than the load requires, so a single failure or maintenance job has no effect. 2N means two fully separate power paths, each large enough for the entire load. 2N is more reliable, but only makes sense for equipment with two power supplies.
How often should a UPS be tested?
A UPS is typically serviced by a specialist at least once a year, including a battery test and a check of the bypass. Many UPS units also run automatic periodic self-tests. A realistic test together with the generator and the shutdown procedure shows whether the whole chain works during a real outage.
📌 In summary
A UPS keeps critical IT and OT systems powered without interruption during mains failures and power disturbances, and buys time for a generator start or a controlled shutdown. For critical OT loads choose an online UPS, size it with headroom, replace batteries on time and treat the network card as an OT asset that must be segmented and patched.
Within OT convergence the UPS grows in importance, because processes depend more and more on networks, servers, real-time communication and recovery under disaster recovery plans. A well-designed and well-secured UPS is the foundation under every availability strategy.