Uninterruptible power supply

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A u nterbrechungsfreie S trom v genaration ( UPS ) provides the supply of critical electrical loads for a fault in the mains sure English Uninterruptible Power Supply (UPS). This is to be distinguished from the emergency power system (also referred to as " backup power supply "), as this has a brief interruption in the power supply when switching.

UPS devices are mainly used in hospitals , control centers , railway signal boxes and data centers due to their cost and additional complexity . In developing countries with poor power supply but also in small offices or at home ( SoHo ).

With simple versions of the UPS, the power supply can be interrupted for a few milliseconds. Most consumers will tolerate this without any loss of functionality.

Example of a small UPS


Depending on the structure, a UPS protects the connected systems from the following faults:

Major power outages are relatively rare in the EU, 14 minutes per year in Germany. Switching large currents causes unwanted repercussions on the power grid. For example, short circuits and the inrush currents from welding power sources or larger electric motors cause voltage drops. Voltage increases occur, for example, when large loads are switched off or when lightning strikes from a distance . Sensitive devices can be impaired or damaged in their function. The energy suppliers regulate the grid voltage and the grid frequency at the feed-in points into the power grid, but this only compensates for the sum of the disturbances. A UPS can compensate for local fluctuations and failures by feeding connected devices with electrical energy from accumulators , which are constantly recharged from the mains.


Battery-backed UPS devices are widespread. There are other types of construction, such as B. rotation-based UPS systems that use flywheel storage . The latter are usually used as part of larger battery-backup systems to bridge short-term failures.

The structure of a UPS

A battery-supported UPS consists of accumulators, in the case of a single-user UPS from lead-fleece batteries (AGM) or lead-gel batteries , in the case of power UPSs from lead-acid batteries , converters and an electronic control . NiCd batteries, which are less sensitive to temperature fluctuations, and in rare cases Li-ion batteries are also used as energy storage devices .

Series-produced UPSs are available from a power of around 300  VA up to several 100 kVA. The power is essentially dependent on the load capacity of the converter . Another essential feature of a UPS is the maximum back-up time, which depends on the capacity of the batteries. It can be a few seconds or several hours , depending on the requirements . A UPS, the bridging time of which can be extended by additional batteries, is available from a power of around 1500 VA. If there is a great need for power and bridging time, power generating sets are also used to recharge the batteries. For this purpose, UPS systems are now available that generate the required electricity from kinetic energy. This is supplied by a flywheel weighing several 100 kg, which is driven by the mains voltage and releases the stored energy again in the event of a power failure. However, this means that bridging a power failure is only possible to a limited extent. This is why these systems are usually supplied in connection with a diesel unit in order to be able to guarantee a time-independent power supply. It should be noted that the UPS system only bridges the period that the diesel unit needs to start up. However, this type of UPS system is only available from a certain size and is not designed for private households.

UPS in a data center

Computers in smaller data centers are automatically shut down in the event of a power failure before the backup time has expired. Open files, for example sensitive databases , are closed in a controlled manner to prevent data loss. For this purpose, the server and UPS communicate as standard via Ethernet or SNMP , but occasionally also via the RS-232 interface or via USB . The UPS can also be monitored, controlled and adjusted via this connection. In the case of a connection via Ethernet, no special software is usually required , just a web browser . The corresponding functions are created in the firmware of the UPS. Larger data centers have backup generators; the supply from accumulators only serves the purpose of bridging the time until the generators are started and brought to their nominal output.

The basic functions of a UPS usually include an automatic load test every 24 hours, during which the accumulators are discharged with the connected load during operation. With 10-year batteries, the rechargeable batteries should be completely replaced after eight years at the latest, with 5-year batteries after four years at the latest, in order to prevent the UPS system from failing due to a defective battery system.

When it comes to switching times, it must be taken into account that the capacitors and coils of the power supply units store sufficient energy for a few milliseconds. For example, ATX power supplies must be able to bridge failures of up to 17 ms.

For sensitive devices that only tolerate very short or no switching times at all, either a line interactive UPS (VI) or an online UPS (VFI) is required.

Extreme caution is required when using a UPS to protect laser printers or copiers. These devices consume several times their normal energy requirement in the warm-up phase. In the worst case, the UPS and the connected devices can be destroyed. If at all, only an online UPS can be used here. However, well-known manufacturers generally advise against the use of a UPS to protect laser printers and sometimes refuse any guarantee.


The IEC 62040-3 standard classifies UPSs in three levels. Several classes are distinguished within each level.

Level 1 - Online, Line-Interactive, Offline

Class 1: VFI ( Voltage and Frequency Independent from mains supply , voltage and frequency independent of the mains)

Schematic structure of an online UPS - VFI

Other terms are “ online ”, “double conversion”, “continuous operation” or “double conversion”.

With a UPS of this category, the input is led directly to a rectifier that feeds the accumulators. The output is exclusively supplied by an inverter, which in normal operation, i.e. when mains voltage is present at the UPS input, draws the necessary energy via the rectifier (GR) and is supplied via the battery system (accumulators) in the event of a power failure.

The AC voltage at the output is always generated - regardless of the quality of the input voltage - from the DC voltage of the so-called intermediate circuit via a downstream inverter (INV). To increase the security of supply, a VFI UPS has a so-called bypass circuit that is connected in parallel to the rectifier / inverter combination. In the event of an overload at the UPS output or an internal fault in the rectifier / inverter branch, the connected consumer is switched over to this bypass branch "without interruption" and thus continues to be supplied. Since rectifiers and inverters are constantly loaded with the full operating current, they must be of particularly high quality and make this design the most expensive. In addition, losses occur in both the direct and the alternating direction, which reduces the efficiency. In the product descriptions of UPS systems, the efficiency is usually given at full load. However, since most UPSs are not used to full capacity, the generally lower efficiencies at part load are decisive for the correct calculation of power consumption and costs. An efficiency of more than 95% at full load can now be described as the standard for a VFI UPS.

In addition to the consequences of a power failure, undervoltage and overvoltage, a VFI UPS also protects against frequency fluctuations and harmonics. They also offer sporadic protection against the effects of lightning and voltage distortion ( burst ). VFI UPSs are preferably used in areas of application that have high criteria for tolerable downtimes, such as B. in the power supply of a data center .

It should be noted that VFI UPSs have the highest internal power requirements compared to VFD or VI UPSs. While z. B. a 650 VA VFD-UPS in idle mode (without connected loads) gets by with approx. 5 W, an 850 VA VI-UPS with approx. 15 W, a VFI-UPS requires considerably more power even when idle (a rule of thumb says, that it needs approx. 10% of its nominal power, i.e. an 850 VA VFI-UPS needs approx. 85 W of own consumption when idling).

Class 2: VI ( Voltage Independent from mains supply , voltage independent of the mains)

Schematic structure of a Voltage Independent UPS (USV VI)

Other terms are “line interactive”, “network interactive”, “single conversion”, “delta conversion” or “active concurrent operation”.

A bidirectional inverter is used as the central component in a UPS of this category . Depending on requirements, it generates the DC voltage for charging the batteries from the AC voltage at the input or the AC voltage at the output from the DC voltage of the batteries. Because the converter also continuously limits the voltage level at the output, this is largely independent of the voltage level at the input. If there is a voltage at the input, its frequency determines the frequency of the voltage at the output. The switchover time in the event of a power failure is shorter than with the VFD-UPS and is around 2 to 4 ms. Switching to mains operation after restoration takes place without any time delay. The input voltage is synchronous with the output voltage.

VI UPSs not only protect against the consequences of a power failure, but also against undervoltage and overvoltage.

Class 3: VFD ( Voltage and Frequency Dependent on mains supply , voltage and frequency dependent on the mains)

Schematic structure of a Voltage and Frequency Dependent UPS - VFD

Other terms are “ offline ”, “stand by” or “passive”.

A UPS of this class forwards the power in normal operation directly from the input to the output. In addition, a rectifier is supplied from the input , which charges the accumulators. Should the mains supply fail, the output is switched to an inverter that is fed from the accumulators. Switching takes place with a delay of up to 10 milliseconds (ms), depending on the model. In addition, voltage fluctuations below 16 ms and voltage peaks between 4 and 16 ms are compensated according to EN 62040-3. For some very sensitive devices, this can be too long. In normal operation, the level and frequency of the output voltage are directly dependent on the input voltage.

Level 2 - Total harmonic distortion of the output voltage

This level indicates the curve shape of the output voltage in normal operation and battery operation with two capital letters.

Letter meaning
S. Sine curve, distortion factor less than 0.08 with linear and non-linear reference load
X Distortion factor less than 0.08 only with linear reference load
Y Shape of the voltage curve is not specified, trapezoidal or rectangular output voltage is also possible.

The SS classification is ideal, but inexpensive VI and VFD UPS only achieve SX or SY.

Level 3 - Interruption when switching in milliseconds

This level indicates the dynamic tolerance curve with three digits.

First digit: Behavior when changing the operating mode, such as switching between mains operation and battery operation.

Second digit: Behavior in the event of sudden load changes with linear load in mains operation and battery operation.

Third digit: Behavior in case of sudden load changes with non-linear load in mains operation and battery operation.

value meaning
1 no interruption
2 Interruption up to 1 ms
3 Interruption up to 10 ms

The best classification 111 is practically reserved for VFI-UPS, while 122 is common for VI-UPS and 333 is typical for VFD-UPS.


  • EN 62040: Uninterruptible Power Supply Systems (UPS)

Web links

Commons : Uninterruptible Power Supply  - collection of pictures, videos and audio files

Individual evidence

  1. 6th CEER Benchmarking Report on the quality of Electricity And Gas supply - 2016 (Annex A to chapter “Electricity - Continuity Of Supply”). Council of European Energy Regulators , 2016, p. 207 , accessed on July 29, 2020 .
  2. Ines Stotz: This is how you calculate the total costs of a UPS correctly. March 19, 2008 .;
  3. UPS classification according to IEC 62040-3. (PDF; 50 kB) Gerhard Muttenthaler, September 2, 2007, accessed on June 21, 2018 .
  4. a b c d e UPS classification. Eaton Corporation , accessed June 22, 2018 .
  5. Electronics tables - energy and building technology . Westermann, Braunschweig 2012, ISBN 978-3-14-245036-0 , p. 173 .
  6. a b c d e f Michael Mutschler: Know-how: UPS classification according to IEC 62040-3. In: ChannelPartner . October 21, 2004, accessed December 1, 2018 .