S 1.28 Local uninterruptible power supply

Initiation responsibility: IT Security Officer, Head of IT, Building Services Manager

Implementation responsibility: Building Services, Administrator

The job of a local uninterruptible power supply (UPS) is to protect individual IT systems or very few IT devices against the consequences of temporary interruptions to the power supply. This is a common goal in smaller IT structures that are not equipped with an emergency power supply.

Central UPS systems are used predominantly for larger IT structures or even to supply an entire building with power (see S 1.70 Central uninterruptible power supply).

Regardless of whether the local UPS comes in the form of a separate device or a 19-inch rack unit, its power output and backup time are defined by the device characteristics and cannot be changed as a rule.

With the local UPS devices available today and the low power outputs usually to be provided by them (in the range up to about 1kVA), it is possible to overcome a power failure for up to 120 minutes without any problems (backup time). The backup time actually required in a specific scenario depends on how long it takes to shut down the devices connected to the UPS (shutdown time) and on the other hand, how long is to be waited for the power supply to come back on line (waiting time). Since a large percentage of power failures only last a few minutes, a waiting time of 15 minutes should usually be sufficient to overcome an interruption of the power supply. If the power supply is interrupted longer than the waiting time and if it is necessary to shut down the supplied IT system in order to avoid losing data, the total backup time should be calculated using the following formula

Backup time = Waiting time plus twice the shutdown time

. Doubling the shutdown time in the calculation provides a safety reserve in case a shutdown takes longer than expected. The backup time must be checked every time after changing or adding IT devices to be supplied by a UPS to make sure the current backup time is sufficient.

There are three basic classes of UPSs:

• VFD-UPS
  
  With a VFD-UPS (VFD stands for Voltage and Frequency Dependent), the connected consumers are supplied directly from the power supply grid in normal operation. It is therefore possible for minor interruptions in the power grid to directly affect the connected consumers. When the power fails, the VFD UPS starts automatically and supplies power. However, it needs up to 10ms to start up (switchover time), which may already be too long for some IT devices. VFD-UPSs also used to be referred to as offline UPSs.
• VI-UPS (Voltage Independent)
  
  In this case, the VI-UPS (VI stands for Voltage Independent) compensates for minor variations in the supply voltage without having the UPS completely take over supplying power to the connected consumers. The frequency on the output of a VI-UPS depends directly on the frequency on the power grid, just like with a VFD UPS. A short interruption in the power supply can also occur when using a VI-UPS when it switches over to the battery mode.
• VFI-UPS (Voltage and Frequency Independent)
  
  On a VFI-UPS (Voltage and Frequency Independent), there is usually no direct dependency between the input and output of the UPS. The electrical energy on the input is rectified and fed into the intermediate circuit. The intermediate circuit then keeps the batteries fully charged and supplies the inverter with power. The inverter then generates the AC power needed to supply the consumers connected.
  
  
  
  Since the output energy is generated permanently and independently from the input by the inverter, there is no switchover time required in this case. VFI-UPSs used to be referred to as online UPSs.

Since the VFI-UPS is the only one of the three systems that really operates without interruption, this type of UPS should always be preferred. When additional features not addressed in this safeguard are taken into account, a UPS classified according to DIN IEC 62040-3 VFI-SS-111 represents the best choice for supplying IT with power.

A UPS does not provide overvoltage protection in the truest sense, regardless of its design. In fact, a UPS needs to be protected against overvoltage using suitable safeguards just like all other electrical equipment (see S 1.25 Overvoltage protection for more information).

To avoid potential problems with protective conductor currents, the IT devices that are backed up by a local UPS should not be connected using shielded cables (e.g. printer cables) to other IT devices that can be supplied via a different route.

Since the batteries of a local UPS are seldom operated within their ideal temperature range (typically around 20°C), the service life of the batteries of a local UPS device is relatively short, at best up to 5 years, but usually much less. During this operating time, the performance of the batteries drops continuously, and after two or three years at most, a local UPS will usually only be able to supply power for half the time it could when it was new. To ensure that the UPS will provide power for the backup time specified, the actual backup time should be determined once per year. Some UPS systems have a built-in mechanism for this purpose. If this is not the case, the backup time can be determined by a load test.

As for all other electrical devices, it is also necessary for UPS systems to make sure they are operated within the temperature ranges specified by the manufacturer. This must be taken into account when determining the required cooling capacity.

To maintain the protective effect of a UPS, maintenance on it must be performed regularly. The maintenance schedule specified by the manufacturer of the UPS must be followed for this reason.

Review questions:

• Has the UPS been dimensioned sufficiently regarding output power and backup time?
• Is the UPS backup time checked again for appropriateness after every change to the consumers?
• Is there a regulation regarding the shut down and proper power down of IT systems in the event of power failure in order to avoid losing data?
• Are the UPS devices and the IT devices provided with overvoltage protection?
• Are connections between UPS-protected IT devices and IT devices supplied otherwise using screen lines avoided?
• Were the temperature ranges of the UPS devices specified by the manufacturer checked when the cooling and/or room temperature were designed?
• Is the actual capacity of the battery, and therefore the backup time of the UPS, tested regularly?
• Is the maintenance schedule of the UPS followed?