S 1.28 Local uninterruptable power supply (ups)

Initiation responsibility: Head of Site/Bldg Technical Service; Head of IT Section; IT Security Management

Implementation responsibility: Head of Site/Bldg Technical Service; administrator

With an uninterruptable power supply (UPS), it is possible to bridge a short-term power failure or maintain the power supply long enough to allow an orderly shut-down of the connected computers. This is particularly expedient

• when large quantities of data are stored temporarily in the computer (e.g. cache memory in the network server) before their transfer to non-volatile storage;

• if, in case of power failure, large quantities of data would be lost and their subsequent re-entry would be required;

• if the stability of power supply is not sufficiently ensured.

Two types of UPS should be distinguished:

• Offline UPS: The connected power consumers are normally fed directly from the supply mains. It is only in the case of failure of the latter that the UPS will automatically switch in and take over the supply function.

• Online UPS: The UPS is permanently switched between the mains and the power consumers. The entire power supply is always provided through the UPS.

In addition to tiding over a complete breakdown of power supply and undervoltage situations, both types of UPS can also serve to smooth overvoltage. In this case, too, the 20 m limit specified in S 1.25 Overvoltage Protection applies with regard to overvoltage protection.

If IT equipment in a building with TN-S network (see S 1.39 Prevention of transient current on shielding) is supplied by a local UPS, the following should be observed: In order to maintain the protective effect of the TN-S network against transient current on shielding of data lines, the PE conductor of the power line must not be connected to the PE conductor of the output side of the UPS.

When dimensioning a UPS system, a normal by-pass time of about 10 to 15 minutes can, as a rule, be assumed. In most cases, a power failure will be remedied within 5 to 10 minutes. This leaves about 5 minutes for the orderly shut-down of the connected IT systems if the power failure should persist for some time. Most modern UPS equipment offers computer interfaces capable of initiating a timely automatic shut-down after a predefined time, in accordance with the time requirements of the IT systems and the capacity of the UPS.

For specific applications (e.g. PBX), the necessary by-pass time may be several hours.

In order to preserve the protective effect, provisions must be made for regular maintenance of the UPS system.

If it is possible to obtain uninterruptable power supply from other sources (e.g. by connection to a central UPS system), this constitutes an alternative to a local UPS system.

Additional controls:

• Are the required intervals for UPS maintenance being observed?

• Have provisions been made for automatic shut-down?

• Is the effectiveness of the UPS system being tested on a regular basis?

• Have any changes taken place with the result that the assigned capacity of the UPS system is no longer sufficient?