S 1.25 Overvoltage protection

Initiation responsibility: Head of Site Technical Services, Head of IT Section; IT Security Management

Implementation responsibility: Head of Site Technical Services; Administrator

Depending on the quality and configuration of the mains supply and the in-house power network, overvoltage peaks can occur in the electricity supply system through induction or lightning, depending on the environment (other power consumers) and on the geographical location. Overvoltage protective measures are aimed at reducing any possible damage to IT equipment in networks caused by direct lightning strike, cross-coupling and switching operations.

A complete overvoltage protection concept envisages three stages which are essentially oriented towards the impulse voltage withstand levels for the overvoltage categories defined in DIN VDE 0110/IEC Publication 664.

• Elementary protection in the building feed is capable of handling overvoltages as they arise through direct lightning strike, confining them to values less than 6000V. With existing external lightning protection the elementary protection must be capable of handling currents of 100 kA as they occur in lightning strikes.

• Medium protection in the floor distributors confines the remaining overvoltages to approx. 1500V and depends on the overvoltages it handles not exceeding 6,000V.

• Fine protection on the power points and plug connections of all the other lines reduces the remaining overvoltages to a level that can be withstood by the connected equipment. Under the Electromagnetic Compatibility Act manufacturers of electric and electronic equipment are obliged to provide their appliances with the fine protection that is necessary for safe operation (the CE marking refers to this).

The protective effect of every stage builds on the preceding stage. If one stage is left out, overvoltage protection will as a whole become virtually ineffective.

If overvoltage protection cannot be ensured throughout the building, as a minimum it is possible to establish a protective perimeter around important IT facilities (server, etc.). To minimise potential damage, networks to which multiple devices are connected can be divided into small sectors protected from each other, using optocouplers or surge arresters. Protected and unprotected areas must be consistently separated, going back to the protection device in which separation occurs. A sufficient distance must be maintained between the lead wires; running them through a single cable channel would cancel out the protective effect. Overvoltage protection devices should be checked periodically and also after known events and, if necessary, they should be replaced.

In particular, when drawing up an overvoltage protection concept for the first time the design and mode of operation of existing uninterruptible power supplies (UPS) and standby generating units should be considered.

In addition to overvoltage protection in the mains supply, safeguards must be implemented in server rooms and in the core units of a computer centre against electrostatic charge. The current flow resistance of floor coverings in such rooms must be between 10 and 100 megaohms. The classification according to DIN standard 4102-1 must be at least "B1 hardly flammable". This applies also for a raised floor or an installation floor.

Irrespective of the extent and development of overvoltage protection, attention must be given to two basic requirements:

• The line between the fine-protection feature and the devices to be protected should not exceed 20 metres. If that length is exceeded, another fine-protection device must be interposed. If a device is provided with fine protection at the entry point, the 20 m upper limit does not apply.

• If overvoltage protection is to be effective, comprehensive potential equalisation is required for all electric resources covered by overvoltage protection! Most damage to IT equipment that is caused by overvoltages is attributable to improperly implemented potential equalisation.

Additional controls:

• Are lightning and overvoltage protection devices checked and replaced as required, both periodically and after known actual incidents?

• Has potential equalisation been implemented throughout?

• In case of retrofitting, is consideration given to the inclusion of potential equalisation?