S 1.25 Overvoltage protection

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

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

Depending on the quality and advancement of the external power supply network and the in-house power network, overvoltage peaks can, depending on the environment (other power consumers) and on the geographical location, be caused in the supply mains by induction or lightning. As a rule, overvoltage due to lightning has quite a destructive potential, whilst that of overvoltage due to other causes is only minor. However, all types of overvoltage can destroy IT systems.

A comprehensive overvoltage protection policy covers three stages:

• elementary protection in the building feeder;

• medium protection in storey distributors; and

• fine protection provided at the respective sockets and the plug-in connections of all other lines.

The design of elementary protection depends on the existence of external lightning arrester. The protective effect of every stage builds on the preceding stage. If one stage is left out, overvoltage protection will, in its entirety, become nearly ineffective.

If overvoltage protection cannot be ensured throughout the building, it will at least be necessary to establish an adequate protective perimeter around important IT facilities (server, etc.). In order to minimise potential damage, networks to which multiple devices are connected can, by means of optocouplers or surge arresters, be divided into small sectors protected from each other.

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 be longer than 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 limitation does not apply.

• If overvoltage protection is to be effective, comprehensive potential equalisation is required for all electric resources covered by overvoltage protection!

Additional controls:

• Are lightning/overvoltage protection devices being checked and, in the appropriate cases, replaced, both periodically and after known actual incidents?

• Has potential equalisation been ensured throughout?

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

© Copyright by Bundesamt für Sicherheit in der Informationstechnik 2000

Last Update on 6 April 2000