S 1.4 Lightning protection devices

Initiation responsibility: Building Services Manager

Implementation responsibility: Building Services

The direct effects of lightning striking a building (damage to the building fabric, fire in the roof structure etc.) can be prevented by installation of a suitable lightning protection system. In addition to this "external lighting protection", the "internal lighting protection", the overvoltage protection, is almost compulsory. This is because the external lighting protection does not protect the electrical equipment in the building. This is only possible by means of an overvoltage protection (see also S 1.25 Overvoltage protection).

Example:

• In the branch office of a service company in southern Germany, lightning caused damage to IT equipment (PCs, servers, laser printers) to an amount of approximately 10,000 Euro. As a consequence of this event, the building was equipped with an external lightning protection without internal lightning protection (overvoltage protection). Despite the external lightning protection, another lightning strike then caused damage to nearly the same amount.

The standard DIN EN 62305 "Protection against lightning" (corresponds to the VDE 0185-305 and IEC 62305 standards) valid since October 2006 completely revised the lightning and overvoltage protection concept. After a transition period of 2 years, all previous standards applying to lightning and overvoltage protection became invalid on 1 October 2008.

Based on the new standard DIN EN 62305, every institution should prepare a lightning protection concept. Part 2 "Risk Management" is the first standard to describe generally binding methods for risk-based lightning and overvoltage protection. Part 3 deals with "Physical damage to structures and life hazard", meaning the external lightning protection.

The external lightning protection, the arresting device (commonly called a lightening rod), is divided into four protection classes with regard to its effectiveness (also referred to as Lightning Protection Level, or LPL for short). Protection class IV (LPL IV) has the lowest level of protection while an arresting device of protection class I offers the best protection. An easily noticeable difference between the 4 protection classes is the mesh size of the arresting devices. This ranges from 20 x 20m for protection class IV in steps of 5m down to 5 x 5m for protection class I. For buildings with comprehensive IT equipment the arresting device should at least comply with protection class II, or preferably protection class 1.

The load-independent lightning current discharged to ground by the arresting device generates a voltage along the arresting device which decreases from the point of strike towards the grounding point. At the highest point of the arresting device, this voltage can be some 100,000 volts. It is therefore necessary to observe that electrically conductive installations (data, electricity, water etc.) must be at a sufficient distance from the arresting devices, especially in the upper floors of a building. This aspect has also been considered in the new standard, referred to as separation distance. This has nothing to do with considerations regarding the protection against compromising induction, even though the aspect of the separation distance has so far often been incorrectly equated to the protection against induction from the data cables located too close to the lightning rod.

Since the voltage drop along the arresting device at the grounding point never drops to 0V due to the remaining earth contact resistance and since the base point of the arresting device must be connected to the main equipotential bonding of the building, the overall PE system of the building and thus also the N conductor are raised to this residual voltage. Voltages in the range of far above 10,000 volts are to be expected here. Therefore, voltages are reached between N/PE conductors and the conductors L1/L2/L3 which significantly exceed the usual value of 230/400V. To ensure that these voltages do not cause damage to the electrical equipment operated inside the building, an essential consequence of the implementation of the external lightning protection is that the internal lightning protection, meaning the overvoltage protection, has to be implemented (see S 1.25 Overvoltage protection).

The arresting device installed must be checked regularly. Arresting devices of protection class I and II must be visually inspected once per year and subjected to comprehensive testing every 2 years. For protection classes III and IV, every 2 and 4 years respectively. For critical systems, meaning such systems for protection of high or highest availability installations, comprehensive tests must be carried out once per year. Any defects discovered must be eliminated immediately. Naturally, the performance of the tests, the observations made and the correction of defects performed must be documented in writing.

Review questions:

• Has a lightning protection system been installed according to the valid standard?
• Is a lightning protection concept in place?
• Do the arresting devices in buildings with comprehensive IT equipment at least comply with protection class II?
• Is the lightning protection system inspected and serviced regularly?