Lightning Protection - Practical application of the BS EN 62305 Zone concept


BS EN 62305 has given us new ways to apply concepts outlined within the standard to tackle problems with a logical, systematic approach. In BS 6651 basic surge protection was mentioned under Annex C. BS EN 62305-4 Electrical and Electronic systems within structures is a complete document dedicated to the problems, and solutions, encountered due to lightning current and induced surges. Mike Forsey, technical manager at DEHN (UK) explains

The new standard calls for a risk assessment calculation to be carried out, part of the assessment determines if surge protection is necessary and if so what type of surge protection device, or devices, needs to be installed.

With the increase of electrical and electronic systems being used within both business and private environments the continuing reliance, functioning and uninterrupted use of these systems is becoming essential. Equipment ranging from the basic power supply and distribution systems to specialist equipment for computer, networks, building management (BMS), telecommunications, control and security, etc now play an essential role in our lives. Damage caused by lightning and switching related events has not only a direct repair cost but also an indirect consequential cost due to down time, data re-instatement, etc.

Using the protection principles outlined in BS EN 62305-4 the protection of such systems against surges is based on the principle of lightning protection zones (LPZs), in which the building or structure being protected is divided subject to the location of the equipment within the structure. Using this approach, suitable zones can be defined according to the number, type, immunity and sensitivity of the electrical and electronic devices or systems present within the structure. Sizes ranging from small local zones to large integral zones that can encompass the whole building can be established. At the boundary of each internal zone, equipotential bonding must be carried out for all metal components and utility lines entering the building or structure. For mains power, data, telecomm, etc this is carried out with the use of suitable surge protection devices.

As can be seen from the diagram below a lightning current arrester, SPD Type 1,  (Waveform 10/350) is required at the interface of zones LPZ0/1 for any cable entering from a zone LPZ0A. At the boundaries of LPZ1/2 and higher a surge arrester, SPD Type 2 (Waveform 8/20) would be used.

The same principle is used for all conductive cables entering the structure be they mains power, telecomm, data, networks or CCTV.

Spatial shielding within the structure also forms part of the protective measures. By correct design and placing of suitable shielding the magnetic fields within the structure can be attenuated.

Protection management
For new buildings and structures, optimum protection of electrical and electronic systems within the structure can be best achieved cost-effectively if these systems are designed together with the building and are taken into account before its construction. For existing buildings and structures, the cost of this protection is usually higher than for new buildings and structures. If however, the LPZs are chosen appropriately and existing installations are used or upgraded, the costs can be reduced. If the risk analysis as specified in the new BS EN 62305 -2 shows that surge protection is required, this is best achieved if:
- The measures are designed by a lightning
protection specialist having knowledge of
electromagnetic compatibility;
- There is close co-ordination on all aspects of
the work between the building experts (e.g.
civil and electrical engineers) and the surge
protection experts;
- An appropriate management plan is
adhered to.