LEMP Protection


The British Standard on lightning protection (BS EN 62305) treats the protection of electrical and electronic systems as an integral part of an overall lightning protection scheme. Samad Khan, Product Manager, Electronic Systems Protection – Furse, reviews the new focus on protection against Lightning Electromagnetic Impulses (LEMP) and the role of Surge Protection Devices.

The importance of LEMP protection
In BS 6651, the protection of electronic and electrical equipment was included as a recommendation only, with its own independent risk assessment. As a result, protection was often fitted after damage had been suffered. With BS EN 62305, this protection has been made an integral part of the standard and is considered within the overall risk assessment detailed in part 2. Structural lightning protection cannot now be considered in isolation from transient overvoltage or surge protection.

The single risk assessment of BS EN 62305 dictates whether structural and/or LEMP protection is required. A structure may need both a structural Lightning Protection System (LPS) and a fully coordinated set of Surge Protection Devices (SPDs) or just transient overvoltage protection alone. Indeed, an LPS system can no longer be fitted without a lightning current or equipotential bonding SPD to incoming metallic electrical services that cannot be bonded directly to earth. Such SPDs (Type I SPDs) protect against flashover caused by a direct lightning strike, preventing fire and electric shock hazards. Type I SPDs form one part of a co-ordinated set of SPDs that include Type II and III overvoltage SPDs - which are needed in total to protect sensitive and electrical and electronic systems.

LEMP damage is identified as one of the specific types to be protected against and that such damage can occur as a result of a strike to, or near any part, of the structure or connected services. This extended approach also takes into account the danger of fire or explosion associated with services connected to the structure, eg power, telecom and other metallic lines.

BS EN 62305 introduces the concept of Lightning Protection Zones (LPZs), a series of zones within a structure created to have, or identified as already having, successively less exposure to the effects of lightning. Successive zones use a combination of bonding, shielding and/or SPDs to achieve a significant reduction in LEMP severity, from conducted surge currents and, transient overvoltages, as well as radiated magnetic field effects. Designers co-ordinate these levels so that the more sensitive equipment is sited in the more protected zones.

Surge Protection Devices
SPDs are key in providing LEMP protection. Superior or enhanced SPDs provide protection against surges in both common mode (between live conductors and earth) and differential mode (between live conductors). As terminal equipment, eg computers, tends to be more vulnerable to differential mode surges, this additional protection can be a vital consideration.

BS EN 62305 focuses on co-ordinating SPDs, where the locations and LEMP handling attributes of a series of SPDs are co-ordinated to nullify the conducted LEMP effects - thereby protecting equipment within their environment. So, for a building with an LPS fitted or an exposed overhead electrical service line, there may be a heavy duty lightning current Type I SPD at the service entrance to handle the majority of the surge energy, plus downstream overvoltage SPDs (Types II and III) to protect equipment from transient overvoltages. Type III SPDs are used close to terminal equipment and can also provide local protection from potential damage by switching sources, eg large inductive motors.
Applying SPDs

A designer must take into account the following when choosing and applying SPDs:
- withstand voltage of protected equipment, ie the level at which permanent damage occurs
- immunity withstand voltage of equipment, ie the level at which system disruption occurs
- additional installation effects, eg voltage drop on connecting leads
- oscillation protective distance, ie if the distance from the SPD to the equipment is over 10m, oscillations could lead to a doubling of the voltage at the “protected” equipment

Here, only using standard SPDs could leave the indirect risk of LEMP damage too high. Also, standard SPDs may only protect against common mode surges, providing effective protection against outright equipment damage but not against downtime due to system disruption. Where continuous equipment operation is critical, the use of enhanced SPDs delivers a lower let-through voltage in both common and differential modes over a standard SPD. Such enhanced SPDs can even offer up to Type I+II+III co-ordinated protection within one unit.

Often, enhanced SPDs are the best choice to achieve cost-effective protection in addition to preventing costly system downtime. Their lower voltage protection in all modes means fewer units are required to achieve protection, which saves on unit and installation costs, as well as installation time.

BS EN 62305 has incorporated the protection of electrical and electronic equipment within the body of the new standard. LEMP protection measures are integral to any lightning protection system. Lightning protection designers and installers need to understand and be confident about this new focus. To help, Furse are offering a series of newly developed, professional resources, from in-depth technical seminars to their Guide to BS EN 62305:2006. For further information, call Furse on 0115 964 3700 or, alternatively, email Furse at enquiry@furse.com.