Most electronic systems and equipment connected to the ac mains supply, telecom and data lines, are at risk from the damaging effects of surges or transient over-voltages. Millions of pounds worth of electronic equipment is destroyed within buildings every year by lightning or other surge phenomena.Lightning Protection Systems (LPS), are designed to safeguard lives and buildings. But beware: a Faraday cage does NOT prevent transient over-voltages from damaging electronic equipment within the building structure; the LPS needs to be supported by Transient Voltage Surge Suppressors (TVSS) or Surge Protective Devices (SPD). The importance of surge protection with regard to its ability to lives, buildings and equipment, has resulted in the preparation of extensive documentation. For guidance, the standards are listed below with extracts reproduced by kind permission of the British Standards Institute (BSI). The full standards documentation can be accessed from the BSI website (see foot of article for details).Surge protection standardsBS6651:1999 Annex C - UK Standard Prior to August 2008 -The BSI “Code of Practice for Protection Against Lightning” provides general advice on the protection of electronic equipment, and was replaced in August 2008 by BS EN 62305-1 to BS EN 62305-5:2006, although as BS6651:1999 is so well established, it is probable that it will still be referred to as a reference source.BS EN 62305-4:2006 – ‘Protection against lightning - Electrical and electronic systems within structures’ addresses the need to provide a surge protection system, and has been complimented by the addition of BSI document BIP2118 – “Protection against lightning - A UK guide to the practical application of BS EN 62305”. For the selection, installation and testing requirements for telecommunication and signalling system SPDs, BS EN 62305-4 refers the reader to BS EN 61643-21 and BS EN 61643-22.BIP2118 - Protection against lightning - a UK guide to the practical application of BS EN 62305. The Risk Assessment to establish the requirement asks the following questions, where is the building geographically situated? Overhead or Underground power feed? Is a Lightning Protection System (LPS) fitted? Are other services bonded to the “Equipotential Earth Bar” and what type of materials have been used (metallic pipes Gas, Water etc)?With this information, it is possible to determine what kA rating if any, is required for the Type 1, Test Class1 10/350 microsecond protector. However, whatever the answer is to the above, an 8/20 microsecond surge protector will still be required to protect sensitive electronic equipment.BS7671:2008 - IEE Wiring Regulations 17th Edition - Section 443: Protection against over-voltages of atmospheric origin or due to switching. This section deals with protection of electrical installations against transient over-voltages of atmospheric origin transmitted by the supply distribution system and against switching over-voltages generated by the equipment within the installation.According to BS7671:2008, the use of surge protection may be based on a risk assessment method; however, if there is a risk or consequence to human life, public services, IT centres, commercial or industrial activity (hotels, banks etc) SPDs at the entrance of the installation are required, and there is no need to perform the risk assessment, as this calculation always leads to the result that the protection is required. These levels directly correspond with those detailed in BS EN 61643-12.BS 7671:2008 incorporating amendment No 1: 2011 - requirements for electrical installations: IET Wiring Regulations, published in July 2011, replaces BS 7671:2008. The newly revised BS 7671 affects all new installations as of January 1 2012. BS 7671 (The IET Wiring Regulations) is the national standard to which all domestic and industrial wiring must conform. The newly amended BS 7671 includes a large number of changes from the original 2008 document and has 70 percent of new content. Amendments to BS 7671:2008 (now incorporated into ‘BS 7671:2008 incorporating amendment No 1: 2011 - requirements for electrical Installations: IET Wiring Regulations‘) include those to Chapter 53, which now contains a new Section 534 - ‘Devices for protection against over-voltage’ - which deals with the installation of SPDs.The requirements of Section 534 are for the selection and erection of SPDs for electrical installations of buildings in order to limit transient over-voltages of atmospheric origin transmitted via the supply distribution system, and against switching over-voltages. The requirements are also intended to protect against transient over-voltages caused by direct lightning strikes or lightning strikes in the vicinity of buildings protected by an LPS. The requirements do not take into account surge protective components, which may be incorporated in the appliances connected to the installation.BS EN 61643-11/12 (Worldwide Standard) - comprehensive globally accepted surge protection standards for the performance of surge protection devices. According to BS EN61643-12, the use of surge protection may be based on a risk assessment method; however, in situations where full analysis is too complex, simplified methods, based on IEC61662 (a former version of IEC62305-2), may be used. The simplified method is as follows, and there are basically two cases:- Where an installation is supplied by, or includes, an overhead line and there are more than 2.24 flashes per square kilometre per year, SPDs at the entrance to the installation shall be provided.- Where one of the above conditions (less than 2.24 flashes per square kilometre per year or underground cables) is not fulfilled, there are different levels of consequences to consider. These risk or consequence have the same content as those detailed in the IEE Wiring Regulations 17th edition, and where risk is identified to human life (for example: safety services, medical equipment in hospitals, public services, IT centres, museums, commercial or industry activity - hotels, banks, industries, commercial markets - and farms), then SPDs at the entrance of the installation are required. There is therefore no need to perform a risk assessment calculation for these levels of consequence, because this calculation always leads to the result that the protection is required. BS EN61643-21/22 - Surge protective devices connected to telecommunications and signalling networks performance requirements and testing methods. The need for protective measures, such as SPDs, for IT systems should be based on a risk assessment, considering the probability of over-voltage and over-current. This takes into account the consequences of the loss of service for the customer and network operator, the importance of the system (hospitals, traffic control, for example), the electromagnetic environment at the particular site (probability of damage) and cost related to repair. These British Standards can be obtained in PDF or hard copy formats from the BSI online shop: http://shop.bsigroup.com/. BS 7671:2008 Incorporating Amendment No 1:2011 can be purchased in hard copy format only from the IET website: http://electrical.theiet.org/ and the BSI online shop: http://shop.bsigroup.comAndy Martin is product marketing manager of PD Devices, a specialist designer and manufacturer of SPD and TVSS components
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