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Like every new edition of the IET Wiring Regulations, the 18th Edition introduced a whole host of changes. Many of these were comparatively minor, although undoubtedly important, but the changes relating to surge protection were large and far reaching. The new requirements for surge protection can, for the most part, be found in Section 443 of the regulations, which deals with transient overvoltages in general, and in Section 534, which discusses the devices that should be used for protection against overvoltages.
It’s worth noting the 17th Edition of the Regulations did, in fact, include detailed requirements for surge protection, but the big change introduced by the 18th Edition is that in many cases the provision of surge protection is now mandatory. Previously, whether or not protection was needed was decided on the basis of a risk assessment.
In Section 443, the new regulations state that protection against transient overvoltages (in other words, surge protection) MUST be provided where the consequences of an overvoltage could result in serious injury or loss of life, interrupt public services or damage cultural heritage, interrupt commercial or industrial activity, or affect a large number of co-located individuals. Most if not all non-domestic installations already fall into one or more of these categories, but the new regulations have even more to say.
For installations outside these categories, a risk assessment must be performed to decide whether surge protection is needed. Alternatively, if no risk assessment is performed, surge protection must be provided except for “single dwelling units where the total value of the installation and equipment therein does not justify such protection.” Given the wide scope of these provisions when they are all taken together, it’s probably safest to assume that surge protection should be fitted to all installations, unless a very good reason can be found for omitting it!
This being so, the logical follow up questions are where should the protection be fitted and what form should it take? To answer these questions, the first port of call is Section 534 of the regulations which, in conjunction with BS 62305, deals with lightning protection zones.
For detailed and definitive guidance, the regulations and standards themselves must always be consulted, but in essence they divide an electrical installation into four zones. Zone 0 is outside a building where there is a risk of direct lightning strikes, Zone 1 is inside the building at the point of common coupling (usually the main distribution board), Zone 2 is inside the building in sub-distribution boards or control panels that are directly connected to the point of common coupling, and Zone 3 is inside the building where connections are made to end devices.
Surge protection devices (SPDs) have to be provided at the origin of the installation (Zone 1) and additional SPDs are needed wherever a cable or service crosses an internal zonal boundary. The type of SPD to be fitted depends on the zone in which it will be installed. When a building has an external lightning protection system or is supplied by overhead lines, a Class I device must be fitted in Zone 0. The purpose of a Class I device is, however, only to minimise the risk of dangerous sparking to protect life. It is not intended to protect downstream electrical or electronic systems.
If a building has no external lightning protection system and is not supplied by overhead lines, a Class II SPD should be installed in Zone 1. In all cases, further Class II SPDs should be fitted in Zone 2, that is, in all sub-distribution boards and control panels that are connected to the main distribution board.
Class II SPDs limit overvoltages and provide protection for downstream devices but this protection may not always be adequate for sensitive equipment such as PLCs. In such cases, Class III SPDs should be installed in Zone 3 – that is, at the point where the sensitive item of equipment is supplied. In some cases, this could be a socket outlet, but it’s equally possible for it to be a fixed connection. Note that some equipment may have built in Class III SPDs, but these can only be effective if the upstream circuits feeding the equipment are correctly protected with Class I and Class II devices.
One more question to be considered is where the SPDs should be physically located. Fortunately, they are small devices so it will almost always be possible to accommodate them in distribution boards and control panels, even if they have to be retrofitted to existing equipment. If this is impractical, the SPDs can be housed in an ancillary enclosure close to the distribution board or control panel. Whichever solution is adopted, the wiring must be kept as short as possible because inductance introduced by long connections impairs the effectiveness of SPDs.
It is important to say that, in this short article, it has only been possible to give a brief overview of the main surge protection requirements that have been brought to prominence by the 18th Edition of the IET Wiring Regulations. In particular, it has not been possible to discuss BS EN 61643, which deals with power, data and signal surge protection, and is closely allied with the new regulations.
All of this may sound rather challenging, but contractors and others involved with designing and implementing electrical installations for buildings should not feel daunted or dismayed; much advice on how to meet the new requirements is readily available. One reliable source is Weidmüller. The company has extensive experience in the field of surge protection and offers a full range of SPDs that are easy to specify and use. Additional guidance is readily available on the Weidmüller website, and the company is always ready to provide advice on devising practical and cost-effective solutions for specific applications.
For further information you can call Weidmuller at 0845 094 2006 or visit www.weidmuller.co.uk
About the author:
Peter Croucher is Product & Sales Specialist at Weidmuller, responsible for the management of the Factory Automation portfolio of products in the UK. An electronics engineer, his 15-year career with Weidmuller started with internal technical and external applications support then for the last 9 years developed into product management for the electronics, automation and modular terminal block portfolios.
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