Surge Protection Devices – An Approach to Risk Assessment | nesik


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The team at NICEIC & ELECSA took a look at the risk assessment approach contained in Regulation 443.5 of BS 7671, to determine whether surge protective devices (SPDs) should be installed. The publication of the eighteenth edition of BS 7671 saw a number of significant changes regarding overvoltage protection and the selection of SPDs. Figure 1 (pictured on the right) shows a chart of the United Kingdom, showing the approximate number of lightning flashes per year. Regulation 443.4 requires protection against transient overvoltages where overvoltages could result in: • serious injury or loss of human life; ● interruption of public services and / or damage to cultural heritage; ● interruption of commercial or industrial activity; or ● a large number of individuals in the same location can be affected. For all other cases, a risk assessment should be carried out in accordance with Regulation 443.5 in order to determine if transient overvoltage protection is required. If a risk assessment is not carried out, protection must be provided to the electrical installation against transient overvoltages. The exception to a risk assessment is for individual dwelling units where it is considered that the total value of the plant and equipment therein does not justify such protection. Any such decision shall be based on discussions between the installer and the customer. Transient overvoltages from switching tend to be of lower amplitude but longer duration than those of atmospheric origin overvoltages. When SPDs are installed to provide protection against atmospheric origin overvoltages, they will generally meet the requirements for transient overvoltages due to high-speed switching. Details of these devices and how to install them are given in Section 534 of BS 7671. Figure 2. Line diagram showing the configuration of supply cables for typical building hazard assessment Risk assessment and management is now more in-depth and effective. It is broad based on the approach used in earlier versions of BS 7671, and is used only when the requirements of Regulation 443.4 do not apply. The risk assessment calculation is based on a formula using the environmental values ​​given in Table 443.1 of BS 7671 which are reproduced in Table 1, and on values ​​related to geographical locations shown in Figure 1, a map of the United Kingdom showing the typical number of lightning flashes per square kilometer. The values ​​shown in Table 1 and the distribution network cable lengths up to the origin of the installation as shown in Figure 2 are used in the equation in Regulation 443.5 to determine the calculated risk level (CRL). If the calculated hazard level is less than 1000, protection against the transient overvoltage of the atmospheric origin is required. Conversely, when the calculation gives a CRL value equal to or greater than 1000, such protection is not required. This risk assessment method is based on the designer having knowledge or access to the supply cable lengths from the lightning arrester on the distribution network to the origin of the installation. Where the cable lengths of the distribution network are unknown or only partially known, Lp shall not exceed 1 km, or the distance from the networks’ overvoltage arrester to the origin of the electrical installation – whichever is less. Examples 1. Will it be necessary to install an SPD in a warehouse located in an urban area where the flash density is 0.8 and the width consists of the lengths shown below? This result greater than 1000 indicates that SPDs are not required. 2. Will it be necessary to install an SPD in a village primary school located in a rural area where the flash density is 0.3 and the width consists of the following lengths? This result is less than 1000 SPD installation requires. This may seem reasonable given the type and occupancy of the building and the equipment likely to be located there. The effect of geographical location and therefore the intensity of the flash has an effect on the result of the calculated risk level. However, the important part of the equation depends on the configuration of the supply cables and whether there is sufficient length to run after a lightning strike in order to mitigate the overvoltage. Summary: There may be some confusion regarding the installation of SPD devices. However, with the introduction of the Eighteenth Edition, the decision to install SPD hardware or to pursue a risk assessment is something that contractors will need to consider when designing new installations or making significant modifications to existing installations. In the event a risk assessment is undertaken to determine that SPD devices do not need to be installed, such paperwork must be attached to the Electrical Installation Certificate or delivered to the customer with other operating documents. Get more details about NICEIC registration by clicking here


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