The team at NICEIC & ELECSA take a look at the risk assessment approach given in Regulation 443.5 of BS 7671, to determine whether surge protection devices (SPDs) should be installed.
Publication of the 18th Edition of BS 7671 saw a number of important changes in respect of overvoltage protection and selection of SPDs.
Fig 1 (pictured right) illustrates an outline of the UK, showing the approximate number of lightning flashes per year.
Regulation 443.4 requires that protection against transient overvoltages shall be provided where such over-voltage occurrences could result in:
● serious injury to, or loss of, human life;
● interruption of public services and/or damage to cultural heritage;
● interruption of commercial or industrial activity;
or
● where a large number of individuals at the same location could be affected.
For all other cases, a risk assessment according to Regulation 443.5 should be performed in order to determine whether protection against transient overvoltage is required. If the risk assessment is not performed, the electrical installation should be provided with protection against transient overvoltage.
An exception to performing a risk assessment is for single dwelling units where it is considered that the total value of the installation and equipment therein does not justify such protection. Any such decision should be based on discussions between the installer and client.
Transient overvoltages due to switching tend to have lower amplitude but a longer duration than those overvoltages of atmospheric origin.
Where SPDs are installed to provide protection against overvoltages of atmospheric origin, they will generally meet the requirements for transient overvoltages due to high speed switching. Details of such devices and how they are fitted are given in Section 534 of BS 7671.
Fig 2. Line diagram showing the make-up of supply cables to a typical premises
Risk assessment
The assessment and management of risk is now significantly more in-depth and extensive than the approach used in earlier editions of BS 7671, and is only used when the requirements of Regulation 443.4 do not apply.
The risk assessment calculation is based upon a formula that uses environmental values that are given in Table 443.1 of BS 7671 which is reproduced as Table 1, and on values relating to geographical locations shown in Fig 1, which is a map of the UK showing the typical number of lightning flashes per km2.
The values shown in Table 1 and the distribution network cable lengths up to the origin of the installation as shown in Fig 2 are used in the equation given in Regulation 443.5 to determine the calculated risk level (CRL).
If the calculated risk level is less than 1 000, protection against transient overvoltage of atmospheric origin is required. Conversely, where the calculation gives a CRL value equal to or above 1 000, such protection is not required.
This method of risk assessment relies on the designer having knowledge of, or having access to, the lengths of supply cable from the surge arrestor on the distribution network up to the origin of the installation.
Where the distribution network cable lengths are unknown or only partially known, then Lp shall not exceed 1 km, or the distance from the networks overvoltage surge arrestor to the origin of the electrical installation – whichever is the lesser.
Examples
1. Would it be necessary to install SPDs at a warehouse situated in an urban area where the flash density is 0.8 and the supply is made up of the lengths shown below?
This result being greater than 1 000 indicates that SPDs are not required.
2. Would it be necessary to install SPDs at a village primary school situated in a rural area where the flash density is 0.3 and the supply is made up of the following lengths?
This result being less than 1 000 requires SPDs to be fitted. This would appear reasonable considering the type and occupancy of the building and equipment likely to be found therein.
The effect of the geographical location and hence the flash density has an impact on the outcome of the calculated risk level.
However, the significant part of the equation is dependent upon the make-up of the supply cabling and whether there is sufficient length of run following a lightning strike for the overvoltage to be attenuated.
Summary
There may be some confusion regarding the installation of SPDs. However, with the introduction of the 18th Edition, the decision to install SPDs or go through the risk assessment process is something contractors will need to consider when designing new installations or carrying out significant alterations in existing ones.
Where a risk assessment has been carried out to determine that SPDs do not need to be installed, such paperwork should be appended to the Electrical Installation Certificate or handed to the client along with other commissioning documents.
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