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Guide to
| Surge Protection Devices
5
Lightning discharges could contain currents of 200,000A
which if struck at or near power transmission lines would
generate a significant voltage transient. This voltage transient
could cause significant damage to both domestic and
commercial electronic equipment.
The UK regional map illustrates the likely lightning activity
caused by the number of thunderstorm days across the
country.
Protection against over-voltages is the subject of section 443
of BS 7671. Here the AQ criteria method is introduced which
is based on the likelihood of the equipment being subjected
to over-voltages caused by lightning strikes, taking account of
the probable number of lightning strikes per year.
For electrical installations in the UK, the map shows that the
probable number of thunderstorm days per year in any given
location is less than 25, and therefore condition AQ1 applies.
BS 7671 and the AQ criteria
method
> 14
12 to 14
10 to 12
8 to 10
6 to 8
4 to 6
< 4
Average value (days)
Average number of days of
thunder during the whole
year (1971 to 2000)
Map courtesyof theMetOffice