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BS 7671 & the AQ criteria method

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.
Where this is the case and for installations being supplied by overhead lines, Regulation 443.2.2 indicates that provided the impulse withstand voltage of the equipment is not less than the values given in Table 44.3 (see Table 1 for installations rated at 230 V to Earth), no additional protection by a SPD is required. However, where higher levels of equipment reliability or higher risks (e.g. fire) are expected, additional protection by an SPD against over-voltage may be required. Similarly, for an installation having overhead lines, no additional protection against overvoltages is required if the equipment meets the minimum voltage withstand values in table 44.3. There are some words of caution in the notes to this section where it is recognised that transient over-voltages transmitted by the supply distribution system are not significantly attenuated. So an induced voltage some distance away could easily manifest itself at the electrical installation and cause potential harm to the equipment within. It is also worth considering that the AQ data is for thunderstorm days NOT lightning strikes. One storm will usually contain many lightning
flashes which could lead to an over-voltage on the installation causing damage to equipment.
Impulse withstand categoryExample of equipment in categor (note 1)Required minimim impulse withstand voltage (note 2)
I
(low impulse voltage)
Sensitive electronic equipment connected to the fixed installation.
1.5 kV
I
(low impulse voltage)
Domestic appliances and portable power tools connected to the fixed installation.
2.5 kV
III
(high impulse voltage)
Equipment intended to be installed in a part of the fixed installation where a high degree of availability of overvoltages is expected, such as distribution boards, circuit-breakers and wiring systems.
4.0 kV
IV
(very high impulse
voltage)
Equipment intended to be installed at or near the intake to the installation, such as the energy meter.
6.0 kV
Table 1: Required minimum impulse withstand voltage for equipment where installation rated voltage is 230V to Earth (based on tables 44.3 44.4 of BS7671)

Notes:
1. Table 44.4 of BS 7671 gives a fuller list of examples of equipment falling into each category.
2. This table applies only for installations of rated voltage (Uo) 230V. For installations of other rated voltages, see Table 44.3 of BS 7671.