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Guide to
| Surge Protection Devices
7
Impulse withstand
category
Example of equipment in category (note 1)
Required minimum impulse
withstand voltage (note 2)
I
(low impulse voltage)
Sensitive electronic equipment connected to the fixed installation.
1.5 kV
II
(normal 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 (U o ) 230V. For installations of other rated voltages, see Table 44.3 of BS 7671.