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Guide to
| Surge Protection Devices
9
BS 7671 section 534 gives the requirements for correct
selection of devices against overvoltages.
Regulation 534.2.1 prescribes that where required by Section
443 or otherwise specified, SPD’s shall be installed:
(i)
near the origin of an installation, or
(ii)
in the main distribution assembly nearest the origin
of an installation
The notes to this regulation give further guidance, stating that
a Type 1 or a Type 2 SPD may be used at the origin whilst
Type 2 and Type 3 are also suited for locations close to the
protected equipment.
Type 1 SPD’s are often referred to as equipotential bonding
SPD’s and are fitted at the origin. A lightning protection
system employing these devices only, offer no effective
protection against failure of sensitive electrical and electronic
systems. In order to achieve this, additional coordinated
devices will have to be employed.
In summary, a Type 1 SPD is used at the origin of the
installation, a Type 2 SPD is used at distribution boards and a
Type 3 SPD is used near terminal equipment.
Surge protection needs to be selected such that their voltage
protection level (U
p
) is lower than the impulse withstand
capability of the equipment to be protected. 534.2.3.1.1
suggests that this value should be referred to category II of
Table 44.3. This for a 230/400V installation suggests that
the value should not exceed 2.5kV. However 534.2.3.1.2
suggests that to protect sensitive and critical equipment, then
consideration should be given to reduce this value to that
required for category 1 equipment (ie 1.5kV).
Selection of suitable devices