Surge protection selection has always been covered in depth within the current UK lightning protection standard BS EN 62305-4. However, as discussed here by DEHN, the latest amendment of the BS 7671 17th Edition Wiring Regulations has recently increased awareness of surge protection devices (SPDs)

Following a risk assessment, the subsequent selection of the correct SPDs need not be a complicated task providing basic requirements are met. If the manufacturer can provide SPDs that already meet these requirements then this gives reassurance to the installer or designer that not only do the SPDs meet the required standards but also offer optimum protection for the installation.

Here it is important to consider some of the criteria in the correct selection of SPDs such as ensuring that the correct Type of SPD, its kA rating, voltage protection level and energy coordination are all selected.


SPD type selection

There are three types of SPDs as defined in BS EN 62305 and EN 61643-11. Additionally, combined SPDs have the distinct advantage of offering both lightning protection and surge protection in one SPD.

Type 1 SPDs are designed to carry partial lightning currents. These Type 1 SPDs must be installed on incoming services (e.g. mains power and data) when an external lightning protection system (LPS) is installed on a building. Where partial lightning currents can also flow due to a direct lightning strike on overhead supplies or externally mounted equipment, Type 1 SPDs should also be used.

Type 2 and Type 3 SPDs are designed to discharge the induced surges created by lightning electromagnetic fields and also other transient surge events such as switching surges, supply faults etc.

For many applications where the requirement is to provide surge protection only, Type 2 and Type 3 SPDs play a critical role in protecting electronic equipment.


kA rating

Once the correct Type of SPD has been established, one of the next critical considerations in the SPD selection process is its kA rating. The kA rating being the maximum amount of current the SPD is designed to discharge and is extremely important when dealing with Type 1 SPDs that have to handle lightning currents.

When an external lightning protection system is installed on a structure it must, as a minimum, also have Type 1 lightning current arrestors installed on the incoming services.

If a risk assessment has been carried out to BS EN 62305, the corresponding lightning protection level (LPL) of the structure will be known and from this the required kA rating and Iimp values can be found.

However, in many cases a risk assessment may not have been carried out on the structure and these important values will not be known. According to BS 7671, ‘Where Type 1 SPDs are required, the value of Iimp shall not be less than 12.5kA for each mode of protection, if Imp cannot be calculated.’

Selecting an SPD with a total kA rating of 100kA such as the DEHNventil will ensure protection to all lightning protection levels even without having to carry out a risk assessment or Iimp calculation.


Voltage protection level (up)

The voltage protection level of an SPD is the maximum instantaneous voltage value on the terminals of the SPD when using standardised tests as defined in EN 61643-11.

When used in power supply systems, the voltage protection level of SPDs should be matched to the impulse withstand voltage (Uw) of the equipment within the installation. There are four categories defined in IEC 60664-1 and BS 7671.

Category IV: (6kV) – equipment used at the origin of the installation, e.g. meter.

Category III: (4kV) – equipment part of the fixed electrical installation, e.g. distribution boards.

Category II: (2.5kV) – equipment connected to the fixed electrical installation, e.g. appliances.

Category I: (1.5kV) – sensitive electrical equipment.

BS 7671 states that SPDs should be selected in accordance with Category II, however, selecting SPDs with a lower voltage protection level suitable for Category I (1.5kV) equipment ensures compatibility with all the above categories.


Coordination of SPDs

Energy coordination of SPDs within an electrical installation must be achieved in order to ensure correct operation of SPDs and protection of the installation. The basic principle of energy coordination is summarised by the fact that each SPD must only discharge the amount of energy that it is designed for.

If there is no energy coordination between SPDs, it is possible that Type 2 SPDs activate before the Type 1 SPDs during a surge event. If this is due to lightning activity, the Type 2 SPD may be damaged and the installation will be at significant risk of damage due to dangerous sparking, fire etc.

BS EN 62305-4: ‘the manufacturer of the SPDs shall prove that coordination is achieved.’

BS 7671: ‘SPDs shall be selected and erected as to ensure coordination in operation.’

SPDs in the DEHN Red/Line family are energy coordinated, ensuring compatibility and avoiding the need for additional testing, decoupling coils or minimum distances between SPDs.


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