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Technical reference

IEC/BS EN 62305-3 - Physical damage to structures & life hazard

The main body of this part of the standard gives guidance on

the design of an external Lightning Protection System (LPS),

internal LPS and maintenance and inspection programmes.

Lightning Protection System (LPS)

IEC/BS EN 62305-1 has defined four Lightning Protection

Levels (LPLs) based on probable minimum and maximum

lightning currents. These LPLs equate directly to classes of

Lightning Protection System (LPS).

The correlation between the four levels of LPL and LPS is

identified in Table 7. In essence, the greater the LPL, the

higher class of LPS is required.

External LPS design considerations

The lightning protection designer must initially

consider the thermal and explosive effects caused at

the point of a lightning strike and the consequences to

the structure under consideration. Depending upon

the consequences the designer may choose either of

the following types of external LPS:

–– Isolated

–– Non-isolated

External LPS design considerations

An Isolated LPS is typically chosen when the structure is

constructed of combustible materials or presents a risk

of explosion.

Conversely a non-isolated system may be fitted where

no such danger exists.

An external LPS consists of:

–– Air termination system

–– Down conductor system

–– Earth termination system

These individual elements of an LPS should be connected

together using appropriate lightning protection components

(LPC) complying (in the case of BS EN 62305) with

IEC/BS EN 62561 series. This will ensure that in the event

of a lightning current discharge to the structure, the

correct design and choice of components will minimize

any potential damage.

Air termination system

The role of an air termination system is to capture the

lightning discharge current and dissipate it harmlessly to

earth via the down conductor and earth termination system.

Therefore it is important to use a correctly designed air

termination system.

IEC/BS EN 62305-3 advocates the following, in any

combination, for the design of the air termination:

–– Air rods (or finials) whether they are free-standing masts or

linked with conductors to form a mesh on the roof

–– Catenary (or suspended) conductors, whether they are

supported by free-standing masts or linked with

conductors to form a mesh on the roof

–– Meshed conductor network that may lie in direct contact

with the roof or be suspended above it (in the event that it

is of paramount importance that the roof is not exposed to

a direct lightning discharge)

The standard makes it quite clear that all types of air

termination systems that are used shall meet the

positioning requirements laid down in the body of the

standard. It highlights that the air termination

components should be installed on corners, exposed

points and edges of the structure.

The three basic methods recommended for

determining the position of the air termination

systems are:

–– The rolling sphere method

–– The protective angle method

–– The mesh method

These methods are detailed over the following pages.

Table 7:

Relation between Lightning Protection Level (LPL) and Class of LPS

(IEC/BS EN 62305-3 Table 1)

LPL

Class of LPS

III

IEC/BS EN 62305-3. This part of the suite of standards deals with protection

measures in and around a structure.