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Guide to BS EN/IEC 62305
BS EN/IEC 62305-3
This part of the suite of standards deals with
protection measures in and around a structure
and as such relates directly to the major part of
BS 6651.
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)
BS EN/IEC 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.
The class of LPS to be installed is governed by the
result of the risk assessment calculation highlighted
in BS EN/IEC 62305-2.
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
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 BS EN 50164 series (note this BS EN
series is due to be superceded by the BS EN/IEC 62561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 vitally important to use a
correctly designed air termination system.
BS EN/IEC 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.
BS EN/IEC 62305-3 Physical damage
to structures and life hazard
LPL
Class of LPS
I
I
II
II
III
III
IV
IV
Table 7: Relation between Lightning Protection Level (LPL)
and Class of LPS (BS EN/IEC 62305-3 Table 1)