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Technical reference
IEC/BS EN 62305-3 - Physical damage to structures & life hazard
Earth termination system
The earth termination system is vital for the dispersion of
lightning current safely and effectively into the ground.
The standard recommends a single integrated earth
termination system for a structure, combining lightning
protection, power and telecommunication systems. The
agreement of the operating authority or owner of the
relevant systems should be obtained prior to any bonding
taking place.
A good earth connection should possess the following
characteristics:
–– Low electrical resistance between the electrode and the
earth. The lower the earth electrode resistance the more
likely the lightning current will choose to flow down that
path in preference to any other, allowing the current to be
conducted safely to and dissipated in the earth
–– Good corrosion resistance. The choice of material for
the earth electrode and its connections is of vital
importance. It will be buried in soil for many years so has to
be totally dependable
The standard advocates a low earthing resistance requirement
and points out that the earthing system should have an
overall resistance to earth path of 10 Ohms or less. Three
basic earth electrode arrangements are used:
–– Type A arrangement
–– Type B arrangement
–– Foundation earth electrodes
Type A arrangement
This consists of horizontal or vertical earth electrodes,
connected to each down conductor fixed on the outside of
the structure.
Type B arrangement
This arrangement is essentially a fully connected ring earth
electrode that is sited around the periphery of the structure
and is in contact with the surrounding soil for a minimum
80% of its total length (i.e. 20% of its overall length may be
housed in say the basement of the structure and not in direct
contact with the earth).
Foundation earth electrodes
This is essentially a type B earthing arrangement. It comprises
conductors that are installed in the concrete foundation of
the structure. If any additional lengths of electrodes are
required they need to meet the same criteria as those for type
B arrangement. Foundation earth electrodes can be used to
augment the steel reinforcing foundation mesh.
Separation (isolation) distance of the external LPS
A separation distance (i.e. the electrical insulation) between
the external LPS and the structural metal parts is essentially
required. This will minimize any chance of partial lightning
current being introduced internally in the structure.
This can be achieved by placing lightning conductors
sufficiently far away from any conductive parts that have
routes leading into the structure. So, if the lightning discharge
strikes the lightning conductor, it cannot ‘bridge the gap’ and
flash over to the adjacent metalwork.