EXTERNAL LIGHTNING
PROTECTION SYSTEM
An external lightning protection system (LPS) is meant to intercept
direct lightning flashes to the structure and flashes to the side of the
structure.
It conducts the lightning current from the point of strike safely into the
ground.
Four different classes: I, II, III, IV, depending on LPS
materials, etc.
PROTECTION ANGLE METHOD
Wire Air-Termination System
- See it as two half-cones with a tent
- The base of these half-cones and tent
gives the protection area
EXAMPLE - PROTECTION ANGLE
METHOD
Given: LPS I,
H = 10 m,
Angle = 45 deg
You can use one single
tall rod as shown.
If not practical,
you may use several
shorter rods as shown
(though some parts
may not be protected)
MESH METHOD
Suitable where plane surfaces are to be protected.
Class of LPS Mesh Size, m
I 5x5
II 10 x 10
III 15 x 15
IV 20 x 20
ROLLING SPHERE METHOD
Roll an imaginary sphere of a given radius around an LPS. One point of
contact with the LPS and one point with the reference plane.
The area of reference plane between the base of the LPS and the base
point of the sphere is the area of protection
Suitable for all cases of LPS
Class of LPS Rolling sphere radius, m
I 20
II 30
III 45
IV 60
ROLLING SPHERE METHOD
Refer to the previous table for the radius of the imaginary sphere for a given LPS
class.
When you roll the sphere around a structure, the sphere should ideally touch only
the LPS and/or the ground.
On structures higher than the rolling sphere
radius, flashes to the sides of the structure
can occur if the structure height is above 60 m.
A lateral air-termination system should be
placed on the top 20% of the height of such
structures.
Note: ‘60m’ is the international standard.
For Singapore, as a rule of thumb, we take
48m as the upper threshold before protection
against side flashes is needed.
COMPONENTS OF AN LPS
The principal components of lightning protection system should
comprise the following:
a. Air termination networks
b. Down conductors
c. Earth termination networks
d. Bonding to prevent side flashing
EARTH TERMINATIONS
The design of the earth termination network should be such that
lightning currents are discharged into the earth in a manner that will
minimize touch and step potentials and risk of side flashing a metal in
or around a structure.
This can be achieved by ensuring that the potential with respect to the
general mass of the earth at each earth termination is limited by a
sufficiently-low resistance to earth and that the discharged current flows
uniformly in all directions away from the structure.
The whole of the interconnected lightning protection system should
have a resistance to earth not exceeding 10 Ω.
PREVENTION OF SIDE FLASHING
There are two ways to prevent side flashing:
- Isolation
- Bonding
i. Isolation
Isolation is the separation or insulation of metal parts in such a way that
electrical breakdown or side flash to them is prevented.
This can be achieved by separation of the lightning protection system from
the structure protected or by separating metal parts and services in a non-
conductive structure from the lightning system.
Obviously, this will not be practical for certain metal installation, e.g. metal
windows.
ii. Bonding
Bonding is the procedure whereby metal parts are positively connected to
one another so as to prevent inadvertent electrical connection occurring
due to side flash. This will eliminate any local potential between the metal
parts that are bonded together.
