Handbook of Electrical Installation Practice - BeKnowledge

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Lightning Protection 211
if down conductors are to be routed externally (Fig. 9.10(d)).
Down conductors must never be routed down lift shafts. If an external route is not
available, an internal continuous, non-conducting, non-flammable duct may be used.
Each external down conductor must incorporate a test clamp installed approximately
3m from the ground.This enables the continuity of the system to be checked
as well as isolating each local earth network for testing purposes.The section of down
conductor from the ground to the test clamp should have a non-metallic guard to
protect from unauthorised interference. To minimise surface voltage gradients the
down conductor should ideally be insulated for the first metre of entry into the soil.
Earth termination networks
Each down conductor should have its own local earth electrode. Copper conductor
should be used as the connection from the test clamp to the earth electrode. Copperbonded
steel-cored rods are by far the most popular form of earth electrode. These
may be driven to considerable depths. Alternatively shallower driven rods interconnected
by copper conductor can be used.
Aluminium and copper, the two metals most commonly used in lightning protection
systems, are not compatible, so great care must be taken when both are used
in a system – particularly where they come into contact with each other.
If aluminium is selected as the material for air termination networks and down
conductors, it has to be converted to copper at or around the test clamp. This is
because both BS 6651 and the Earthing Code BS 7430 do not permit aluminium to
be buried underground due to the likelihood of corrosion.
A simple and effective means of joining the aluminium and copper conductors is
with a friction-welded bi-metal clamp. This termination, if used in conjunction with
an inhibitor grease, minimises the effect of corrosion.
The contact surfaces of dissimilar metals should be kept completely dry and protected
against the ingress of moisture, otherwise corrosion will occur. A particularly
effective means of excluding moisture is to use inhibitor pastes, bitumastic paint, or
approved protective wrappings.
As aluminium is prone to corrosion when in contact with Portland cement and
mortar mixes, aluminium conductors need to be fixed away from the offending
surface with an appropriate fixing.
There are two stages in testing an earth network for satisfactory resistance:
(1) With the test link removed from the down conductor and without any bonding
to other services, etc., the earth resistance of each individual earth electrode
should be measured. The resistance, in ohms, should not exceed 10 times the
number of down conductors on the structure. For example, if there are 15 down
conductors equally spaced around a building, then the resistance of each electrode
with the test link removed should not exceed 10 ¥ 15 = 150 W.
(2) With the test links replaced, the resistance to earth of the complete lightning
protection system is measured at any point on the system.The reading from this
test should not exceed 10 W.This is still without any bonding to other services.