Earthing plays a vital
role in all electrical systems. The correctly designed and
installed earthing system i.e. earthing rods or grounding rods
will safeguard both lives & equipment. A Good Earth
Connection should have :
- Low electrical resistance to earth.
- Good Corrosion resistance.
- Ability to carry high currents repeatedly.
- A reliable life of at least 30 years.
Soil Conditions
Achieving a good earth will depend on local soil conditions. A
low soil resistivity is the main Aim. The factor affecting the
resistivity are :
- Moisture content of the soil
- Chemical composition of the soil e.g. Salt content.
- Temperature of the soil.
The moisture content in the soil decreases the resistivity of
the soil. Similarly, increase in the salt content reduces the
resistivity, while reduction in temperature of the soil
increases the resistivity.
Selection of the Correct Earth Electrodes
A soil resistivity survey indicates lower resisitivity at
greater depths. Often these levels are some meters below the
surface. The most economical way of reaching them is by Deep
Driven Earth Rod Electrodes. When deep driving is not possible
mainly due to rocks, underneath, parallel driven shorter rods,
plates, mates or buried conductors or a combination of three can
be used.
Earth Rod Electrodes
Earth rods are commonly made from solid copper or stainless
steel with copper bonding. Copper Bonded Earthing Rods or Copper
Bonded Grounding Rods are commonly used due to strength,
corrosion resistance and comparatively low cost. Earth Rod
Copper Bonded Earth Rod System
This
system includes rod along with accessories.
- Driving Stud : These are made from steel &
suitable for use with Power Hammers.
- Coupling : These are made from copper alloy &
counter bored to enclose threads. Coupling is high strength
& also highly corrosion resistant. These are used to
connect driving stud to rod or rod to rod.
- Rod : Steel rod with core of low carbon provides
tensile strength at least of 600 N/mm2. These rods are
molecularly bonded with 99.9 % pure electrolytic copper up
to thickness of 0.25mm, Rolled threads at each end gives
strength than cut threads.
- Clamp : These are made from high copper content
alloy strong resistance to corrosion.
Lightning Protection
BS6651:1992 clearly advices strict adherence to the provision
of conventional lighting protection system.
Design Consideration
The principle competence of a conventional lighting system in
accordance with BS 6651 are :
- Air Termination Network
- Down Conductors
- Earth Termination Network
- Bonding
Air Termination Network
On high-risk structures such as, explosives factories, nuclear
power plants, no part of roof should be more than 2.5m from an
air termination conductor. This is generally achieved by
applying a 5m x 10m mesh to the roof. 10m x 20m Mesh 60 m
For most structures, a mesh of 10m x 20m is considered
sufficient. The zone of protection offered by an air termination
network is considered to be 45 for heights up to 20m. Above this
height the zone of production is determining by the 'Rolling
Sphere Method'. 140 m
Down Conductors
The architectural design affects the location of down
conductor. These should be one down conductor for every 20m.
These should be evenly spaced & distances not mere that 20m.
They should be routed as directly as possible from the air
termination network to the earth termination network to length
of conductor forcing the loop should not exceed eight times the
width of its open side.
Earth Termination Network
Each down conductor must have a separate earth termination. The
provision should be made in each down conductor for
disconnection from the earth for testing purposes.
Bs6651 states that the resistance to earth of the complete
lightning protection system measured at any point should not
exceed 10 ohms. BS 6651 also states that the combined earth rod
length of system should be no less then 9m, whilst each
individual earth rod should be not less than 1.5m in length.
Bonding
When a lightning protection system is struck, its electrical
protection with respect to earth is significantly raised and if
resistance offered by the down conductor becomes more then the
other metalwork on the structure, the discharge seeks
alternative path to earth by side flashing.
Hence, it is vital that all the exposed metal work is bonded
into the lightning protection installation.
