Surge Protec Blog
Principles of Lightning and Surge
Protection
Lightning strokes, which amount to 200 kA or 300 kV, cause hazards to the equipment or location, so
lightning protection is crucial for operation.
Let us start with the initial note as to what is lightning and why lightning protection is so important. The
basic phenomenon behind lightning is that charges accumulated from the cloud and the earth are equal
and opposite. This forms a non-uniform potential gradient surface in the air. When the gradient is
greater than the potential of the surface, the breakdown occurs and a “streamer” flows from the cloud
towards the earth.
A direct stroke occurs when the lightning hits the power systems directly that the immense potential will
cause destruction of the equipment or the facility. In contrast, an indirect stroke occurs from the
lightning discharges in the proximity of the power line or from electrostatic discharge on the conductor
due to the charged clouds.
The main power system elements requiring lightning protection are power feeds, security systems,
telephone lines, data and control systems and RF cables.
Methods of Lightning Protection
The rolling sphere method is used for identifying the exact placement of the lightning and surge
protection devices near the equipment under operation.
Protection of the power line against direct strokes is through a ground wire or protector tube. The
former produces electrostatic screening, which is affected by the capacitances of the cloud to line and
the line to ground. The latter forms an arc between the electrodes, causing gas deionisation.
Rooftop/Frame Protection
It is interesting to note that the building and rooftop frame or cladding is preferably metal than
insulation type.
Installation of a finial at the top of the power tower should have a minimum distance of 1.5 m above the
highest antenna or lights. Such a rooftop or building frame is made of reinforced steel for protection
purpose.
Wooden towers without downconductors may cause a fire hazard, as they route the incoming charges to
ground. In principal, for non-metallic roofs, proper downconductors should be installed at the
appropriate location and height.
Device Protection
Antenna lightning protection is provided through spark gap, the gas discharge tube and quad-wavelength
shorted stub. The first method uses ball points so that if a strike occurs, high potential forms between
them and the ground. The second method causes gas deionisation through arc formation between the
electrodes. The last method uses a coax transmission line across the transmission line so that system
bandwidth is narrow.
A lightning arrester is a device offering lightning protection by regulating spark gaps.The device
classification may range from rod gap, horn gap and valve type to metal oxide lightning arresters.
Earthing and Bonding Solutions
Now let us discuss how earthing and bonding solutions for lightning protection should be afforded. The
design of earth rods, terminals or clamps should be in a way to route the incoming transients to earth to
minimise step and touch potentials. The geometric measurements chosen should comply with the IEEE
and NFPA standards. Any earthing system should have proper bonding, as ground potential rise cannot
be compensated. Again, the number of interconnects and spacing should be designed per the lightning
standards.
Surge Protection
The device ideal for protection against travelling waves is a surge diverter, connected between line and
earth at the substation. Its purpose is to divert the excessive incoming voltage to ground by developing
low impedance between the line and earth. Surge protection is essential as the overvoltage may damage
the lightning protection devices and others across the line. Surge measurement can be performed based
on the Faraday principle or remote monitoring with sensors.
If there are overvoltage devices, they are placed between surge arresters or diverters and the control
equipment.
Surge protection for telephone cables is through a setup of a gas arrester, metal oxide varistors and
suppressor diodes.
The primary protection is for the mains on which surge diverters need to be installed. The secondary
protection is for the lines, field apparatus and outdoor devices (CCTVs, security devices or towers). In
addition, secondary stage protection can be provided with rod gap arresters and the like.
Concerning lightning and surge protection, it is true that the engineers will implement the
aforementioned solutions.
Please use the following links for more information on the principles of lightning protection:
https://books.google.co.in/books?id=Cr6KrKUJLesC&printsec=frontcover#v=onepage&q&f=false
https://www.dehninternational.com/sites/default/files/uploads/dehn/pdf/blitzplaner/bpl2015/lpg_2015_e_complete.pdf
Tags: lightning stroke, direct stroke, surge protection.