Principles of Lightning and Surge Protection
By Sulabha Kulkarni | Submitted On September 26, 2017
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.
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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.
Surge Protection - How To Protect You and Your Expensive Equipment From Power Surges
By Trevor Crunkhorn | Submitted On April 16, 2014
With the high level of sensitivity, today's technology is very susceptible to power surges causing them to become a very real problem. The origin of these surges can be from a number of different events ranging from lightning to a car crashing into a power pole. Lightning however is the most devastating as it can deliver a surge, which can literally blow your equipment apart. Lightning surges can range from a few Amps to over 300,000 Amps. When lightning strikes, there are two events which will cause surges.
This is where Lightning directly strikes a structure or cable feeding into that structure with the resulting energy finding its way to earth through the structure or equipment contained within. Unfortunately with a direct strike, both the direct effect and the indirect effect will occur.
The effects of an indirect lightning strike can be more devastating as it may not be as obvious as a direct strike to the structure. This is where lightning will strike an object and the resulting EMF induces into all cable within a certain distance from the injection point. The induced power surges travel down the cable until they reach an earth. This can be through sensitive electronic equipment. It is important to remember that it does not have to be a direct strike to cause a major problem!
Many power surges are caused by man, with industrial switching accounting for approximately 60% of all surges and transients. Lightning accounts for approximately 20% of power surges and is by far the most spectacular.
Man made events include:
• Substation faults,
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• Internal "plant' equipment failure causing surges to other areas of the factory,
• Inductive loads such as the starting of motors, generators and compressors. This has the potential to cause catastrophic PLC failure and transformer burnout.
• Being located at the end of the grid which opens you up to large voltage variances, over and under voltage situations and general interference.
These include all weather related events, earthquakes and tremors etc.
These are those events with no real known cause.
All surges and transients have access to your equipment via the following:
• Power Lines - Main power feed to the structure
• Data/Signal Lines - Any RS232/422/485, 4-20mA loop, CAT5 or 6 cables.
• Telecommunication Lines - Phone lines, CCTV, MATV, Satellite, etc.
• Poor earthing.
So how do we protect our expensive equipment?
The common advice given in the past was to disconnect your equipment from the power point when thunder is heard. This advice is still good if you are at home when a thunderstorm appears, but it will only protect your equipment from 20% of the damaging surges that are out there. Therefore, while this advice is still good and is still being dispensed today, is no longer feasible as much of our technology is hard-wired. C-Bus & CCTV are good examples as you can not simply disconnect them when you hear thunder. Fortunately there are very effective solutions available today which will give surge protection for all equipment without having to disconnect it.
There are two main types of surge protection available.
• Surge Diverters
• Surge Filters
Surge Diverters: Also known as a shunt or parallel connected protector.
The Surge Diverter is the basic form of protection designed to divert to earth a large portion of the energy from the incoming surge. Diverters can be mounted on the main switchboard or at distribution boards.
Surge Filters: Also known as a series connected protector.
This is where you have to cut the incoming cable and wire it through the filter. Due to this, the filter is rated to reflect the load rating of the equipment, if it is being installed on a power circuit. Earlier we explained that a diverter removes the energy from the incoming surge, well were the diverter leaves off, the filter takes up and filters even more of the surge out of the incoming sine wave or signal. Surge filters should always be used where sensitive electronics are to be protected.
It is always good practice to place the surge protection as close as possible to the equipment being protected.