Lightning is an atmospheric discharge of electricity from thunder, occurs during thunderstorms. The atmospheric discharge lightning can travel at speed of light and its temperature can reach up to 30,000 degree Celsius. The effect of lightning strike may cause thousands of death every year. It is a natural phenomenon occurring at any day and this discharge produce a wide range of electromagnetic radiation.
Lightning protection unit are designed to protect the building structures, industrial appliances, commercial building, HT substation, electrical equipments, communication transmission lines etc. from damage effect of lightning strike. It play a very important role in safety from lightning storms. A lightning strike could bring thousands of mega ampere current within milliseconds. As a result, failure of home and industrial appliances, fire in building structures, home etc. and may cause death also.
Fig: Effect of lightning strike on building
Fig: Building with lightning protection unit
As we can relate with both of the figure above , effect of lightning on building without and with light protection unit.
Components of lightning protection devices
- Air terminal
Air terminal is used to intercept lightning strike. It provide a low resistance path to lightning strike and it is connected to down conductor and down conductor is connected with earthing electrode which is directly kept into soil mud. Whenever lightning strike hits, the whole current is discharged through air terminal- which is connected to ground electrode instead of building structure.
- Down conductor
Down conductor provide a low resistive path to lightning current from air terminal to ground electrode. It should be installed vertically and straight to avoid the bending of down conductor. This shall ensure minimum inductance because of resistance in the path of discharging current and surges.
It play a very important role in discharging lightning current. It dissipates all the lightning current into a big mass of soil. The soil resistivity should be less than 10 ohm for comfortable discharge of current, according to electrical standard. A typically earth electrode of copper alloy is deep driven vertically into the soil. Soil have finit conductivity so that current discharges easily. The mixing of charcoal and salt is done in soil pit to decrease the resistivity of soil.
These devices are designed to protect the transmission line, insulations, switchgears, transformers communication line by the effect of lightning strike. They directly installed to power substation , distribution system or telecommunication line. It has ground termininal and high voltage terminal and whenever lightning travel along the power line to arrestor then current through surges diverted by arrestor to earth. In telegraphy and telecommunication, it is placed where wires enter a structure so that preventing damage to electronic instrument and ensuring safety near to them. Their purpose is basically to limit the rise in voltage when power lines are struck by lightning.
Fig: Lightning arrestors
Small version of light arrestor is called surge arrestor. That are generally used for protection from surges produced in transient condition and they are not used directly to protect from lightning. The most common surge arrestor is non linear metal oxide resistor type in porcelain or silicon rubber housing that are fitted parallel to circuit and connected to earth griding.
Types of Lightening Arrestors
- Station class
Typically used in power stations or substations and other high voltages structure and areas. It is designed to protect equipment above 20MVA range.
- Intermediate class
Designed to used in medium voltage equuipment areas, electrical substaions, transformer or other substation equipment and it is designed in range of 1 to 20 MVA.
- Distribution class
Most commonly found on transformers and are commonly used in equipments rated at 1000kva or less.
- Secondary class
Found in homes and commercial buildings and provide least amount of protection to electrical system.
All electrical equipment in an electrical system needs to be protected from voltage surges. The rating of arrestor, the class of arrestor and location of arrestor all play a important role in the surge protection. In protection of substation, we use different class of lightning arrestors to protect the electrical equipment and In ships, water is used to discharge voltage.