Lightning is a natural phenomenon which takes place due to discharge of heavy electrical charges between clouds and the earth ( induced charges due to the charges accumulated on the lower surface of the cloud) . The discharge of energy, when oppositely charged ions come sufficiently closer to each other , takes place in the form of lightning strike which carries huge amount of current even up to 200 KA or more. In electrical terms lightning is defined as a wave shape, with rise time of 10 μs and time to fall to half of the peak is 350 μs (known as 10/350 μs wave). Hence in one side the lightning carries very high KA amplitude of current causing enormously huge I²R (heat energy) in the conductor ( e.g. Building, Tree etc ) and at the same time creates huge induced electromagnetic field ( directly proportional to di/dt ) due to very high rate of change of current ( di/dt). In addition, switching surges ( 8/20 μs) are being generated more frequently due to switching ON/OFF functions of machinery during routine plant operation, welding, overload conditions, short circuits etc. Normally tall metal structures e.g Oil rigging platforms, Telephone / Microwave towers, Windmills,Tanks, Chimney, Distillation columns etc are more prone to direct lightning strikes.
Apart from direct lightning strike, there are huge impact of distant/indirect lightning. There are following two major effects which develops surge threats to electrical equipments.
When a lightning strikes a huge current is passing through the associated soil or earth pit. Even if a very moderate level of lightning current (e.g. 100KA) is passing through a very good earth pit (1 Ohm) , it will raise the ground potential around the earth pit to a level close to 100KA x 1 Ohm= 100 kV due to Ohm's law effect. Ground potential will get reduced gradually as we measure it for more and more distance from the point of strike. This will create a ground potential imbalance within a distributed area industrial plant scenario. If any metal wire or pipe is connected between lbsûsi
two physically distant locations ( pls refer example below) then a surge current will flow due to this ground potential difference between the two locations.
Electromagnetic induction – (EMI)
Due to very high steepness (10/350 μs) of current and extremely high KA amplitude of current flow, very high induced surges are generated during lightning. These surges gets coupled through induction on power and data cables connected to all equipments placed in and around the point of lightning strike.
In general, any industrial plant, houses number of sophisticated motors, generators, electrical and electronic devices, PLC & DCS and field mounted electronic sensors / transmitters, Weigh bridge ,CCTV system etc. Protection of these equipments are most important for uninterrupted operation (production) of the plant.
The latest standard applicable now in India is IS / IEC 62305, having four chapters to discuss different aspects of Lightning protection very thoroughly. Four Lightning protection levels (LPL) has been defined in the standard. Three methods [ Rolling sphere, Angle of protection, Mesh ] of designing of Lightning arrester system has been recommended in the standard. In the light of IS/IEC 62305 standard, National building code (NBC 2016) has been upgraded in 2017 with various important changes e.g. need of risk assessment, non-approval of Early Streamer Emission (ESE) terminals, usage of Surge Protection Devices (SPD) for power supply line and communication line, maintaining calculated separated distance for the down conductor etc.
Risk analysis as per chapter 2 helps us to decide about the required level of lightning protection for a particular application. If the determined risk is higher than the acceptable values specified in the standard, measures like external lightning arrester system, surge protection, equipotential bonding , earthing and fire protection arrangements need to be considered to bring down the existing risk level to acceptable level. In case of explosive environment industry, the necessity of lightning protection system need to be determined on the basis of the risk analysis according to the operational safety regulations IEC 60079 sub-clause 16.3, OISD-STD-244 guideline for handling petroleum products at Refineries, Oil depots/terminals and fire protection measure as per NFPA70. If surge protection devices are installed inside of explosive zones, they have to be certified appropriately that any chance of potential ignition is avoided.
The detail guideline for protection of building & structures against direct lightning strike has been discussed in part 3 of the IS/IEC 62305 standard and lightning arrester system needs to be carefully installed as per the guideline of the standard. Special care needs to be taken to avoid any uncontrolled flashover between down conductor and other metal part of the building/equipment. Either sufficient separation distance ( as per calculations provided in IEC 62305-3) between the down conductor and any other metal body / cable needs to be maintained or specially designed High Voltage Insulated cable has be used as down conductor to avoid such flashover issue during lightning .
Adequate measure needs to taken with respect to expected Touch Potential and Step Potential too during lightning, as these parameters are directly related to human safety within the plant . As earth is the final and only dumping place for the lightning current , we need to have very good earthing system ( Resistance as low as possible ) and equipotential bonding between all earth pits and grids. In case , permanent bonding between two earth grids ( electrical earth and instrumentation system earth) is not possible due to operational constraints, then Isolating Spark gap device needs to be used for temporary equipotential bonding arrangement during lightning .
All in all, a comprehensive lightning protection system ( in line with IS/IEC 62305) appropriately designed and installed as per the calculated risk level reduces the risk of damages of the buildings, equipments and operators to an acceptable limit.