Lightning Protection Standards
Lightning is one of the most widely studied and documented natural phenomena. It is also one of the main causes of transient over-voltages in electrical systems. A proper understanding of lightning is essential for planning protection against lightning strikes so that no untoward damage is caused to buildings and electrical installations.
A lot of research has been done over a number of years worldwide and several publications as well as national and international standards have evolved which give us a good insight into this phenomenon.
Some of these are:
- AS 1768 Australian standard on lightning protection
- ANSI/NFPA 780 National lightning protection code
- IEEE 142 IEEE green book
- IEC 1024 Protection of structures against lightning.
Lightning is the sudden draining of charge built up in low-cloud systems. The flow of charge creates a steep fronted current waveform lasting for several tens of microseconds. The flow is more usually that of negative charges though at times it may involve positive charge flow too. The latter are generally of lower magnitude.
The occurrence of lightning flash starts with a buildup of charge in a cloud system close to the ground. This charge is usually of the order of several million volts and usually of negative polarity at the bottom. Though the exact mechanism of charge separation is not clear, observations indicate that the ice particles in the top portion of the cloud are positively charged whereas the heavier water particles in the bottom portion of the cloud carry a negative charge.
The high electrical field causes ionization of air and creates a conducting path. This usually happens near the cloud and the ionized path is called the downward leader. The leader precedes in steps of 20Â–30 m toward the ground, each step forming further ionization of the subsequent step. Simultaneously, from the high points or structures on the ground upward leaders of positive charges start forming. The interception of downward leaders and upward leaders completes the conducting path between the cloud and the ground and results in a lightning strike.
The lightning flash along the ionized path causes a very high peak of current amounting to several kilo amperes and dissipates its energy in the form of heat (temperatures up to 20 000Â°C for a few microseconds), sound and electromagnetic waves (light, magnetic fields and radio waves).