Thunderstorm electrical hazards – threats and mitigation
16 June 2016
Thunderstorms bring a number of distinct electrical hazards to manufacturing and storage facilities and not just the most obvious – lightning strike. In this article, Dr Alec Bennett of Biral looks at these hazards and suggests ways to protect plant and personnel from the risks present when a cumulonimbus cloud is lurking nearby.
Flammable gas, liquids and fine powders such as flour burn very rapidly when suspended in air, causing explosions if they come into contact with a source of ignition. Whilst obvious ignition sources such as flames or heating elements are straightforward considerations, other potential sources such as electrical sparks require more careful planning, especially when the electricity is generated by processes such as the movement of the product itself.
Sparks from static electricity produced by material transportation and storage is a recognised hazard, so is considered when choosing materials for use with potentially flammable material, along with good electrical grounding. There is however one major source of ignition outside of industrial control – thunderstorms.
Thunderstorms bring with them two significant electrical hazards; the first and most obvious is lightning. Whilst the typical lightning flash only lasts a fifth of a second, it can produce currents exceeding 100,000 amps, heating its surroundings to over 30,000°C. Even if the lightning does not strike a site directly, nearby flashes pose a threat by inducing strong electrical surges through the ground and electrical wiring, damaging unprotected equipment and causing electrical sparks.
The threat to personnel at the site must also be considered during thunderstorms, especially those working outside or amongst tall objects.
The second electrical hazard exists even without lightning; the strong electric field of an overhead thunderstorm. When a thunderstorm cloud is nearby it increases the strength of the atmospheric electric field near the ground, causing the air to break down around tall, grounded objects such as the tops of chimneys, masts or the edges of metal storage silos.
This electrical breakdown is called corona discharge, and can even be visible as a weak blue-green or purple glow when seen on dark nights. Whilst usually too weak to pose an ignition threat in itself, the sparks which arc between objects energised by corona can be significant, increasing the threat of ignition even before the first lightning flash is produced.
The occurrence of thunderstorms varies with both geographical location and season. They can be a daily occurrence in the tropics, with about one per month in north-west Europe.
Local thunderstorms are typically much more frequent than you might expect, since by the time thunder is heard the storm is less than a few miles away and the threat of lightning is already present. Most thunderstorms are undetected by workers on site, just outside audible range of the thunder. Whilst no one can prevent thunderstorms rolling over their site, their arrival can be detected up to an hour in advance by warning systems located at the site itself.
Like all operational changes, sufficient time is required to re-schedule on-site operations. The threat posed by lightning requires a warning system capable of continuously monitoring the atmosphere, scanning for distant lightning activity or possible nearby thunderstorm development. Sensitivity to all forms of lightning, including weak intra-cloud activity is of particular benefit, since although these flashes do not strike the ground, they can be the first indicators of imminent ground strikes in the local area.
However, using a thunderstorm warning system based only on lightning detection has a significant disadvantage if the first lightning flash strikes the site! It is therefore important that a warning system is also sensitive to other indicators of a developing storm in addition to lightning, such as the increase in atmospheric electric field.
Whilst such indicators provide a shorter warning time than distant lightning from an already developed storm, even a few minutes early warning is sufficient to advise those working outside to seek shelter or delay a spark-sensitive procedure.
Whilst thunderstorm locations can be identified using data supplied by third-party lightning detection networks, this relies on continuous communication links which are not always practical for remote sites or might suffer outages, especially during thunderstorms!
Such data requires ongoing subscription costs and cannot warn until the first lightning flash is detected and details are transmitted to the site, which may be too late.
One effective alternative is a standalone thunderstorm warning system, which does not have subscription charges or communication links to third-parties. In the past, standalone lightning detectors had a poor reputation because of high false alarm rates caused by the detection of local electrical sparks and arcs.
This has now changed. The latest systems using quasi-electrostatic technology and operate at frequencies well below the sources of man-made radio interference common at industrial units, which much reduce false alarms and increase sensitivity to lightning.
These systems are capable of providing warnings from both lightning and non-lightning sources, sampling the atmospheric electric field one hundred times per second and able to detect lightning up to 50 miles (80 km) away. They then trigger alerts if the conditions indicate an increased risk of impending overhead thunderstorm activity.
This article has concentrated on electrical hazards, but there are many others associated with thunderstorms, including flash flooding, hail and strong winds. With the latest generation of thunderstorm detection systems, plant operators now have a reliable and cost-effective way to forecast thunderstorm activity early, enabling them to take steps to reduce risk in a timely manner.
About the author
Dr Alec Bennett (PhD MInstP FRMetS CMet) is responsible for the development of meteorological sensors at Bristol Industrial & Research Associates Ltd (Biral). He is a Visiting Research Fellow at the University of Bath and an active researcher, keeping up to date with the latest advances in applied meteorology.
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