WHAT CAUSES LIGHTNING STRIKE?
What causes a lightning strike? A quick refresher course is in order. Within the cloud formation, strong updrafts and downdrafts generate high electrical charges. When the voltage reaches a sufficiently high level both cloud to cloud and ground discharges occur.
Strikes occur when the ground is at positive polarity and the cloud negative region attempts to equalize with ground. Alternatively a strike can occur when the positively charged cloud top equalizes with the negative ground, or when the positive charged ground equalizes with the negative charge cloud or the negatively charged ground equalizes with the positive charged cloud top.
Lightning consists of a number of components which form a multidirectional flow of charges. Peak currents can exceed 200,000 amperes (200kA) at over 30,000°C for a matter of milliseconds (25-100 mS). The positively charged ions rise to the cloud top, and the negative ions migrate to the cloud base. Regions of positive charged ions also form at the cloud base. Eventually the cloud charge levels have sufficient potential difference between ground or another cloud to discharge. The processes are as follows:
The Leader The leader consists is a negative stream of electrons consisting of many small forks or fingers that follow and break down the air paths offering the least resistance. The charge follows the fork finding the easiest path as each successive layer is broken down and charged to the same polarity as the cloud charge.
Upward Positive Leader This is a positive charge that rises some 50 meters above the ground. (sometimes from your mast head)
Channel When the two meet a channel is formed.
Return Stroke This path is generally much brighter and more powerful than the leader, and travels upwards to the cloud partially equalizing the potential difference between ground and cloud. (often directly through your vessel)
Dart Leader In a matter of milliseconds after the return stroke, another downwards charge takes place following the same path as the stepped leader and return stroke, sometimes followed by multiple return strokes (multi-pulse surges).
Multi-pulse Surges These occur in over 70% of strikes. This phenomenon is where up to 20 re-strikes follow the initial strike at intervals of around 10-200 milliseconds apart. In addition continuing currents of 200-500 amps with durations of up to 1-2 seconds may also occur. The movements happen so fast that it appears to be a single event. This sequence can continue until the differential between cloud and ground has been equalized.
St Elmo’s Fire or Brush Discharge is a lightning phenomenon that is more common on steel vessels and when it occurs usually precedes a strike, although the effect does not occur all the time. The vessel in effect becomes a large ground mass. The discharge is characterized by ionized clouds and balls of white or green flashing light that polarizes at vessel extremities.
The discharge of negative ions reduces the potential intensity of a strike. Damage to electrical systems is usually induced into external wiring, as the steel hull itself acts as a large Faraday cage. Lightning strike protection starts here!
Electromagnetic Pulse is also a major effect. A vessel can have damaged equipment from a strike within a few hundred meters. Insurance companies do not like to accept claims on damage unless you can show total damage to external masthead or other strike damaged systems. A strike sends out a very large electromagnetic pulse, which is a strong magnetic field.
This field is induced into wiring and systems as a high voltage spike, doing just as much damage. If you suspect damage from an induced electromagnetic pulse from a localized lightning strike, check with all vessels adjacent to yours, and get statements to support the contention. Generally all the electronics will be out if this is the case as any external wiring acts as a large aerial. Lightning strike protection starts here!
Do you want more lightning protection and related boat electrical systems, such as corrosion? This article was extracted from The Marine Electrical & Electronics Bible and this is available through West Marine and Amazon.com Lightning strike protection starts here!