Lightning can be considered as the mother of all electric arcs; a tremendously powerful electric discharge between two environments with opposite electrostatic charges. It is a spontaneous event, aiming to restore the equilibrium between the earth and sky. Despite popular belief, all lightning strikes do not necessarily occur between the clouds and the ground, cloud to ground; a large amount of the electrical discharges occur between clouds themselves, cloud to cloud, and sometimes even from the ground towards the clouds, ground to cloud. According to Environment Canada, an average of 2.35 million lightning strikes touch the earth each year in Canada and the highest frequency is in July. According to NFPA (National Fire Protection Association), approximately 24,600 fires are caused by lightning in the United States each year.
The electric potential difference between two environments can reach 100 million volts and the intensity of the current generated varies from 50 kiloamperes to 200 kiloamperes for most lightning strikes. Strikes usually spread by the least resistive path. When a strike initiates, a less intense tracer moves in front of the main arc with an average speed of 200 km/h. The tracer thus creates an ionized path in the air, which is afterward followed by the lightning strike. The temperature of the electrical arc can reach 30,000 degrees Celsius, which is higher than the temperature on the surface of the sun (the temperature of the photosphere is somewhere around 10,000 degrees Celsius).
When lightning hits the ground, in most cases it will be on an elevated point, such as the top of a mountain, a high-rise structure, a component of an electric distribution network, etc.
In a spectacular fashion, those lightning strikes can also cause severe damage. The consequences are various, ranging from the simple damage of an electronic device to a major fire.
Electronic devices are extremely sensitive to electric and magnetic disturbances created by lightning. Thus, they may sustain damage, leaving them non-functional or even cause intermittent problems. The electric circuits of those devices usually operate at low voltage. Yet, the difference of potential generated by lightning is hundreds of thousands of times higher than the voltage for which those devices are designed. This difference leaves the low voltage electrical devices highly vulnerable to the high voltage lightning. We can compare this phenomenon to a water pipe network that would burst under huge pressure.
When lightning directly strikes on an electric distribution network, it inevitably generates devastating consequences. First, network power distribution equipment can be damaged. Some of you may have already seen bluish lightning, characteristic of transformer failure. Most of the regular protection mechanisms will not be able to withstand the power generated by the electrical discharge of a lightning strike. Remember that the electrical arc has often already traveled many meters, or even kilometers; thus, it can easily continue its path through the protection devices.
For most of us, the problems become realistic when lightning strikes our home. In addition to causing damage to electrical devices, as mentioned above, this situation creates a real risk of fire. Usually, the power generated by an electrostatic discharge will find its way to the ground through the electrical wiring network installed inside the walls of residences. Consequently, those conductors, which are not designed for such high current, will overheat almost instantaneously and very severely. In some cases, the copper of the conductors can even vaporize! The risk of fire then comes from the contact between combustibles and conductors or melted copper particles.
It is not always easy to detect signs of a lightning strike during a fire investigation, but some are sometimes clear, such as melting traces on gutters and chimneys or severe electrical damage. Some companies provide detection services offering the possibility to precisely determine the moment, the position and the power of each event of lightning that strikes the ground. Those tools then can confirm, with reliable scientific data, the observations made during a site investigation. The engineer conducting the investigation can then confirm or eliminate the possibility that the fire was caused by lightning.