What is Lightning?
Lightning is a natural phenomenon of electrostatic discharge of very short duration but of great power. Lightning is the luminous phenomena while thunder is the sound manifestation.
Within certain types of clouds, an unbalance of the electrostatic discharge is then produced between different areas (i.e.: low part of the cloud and the ground). This unbalance generates a difference in the electrical potential (tension). While the tension between two distinct areas is big enough and exceeds the dielectric characteristic tension in the air, there is then an electrical discharge (electrical breakdown) which rebalances the electrostatic voltage locally.
The electrical breakdown is produced if the air is humid. It is seen in different ramifications. The electrostatic tension may reach several millions of volts, with the discharge of the typical current of tens of thousands of amps.
Some Statistics on Thunder
Although thunder strikes the planet almost continuously, all lightning does not strike the ground. There are several types of lightning. Most of the lightning (about 75%) occurs between the clouds themselves. Other lightnings (about 25%) are produced between a cloud and ground (or a structure).
Contrary to popular belief, lightning can strike the same place more than once. Indeed, in New York, the Empire State Building might be hit dozens of times during a single storm.
Around 2 000 to 5 000 thunderstorms are constantly formed on our planet. About 32 million lightning bolts would strike the ground each year. Generally, regions near the poles are less affected by lightning than tropical regions.
The average number of lightning bolts that would strike annually the Canadian ground is about 2 million.
Lightning Effects on Structures
The most known dangers caused by lightning are forest fires, as well as electrocution for living beings who are outdoors. However, structures and electrical equipment can also be affected.
The effects of lightning on structures, specifically on their electrical infrastructures, are various. For example, lightning may be at the origin of the following consequences:
• Fire in buildings (dwelling units, commercial or industrial);
• Damages in certain structural components;
• Damaging electrical equipment (transformers, electrical cables, appliances, electronics, etc.);
• Disturbance caused to computers and communication networks;
• Failures in automation software or in control systems;
• Loss of production for companies.
The effects on the structures depend on the type of impacts, current intensities, existing trajectory, and protections. The main types of impacts that may affect a structure are:
• Direct impact on the structure;
• Direct impact on aerial power line;
• Indirect impact near a power line (by induction);
• Indirect impact near a structure (ground potential gradient).
Regarding the direct impact, lightning borrows structural elements or electrical conductors to reach the ground. The abnormally elevated current flowing through these components may then provoke important physical damages, even fires, by igniting combustible material located nearby.
Figure 1: Residential electrical cable where lightning current flowed through. Some sections of the copper conductors have been sublimated by current intensity.
Moreover, potential rise during a direct or indirect lightning impact frequently exceeds the insulation of conductors (electrical and communication networks) and electrical equipment. These components were not designed to support the overvoltage generated by lightning, could be damaged permanently and provoke fires by igniting nearby combustibles.
Among the different electrical disturbances generating overvoltage, lightning is probably the cause of the most destructive overvoltage. So, to confirm or invalidate a scenario involving lightning during an expert appraisal, to find the cause of a fire or the failure of a piece of equipment, it is possible to rely on several types of information, such as:
• The analysis of the damages to the structure and to the electrical equipment;
• Lightning report: such report lists the detected electrostatic discharge in a defined area and during a given period.
Protection and Regulation
The basic idea concerning the protection against lightning is to avoid disturbing energy to reach the sensitive equipment. Therefore, it must:
• Capture and channel current of lightning diverting into the ground through the most direct path (lightning protection system) by:
– capture devices (lightning rod),
– ground conductor,
– earth electrode,
– connection between the metallic masses (equipotentiality network);
• Minimize direct and indirect effects (protection system of electrical installations) by protection devices against the effects of lightning.
Protection and regulation
In Quebec the installation of a protection system against lightning is not mandatory. Nevertheless, the Article 126.96.36.199.(1) of Chapter I, Building, of the Quebec Construction Code specifies ‟Every lightning protection system shall comply with CAN/CSA-B72-M, “Installation Code for Lightning Protection Systems”.
As per Chapter V, Electricity, Quebec Construction Code (C22.10-18), it includes provisions concerning the installation of lightning arrester and other protector devices against the effect of lightning (ex.: surge protectors), as well as requirements for the installation of equipment close to the components of lightning protection system, such as ground conductors of lightning conductors and grounded sockets dedicated to lightning protection.
Other national and international industrial standards concerning lightning protection may also be relevant, particularly in the case of partial protections.
Regulation at the Level of Electrical Distribution Network
Concerning the electricity distribution systems, the Hydro-Québec Red Book (Medium-Voltage Electrical Service – Standard E.21-12) indicates that outdoor electrical installations shall be equipped with surge arresters at the junction of air-to-air, air-to-underground and underground-to-air connections.