Lightning has already been a subject of last January newsletter. We had then discussed the development of this event and its potential impact on structures (this newsletter can be (re)read here).
We shall now be addressing two other lightning aspects:
- The detection of this event; and
- Lightning as a cause of fire.
Lightning is a phenomenon where a great power is released over a very brief period of time. The electromagnetic pulses generated by the flow of the current as lightning strikes could be detected at a distance and located with special sensors. Such sensors constitute a monitoring network in North America: The National Lightning Detection Network (which includes the Canadian Lightning Detection Network).
This network detects and provides the locations of the impacts of cloud-to-ground lightning. You may accordingly obtain a lightning report that identifies the impacts in a specific area, over a given period of time.
Before proceeding any further, we should establish that a lightning flash usually comprises several lightning strokes. An “average” lightning flash would then consist of three to five lightning strokes. The latter only last a few milliseconds, and the location of the impact with the ground can move between lightning strokes. The monitoring network’s sensors detect each one of these lightning strokes independently and one single lightning flash may accordingly be represented by several points on a lightning report.
There is a 70% chance that a lightning strike gets detected and once it is, the location of the impact to the ground is calculated and the map would then indicate the confidence ellipse for this finding. The ellipse represents the area where the impact took place, with a 99% certainty.
Figures 1 and 2—Examples of StrikeNet Lightning Reports—Highlighted in yellow are two lightning strokes of the same flash
To sum up, lightning flashes are almost all detected (at more than 95%). It is, however, possible that one of the lightning strokes, of which a lightning flash consists, goes under the radar. The recorded location for the lightning strokes that do get detected is, on the other hand, quite accurate.
Cause of Fire
Lightning is indeed an extreme weather phenomenon that could cause a fire. The damages produced by lightning and the risk of fire resulting thereof are contingent on the impact’s location:
- Direct impact on a building;
- Impact on a building’s overhead power line;
- Indirect impact in the vicinity of a building.
Upon a direct impact on a building, lightning shall strike an item placed at an elevated height before finding its way to the ground. In view of the high intensity of energy carried by the lightning stroke, most of the construction materials shall in this case conduct the current. As the current flows through them, some elements shall get heated enough to ignite or induce the ignition of adjacent combustible materials. Short-circuit electric arcs might also occur within metal components.
- While investigating, the following elements could indicate a direct impact:
- Amounts of ripped apart wood: water within the wood evaporates almost instantly as the current flows through. This expansion results in the explosion of a certain amount of wood, or even of a whole section of the wall;
- Traces of electric arcs on cables or on external metallic surfaces;
- Visible charring traces near the metallic ducts (ex: water pipes);
- Cables sublimated by the passage of current (“vaporized”) and charring traces visible nearby;
- Damaged permanent electrical equipment in the whole building.
Figures 3 to 7 — Damages to a building following a direct impact
Impact on a Power Line
Following an impact on the building’s overhead power line (or in proximity of the latter), a very intense current can pass through the cables and equipment of the building itself. Such components, not designed to absorb such amounts of energy, may overheat and get damaged. Electric arcs might also be generated. These damages might be comparable to those found when a current is injected by a medium or high-voltage line. In some cases — for example, when several intense lightning strokes take place at short intervals — also protective equipment, such as lightning arresters, may get damaged.
- While investigating, the following aspects could indicate an impact on a power line serving a building:
- Electrical cables heated internally or sublimated by the passage of a current. Visible charring traces nearby;
- Damaged electrical equipment in the whole building;
- Damages to the power system protections and/or to the transformer serving the building.
Figure 8 — Damages to the installations of Hydro-Québec
In case of an indirect impact, some of the energy released as it occurs could migrate to the building through the ground cable. This cable could then overheat, and electric arcs may be generated. In some cases, the current could also migrate through communication cables (such as a coaxial cable).
- While investigating, the following elements could indicate an indirect impact:
- Bonding or neutral cables heated from the inside due to the passage of the current, less often sublimated. Visible charring traces nearby;
- Damaged electrical equipment in the whole building.
In case lightning is suspected of being the cause of a fire, it is recommended to obtain the lightning report which would include all the impacts detected in the vicinity of the building during a particular period of time.
If the lightning report reveals that the building is indeed in an area where impacts had been detected, even if the building is not specifically located within the confidence ellipse of the lightning stroke, this fire scenario should be taken into consideration. In view of the lacking accuracy in the detection of lightning strokes (only 70% of them are detected), it is indeed impossible to reach a definitive conclusion while relying solely on the lightning report. Such report shall remain a mere supplementary information that will be considered in the analysis to determine the probable cause of the fire.
Do you have questions about this article? Contact the author!