Protection During Thunderstorms
Author: Michael Vogel
Lightning conductors ensure that lightning strikes have no negative consequences. The need for external and internal lightning protection depends on what a building is used for and its specific structural conditions.
Last year, lightning struck around 491,000 times in Germany alone, according to the Siemens Lightning Atlas. Statistically, that’s 1.4 lightning strikes per square kilometer, which ranks in midfield compared to other European countries. Bosnia-Herzegovina, Europe’s country with the most lightning strikes in 2021, was hit a good four times as often. It was followed by Slovenia and Montenegro. Bringing up the rear in the statistics behind Norway and the United Kingdom is Ireland. Lightning strikes Bosnia-Herzegovina 77 times as often as it does Ireland.
Lightning forms in rain clouds. When raindrops are swirled around, they collide repeatedly and become electrically charged. The heavier drops are negatively charged and sink downward in the cloud, while the lighter – positively charged – drops rise upward. Electrical voltage builds up between them. Lightning flashes when a short circuit occurs, i.e. when an electrically conductive connection is formed between the different charges. This might happen between clouds, which can be perceived as sheet lightning. Or a short circuit occurs between the cloud and the ground – and lightning strikes.
A flash of lightning discharges in less than a thousandth of a second
Very high electrical currents flow briefly in a flash of lightning: 10,000 to 40,000 amperes, sometimes more than 200,000 amperes. By comparison, a household outlet’s maximum is 16 amperes. A lightning bolt discharges in less than a thousandth of a second. So within this short period of time, it releases all of its energy. A lightning strike can melt a thin wire. In a tree or wooden frame, the contained moisture vaporizes explosively, causing damage through sheer mechanical force. Highly flammable materials can also catch fire from lightning strikes.
So lightning can cause a lot of damage. According to the German Insurance Association, this damage amounted to around 200 million euros in Germany in 2021. However, statistically speaking, the risk of a lightning strike is not so great that there is a general lightning protection obligation in European countries. Private houses, for example, are exempt. “The obligation is limited to special buildings, for example kindergartens, nursing homes, churches, hospitals, or wind turbines,” explains Michael Ringleb, recognized expert for electrical engineering at DEKRA. “The gist of what it says in the state building codes is that elevated buildings in towns and cities must be equipped with a lightning protection system.” Ultimately, the responsible building authority always has the option of explicitly requiring lightning protection before issuing a building permit.
Risk analysis for lightning protection obligation
Obligatory lightning protection requires a risk analysis – which DEKRA also conducts, sometimes in cooperation with partners. “To assess the risk, we determine things like the availability requirement of an industrial plant, the value of a building, and the value of stored goods and files,” says Ringleb. “We determine the risk for lightning strikes based on the structural conditions of the building under construction and historical data on lightning strikes at that location.” In European countries, there are so-called lightning services that record all lightning events and make them available to interested organizations as processed data.
A lightning protection system consists of two components, the external and the internal lightning protection. The external lightning protection corresponds to what is commonly referred to as a lightning conductor or lightning rod. Essentially, it consists of sufficiently thick metal wires that are attached to the highest points of a building or installation and run from there down to the ground. How deep depends on the local conditions, specifically the electrical conductivity of the ground.
There are four lightning protection classes in Europe
In planning, the danger posed by a lightning strike with a specific peak current intensity can be illustrated with a sphere. The sphere’s radius results from the distance that the lightning no longer travels randomly but purposefully to the exact point of impact. The greater the peak current intensity of a lightning bolt, the larger this sphere. This makes it possible to determine where lightning rods – air-termination systems – should be installed. A European standard defines four lightning protection classes to classify buildings or installations. These protection classes indicate which current strengths an air-termination system must be able to safely discharge and which proportion of the locally expected lightning strikes can be safely controlled. For example, 99 percent of lightning strikes are safely controlled in “Lightning Protection Class I” and 84 percent in “Lightning Protection Class IV”.
Not only can lightning cause damage by directly striking a building, but also by striking nearby. In this case, overvoltages may occur in a building, causing damage to connected electrical devices. Therefore, internal lightning protection is important in addition to external lightning protection. The external lightning protection and all lines that can carry an electrical current must be connected to a common electrical equipotential bonding system. This includes all lines for electricity, telephone, gas, and water.
If lightning protection is mandatory, a structure’s assignment to a lightning protection class results in regular testing. “The frequency and scope depend greatly on the lightning protection class,” says expert Ringleb. DEKRA offers such lightning protection inspections for any type of building or installation. Thus equipped, you can hopefully enjoy the spectacle of lightning without worries. Of course, there is no such thing as 100 percent safety.