This week in the second edition of our technical blog we will demystify the theory of a strike risk assessment and highlight the importance of this study in the design phase of a building development to reduce risks and probability of damage associated with lightning strikes.
What is a Strike Risk Assessment
A lightning protection system is an important component in protecting human life, the building structure, electrical systems and critical business processes. In the design stage of any given project, when it comes to lightning protection, a strike risk assessment is the first fundamental step that must be undertaken by your chosen lightning protection specialist. This study determines the number of risks and the protection measures needed to reduce these risks to acceptable limits. These findings then enable the designer to establish the parameters that must be considered during the design of the lightning protection system for any building or structure.
Theory of a Strike Risk Assessment
Lightning protection design requirements are closely related to the extent of risk reduction, while the risk amount is directly related to the amount of loss. Risks and losses can be related to human life, service to the public, cultural heritage and economic value.
The amount of Risk (R) associated with a project is established during a strike risk assessment by multiplying the following three key parameters:
- Dangerous events? (N): How many events could be considered dangerous?
- Probability of damage (P): What percentage of the threats (N) will lead to damage (P)?
- Loss Amount (L): What percentage of damage (P) will lead to a loss(L)?
These parameters must be closely considered by a certified lightning protection system specialist using reliable strike risk assessment software.
The general formula used within the strike risk assessment software: R = N x P x L
In order to reduce the number of threats on a building, lightning protection specialists will aim to reduce the number of dangerous events (N).
To highlight this in practice figures 1 and 2 outline where top installed earth wires on a building can reduce the number of strikes that may travel and reach the bottom installed power and signal cables. This demonstrates how earth wires act as a protective barrier to any destructive currents that threaten the building and its contents.
If we can diminish the possibility of damage (P) at the first exposure points of lightning strikes, then the number of threats will be decreased. For example, by employing overvoltage protective devices along the length of the telecommunication system network, the probability of damage will be reduced.
As shown in figure 3, in a sample facility with a certain value of tolerable frequency of damage (FT), and an expected number of direct flashes (ND), by installing surge protective devices compatible with a level 1 lightning protection systems. This will decrease the threat of a direct flash to approx. 0.01 of the base value.
The standard formula commonly referenced in the design of telecommunication facilities is ITU-TK. 97: ND X PSPD = FT
Reducing Risks Associated with a Lightning Strike
In the initial communication stage between the client and the lightning protection designer, it is established whether any form of lightning protection is already present on the building/structure or on a surrounding building nearby (zone of protection). As outlined above, there are several various risks associated with lightning strikes, which makes it the ultimate aim of the lightning protection specialist to deliver a system design and solution that reduces the risks associated with a lightning strike. These will be decreased, once either one or all of; Dangerous Events (N), Probability of Damage (P) or Loss Amount (L) are reduced.
Dangerous Events (N)
When considering a direct lightning strike to a building, the number of Dangerous Events (N) can be controlled once an external lightning protection system is considered for the building. In other words, the number of dangerous events depends on the dimension of the Collection Area (A1) and the Geographic Condition (Ng) of the structure’s location.
In the case of a direct strike to the power supply lines entering the building, if a Medium Voltage (MV)/ Low Voltage (LV) Transformer is included on the power supply line at its point of entry to the facility, the number of dangerous events will be reduced to 0.2 of its base value (see figure 4).
Probability of Damage (P)
The Probability of Damage (P) can be reduced through various lightning protection standard methods, such as bonding, shielding and the use of overvoltage protection devices. As stipulated above, these types of protection measures can reduce the number of threats to the building, even if no strike risk assessment study has been conducted.
In buildings, such as telecommunications, where an antenna tower situated, at the beginning of the strike risk assessment study some protection measures are required to reduce the number of threats and ensure compliance with lightning protection standard requirements. This is due to the high vulnerability nature of the facility. For example, in standard ITUTK.56 part 6.2, some bonding obligations have been introduced due to the vulnerability level of the waveguide cable and connected equipment found within telecommunication facilities.
Loss Amount (L)
Generally, the loss amount is related to the number of humans and their presence time and duration within an area of a building. There are several lightning protection standard methods used to reduce the loss amount (L), such as utilising a certain type of surface soil or floors, or a fire extinguisher. In addition, as outlined in the lightning protection standard IEC62793, a thunderstorm warning system can be used to reduce the loss amount. In the event of a thunderstorm, this convenient alarm system can prevent the presence of individuals from being exposed to harmful lighting strikes.
Through close collaboration with your lightning protection system provider and the undertaking of a strike risk assessment study, by reducing one or all of;
1) Number of Threats, 2) Number of Dangerous Events (N), 3) Probability of Damage (P), or 4) Loss Amount (L), the overall risk associated with a lightning strike can be successfully reduced.
Types of Risks a Lightning Strike Can Produce
As stipulated in the standards IEC62305-2, the following describes the risks associated with a lightning strike:
R 1: Risk associated with the loss of life or injury.
R 2: Risk associated with the loss of service.
R 3: Risk associated with the loss of historical significance.
R 4: Risk associated with the loss of economic value.
In every project there is an accepted value of risk, for instance, the following figure 5 presents the allowable risk limits that are mentioned in lightning protection standards as a typical value.
The above-listed risk values are the typically referenced values, however, in some vital circumstances, lightning protection designers may use different risk values. For example, in vital network and telecommunication buildings, the tolerable loss of service to the public may be considered 10-4 .
In some lightning protection standards and guidelines related to telecommunication buildings, the tolerable frequency of damage value should be defined by the network operator. For example, FT = 0.05 means that on average, 1 damage in 20 years (1/20) is acceptable.
As referenced in IEC62305-2, the following are definitions of different types of loss:
- L1 (Loss of human life, including permanent injury): the endangered number of (victims).
- L2 (loss of public service): the number of users not served.
- L3 (loss of cultural heritage): the endangered economic value of structure and content.
- L4 (Loss of economic values): the endangered economic value of animals, the structure (including its activities), content and internal systems.
There are typical loss amounts as shown in following figures 6 and 7:
Figure 6: General typical value of loss based on standard NFPA780.
Figure 7: Typical value of loss used in the calculation of the risks related to the loss of human life based on standard IEC62305-2.
Figure 8: Typical value of loss used in the calculation of the risks related to the loss of service to the public based on standard IEC62305-2.
Figure 9: Typical value of loss used in the calculation of the risks related to the loss of economic value based on standard IEC62305-2.
Risks and Probable Strike Points
Each risk could be caused by different kinds of probable strike points as shown in figure 10.
We can conclude that each area on a building exposed to a lightning strike is at risk. Therefore, to assess the overall risk, we need to establish what kind of strike will cause which type of risk (figure 11). The strike risk assessment software is important for lightning protection specialists to understand the various strike types and determine the appropriate protection measures to reduce the risks.
It is important to note, that before conducting a strike risk assessment, the tables shown in figure 11 and the area defined in figure 12 will be closely considered by your designer.
Lightning affects European territory on an average of 350 days per year, meaning that there is at least one lightning strike somewhere in Europe every day. Given this stark figure, it is more important to understand the need to conduct a strike risk assessment at the earliest possible design phase. This determines the appropriate level of lightning protection to protect the full building envelope from potential risks and losses caused by lightning strikes.
The LPI Group team of technical design experts located globally can actively assist our clients in establishing their lightning protection requirements by utilising the latest, innovative software to reduce risks and losses and deliver a bespoke code-compliant lightning protection design.
If you have any questions on the topics covered in this week’s technical blog or would like to discuss lightning protection for an upcoming project, please leave a comment below or email firstname.lastname@example.org and our team will get back to you promptly.
By Hadi Beik Daraei, Technical Designer at LPI Group