IEC 62305-2 calculates the risk R for each type of loss as a sum of components and compares it with a tolerable risk RT. If R does not exceed RT for loss of life (R1), public service (R2) and cultural heritage (R3), no protection is needed. Typical RT values are 10⁻⁵, 10⁻³ and 10⁻⁴ per year, but they are set by the competent authority.
Short answer
Part 2 of IEC 62305 answers a specific question: does the risk of lightning losses in this structure exceed the tolerated value? For each type of loss a risk R is calculated by adding up its components, and it is compared with the tolerable risk RT. If R is less than or equal to RT for all the risks that apply, the structure is protected without new measures. If R exceeds RT, measures must be adopted until it no longer does.
Which risks are assessed
The standard distinguishes four risks, one per type of loss:
- R1: loss of human life, including permanent injury.
- R2: loss of service to the public.
- R3: loss of cultural heritage.
- R4: loss of economic value.
To decide whether protection is needed, R1, R2 and R3 are compared with their RT. R4 is assessed when protection is to be justified economically. If several types of loss can occur, the condition R ≤ RT must be met for each one.
What the assessed structure is
The structure considered includes the building, its installations, its contents, the people inside or up to 3 m outside, and the environment affected if the structure is damaged. The protection does not include connected lines outside the structure. A structure may be part of a larger one, and may be subdivided into zones ZS.
Tolerable risk RT
The RT value is identified by the competent authority. Table 4 of Part 2 gives typical values per year:
| Type of loss | Typical RT |
|---|---|
| L1: human life or permanent injury | 10⁻⁵ |
| L2: service to the public | 10⁻³ |
| L3: cultural heritage | 10⁻⁴ |
| L4: economic value, only if there are no data for the cost-benefit analysis | 10⁻³ |
For L4 the principal route is the comparison of cost and benefit.
How each risk is calculated
Each risk is the sum of its components. Each component RX has the general form:
RX = NX × PX × LX
- NX: number of dangerous events per year, which depends on the ground flash density and on the physical characteristics of the structure, the lines and the terrain.
- PX: probability that the event causes damage, which depends on the structure, the lines and the protection measures.
- LX: consequent loss, which depends on the use of the structure, the presence of people, the service it provides and the value of what is affected.
These three factors are calculated with Annexes A, B and C of Part 2. Each is explained in its own guide: number of strikes ND, probability of damage and losses LX.
Decision procedure
Figure 1 of Part 2 orders the decision as follows:
- Does the risk R exceed the tolerable risk RT?
- No: The structure is protected; no new protection is needed
- Yes: Is an LPS already installed on the structure?
- No: Does RA + RB + RU + RV exceed RT?
- Yes: Install an LPS of a suitable type
- No: Install suitable LEMP protection measures (SPM)
- Yes: Are SPM already installed?
- No: Install suitable SPM
- Yes: Install other protection measures (Table 3 of Part 2)
- No: Does RA + RB + RU + RV exceed RT?
After each measure the risk components are recalculated and compared with RT again. The sum RA + RB + RU + RV groups the components of electric shock to living beings and physical damage; for that reason it decides whether a building protection system (LPS) is needed. The components of failure of internal systems are resolved with the LEMP measures.
If the risk cannot be reduced to a tolerable level, the standard indicates informing the owner and giving the installation the highest level of protection.
How the measure is selected
The choice is made by the designer according to the weight of each component in the total risk and the technical and economic aspects of each measure. For each type of loss there are several measures, alone or combined, that achieve R ≤ RT; the standard asks for the most critical components to be identified and reduced first. A measure counts as effective only if it meets the applicable standards: IEC 62305-3 for physical damage and electric shock to living beings, and IEC 62305-4 for failure of electrical and electronic systems.
Structures with an explosion risk
When the authority requires lightning protection for a structure with an explosion risk, Part 2 recommends at least a class II LPS. The standard allows exceptions that are technically justified and authorized by the authority: level I is always valid, and the authority may accept level III where lightning activity is infrequent or the contents are not very sensitive.
The economic assessment
Besides the need, it can be assessed whether the protection pays for itself. Part 1 says protection is cost-effective if the cost of the residual loss with the measures (CRL) plus the cost of the measures (CPM) is less than the cost of the total loss without protection (CL). Part 2 formulates it as an annual saving that must be greater than zero. The full procedure is explained in the guide on cost-benefit analysis.
What is needed to calculate it
The calculation requires gathering the data on the structure, the incoming lines, the activity and the internal equipment. The usual approach is to divide the structure into zones and the lines into sections, as explained in the guide on lightning losses and zones.
Before starting, ask the applicable authority which RT values it uses and whether it requires a protection level without calculation. For a photovoltaic plant, the guide on SPDs in photovoltaic systems covers the overvoltage side and the guide on solar plant engineering and design cites the risk assessment within the design.
Guides in the same series
The detail of each component is in risk components RA to RZ. To decide the protection level see protection levels; for the types of damage and loss, lightning damage.
When to contact us
If you need to justify whether your control building, plant or solar farm needs lightning protection, or support a protection level before the authority, tell us the scope. Learn about our engineering design and studies service and write to us through contact.
Related guides
- IEC 62305 lightning protection: what each part covers
- Lightning risk components in IEC 62305-2: RA to RZ
- Number of strikes on a structure: ND and collection area
- Probability of lightning damage: PB, PSPD and KS factors
- Transient overvoltages in the plant
References
- IEC 62305-1:2010, Protection against lightning – Part 1: General principles, clauses 6.1 and 6.2.
- IEC 62305-2:2010, Protection against lightning – Part 2: Risk management, clauses 4.2, 4.3, 5.1 to 5.7 and 6.1, table 4 and figure 1.
Frequently asked questions
Who decides the tolerable risk RT?
The competent authority. Part 2 gives typical values for reference only: 10⁻⁵ per year for loss of human life, 10⁻³ for loss of public service and 10⁻⁴ for loss of cultural heritage. For loss of economic value the route is cost-benefit analysis; without data to do it, 10⁻³ may be used.
Can a structure need protection if the local code does not require it?
Yes. The risk assessment is what says whether the risk exceeds the tolerable value. At the same time, an authority may require protection in certain cases without asking for the calculation; the authority then states the required protection level, and the assessment can be used to justify an exception.
What class of protection is used in a structure with an explosion risk?
If the authority requires lightning protection in a structure with an explosion risk, the standard recommends at least a class II LPS. Exceptions that are technically justified and authorized by the authority are allowed; for example, level I is always acceptable.