The probability that lightning causes damage depends on the measures installed. The lightning protection system sets PB according to its class (from 0.2 for class IV to 0.02 for class I). A coordinated SPD system sets PSPD according to its level (from 0.05 to 0.01). Shielding and internal wiring set PMS through the factors KS1 to KS4. Line shielding and equipotential bonding define CLD, PLD and PEB.
Short answer
Each risk component in IEC 62305-2 carries a probability of damage P that ranges from 1 (damage always occurs when lightning strikes) to close to 0. The protection measures are reflected here: the lightning protection system (LPS) lowers PB, a coordinated SPD system lowers PSPD, shielding and internal wiring lower PMS, line shielding and isolating interfaces define CLD, PLD and CLI, and lightning equipotential bonding defines PEB. The Annex B values are valid if the measures comply with Part 3 and Part 4.
General rules
- A probability below 1 is assigned only if the measure or characteristic applies to the whole structure or zone and to all relevant equipment.
- Other values can be used if justified, for example by a detailed investigation.
- The values depend on the measures complying with IEC 62305-3, for physical damage and electric shock to living beings, and IEC 62305-4, for failure of internal systems.
Strike to the structure
Electric shock to living beings: PA
PA = PTA × PB. PTA depends on the additional measures against step and touch voltages:
| Additional measure | PTA |
|---|---|
| None | 1 |
| Warning notices | 10⁻¹ |
| Electrical insulation of exposed parts, such as the down-conductors (for example, at least 3 mm of cross-linked polyethylene) | 10⁻² |
| Effective ground equipotentialization | 10⁻² |
| Physical restrictions, or the building frame used as a down-conductor system | 0 |
If several measures are taken, PTA is the product of the values. These measures are effective only in structures with an LPS, or with a continuous frame acting as a natural LPS.
Physical damage: PB
The LPS is the measure that lowers PB. Its value depends on the class:
| Characteristic | PB |
|---|---|
| No LPS | 1 |
| LPS class IV | 0.2 |
| LPS class III | 0.1 |
| LPS class II | 0.05 |
| LPS class I | 0.02 |
| Class I air-termination and continuous metal or reinforced-concrete frame as a natural down-conductor | 0.01 |
| Metal roof with air-termination, complete protection of the rooftop installations and continuous frame as a natural down-conductor | 0.001 |
Failure of internal systems: PC
PC = PSPD × CLD. A coordinated SPD system conforming to IEC 62305-4 lowers PC. PSPD depends on the protection level for which the SPDs are designed:
| SPD protection level | PSPD |
|---|---|
| No coordinated SPD system | 1 |
| III or IV | 0.05 |
| II | 0.02 |
| I | 0.01 |
| Better characteristics than level I | 0.005 to 0.001 |
CLD depends on the shielding, earthing and isolation of the line the system is connected to. It is 1 for unshielded lines and for shields not bonded to the same bar as the equipment, and 0 when the line is a lightning protection cable or duct with its shield bonded to the same bar as the equipment, when the system has no external line or when an isolating interface conforming to Part 4 is used.
Strike near the structure: PM
If there is no coordinated SPD system conforming to Part 4, PM = PMS. If there is, PM = PSPD × PMS.
PMS depends on four factors:
PMS = (KS1 × KS2 × KS3 × KS4)²
- KS1: effectiveness of the shielding of the structure, the LPS or other shields at the LPZ 0/1 boundary.
- KS2: effectiveness of the internal shields at the boundaries between subsequent zones.
- KS3: characteristics of the internal wiring.
- KS4: impulse withstand voltage of the system being protected.
Inside a zone, at a distance from the screen equal to or greater than the mesh width, KS1 and KS2 can be evaluated with:
KS1 = 0.12 × wm1 and KS2 = 0.12 × wm2
where wm1 and wm2 are, in meters, the mesh width of the spatial shield or of the meshed LPS down-conductors, or the spacing between metal columns or between frame rods. For a continuous metal shield at least 0.1 mm thick, KS1 = KS2 = 10⁻⁴. If there is a meshed bonding network conforming to Part 4, the values can be halved. And close to the screen, within the safety distance, the values increase: the standard doubles them when the distance is between 0.1 and 0.2 times the mesh width.
KS3 depends on the internal wiring:
| Type of internal wiring | KS3 |
|---|---|
| Unshielded, no routing precautions to avoid loops | 1 |
| Unshielded, with routing that avoids large loops | 0.2 |
| Unshielded, with routing that avoids loops (conductors in the same cable) | 0.01 |
| Shielded cables and cables in metal conduit | 0.0001 |
KS4 = 1 / UW, with UW the impulse withstand voltage of the system in kV. If there is equipment with different withstand voltages, the lowest is taken. KS1, KS2 and KS4 are limited to a maximum of 1. When there are isolating interfaces with isolating transformers with an earthed screen between windings, fiber optic cables or optocouplers, PMS = 0 is assumed.
Strike to a line and near it
The other probabilities combine the factors above with those of the line:
| Component | Probability | Which measures lower it |
|---|---|---|
| RU | PU = PTU × PEB × PLD × CLD | Restrictions and notices, equipotential bonding with SPDs at the entrance, line shielding |
| RV | PV = PEB × PLD × CLD | Equipotential bonding with SPDs at the entrance, line shielding |
| RW | PW = PSPD × PLD × CLD | Coordinated SPD system, line shielding |
| RZ | PZ = PSPD × PLI × CLI | Coordinated SPD system, line shielding |
- PEB depends on the lightning equipotential bonding and the protection level of its SPDs: 1 without SPDs, 0.05 for levels III and IV, 0.02 for II, 0.01 for I and between 0.005 and 0.001 with better characteristics.
- PLD depends on the routing and shielding of the line, the cable shield resistance (for example, between 1 and 5 Ω/km, or less than 1 Ω/km) and the equipment withstand voltage UW, between 1 and 6 kV. PLI depends on the line type (power or telecommunications) and on UW. The higher the UW and the lower the shield resistance, the lower the probability.
The full values are in tables B.4, B.6, B.7, B.8 and B.9 of Part 2.
What this logic shows
- For physical damage, the LPS class raises protection in steps. Going from class IV to class I lowers PB from 0.2 to 0.02.
- For failure of internal systems from nearby strikes, there are two levers: the SPDs (PSPD) and the electromagnetic environment (PMS). PM is the product of both, so each measure multiplies the effect of the other.
- Equipment with a higher withstand voltage improves the probability. UW enters KS4, PLD and PLI.
Guides in the same series
The probability is combined with ND and with the loss in the risk components. The measures that reduce it are described in LPS, classes I to IV, magnetic shielding and cable routing and coordinated SPDs.
When to contact us
If you need to know which combination of measures brings the risk of your building or plant down to the tolerable value, or verify that those you already have are correctly reflected in the calculation, write to us. 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
- Coordinated SPDs: classes I, II and III and Iimp current
- Magnetic shielding and cable routing against lightning
- Transient overvoltages in the plant
References
- IEC 62305-2:2010, Protection against lightning – Part 2: Risk management, Annex B, clauses B.1 to B.9, equations B.1 to B.11 and tables B.1 to B.9.
Frequently asked questions
Why does a class I LPS give PB = 0.02 and not zero?
Because the system is effective only against lightning within the current range of its protection level. The standard assigns each class a probability of physical damage that reflects this residual risk: 0.2 for class IV, 0.1 for III, 0.05 for II and 0.02 for I. With an LPS plus a continuous steel or metal frame acting as a natural down-conductor, PB drops to 0.01 or 0.001.
Does an SPD reduce the probability of damage in any building?
Only in structures protected by an LPS, or with a continuous metal or reinforced-concrete frame that acts as a natural LPS and meets the bonding and earthing requirements of Part 3. The standard states this in the notes to table B.3.
What happens to the probability if the equipment does not meet the withstand voltage in its product standard?
PM = 1 is assumed. The probability for nearby strikes only drops with the Part 4 measures when the equipment conforms to the withstand or impulse voltage levels of its product standards.