Lightning Protection (BS EN 62305)
Lightning protection systems — risk assessment, protection levels, and installation requirements
BS EN 62305 Overview
BS EN 62305 is the four-part standard governing lightning protection in the UK, covering risk assessment, physical protection, electrical systems protection, and electronic systems.
BS EN 62305 parts
BS EN 62305:2011| Part | Title | Scope |
|---|---|---|
| Part 1 | General principles | Lightning parameters and damage types |
| Part 2 | Risk management | Risk assessment methodology |
| Part 3 | Physical damage and life hazard | LPS design and installation |
| Part 4 | Electrical and electronic systems | SPD selection and coordination |
A lightning protection system (LPS) consists of three elements: air termination (rods, mesh, or catenary wires on the roof), down conductors (connecting air terminations to earth), and earth termination (electrodes or ring earth providing a low-impedance path to ground).
Lightning Protection Levels
LPS classes and protection efficiency
BS EN 62305-3, Table 2| LPS Class | Protection Level | Efficiency | Typical Application |
|---|---|---|---|
| I | Highest | 98% | Explosive storage, hospitals, data centres |
| II | High | 95% | Schools, offices, shopping centres |
| III | Standard | 90% | Standard commercial, residential towers |
| IV | Basic | 80% | Low-risk buildings, agricultural |
Efficiency represents the probability of protection against a direct strike.
Risk Assessment First
Surge Protection Devices (SPDs)
Risk R = N x P x L- R
- = Calculated risk of loss
- N
- = Number of dangerous events per year
- P
- = Probability of damage
- L
- = Consequent loss (fraction of total)
BS EN 62305-2 simplified risk formula
SPD type selection and location
BS EN 62305-4 and BS 7671| SPD Type | Location | Function | MCB Required |
|---|---|---|---|
| Type 1 | Main intake / origin | Direct lightning current | Yes (63-125A gG) |
| Type 2 | Main distribution board | Indirect surges, switching | Yes (32-40A Type B) |
| Type 3 | Near sensitive equipment | Fine protection, residual surges | No (fed via Type 2) |
SPD Coordination
Earthing for Lightning Protection
The LPS earth termination must achieve a resistance of 10 ohms or less. A ring earth electrode (bare copper strip buried at least 500mm deep around the building perimeter) is the preferred method. The LPS earth must be bonded to the electrical installation main earthing terminal to create an equipotential zone.
Separation Distance