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BS EN 62305 – Protection Against Lightning Strikes & Risk Management Notes

Published 11 Jun 2018

BS EN 62305

BS EN 62305

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BS EN 62305

The Part 2 of BS EN 62305 provides a comprehensive mathematical model to evaluate and manage the risk posed by lightning strikes and measures to alleviate the risk associated with earthing system designs.

The risk assessment is extremely time consuming to calculate by hand so Wallis, working in conjunction with their distributor Thorne & Derrick International, can offer to do the assessment using bespoke computer software.

The basics of the calculation can be briefly explained as follows taking into account:

The Sources of Lightning

  • S1: flashes to a structure
  • S2: flashes near a structure
  • S3: flashes to a line
  • S4 flashes near a line

The Likely Damage

  • D1: injury to living beings by electric shock
  • D2: physical damage
  • D3: failure of electrical and electronic systems

The Likely Losses

  • L1: loss of human life (including permanent injury)
  • L2: loss of service to the public
  • L3: loss of cultural heritage
  • L4: loss of economic value (structure, content, and loss of activity).

The following Table 2 from BS EN 62305 explains the link between Sources, Damages & Losses.

Table 2 – BS EN 62305

Sources of damage, types of damage and types of loss according to the point of strike:

Lightning Flash Structure
Source of Damage Type of Damage 1 Type of Loss
S1 D1D2D3 L1, L4aL1, L2, L3, L4 L1b, L2, L4
S2 D3 L1b, L2, L4
S3 D1D2D3 L1, L4a,L1, L2, L3,L4  Lib, L2, L4
S4 D3 L1b, L2, L4

Note:

a) Only for properties where animals may be lost.

b) Only for structures with risk of explosion and for hospitals or other structures where failures of internal systems immediately endangers human

Risk: The risks to be evaluated in a structure may be as follows

  • R1: risk of loss of a human life (including permanent injury)
  • R2: risk of loss of service to the public
  • R3: risk of loss of cultural heritage
  • R4: risk of loss of economic value.

Risk Management

  1. Basic Procedure

The following procedure shall be applied:

  1. Identification of the structure to be protected and its characteristics
  2. Identification of all the types of loss in the structure and the relevant corresponding risk R (R1 to R4); ­evaluation of risk R for each type of loss R1 to R4
  3. Evaluation of need of protection, by comparison of risk R1, R2 and R3 with the tolerable risk RT
  4. Evaluation of cost effectiveness of protection by comparison of the costs of total loss with and without protection measures. In this case, the assessment of components of risk R4 shall be performed in order to evaluate such costs.
  1. Structure to be considered for risk assessment includes
  1. The structure itself
  2. Installations in the structure
  3. Contents of the structure
  4. Persons in the structure or in the zones up to 3 m from the outside of the structure
  5. Environment affected by damage to the structure.

Protection does not include connected lines outside of the structure.

  1. Tolerable Risk RT

The calculated risks R1, R2 & R3 shall be compared with tolerable risk. The permissible values for the tolerable risk are mentioned in the Table 4 BS EN 62305.

Table 4 – Typical values of tolerable risk RT

Types of Loss RT (y-1)
L1 Loss of human life or permanent injuries 10-5
L2 Loss of service to the public 10-4
L3 Loss of cultural heritage 10-4

If R < RT, lightning protection is not necessary. If R > RT, protection measures shall be adopted in order to reduce R RT for all risks to which the structure is subjected.

The manual calculations are time consuming. It is recommended to use available Lightning Protection Risk Management Software to perform these calculations – contact T&D.

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