Tolerable Risk – IEC 62305-2

Tolerable Risk

In performing risk assessment as an essential step in the application of IEC 62305, the calculated risk of loss due to lightning shall be below the tolerable level.

Tolerable risk (RT) as defined in IEC 62305-2 is

tolerable risk RT
maximum value of the risk which can be tolerated for the structure to be protected
Also referred to as acceptable risk, being the maximum value of risk acceptable based on community expectations.

In terms of the risk of various types of losses due to lightning, typical values of the tolerable risk, RT are specified in IEC 62305-2 Table 4. For each type of loss due to lightning, RT represents the tolerable probability of that loss occurring over the period of a year.

It is the responsibility of the authority having jurisdiction to identify the value of tolerable risk. Representative values of tolerable risk RT, where lightning flashes involve loss of human life or loss of social or cultural values, are given in below table.

Table 4 - Typical values of tolerable risk RT
Type of loss RT(y-1)
L1 Loss of human life or permanent injuries 10-5
L2 Loss of service to the public 10-3
L3 Loss of cultural heritage 10-4
In principle, for loss of economic value (L4), the route to be followed is the cost/benefit comparison given in IEC 62305-2 Annex D Evaluation of costs of loss. If the data for this analysis are not available the representative value of tolerable risk RT = 10-3 may be used.

Whether or not there is need to determine protection to reduce risks R1, R2, and R3, it is useful to evaluate the economic justification in adopting protection measures in order to reduce the risk R4 of economic loss. The items for which the assessment of risk R4 is to be performed shall be defined from

  • the whole structure,
  • a part of the structure,
  • an internal installation,
  • a part of an internal installation,
  • a piece of equipment,
  • the contents in the structure.

For a loss of economic value, the tolerable risk RT may be fixed by the facility owner or user, often in consultation with the designer of the protection measures, based on economic or cost/benefit considerations.

To illustrate this for a particular facility, it may be considered that a chance of 1 in 1000 of economic loss due to lightning occurring over a period of a year is tolerable. Alternatively, this would mean that it is considered acceptable for such a loss to occur, on average, once every 1000 years. In such a case the tolerable risk, R4 for loss of economic value would be set at 10-3. Similarly, if it were considered acceptable for such a loss to occur, on average, once every 100 years, R4 for loss of economic value would be set at 10-2.

The calculated risk is the sum of R1, R2, R3 and R4. The value of calculated risk shall be below the tolerable risk RT.