Handbook of Electrical Installation Practice - BeKnowledge

Lightning Protection 231
Table 9.13 Classification of structures and contents.
Reproduced from BS 6651: 1999.
Note: the examples of structure usage are only intended to give greater
meaning to the descriptions of consequential effect – they should not be
seen as binding.
The average number of years between transient overvoltages caused by lightning is
given by 1/R. It should be noted that average values such as these are based on
periods of many years. This risk assessment R is intended only to guide the user’s
protection decision. Both the commercial impact of systems damage and downtime
and the health and safety implications should be considered (see earlier). Often
the decision to protect will be based on a simple comparison of the cost of damage
and downtime to computers and plant systems, with the cost of protection and
prevention.
Protector exposure levels
This risk assessment can also be used to indicate the type of protector required at
different types of installation. BS 6651 provides a four tier classification of installations
based on the consequential effects of damage to the installations’ contents.
This is shown in Table 9.13. Combining together the risk R with the consequential