Handbook of Electrical Installation Practice - BeKnowledge

228 Handbook of Electrical Installation Practice
Note, the four quarter circle areas have been combined to give a circular area which
can be calculated using pr 2 where the radius r is equal to D. If the structures height
h exceeds D, the collection distance is assumed to be h.Where a complex shaped
building is being assessed, approximations and rough graphical methods will allow
the area to be calculated with sufficient accuracy.
Collection area of adjacent associated structures and their surrounding ground (CAS).
Where there is a direct or indirect electrical connection to an adjacent structure,
the collection area of this structure should be taken into account. Typical examples
are lighting towers (supplied from the main building), radio transmission
towers, and other buildings with computer control and instrumentation equipment.
Structures within a distance of 2D from the main building are considered to be
adjacent. As before, the collection area is calculated as the area of the structure plus
the area extending D m all around the building. Once again if the height of the
structure h exceeds D, the collection distance is assumed to be h. Any part of
this area which falls within the collection area of the main structure CSG should
be disregarded.
Collection area of incoming/outgoing mains power supplies (CMP). The effective
collection areas of all incoming and outgoing power supplies should be considered.
This includes outgoing supplies to neighbouring buildings, CCTV equipment,
lighting towers, remote equipment and the like. The effective collection areas of
different types of mains power supplies are shown in Table 9.8.
Collection area of data lines leaving the earth reference of the building (CDL). The
effective collection areas of all incoming and outgoing data lines should be considered.
This includes data lines to other buildings, security equipment, remote data
logging equipment and the like. The effective collection areas of different types of
data lines are shown in Table 9.9.
Vulnerability of system configuration
The overall risk to electronic equipment from the secondary effects of lightning will
therefore depend on P (the probability of a strike) and the type of structure F, the
degree of isolation G, the type of terrain H.Weighting factors based on the relative
degrees of risk are assigned for F, G and H in Tables 9.10, 9.11 and 9.12,
respectively.
Calculation of overall risk
We described earlier how the threat to electronic equipment from the secondary
effects of lightning depends on the probable number of lightning strikes to the area
of influence, and the vulnerability of the system configuration. We have now seen
how the former can be calculated from P = Ae ¥ N g ¥ 10 -6 and how the latter is given
by the weighting factors F, G, and H.
Thus, the risk R of a lightning strike coupling into electrical or electronic systems
through incoming/outgoing mains power supplies or data lines is:
R= P ¥ F ¥ G¥ H