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Down
conductor positioning and distancing is often dictated by architectural
constraints. There should be one down conductor for every 20m or part
there of of the building perimeter at roof or ground level (whichever
is greater).These should be evenly spaced and distances apart of more
than 20m avoided if possible.
If the building is above 20m in height or of an abnormal risk this distance should be reduced to 10m.
They
should be routed as directly as possible from the air termination
network to the earth termination network to avoid risks of side
flashing. Re-entrant loops are also to be avoided. BS 6651 recommends
that the length of conductor forming the loop should not exceed eight
times the width of its open side.
BS 6651 allows the use of `natural conductors' such as rebars and structural
steelwork, provided that they are electrically continuous and adequately
earthed.
| click image to enlarge
Figure 3 - Lightning Protection Scheme to BS 6651 using the reinforced
concrete within the structure for down conductors.
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BS 6651 allows the use of `natural conductors'such as rebars and
structural steelwork, provided that they are electrically continuous
and adequately earthed.
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Each
down conductor must have a separate earth termination. Moreover
provision should be made in each down conductor, for disconnection from
the earth for testing purposes.This is achieved with a test clamp (see
Figure 4).
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Figure 4 - Oblong test or junction clamp
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6651 stipulates that the resistance to earth of the lightning
protection system measured at any point, should not exceed 10 ohms.
With the test clamp
disconnected, the resistance of each individual earth should be no more
than ten times the number of down conductors in the complete system. eg
for a system with 15 down conductors, the individual earth readings
should be no more than 10 x 15 = 150 ohms.
Several
types of earth electrode are permissible, but by far the most commonly
used are deep driven earth rods. BS 6651 states that the combined earth
rod length of a system should be no less than 9m whilst each individual
earth rod should be no less than 1.5m in length.
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Figure 5 - Deep driven earth electrode
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