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Structural lightning protection
design considerations
BS 6651 (Protection of structures against lightning)
clearly advises strict adherence to the provision of
a conventional Lightning Protection System (LPS) - to
the total exclusion of any other device or system for
which claims of enhanced protection are made.
The principle components of a conventional structural
lightning protection system, in accordance with BS 6651
are:
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Other
areas that need to be looked at: |
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Air termination network
On
high risk structures such as explosive factories, no part
of the roof should be more than 2.5m from an air termination
conductor. This is generally achieved by applying a 5m
x 10m mesh to the roof.
However,
for most structures, a mesh of 10m x 20m is considered
sufficient, giving a maximum distance from any part of
the roof to the nearest conductor of 5m.
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Air
terminations for tall
conducting structures
The zone of protection does not seem to be applied
because of the need to interconnect the down conductors
of the tall block to the air termination of the
lower block.
In such cases it is necessary to connect the lower
air termination up to the lower down conductors
to facilitate this inter connection, even though
this extension is within the zone of protection
of the tower. |
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The 'Zone of Protection' offered by an air termination
network is considered to be 45º for heights up
to 20m. Above this height, the zone of protection is
determined by the 'Rolling Sphere Method'.
This
involves rolling an imaginary sphere of 60m radius over
a structure. The areas touched by the sphere are deemed
to require protection. On tall structures, this can
obviously include the sides of the building.
Zones of protection
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Down
conductors
Down conductor siting and distancing is often dictated
by architectural circumstances. There should be one down
conductor for every 20m or part thereof 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.
Lightning Protection Scheme to BS 6651 using the reinforced
concrete within the structure for down conductors |
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Inner area requires no conductors as it is
within the zone of protection determined by
the rolling sphere |
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Earth
termination network
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 below).
BS
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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Deep
driven
earth electrode |
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Oblong
test or
junction clamp |
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Parallel
earth rod electrodes
Where ground conditions make deep driving of earth rods
impossible, a matrix arrangement of rods coupled to one
another by conductors can be used. If possible, the earth
rods must be spaced at a distance at least equal to their
driven depth.
If
earth rods cannot be driven in a parallel line a "Crows
Foot" configuration can be used, ensuring that the
spacing/depth ratio is still maintained.
High
resistivity soil conditions can be overcome by backfilling
earth rods with a suitable medium such as Marconite conductive
concrete which effectively increases the diameter of the
earth rod and hence its surface area, thus lowering resistance
to earth.
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Spacing
of parallel
earth rod electrode |
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Bonding
All metal work, including water pipes, gas pipes, handrails,
air conditioning units, metal cladding, metal roofs etc,
in the vicinity of the LPS must be bonded to it, to avoid
the danger of side flashing.
For the same reason, the LPS earth should be bonded to
the main electrical earth, as well as any other earthing
system present in the structure.
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Example
of side flashing
If the lightning protection system on a structure
is hit by lightning, then the current flowing through
the system and the resistance/impedance offered
by the conductor path will determine themagnitude
of the potential difference seen by the lightning
conductors with respect to true earth.
The lightning conductors can, instantaneously, have
a potential magnitude of megavolts (1,000,000V)
with respect to true earth.
Typically,
at instant of discharge:
Potential difference at A = 1,500,000V
Potential difference at B = 0V |
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| Bonding
to prevent side flashing |
| 1 |
Air
termination |
| 2 |
Down
conductor |
| 3 |
Bond
to aerial |
| 4 |
Bond
to vent |
| 5 |
Bond
to re-bar |
| 6 |
Bond
to metal staircase |
| 7 |
Bond
to metal window frame |
| 8 |
Bond
to vent pipe |
| 9 |
Bond
to steel door/frame |
| 10 |
Test
clamp |
| 11 |
Indicating
plate |
| 12 |
Main
earthing terminal of electrical installation |
| 13 |
Earth
termination point |
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Corrosion
BS 6651 contains tables of materials suitable for use
in lightning protection system components. Adherence to
these requirements is vital to avoid corrosion problems.
The
correct choice of material and installation design should
ensure a life span of 30 years for the earth electrode
system. |
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Inspection,
testing, records and maintenance
The code adequately details the requirement for inspecting
a lightning protection system, the testing required
and the detailed records that should be maintained.
Observance
of clauses 32 to 34 of the code will highlight any maintenance
of the system required. Of particular importance is
the regular detailed examination of the complete lightning
protection system for any evidence of corrosion. If
this check is not carried out then vital components
within the LPS, which may have suffered from corrosion
and which could exhibit a high resistance could be missed.
This will have a detrimental effect on the whole lightning
protection system making it an unattractive high impedance
path for the lightning current to follow.
To
minimise this problem, along with regular inspections,
the selection of the correct materials should be made
in accordance with the recommendation of BS6651.
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