Chapter 14
Lightning protection
Lightning strokes can be of two kinds. In the first, a charged cloud induces a charge of
opposite sign in nearby tall objects, such as towers, chimneys and trees. The electrostatic
stress at the upper ends of these objects is sufficiently great to ionize the air in the
immediate neighbourhood, which lowers the resistance of the path between the cloud and
the object. Ultimately, the resistance is lowered sufficiently for a disruptive discharge to
occur between them. This type of discharge is characterized by the time taken to produce
it, and by the fact that it usually strikes against the highest and most pointed object in the
area.
The second kind of stroke is a discharge which occurs suddenly when a potential
difference between a cloud and the earth is established almost instantly. It is generally
induced by a previous stroke of the first kind; thus if a stroke of this kind takes place
between clouds 1 and 2 (Figure 14.1), cloud 3 may be suddenly left with a greater
potential gradient immediately adjacent to it than the air can withstand, and a stroke to
earth suddenly occurs. This type of stroke occurs suddenly and is not necessarily directed
to tall sharp objects like the first kind of stroke. It may miss tall objects and strike the
ground nearby. Figure 14.2 shows other ways in which this. kind of stroke may be
induced. In each case, A is a stroke of the first kind and B is the second type of stroke
induced by A. In each case the first stroke from cloud 1 changes the potential gradient at
cloud 2 and thus produces the second stroke.
The current in a discharge is uni-directional and consists of impulses with very steep
wave fronts. The equivalent frequency of these impulses varies from 10kHz to 100kHz.
While some lightning discharges consist of a single stroke, others consist of a series of
strokes following each other along the same path in rapid succession. The current in a
single stroke can vary from about 2000A to a maximum of about 200000A, with a
statistical average of 20000A. It rises to a peak value in a few microseconds. When a
discharge consists of several successive strokes, each stroke rises and falls in a time and
to an amplitude of this order so that the whole discharge can last up to a second.
Figure 14.1 Induced lightning stroke
The effects of a discharge on a structure are electrical, thermal and mechanical.
As the current passes through the structure to earth it produces a voltage drop which
momentarily raises the potential of part of the structure to a high value above earth. One
function of a lightning conductor is to keep this potential as low as possible by providing
a very low resistance path to earth. It is recommended in the British Standard Code of
Practice (BS 6651:1999) that the resistance to earth of the protective system should not
exceed 10ohms. The sharp wave front of the discharge is equivalent to a high-frequency
current and, therefore, there is also an inductive voltage drop which has to be added
phasorially to the resistive drop. Part of the lightning conductor is thus inevitably raised
to a high potential. This brings with it a risk of flashover from the conductor to other
metal in the structure, such as water and gas pipes and electrical cables. These in turn
would then be raised to high potential which could bring danger to occupants of the
building, and it is necessary to guard against such flashovers. Bonding the lightning
Figure 14.2 Induced lightning strokes
Lightning protection 213
protection system to the main earthing terminal of the installation does this. The
discharge of the lightning stroke to earth can also produce a high potential gradient in the
ground around the earthing electrode, which can be lethal to people and to animals. The
resistance to earth of each earthing electrode should be kept as low as is practicable.
The duration of a lightning discharge is so short that its thermal effect can in practice
be ignored.
When a large current of high frequency flows through a conductor which is close to
another conductor, large mechanical forces are produced. A lightning conductor must,
therefore, be very securely fixed.
A lightning conductor works by diverting to itself a stroke which might otherwise
strike part of the building being protected. The zone of protection is the space within
which a lightning conductor provides protection by attracting the stroke to itself. It has
been found that a single vertical conductor attracts to itself strokes of average or above
average intensity which in the absence of the conductor would have struck the ground
within a circle, having its centre at the conductor and a radius equal to twice the height of
the conductor. For weaker than average discharges the protected area becomes smaller.
For practical design it is therefore assumed that statistically satisfactory protection can be
given to a zone consisting of a cone with its apex at the top of the vertical conductor and
a base of radius equal to the height of the conductor. This is illustrated in Figure 14.3. For
structure of a complicated nature a ‘rolling sphere’ of 6m diameter method is used.
A horizontal conductor can be regarded as a series of apexes coalesced into a line, and
the zone of protection thus becomes a tent-like space (Figure 14.4). When there are
several parallel horizontal conductors the area between them has been found by
experience to be better protected than one would expect from the above considerations
only. On the basis of experience the recommended design criterion is that no part of the
roof should be more than 5m from the nearest horizontal conductor except that an
additional 1.0m may be added for each 1.0m by which the part to be protected is below
the nearest conductor.
Figure 14.3 Protected zone
Design of electrical services for buildings 214
Figure 14.4 Protected zone—
horizontal conductor
Whether or not a building needs protection against lightning is a matter of judgement. It
obviously depends on the risk of a lightning stroke and also on the consequence of a
stroke. Thus a higher risk of a strike can probably be accepted for an isolated small
bungalow than for, say, a children’s hospital. While no exact rules can be laid down that
would eliminate the designer’s judgement entirely, some steps can be taken to objectify
the assessment of risk and of the magnitude of the consequences. The method
recommended in BS 6651:1999 is to determine the probable number of strikes per year,
apply a weighting factor to this, and see if the result is more or less than an acceptable
level of risk. The weighting factor is the product of individual factors which take into
account the use of the structure, the type of construction, the consequential effects of a
strike, the degree of isolation and the type of country.
The probable number of strikes is given by
P=Ac×Ng×10−16
where
P=probable number of strikes per year
Ac=area protected by conductor, m2
Ng=lightning flash density, i.e. the number of flashes to ground per km2 per year.
A map showing values of Ng for different parts of the UK is shown in Figure 14.5.
This and other extracts from BS 6651 are reproduced here by permission of the British
Standards Institution (BSI). Complete copies can be obtained from BSI at Linford Wood,
Milton Keynes, MK14 6LE. It should be noted that the area protected depends on the
height of the conductor,
Lightning protection 215
Figure 14.5 Number of lightning
flashes to the ground per km2 per year
for the UK
which is normally the height of the building, and the latter is thus allowed for in the
calculation of P.
The individual weighting factors are reproduced from BS 6651 in Table 14.1. The
previously calculated value of P is multiplied by the product of
Design of electrical services for buildings 216
Table 14.1 Need for lightning protection
Weighting factor Factor
A Use of structure
Houses and similar buildings 0.3
Houses and similar buildings with outside aerial 0.7
Factories, workshops, laboratories 1.0
Offices, hotels, blocks of flats 1.2
Places of assembly, churches, halls, theatres, museums, department stores, post offices,
stations, airports, stadiums
1.3
Schools, hospitals, children’s and other homes 1.7
B Type of construction
Steel framed encased with non-metal roof 0.2
Reinforced concrete with non-metal roof 0.4
Steel framed encased or reinforced concrete with metal roof 0.8
Brick, plain concrete, or masonry with non-metal roof 1.0
Timber framed or clad with roof other than metal or thatch 1.4
Brick, plain concrete masonry, timber framed, with metal roof 1.7
Any building with a thatched roof 2.0
C Contents or effects
Contents or type of building
Ordinary domestic or office building, factories and workshops not containing valuable
materials
0.3
Industrial and agricultural buildings with specially susceptible contents 0.8
Power stations, gas installations, telephone exchanges, radio stations 1.0
Industrial key plants, ancient monuments, historic buildings, museums, art galleries 1.3
Schools, hospitals, children’s and other homes, places of assembly 1.7
D Degree of isolation
Structure in a large area of structures or trees of same height or greater height, e.g.
town or forest
0.4
Structure in area with few other structures or trees of similar height 1.0
Structure completely isolated or twice the height of surrounding structures of trees 2.0
E Type of country
Flat country at any level 0.3
Lightning protection 217
Hill country 1.0
Mountain country between 300m and 900m 1.3
Mountain country above 900m 1.7
all the individual factors to give an overall risk factor, Po. The standard recommends that
protection is needed if Po is greater than 1×10−5 per year.
A complete lightning protective system consists of an air termination network, a down
conductor and an earth termination. The air termination network is that part which is
intended to intercept lightning discharges. It consists of vertical and horizontal
conductors arranged to protect the required area in accordance with the empirical rules
which we have given above. Typical arrangements are shown in Figure 14.6.
The earth termination is that part which discharges the current into the general mass of
the earth. In other words, it is one or more earth electrodes. These have already been
discussed in Chapter 9; earth electrodes for lightning protection are no different from
earth electrodes for short-circuit protection systems. The total resistance of an earthing
system, with all electrodes in parallel, should not exceed 10ohms. It is clearly safer to
ensure that the resistance of each electrode is less than 10ohms. It is also recommended
that the same earth termination system should be used for lightning protection as for all
other services. The electrodes should be the rod or strip type, and should be either
beneath or as near as possible to, the building being protected. Plate electrodes are
expensive and come into their own only when large current-carrying capacity is
important. Because of the short duration of a lightning stroke, this is not a consideration
for lightning electrodes. The practice sometimes adopted of putting the electrode some
distance away from the building is both unnecessary and uneconomical, and may increase
the danger of voltage gradients in the ground.
The down conductor is the conductor which runs from the air termination to the earth
termination. There should be one down conductor for every 20m of perimeter. For
buildings higher than 20m there should be one down conductor for every 10m of
perimeter. A tall non-conducting chimney should have two down conductors equally
spaced, with metal conductors joining the two down conductors round the top and bottom
of the chimney and at intervals along its height. The down conductors should preferably
be distributed round the outside walls of the building. If this is for any reason not
practicable a down conductor can be contained inside a non-metallic and noncombustible
duct. It can, for example, run inside a service duct provided the service duct
does not contain any non-metal-sheathed cables. Sharp bends, as for example at the edge
of a roof, do not matter, but re-entrant loops can be dangerous. A re-entrant loop
produces a high inductive voltage drop which can cause the lightning discharge to jump
across the loop. The discharge can, for example, go through the masonry of a parapet
rather than around it. On the basis of experience it can be said that this danger may arise
when the perimeter of the loop is more than eight times the length of the open side. This
is illustrated in Figure 14.7. If a parapet is very narrow the problem can be solved by
taking the conductor through a hole in the parapet as shown in Figure 14.8.
Sometimes a building is cantilevered out at a level above the ground. If the down
conductor followed the contour of the building, there would be a real risk of flashover
Design of electrical services for buildings 218
down inside the ducts within the building. This problem and its solution are illustrated in
Figure 14.9.
The material used for lightning conductors is normally aluminium or copper. The
criterion for design is to keep the resistance from air termination to earth to a minimum.
Since the bulk of resistance is likely to occur at the
Design of electrical services for buildings 220
Figure 14.7 Re-entrant loops
Figure 14.8 Parapet
earth electrode the resistance, and therefore the size, of the down conductor would not
appear to be critical. Recommended dimensions are given in Table 14.2. Larger
conductors should be used if the system is unlikely to receive regular inspection and
maintenance.
External metal on a building should be bonded to the lightning conductor with bonds
at least as large as the conductor.
When a lightning conductor carries a stroke to earth, it is temporarily raised to a
considerable potential above earth. There is, therefore, a risk that the discharge will flash
over to nearby metal and cause damage to the intervening structure or occupants. This
can be prevented either by providing sufficient clearance between conductor and other
metal or by bonding them to ensure that there can be no potential difference between
Lightning protection 221
them. The clearance required depends on the voltage to which the lightning system rises,
which in turn depends on the current and the impedance. The impedance has
Figure 14.9 Cantilevered building
Table 14.2 Lightning conductors
Components Minimum
dimensions
Air terminations mm
Aluminium and copper strip 20×3
Aluminium, aluminium alloy, copper and phosphor bronze rods 10 diam.
Stranded aluminium conductors 19/2.50
Stranded copper conductors 19/1.80
Down conductors
Aluminium and copper strip 20×3
Aluminium, aluminium alloy and copper rods 10 diam.
Earth terminations
Hard drawn copper rods for driving into soft ground 12 diam.
Hard drawn or annealed copper rods for indirect driving or laying in
ground
10 diam.
Phosphor bronze for hard ground 12 diam.
Copper clad steel for hard ground 10 diam.
a resistive component and an inductive one; in the worst case, which is the one which
should be designed for, the two components add linearly. The induced voltage arises in a
Design of electrical services for buildings 222
loop formed by the down conductor and other metalwork so that the coupling is
generated by the self inductance minus the mutual inductance to this metal work. This
quantity is termed the transfer inductance and is given by the expression
where
MT=transfer inductance, μH m−1
S=distance between centre of down conductor and centre of nearest vertical metal
component, m
re=equivalent radius of down conductor, m.
For a circular down conductor re is the actual radius. For the more usual case of a
rectangular strip down conductor,
where
w=width, m
t=thickness, m.
The inductive voltage is proportional to the rate of change of current, and for design
purposes this must be taken as the maximum likely to occur, which is 200kA s−1. The
voltage is therefore calculated from the formula
where
VL=inductive voltage, kV
l=length of inductive loop, m
MT=transfer inductance μH m−1
n=number of down conductors.
The length of the loop is the distance over which the down conductor and other metal
run in parallel. The number of down conductors is brought into the formula because the
total current is assumed to be shared between all of them, and if the peak current reached
in one down conductor is 1/n times the total peak current, then the rate of change of
current in one conductor is also 1/n times the maximum rate of change assumed. This
assumption is not entirely valid, but it can be corrected for by the addition of 30 per cent
to the calculated voltage for the down conductor at a corner of a rectangular or square
building which has more than four down conductors, and a corresponding deduction of
30 per cent from the calculated voltage for a down conductor in the central area of such a
building.
The resistive voltage is the total maximum current, assumed to be 200kA, divided by
the number of down conductors and multiplied by the permitted resistance to earth of the
down conductor. The latter is the combined resistance of all down conductors, which is
allowed to be 10ohms, times the number of down conductors, so that the number of down
conductors in fact cancels out the equation.
Lightning protection 223
The sum of the inductive and resistive voltages is the voltage which could occur
between the down conductor and the adjacent metalwork. Figure 14.10 shows the spacing
required to avoid flashover for a given voltage. If the distance between the down
conductor and the adjacent metalwork is less than this, the metalwork should be bonded
to the down conductor. It will be found that the critical factors determining whether or
not bonding is required are usually the number of down conductors and the resistance to
earth.
Metal services entering the building should be bonded as directly as possible to the
earth termination. Large masses of metal, such as a bell frame in a church tower, should
be bonded to the nearest down conductor as directly as possible. Short isolated pieces of
metal like window frames may be ignored and do not have to be bonded. Similarly, metal
reinforcement in a structure which cannot easily be bonded and which cannot itself form
Figure 14.10 Flashover voltage in air
as a function of spacing
part of a down conductor can also be ignored. The danger from such metal is best
minimized by keeping it entirely separate from the lightning protection system.
It is perfectly in order for metal cladding or curtain walling which has a continuous
conducting path in all directions to be used as part of a lightning protection system. In the
Design of electrical services for buildings 224
extreme case, a structure which is itself a complete metal frame, such as a steel chimney,
needs no lightning conductor other than itself. It is enough to earth it effectively.
A structure having reinforcement or cladding forming a close metal mesh in the form
of internal reinforcement or screen approaches the conditions of a Faraday Cage, in
which any internal metal assumes the same potential as the cage itself. The risk of side
flashing is thereby reduced and the recommendations for bonding need not be so strictly
adhered to.
The metal bars of concrete reinforcement are tied together by binding cable. Both the
bars and the binding cable are usually rusty, so that one does not expect a good electrical
contact. Nevertheless, because there are so many of these joints in parallel the total
resistance to earth is very low, and experience has shown that it is quite safe to use the
reinforcement as a down conductor. Naturally the resistance from air termination to earth
must be checked after the structure is complete and if it is too high a separate down
conductor must after all be installed.
A building containing explosive or highly flammable materials may need more
thorough protection. An air termination network should be suspended above the building
or area to be protected, and the conductors should be spaced so that each protects a space
formed by a cone having an apex angle of 30°, i.e. a smaller zone than is adopted for less
hazardous buildings. The height of the network should be such that there is no risk of
flashover from the network to the building, and the down conductors and earth
terminations should be well away from the building. All the earth terminations should be
interconnected by a ring conductor buried in the ground. All major metal inside or on the
surface of the building should be effectively bonded to the lightning protection system.
It may be difficult to put a radio or television aerial on a roof so that it is within the
space protected by the air termination network, and this may present something of a
problem. If the down lead is concentric or twin screened, protection can be obtained by
connecting the metallic sheath of the cable to the lightning conductor. With a single or
twin down lead it is necessary to insert a discharge device between the conductors and an
earth lead. In either case metal masts, crossarms and parasitic elements should be bonded
to the lightning conductor.
As an example of the calculations described in this chapter consider a large factory in
a built-up area within Greater London. It is assumed to be 80m long by 15m wide and to
be 6m high. Vertical rods on the roof to give protection over such an area would be
impracticably high, so the air termination
Lightning protection 225
Figure 14.11 Zone of protection and
spacing of air termination
must be a network of conductors on the roof. One strip will run round the perimeter, and
an additional lengthwise strip down the centre of the roof will ensure compliance with the
requirement that no part of the roof is more than 5m from the nearest horizontal
conductor. This is shown in Figure 14.11 which also shows the area protected; the latter
extends outside the building by a distance equal to the height.
This amount of preliminary design had to be done to establish the area protected,
which is needed to determine whether or not protection is necessary. The protected area
Ac is (80+6+6)×(15+6+6)+π62=2600m2. From Figure 14.5 it is seen that for Greater
London Ns=0.6.
P=2600×0.6×10−6=1560×10−6
The weighting factors are as follows:
A: factory 1.0
B: steel frame encased with non-metal roof 0.2
C: normal factory, no special contents 0.3
D: structure in large area of structures of same height 0.4
E: flat country 0.3
Po=1560×10−6×1.0×0.2×0.3×0.4×0.3=11×10−6=1.1×10−5
This is greater than 1.0×10−5 and therefore protection is needed. The building perimeter is
(2×80)+(2×15)=190m and the number of down conductors required will therefore be
190/20=10. Each will terminate in a rod type earth electrode.
There are metal rainwater pipes running down the building, and it is necessary to
consider whether they should be bonded to the down conductors. The down conductors
may be 20mm×3mm
Design of electrical services for buildings 226
Suppose there is a rainwater pipe 1.5m from a down conductor. Then
Since both rainwater pipe and down conductor run the full height of the building, l=6 and
the number of down conductors is 10. Then
The down conductor is not at a corner, so this figure can be reduced by 30 per cent.
VL=130×0.7=90.7kV
The resistive voltage VR is 200×10=2000kV.
The flashover voltage VL+VR=90.7+2000=2100kV.
From Figure 14.10 the safe spacing for 2100kV is 5m. The rainwater pipe is less than
this distance from the down conductor and therefore bonding is required. Without
bonding, the flashover voltage arises almost completely from the resistive component and
in order to eliminate this along the whole length of the pipe bonding is required at both
top and bottom.
Standards relevant to this chapter are:
BS 6651 Code of practice for protection of structures against lightning
Lightning protection 227
Lightning protection
Lightning strokes can be of two kinds. In the first, a charged cloud induces a charge of
opposite sign in nearby tall objects, such as towers, chimneys and trees. The electrostatic
stress at the upper ends of these objects is sufficiently great to ionize the air in the
immediate neighbourhood, which lowers the resistance of the path between the cloud and
the object. Ultimately, the resistance is lowered sufficiently for a disruptive discharge to
occur between them. This type of discharge is characterized by the time taken to produce
it, and by the fact that it usually strikes against the highest and most pointed object in the
area.
The second kind of stroke is a discharge which occurs suddenly when a potential
difference between a cloud and the earth is established almost instantly. It is generally
induced by a previous stroke of the first kind; thus if a stroke of this kind takes place
between clouds 1 and 2 (Figure 14.1), cloud 3 may be suddenly left with a greater
potential gradient immediately adjacent to it than the air can withstand, and a stroke to
earth suddenly occurs. This type of stroke occurs suddenly and is not necessarily directed
to tall sharp objects like the first kind of stroke. It may miss tall objects and strike the
ground nearby. Figure 14.2 shows other ways in which this. kind of stroke may be
induced. In each case, A is a stroke of the first kind and B is the second type of stroke
induced by A. In each case the first stroke from cloud 1 changes the potential gradient at
cloud 2 and thus produces the second stroke.
The current in a discharge is uni-directional and consists of impulses with very steep
wave fronts. The equivalent frequency of these impulses varies from 10kHz to 100kHz.
While some lightning discharges consist of a single stroke, others consist of a series of
strokes following each other along the same path in rapid succession. The current in a
single stroke can vary from about 2000A to a maximum of about 200000A, with a
statistical average of 20000A. It rises to a peak value in a few microseconds. When a
discharge consists of several successive strokes, each stroke rises and falls in a time and
to an amplitude of this order so that the whole discharge can last up to a second.
Figure 14.1 Induced lightning stroke
The effects of a discharge on a structure are electrical, thermal and mechanical.
As the current passes through the structure to earth it produces a voltage drop which
momentarily raises the potential of part of the structure to a high value above earth. One
function of a lightning conductor is to keep this potential as low as possible by providing
a very low resistance path to earth. It is recommended in the British Standard Code of
Practice (BS 6651:1999) that the resistance to earth of the protective system should not
exceed 10ohms. The sharp wave front of the discharge is equivalent to a high-frequency
current and, therefore, there is also an inductive voltage drop which has to be added
phasorially to the resistive drop. Part of the lightning conductor is thus inevitably raised
to a high potential. This brings with it a risk of flashover from the conductor to other
metal in the structure, such as water and gas pipes and electrical cables. These in turn
would then be raised to high potential which could bring danger to occupants of the
building, and it is necessary to guard against such flashovers. Bonding the lightning
Figure 14.2 Induced lightning strokes
Lightning protection 213
protection system to the main earthing terminal of the installation does this. The
discharge of the lightning stroke to earth can also produce a high potential gradient in the
ground around the earthing electrode, which can be lethal to people and to animals. The
resistance to earth of each earthing electrode should be kept as low as is practicable.
The duration of a lightning discharge is so short that its thermal effect can in practice
be ignored.
When a large current of high frequency flows through a conductor which is close to
another conductor, large mechanical forces are produced. A lightning conductor must,
therefore, be very securely fixed.
A lightning conductor works by diverting to itself a stroke which might otherwise
strike part of the building being protected. The zone of protection is the space within
which a lightning conductor provides protection by attracting the stroke to itself. It has
been found that a single vertical conductor attracts to itself strokes of average or above
average intensity which in the absence of the conductor would have struck the ground
within a circle, having its centre at the conductor and a radius equal to twice the height of
the conductor. For weaker than average discharges the protected area becomes smaller.
For practical design it is therefore assumed that statistically satisfactory protection can be
given to a zone consisting of a cone with its apex at the top of the vertical conductor and
a base of radius equal to the height of the conductor. This is illustrated in Figure 14.3. For
structure of a complicated nature a ‘rolling sphere’ of 6m diameter method is used.
A horizontal conductor can be regarded as a series of apexes coalesced into a line, and
the zone of protection thus becomes a tent-like space (Figure 14.4). When there are
several parallel horizontal conductors the area between them has been found by
experience to be better protected than one would expect from the above considerations
only. On the basis of experience the recommended design criterion is that no part of the
roof should be more than 5m from the nearest horizontal conductor except that an
additional 1.0m may be added for each 1.0m by which the part to be protected is below
the nearest conductor.
Figure 14.3 Protected zone
Design of electrical services for buildings 214
Figure 14.4 Protected zone—
horizontal conductor
Whether or not a building needs protection against lightning is a matter of judgement. It
obviously depends on the risk of a lightning stroke and also on the consequence of a
stroke. Thus a higher risk of a strike can probably be accepted for an isolated small
bungalow than for, say, a children’s hospital. While no exact rules can be laid down that
would eliminate the designer’s judgement entirely, some steps can be taken to objectify
the assessment of risk and of the magnitude of the consequences. The method
recommended in BS 6651:1999 is to determine the probable number of strikes per year,
apply a weighting factor to this, and see if the result is more or less than an acceptable
level of risk. The weighting factor is the product of individual factors which take into
account the use of the structure, the type of construction, the consequential effects of a
strike, the degree of isolation and the type of country.
The probable number of strikes is given by
P=Ac×Ng×10−16
where
P=probable number of strikes per year
Ac=area protected by conductor, m2
Ng=lightning flash density, i.e. the number of flashes to ground per km2 per year.
A map showing values of Ng for different parts of the UK is shown in Figure 14.5.
This and other extracts from BS 6651 are reproduced here by permission of the British
Standards Institution (BSI). Complete copies can be obtained from BSI at Linford Wood,
Milton Keynes, MK14 6LE. It should be noted that the area protected depends on the
height of the conductor,
Lightning protection 215
Figure 14.5 Number of lightning
flashes to the ground per km2 per year
for the UK
which is normally the height of the building, and the latter is thus allowed for in the
calculation of P.
The individual weighting factors are reproduced from BS 6651 in Table 14.1. The
previously calculated value of P is multiplied by the product of
Design of electrical services for buildings 216
Table 14.1 Need for lightning protection
Weighting factor Factor
A Use of structure
Houses and similar buildings 0.3
Houses and similar buildings with outside aerial 0.7
Factories, workshops, laboratories 1.0
Offices, hotels, blocks of flats 1.2
Places of assembly, churches, halls, theatres, museums, department stores, post offices,
stations, airports, stadiums
1.3
Schools, hospitals, children’s and other homes 1.7
B Type of construction
Steel framed encased with non-metal roof 0.2
Reinforced concrete with non-metal roof 0.4
Steel framed encased or reinforced concrete with metal roof 0.8
Brick, plain concrete, or masonry with non-metal roof 1.0
Timber framed or clad with roof other than metal or thatch 1.4
Brick, plain concrete masonry, timber framed, with metal roof 1.7
Any building with a thatched roof 2.0
C Contents or effects
Contents or type of building
Ordinary domestic or office building, factories and workshops not containing valuable
materials
0.3
Industrial and agricultural buildings with specially susceptible contents 0.8
Power stations, gas installations, telephone exchanges, radio stations 1.0
Industrial key plants, ancient monuments, historic buildings, museums, art galleries 1.3
Schools, hospitals, children’s and other homes, places of assembly 1.7
D Degree of isolation
Structure in a large area of structures or trees of same height or greater height, e.g.
town or forest
0.4
Structure in area with few other structures or trees of similar height 1.0
Structure completely isolated or twice the height of surrounding structures of trees 2.0
E Type of country
Flat country at any level 0.3
Lightning protection 217
Hill country 1.0
Mountain country between 300m and 900m 1.3
Mountain country above 900m 1.7
all the individual factors to give an overall risk factor, Po. The standard recommends that
protection is needed if Po is greater than 1×10−5 per year.
A complete lightning protective system consists of an air termination network, a down
conductor and an earth termination. The air termination network is that part which is
intended to intercept lightning discharges. It consists of vertical and horizontal
conductors arranged to protect the required area in accordance with the empirical rules
which we have given above. Typical arrangements are shown in Figure 14.6.
The earth termination is that part which discharges the current into the general mass of
the earth. In other words, it is one or more earth electrodes. These have already been
discussed in Chapter 9; earth electrodes for lightning protection are no different from
earth electrodes for short-circuit protection systems. The total resistance of an earthing
system, with all electrodes in parallel, should not exceed 10ohms. It is clearly safer to
ensure that the resistance of each electrode is less than 10ohms. It is also recommended
that the same earth termination system should be used for lightning protection as for all
other services. The electrodes should be the rod or strip type, and should be either
beneath or as near as possible to, the building being protected. Plate electrodes are
expensive and come into their own only when large current-carrying capacity is
important. Because of the short duration of a lightning stroke, this is not a consideration
for lightning electrodes. The practice sometimes adopted of putting the electrode some
distance away from the building is both unnecessary and uneconomical, and may increase
the danger of voltage gradients in the ground.
The down conductor is the conductor which runs from the air termination to the earth
termination. There should be one down conductor for every 20m of perimeter. For
buildings higher than 20m there should be one down conductor for every 10m of
perimeter. A tall non-conducting chimney should have two down conductors equally
spaced, with metal conductors joining the two down conductors round the top and bottom
of the chimney and at intervals along its height. The down conductors should preferably
be distributed round the outside walls of the building. If this is for any reason not
practicable a down conductor can be contained inside a non-metallic and noncombustible
duct. It can, for example, run inside a service duct provided the service duct
does not contain any non-metal-sheathed cables. Sharp bends, as for example at the edge
of a roof, do not matter, but re-entrant loops can be dangerous. A re-entrant loop
produces a high inductive voltage drop which can cause the lightning discharge to jump
across the loop. The discharge can, for example, go through the masonry of a parapet
rather than around it. On the basis of experience it can be said that this danger may arise
when the perimeter of the loop is more than eight times the length of the open side. This
is illustrated in Figure 14.7. If a parapet is very narrow the problem can be solved by
taking the conductor through a hole in the parapet as shown in Figure 14.8.
Sometimes a building is cantilevered out at a level above the ground. If the down
conductor followed the contour of the building, there would be a real risk of flashover
Design of electrical services for buildings 218
down inside the ducts within the building. This problem and its solution are illustrated in
Figure 14.9.
The material used for lightning conductors is normally aluminium or copper. The
criterion for design is to keep the resistance from air termination to earth to a minimum.
Since the bulk of resistance is likely to occur at the
Design of electrical services for buildings 220
Figure 14.7 Re-entrant loops
Figure 14.8 Parapet
earth electrode the resistance, and therefore the size, of the down conductor would not
appear to be critical. Recommended dimensions are given in Table 14.2. Larger
conductors should be used if the system is unlikely to receive regular inspection and
maintenance.
External metal on a building should be bonded to the lightning conductor with bonds
at least as large as the conductor.
When a lightning conductor carries a stroke to earth, it is temporarily raised to a
considerable potential above earth. There is, therefore, a risk that the discharge will flash
over to nearby metal and cause damage to the intervening structure or occupants. This
can be prevented either by providing sufficient clearance between conductor and other
metal or by bonding them to ensure that there can be no potential difference between
Lightning protection 221
them. The clearance required depends on the voltage to which the lightning system rises,
which in turn depends on the current and the impedance. The impedance has
Figure 14.9 Cantilevered building
Table 14.2 Lightning conductors
Components Minimum
dimensions
Air terminations mm
Aluminium and copper strip 20×3
Aluminium, aluminium alloy, copper and phosphor bronze rods 10 diam.
Stranded aluminium conductors 19/2.50
Stranded copper conductors 19/1.80
Down conductors
Aluminium and copper strip 20×3
Aluminium, aluminium alloy and copper rods 10 diam.
Earth terminations
Hard drawn copper rods for driving into soft ground 12 diam.
Hard drawn or annealed copper rods for indirect driving or laying in
ground
10 diam.
Phosphor bronze for hard ground 12 diam.
Copper clad steel for hard ground 10 diam.
a resistive component and an inductive one; in the worst case, which is the one which
should be designed for, the two components add linearly. The induced voltage arises in a
Design of electrical services for buildings 222
loop formed by the down conductor and other metalwork so that the coupling is
generated by the self inductance minus the mutual inductance to this metal work. This
quantity is termed the transfer inductance and is given by the expression
where
MT=transfer inductance, μH m−1
S=distance between centre of down conductor and centre of nearest vertical metal
component, m
re=equivalent radius of down conductor, m.
For a circular down conductor re is the actual radius. For the more usual case of a
rectangular strip down conductor,
where
w=width, m
t=thickness, m.
The inductive voltage is proportional to the rate of change of current, and for design
purposes this must be taken as the maximum likely to occur, which is 200kA s−1. The
voltage is therefore calculated from the formula
where
VL=inductive voltage, kV
l=length of inductive loop, m
MT=transfer inductance μH m−1
n=number of down conductors.
The length of the loop is the distance over which the down conductor and other metal
run in parallel. The number of down conductors is brought into the formula because the
total current is assumed to be shared between all of them, and if the peak current reached
in one down conductor is 1/n times the total peak current, then the rate of change of
current in one conductor is also 1/n times the maximum rate of change assumed. This
assumption is not entirely valid, but it can be corrected for by the addition of 30 per cent
to the calculated voltage for the down conductor at a corner of a rectangular or square
building which has more than four down conductors, and a corresponding deduction of
30 per cent from the calculated voltage for a down conductor in the central area of such a
building.
The resistive voltage is the total maximum current, assumed to be 200kA, divided by
the number of down conductors and multiplied by the permitted resistance to earth of the
down conductor. The latter is the combined resistance of all down conductors, which is
allowed to be 10ohms, times the number of down conductors, so that the number of down
conductors in fact cancels out the equation.
Lightning protection 223
The sum of the inductive and resistive voltages is the voltage which could occur
between the down conductor and the adjacent metalwork. Figure 14.10 shows the spacing
required to avoid flashover for a given voltage. If the distance between the down
conductor and the adjacent metalwork is less than this, the metalwork should be bonded
to the down conductor. It will be found that the critical factors determining whether or
not bonding is required are usually the number of down conductors and the resistance to
earth.
Metal services entering the building should be bonded as directly as possible to the
earth termination. Large masses of metal, such as a bell frame in a church tower, should
be bonded to the nearest down conductor as directly as possible. Short isolated pieces of
metal like window frames may be ignored and do not have to be bonded. Similarly, metal
reinforcement in a structure which cannot easily be bonded and which cannot itself form
Figure 14.10 Flashover voltage in air
as a function of spacing
part of a down conductor can also be ignored. The danger from such metal is best
minimized by keeping it entirely separate from the lightning protection system.
It is perfectly in order for metal cladding or curtain walling which has a continuous
conducting path in all directions to be used as part of a lightning protection system. In the
Design of electrical services for buildings 224
extreme case, a structure which is itself a complete metal frame, such as a steel chimney,
needs no lightning conductor other than itself. It is enough to earth it effectively.
A structure having reinforcement or cladding forming a close metal mesh in the form
of internal reinforcement or screen approaches the conditions of a Faraday Cage, in
which any internal metal assumes the same potential as the cage itself. The risk of side
flashing is thereby reduced and the recommendations for bonding need not be so strictly
adhered to.
The metal bars of concrete reinforcement are tied together by binding cable. Both the
bars and the binding cable are usually rusty, so that one does not expect a good electrical
contact. Nevertheless, because there are so many of these joints in parallel the total
resistance to earth is very low, and experience has shown that it is quite safe to use the
reinforcement as a down conductor. Naturally the resistance from air termination to earth
must be checked after the structure is complete and if it is too high a separate down
conductor must after all be installed.
A building containing explosive or highly flammable materials may need more
thorough protection. An air termination network should be suspended above the building
or area to be protected, and the conductors should be spaced so that each protects a space
formed by a cone having an apex angle of 30°, i.e. a smaller zone than is adopted for less
hazardous buildings. The height of the network should be such that there is no risk of
flashover from the network to the building, and the down conductors and earth
terminations should be well away from the building. All the earth terminations should be
interconnected by a ring conductor buried in the ground. All major metal inside or on the
surface of the building should be effectively bonded to the lightning protection system.
It may be difficult to put a radio or television aerial on a roof so that it is within the
space protected by the air termination network, and this may present something of a
problem. If the down lead is concentric or twin screened, protection can be obtained by
connecting the metallic sheath of the cable to the lightning conductor. With a single or
twin down lead it is necessary to insert a discharge device between the conductors and an
earth lead. In either case metal masts, crossarms and parasitic elements should be bonded
to the lightning conductor.
As an example of the calculations described in this chapter consider a large factory in
a built-up area within Greater London. It is assumed to be 80m long by 15m wide and to
be 6m high. Vertical rods on the roof to give protection over such an area would be
impracticably high, so the air termination
Lightning protection 225
Figure 14.11 Zone of protection and
spacing of air termination
must be a network of conductors on the roof. One strip will run round the perimeter, and
an additional lengthwise strip down the centre of the roof will ensure compliance with the
requirement that no part of the roof is more than 5m from the nearest horizontal
conductor. This is shown in Figure 14.11 which also shows the area protected; the latter
extends outside the building by a distance equal to the height.
This amount of preliminary design had to be done to establish the area protected,
which is needed to determine whether or not protection is necessary. The protected area
Ac is (80+6+6)×(15+6+6)+π62=2600m2. From Figure 14.5 it is seen that for Greater
London Ns=0.6.
P=2600×0.6×10−6=1560×10−6
The weighting factors are as follows:
A: factory 1.0
B: steel frame encased with non-metal roof 0.2
C: normal factory, no special contents 0.3
D: structure in large area of structures of same height 0.4
E: flat country 0.3
Po=1560×10−6×1.0×0.2×0.3×0.4×0.3=11×10−6=1.1×10−5
This is greater than 1.0×10−5 and therefore protection is needed. The building perimeter is
(2×80)+(2×15)=190m and the number of down conductors required will therefore be
190/20=10. Each will terminate in a rod type earth electrode.
There are metal rainwater pipes running down the building, and it is necessary to
consider whether they should be bonded to the down conductors. The down conductors
may be 20mm×3mm
Design of electrical services for buildings 226
Suppose there is a rainwater pipe 1.5m from a down conductor. Then
Since both rainwater pipe and down conductor run the full height of the building, l=6 and
the number of down conductors is 10. Then
The down conductor is not at a corner, so this figure can be reduced by 30 per cent.
VL=130×0.7=90.7kV
The resistive voltage VR is 200×10=2000kV.
The flashover voltage VL+VR=90.7+2000=2100kV.
From Figure 14.10 the safe spacing for 2100kV is 5m. The rainwater pipe is less than
this distance from the down conductor and therefore bonding is required. Without
bonding, the flashover voltage arises almost completely from the resistive component and
in order to eliminate this along the whole length of the pipe bonding is required at both
top and bottom.
Standards relevant to this chapter are:
BS 6651 Code of practice for protection of structures against lightning
Lightning protection 227
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