The presentation deals with principles of protecting buildings/structures and and power systems from effects of lightning.It also deals with protecting the power systems from over voltages arising from lightning and switching.

1. LIGHTNING PROTECTION
TO STRUCTURES AND
POWER SYSTEMS
Prof. Suryanarayana Danturti

2. What is Lightning?
 Lightning is a discharge of electricity between two
clouds or a cloud and earth.
 The magnitude of current is in the range of about
2kA to 200kA.
 The current rises rapidly typically in about 5-10
microseconds and falls to a low value in about
100microseconds.

3. EFFECTS OF LIGHTNING
 When the lightning discharge passes through a
conducting path like a lightning conductor heat is
produced but the amount of heat produced is very
small as the phenomena is short lived and lasts for
100 micro seconds or less.

4.  When the discharge current (i) passes through the
lightning conductor a large voltage drop is produced
as the magnitude of the current and also the rate of
rise of current (di/dt) are very high .
 The voltage drop is given by iR+ l di/dt
 Even though the resistance(R) and the inductance
of the lightning conductor(l) are small, the voltage
drop, as given above, is high as i and di/dt are
very high. This rises the potential of the lightning
conductor momentarily to a very high value during
discharge and may cause arcing at sharp bends in
the lightning conductor.

5.  When the lightning strikes a structure made up of
non conducting materials like brick work, masonry or
concrete, it is shattered.
 Since most of the structures are made up of non
conducting materials , structures which are prone to
lightning are provided with a lightning conductor
system to protect the structure against lightning.

6. PRINCIPLE OF LIGHT PROTECTION
 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 with in which a
lightning conductor provides protection by attracting
the stroke to itself.

7.  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. The protective angle works out to 63.5
degrees as shown in the next slide.
 For weaker than average discharges the protective area
becomes smaller.

9. Practical design rule
 Statically satisfactory protection can be provided 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. Thus the protective
angle becomes 45 degrees.
 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 shown in
the next slide.

10. Protective zone of a single vertical conductor
Courtesy: IS 2309:1989

11. Protective zone of a single horizontal conductor
Courtesy: The Design of Electrical Services for Buildings by
F.Porges

12.  Zone of protection for a building provided with a
lightning conductor of square shape laid along the
periphery of the roof is shown in the next slide

13. Courtesy: IS 2309:1989

14. RISK INDEX
 Whether or not a building needs protection is
decided based on probability of lightning stroke and
its consequences. The risk index is calculated based
on the following factors.
 Index numbers 1 to 10 are allotted for each factor.
Higher index figure indicates greater risk.

15.  Use of Structure :- Residential buildings or houses
in a large colony of similar houses have lower risk
index (2to4).
 Public places like big office buildings, churches,
assembly halls, auditoriums, post offices, stadiums,
hospitals and schools etc have higher risk index (6 to
10).
 Type of construction:- Buildings made up of
concrete, masonry, brick work have lower risk index
(2 to 4) while made up of timber or metal roofs have
larger risk index (7to9).

16.  Contents or Effects :-Domestic buildings, small
workshops or factories have smaller risk index(2 to
4), while power stations, telephone exchanges,
petroleum and gas installations, key industrial plants,
monuments, historical buildings, schools, hospitals
have greater risk index( 6 to10).
 Degree of Isolation:-buildings surrounded by
buildings of similar height have lesser risk index(2 to
4),while isolated buildings or buildings twice the
height of surrounding buildings or trees have higher
risk index (8to10).

17.  Type of Country:- Flat country or hill country at
lower altitude below 300 Mtrs has a lower index (2 to
6),while mountainous country at higher altitudes
have higher index ( 8 to 10).
 Height of structures:- Risk index increases for
higher structures from 9 to 53 Mtrs (from 2 to
30).Higher than 53 Mtr structures require
protection in all cases.
 Lightning prevalence:-Number of thunder storm
days in a year varies from 3 to 200 or more. Risk
index is equal to the number of thunder storm days
up to 21. Above 21 days risk index remains 21.

18.  Total risk index based on above factors is calculated.
If the total is more than 40 lightning protection is
generally recommended.

19. DESIGN LIGHTNING PROTECTION SYSTEM
 Hence a structure can be protected by providing a
set of horizontal conductors to attract the lightning
strokes and a set of down conductors which are
earthed to discharge the current to the earth.
 Number of horizontal conductors:- For buildings
with flat roof no part of the building should be more
than 9 m from the nearest horizontal conductor .In
case of buildings with inclined roof an additional 0.3
m may be added for each 0.3 m by which the part to
be protected is below the nearest conductor.

20.  Hence the maximum spacing between conductors
shall not be more than 18 m. for flat roofs
 For inclined roofs, the spacing can be beyond 18 m
as per rule stated above.
 Normally lightning conductor is provided along
the periphery of the roof. Extra horizontal
conductors are fixed if required as shown

21. Horizontal conductors for a flat roof
 Arrangement of horizontal conductor system for
buildings with flat roof are shown in the following
slides. These arrangements ensures no part of the
building is more than 9 Mtrs from the nearest
horizontal conductor.

22. Courtesy: IS 2309:1989
Elevation showing protective angle

23. 18m
(max.)
Courtesy: IS 2309:1989
Plan showing zone of protection at ground level

24. 18m
(max.)
Courtesy: IS 2309:1989
Isometric View of building showing horizontal
conductors

25. Courtesy: The Design of Electrical Services for Buildings by
F.Porges
Plan of the building showing horizontal
conductors

26. Courtesy: The Design of Electrical Services for Buildings by
F.Porges
Part elevation of the building

27. Courtesy: The Design of Electrical Services for Buildings by
F.Porges
Horizontal conductor system for low factory building with large horizontal roof

28. Horizontal conductors for a inclined roof
 Following slides show two examples of common
profiles for inclined roofs covering large areas.
 Horizontal conductors are fixed along the ridges
bounded at either end by conductors following the
roof profile.

29.  A 10 Mtrs horizontal spacing is adopted between
conductors because of site conditions as the same
can be laid along the ridges only.
If S> 10+H additional longitudinal conductors are
required.

30. Industrial building with inclined roof
Courtesy: IS 2309:1989

31. Courtesy: IS 2309:1989

32. Courtesy: IS 2309:1989
Industrial building with a different roof profile

33.  If the length of the roof exceeds 20 Mtrs additional
transverse conductors are required as shown in the
next slide.

34. Courtesy: IS 2309:1989

35.  Horizontal conductor system is quite effective in
attracting the lightning strokes and use of pointed air
terminations or vertical finials is, therefore not
required as essential .
 However a vertical conductor may be used to protect
a special structure like a chimney or a narrow vertical
structure like a pent house.(illustrated in the
following slides)

36. Courtesy: The Design of Electrical Services for Buildings by
F.Porges
Plan showing a building with penthouse

37. Courtesy: The Design of Electrical Services for Buildings by
F.Porges
Elevation showing a building with penthouse

38. Number of Down Conductors
 One conductor if the base area is not more than 100
Sqm.
 If the base area is more than 100 Sqm adopt
smaller of the following
 One plus an additional one for each 300 Sqm or part
there of
 One for each 30 m of the perimeter of the structure.

39.  A down conductor should follow the most direct path
between the horizontal conductor network and the
earth terminal network.
 Where more than one down conductor is used the
conductors should be arranged as evenly as
practicable around the periphery of the structure.

40.  Each down conductor is connected to a separate
earth electrode.
 Where there are more than one earth electrode, all
the earth electrodes are interconnected by a ground
ring . This is called earth terminal network.
 The total earth resistance of the complete lightning
protection system to earth should not exceed 10
ohms.

41. Protection to Buildings where Explosives
are Manufactured, Processed or Stored
 For Such buildings, smaller spacing between
horizontal conductors, larger number of down
conductors are adopted. Refer BS /BIS code of
practice for details.

42. Protection to Power systems
Transmission lines are to be protected against
1. Direct strokes
2. Over voltages induced in the power system due to
indirect strokes

43.  Protection against direct strokes is by shielding.
 A shielding angle of 30 degrees is adopted which
means , all the live conductors below should be
within an angle of 30 degrees as shown in the next
slide
 Field studies have indicated that a shielding angle of
30 degree provides 0.1 % exposure. This means out
of 1000 strokes 999 strokes will be shielded and only
stroke may hit the protected object.

45.  However the presence of shield wire does not provide
protection against induced over voltages occurring in the
power system due to indirect strokes.
 Also over voltages occur in the power system due to
switching.
 As components of power system i.e. transformers,
transmission lines, cables etc possess both inductance
and capacitance and transient over voltages are
produced when ever such circuits are either switched on
or off.

46. Transient Over Voltages
 The frequency range of the switching transients are
in the range of a few kilo cycles and last for a few
hundred milliseconds.
 The magnitude of the switching over voltages are in
the range of 3 to 4 times the magnitude of the
system voltage.
 In the case of lightning over voltages the magnitude
is independent of the system voltage and may reach
values up to 1500 to 2000 KV and above.
 The frequency range is in the range of a few
hundred mega cycles and may last up to a few
hundred micro seconds.

47.  The most commonly used device for protection
against high voltage surges is called a surge
diverter. It is connected between each line and the
earth
 For a three phase system a set of three surge
diverters will be connected between each line and
earth as shown in the next slide.

49.  The surge diverter diverts to earth any voltage surge
in excess of a pre fixed voltage wave. When a
voltage surge reaches the diverter, it breaks down
and sparks over at a certain prefixed voltage as
shown by point A in the next slide and provides a
conducting path of low impedance to earth.

51. OPERATION OF SURGE DIVERTOR
 The voltage surge is, diverted to the earth and this
results in flow of current to earth through the diverter.
 Since the impedance of the path is low and the
voltage is very high, a very high current flows which
is in the range of a few kilo amperes for the duration
of the surge which may last for a few microseconds
to few milliseconds.
 The voltage drop across the surge diverter due to
flow of heavy current and due to the finite impedance
of the diverter is called residual voltage which is
shown in the previous slide.

52.  In other words the insulation of the equipment ‘sees’
only the residual voltage as against being subjected
to the peak voltage ‘P’ of the surge shown in dotted
line in the previous slide. The figure also shows the
resulting current wave.
 It is very important to note that this low impedance
path should not exist before the over voltage
appears and it should cease to as soon as the
voltage returns to normal.

53. REQUIREMENTS OF IDEAL SURGE DIVERTOR.
 No current should flow through the diverter during the
normal system voltage .i.e its power frequency break
down (or spark over) voltage must be above any normal
or abnormal power frequency voltage that may occur.
 Any voltage surge with a peak voltage exceeding the
spark over voltage of the surge diverter must cause it to
break down as quickly as possible and provide low
impedance path to ground.

54.  Having broken down, it must be capable of carrying
the resulting discharge current which shall be in the
range of a few kilo amperes for the duration of the
surge ,without damage to itself and without the
voltage across (residual voltage) it exceeding the
breakdown voltage of the insulation of the equipment
it is protecting.
 The surge diverter must regain its high impedance
as soon as the surge has passed to interrupt the flow
of power frequency current due to normal system
voltage. An ideal surge diverter shall offer infinite
impedance under normal conditions and quickly
breaks down and offers zero impedance as soon as
a high voltage surge reaches it.

55.  Surge diverters are also called lightning arrestors. As
the device actually diverts the surge to ground and
as it cannot arrest (stop) the surge ,the term surge
diverter is technically correct.
 METAL oxide type arrestors are used for protecting
the power systems against over voltages induced
due to lightning strokes or switching.

56.  For selecting the voltage rating and current rating of
surge diverters, location and method of installation
and for other details any book on Power System
Protection may be referred.

57. REFERENCES
 The Design of Electrical Services for Buildings by
F.Porges
 B S Code of Practice 326 of 1965
 B I S Code of Practice 2309 of 1989
 Power System analysis and Design by Glover and
Sharma

58.  For suggestions and clarifications contact
 dsurya1@rediffmail.com