In Article " Introduction to Lightning System Design- Part One ", I listed all terms, abbreviations and Symbols used in lightning field.
Also, in Article " Introduction to Lightning System Design- Part Two ", I answered the following questions:
- What is Lightning?
- What are the types of Lightning flashes?
- What is the shape of The Lightning Waveform?
- How Lightning strikes can affect the electrical and/or electronic systems of a building?
- What are the main effects of Lightning?
And in Article " Types Of Lightning Protection Systems LPS ", I list the main types of Lightning Protection Systems as follows:
Types of Lightning
Protection Systems LPS
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Types of Lightning Protection Systems LPS
Lightning protection systems for buildings and installations may be divided into three principal types as follows:
1- LPS for Protection for buildings and installations against direct strike by lightning, which includes:
A- Conventional lightning protection system, which includes:
B- Non-Conventional lightning protection system, which includes:
a- Active Attraction LPS, which includes:
b- Active Prevention/Elimination LPS, which includes:
2- LPS for Protection against overvoltage on incoming conductors and conductor systems,
3- LPS for Protection against the electromagnetic pulse of the lightning.
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And in Article " Conventional Lightning Protection System Components – Part One ", I indicated the Conventional Lightning Protection System parts and components as follows:
Conventional Lightning Protection System LPS Components
The Conventional Lightning Protection System consists of two main parts:
1- The External Lightning Protection System, which includes:
2- The Internal Lightning Protection System, which includes:
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And I explained the Strike Termination Subsystem in this Article.
Also, in Article " Conventional Lightning Protection System Components – Part Two ", I began explaining the Conductor Subsystem through the following points:
- Function of Conductor Subsystems,
- Effects of Lightning Strikes on Conductor Subsystems,
- Conductor Subsystem Material Requirements.
And in Article " Conventional Lightning Protection System Components – Part Three ", I explained Types of Lightning Conductors and Installation Requirements for Down Conductors.
And in Article " Conventional Lightning Protection System Components – Part Four ", I explained How to use Natural Structure Components as down Conductors.
Today, I will explain The Third Subsystem of the External Lightning Protection System; Grounding Electrode Subsystem.
1- Grounding Electrode Subsystem (Earth Termination System)
The reliable
performance of the entire lightning protection system is dependent upon an
effective earthing system. The grounding electrode subsystem to be effective,
it must has:
Notes:
Functions of
Grounding
Electrode Subsystem:
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2- Grounding System – General Overview
The grounding system
must have a low impedance to disperse the energy of the lightning strike.
Grounding systems are highly variable from site to site due to geographical
considerations. However, The Variables that affect the design,
performance and Impedance of the grounding system are:
Notes:
In the following
Articles, you can review The Introduction And Components Of Earthing
Systems:
And In the following Articles, you can
review The Design Steps Of Earthing Systems:
Also I explained the
Methods of Grounding Design Calculations of Domestic, commercial
and industrial premises in the following Articles:
And I explained The Grounding Design Calculations for second type of buildings: AC
Substations in the following Articles:
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3- Resistance value for Grounding Electrode Subsystem
Note:
The
resistance to earth of the complete lightning protection system (L.P.S) must
be measured at a frequency of, or multiple of the power system frequency.
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4- Grounding Electrode Subsystem Types
Three basic
earth electrode arrangements are used:
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Type A arrangement
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4.1 Type A arrangement
This consists of
horizontally star-type earth electrodes or vertical earth electrodes installed outside the
structure footprint (see Fig.1), There must be earth electrodes installed at the base of
each down-conductor fixed on the outside of the structure. A minimum of two electrodes must
be used.
Type A Arrangement
Criteria:
Table: Earth rods required to achieve 10 ohms
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4.1.1 Calculating the minimum total length of electrode at
each down-conductor for Type A arrangement
The minimum total
length of electrode at each down-conductor is:
Where L1 is obtained from Fig.3 (Figure 2 of BS EN
62305-3).
Notes:
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Example#1:
A Class I
lightning protection system is installed with 6 down conductors in 2000 ohm.m
soil. What will be the effective design and configuration of the earth termination
system?
Solution:
From
Fig.3 it is
seen that L1 = 50 m.
So, Requirements could be met with:
6 x 50 m
horizontal conductors, or
6 x 25 m
vertical electrodes.
Alternative
scenarios for each down-conductor by using 1.2 m driven ground rods are as
follows:
1- Use of single
vertical electrode:
25 m of
electrode would need to be installed (25.5 m for sites affected by freezing
soil), with the top of electrode buried at a depth of at least 0.5 m. (Total
requirements 21 x 1.2 m ground rods, 20 couplers and 1 rod clamp)
The
installation of a single deep electrode could be difficult to install in many
soil types without specialist driving equipment. More practical would be a
greater number of parallel connected shorter rods.
2- Use of multiple vertical
electrodes:
For sites
not affected by freezing soil, a solution could be achieved by 11 parallel
electrodes connected with vertical conductors (the effect of the vertical
conductor interconnecting these rods was not considered).
Each
electrode could be comprised of 2 x 1.2 m coupled ground rods. For optimum
effectiveness these electrodes should be spaced apart a distance equal to 1
to 2 times their length, i.e. 2.4 to 4.8 m apart.
(Total
requirements 22 x 1.2 m ground rods, 11 couplers, 11 rod clamps & 24 to
48 m conductor).
3- Use of multiple
vertical electrodes connected with horizontal bare conductors:
Assuming vertical
electrodes were placed at 2.4 m apart, and bare conductor interconnects the
rods, then the presence of horizontal conductor has the same effect as adding
1.2 m to each vertical electrode.
11 x2x
1.2 m required ground rods in case#2 above will be minimized to 8 x2x 1.2 m
electrodes would be required.
(Total requirements
16 x 1.2 m ground rods, 8 couplers, 8 rod clamps & 17 m of conductor).
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4.1.2 Type A Arrangement For Sites With Extreme
Weather Conditions
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Example#2:
Considering case in
example#1, and for sites affected by freezing soil. What
will be the effective design and configuration of the earth
termination system?
Solution:
If using parallel 2.4
m electrodes, only the lower 1.9 m would be “effective”, requiring 14
electrodes. A more appropriate configuration may be 6 x 4.8 m electrodes
(total requirements: 24 x 1.2 m ground rods, 18 couplers, 6 rod clamps and 24
to 48 m conductor).
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Type B arrangement
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4.2 Type B arrangement
This arrangement is
essentially a ring earth electrode or natural elements within the foundation
that is sited around the periphery of the structure and is in contact with
the surrounding soil for a minimum 80% of its total length (i.e. 20% of its
overall length may be housed in say the basement of the structure and not in
direct contact with the earth).(see fig.4)
Type B Arrangement
Criteria:
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4.2.1 Calculating the minimum length of the ring earth
electrode
for Type B
arrangement
For earth electrodes Type B arrangement, the average radius re of the area enclosed by the
earth electrode must be not less than the given minimum length L1 provided from Fig.3 (Figure 2 of BS EN
62305-3).So:
re ≥ L1
To determine the average
radius re, the area under
consideration is transferred into an equivalent circular area and the radius
is determined as shown in Fig.5.
Notes:
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Example#3:
A class I lightning
protection system is installed with 6 down-conductors in 2000 ohm.m soil. The
building perimeter is 60 m (20 x 10 m).
Determine if
additional electrodes are required or not?
If additional
electrodes are required, determine numbers and quantity for horizontal
radials, or vertical electrodes?
Solution:
From Fig.3, it is seen that L1 = 50 m (where re is the mean radius of
the area enclosed by the ring).
Installing a ring
electrode at 1 m from building perimeter would require 68 m of conductor, and
the ring would contain an area of 264 m2 (22 x 12 m). The equivalent radius
of this ring would be
re = √Area/∏ = 9.16 m
Since, re < L1 Therefore, in
addition to the ring, we require additional electrodes at the base of each
down-conductor. Either:
Lr = L1 – re = 50 – 9.16 = 40.84 m
or
Lv = ( L1 – re ) / 2 = (50 – 9.16)/2 = 40.84/2
= 20.42 m
So, additional electrodes will be:
6 x 40.84 m
horizontal radials, or
6 x 20.42 m vertical
electrodes.
Example#4:
Residential building,
LPS Class III, L1 = 5 m with dimensions as shown in below:
Determine if
additional electrodes are required or not?
If additional
electrodes are required, determine numbers and quantity for horizontal
radials, or vertical electrodes?
Solution:
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4.3 Comparison of Type A and Type B arrangements
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Type C arrangement
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4.4 Type C arrangement: Foundation earth electrodes
This is
essentially a type B arrangement. It comprises conductors that are installed
in the concrete foundation of the structure (see fig.7).
Type C Arrangement
Criteria:
Table: Earthing material
requirements
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5- Earth Termination System Testing
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In the next Article, I will explain the second part of Lightning Protection System; The Internal Lightning Protection System. Please, keep following.
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