SURGE AND SURGE PROTECTION DEVICES
Transcript of SURGE AND SURGE PROTECTION DEVICES
SURGE AND SPDs VER 1.0 1
TC-5
(Part-1)
SURGE AND
SURGE PROTECTION DEVICES
JAYARAJAN.D
INSTRUCTOR (TELE)
SURGE AND SPDs VER 1.0 2
What is Surge?
• Surges are fast, short duration electrical transients in
voltage (voltage spikes), current (current spike), or
transferred energy (energy spikes) in an electrical circuit.
• A surge is an electrical transient causing total or partial
damage to electrical and electronic equipment.
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Surges are caused by:
• Lightning Discharges.
• Switching on/off of inductive loads (for example
transformers, coils, motors).
• Ignition and interruption of electric arcs (for example
welding process).
• Tripping of fuses.
• Short circuits, etc.
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Losses (Cost of Damages) due To Various Reasons
Cause Loss as % of Total Loss
Negligence 36.1%
Surges 27.4%
Burglaries 12.9%
Floods/Storms 6.9%
Others 16.7%
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Lightning Surges
Lightning is the most prominent and damaging
among surges.
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• Lightning takes place due to accumulation of electric
charges in cloud mass in atmospheric conditions.
• Violent up-draught of air through centre of cloud cell
causes the following:
1. Ice crystals are positively charged
2. Water droplets negatively charged
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• Positive charge centre lies at about 10000 m &
negative charge centre at about 5000 m in tropical
regions.
• Positive charge centre lies at about 6000 m &
negative charge centre at about 2500 m in temperate
regions.
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• The negatively charged part of the cloud electro-statically
induces positive charge on the ground directly below it.
• The negative charge will be accelerated towards the
ground and it is called as the ‘Stepped leader’.
• When the step leader comes close to the ground a strong
electric field is created which drives the positive charge on
the ground to neutralize the negative charge.
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• This is called the returning stroke which is also
called as the ‘Streamer’.
• This returning stroke is much brighter than the
step leader.
• The returning stroke is the origin of intense light,
heat and sound in lightning.
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Physical Effects of Lightning
Lightning has the following physical effects:
• Heating of air up to 30000 0 K
• Creation of pressure shock wave
• Flow of current of magnitudes 10 kA to 200 kA
• Heavy potential difference of the order of 1 to 10
Million Volts
The above effects are transients and are to be
discharged through suitable protection mechanisms to
safeguard electrical and electronic installations.
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Surge Currents
Surge current due to lightning is expressed in 3 parameters:
• Surge amplitude
• Time taken by the surge to reach it’s maximum value.
• Time taken by the surge to fall to it’s half max. value.
Normally, we encounter surges of
• 200kA 10/350 micro sec.
• 50 kA 10/350 micro sec., and
• 15 kA 8/20 micro sec., as per severity of lightning.
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Surge Pulse
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Concept of Lightning Protection Zones (LPZ)
Lightning strike can be -
1. Direct strike on equipment room.
2. Indirect strike
• Galvanic coupling, through metallic conductor coming
in contact with surge.
• Capacitance coupling due to capacitive effect caused by
parallel surface.
• Inductive coupling due to conductors, parallel to surge
movement / discharge path.
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ZoneConcept Details
LPZ 0A Direct strikes; Full lightning current; Full
magnetic field
LPZ 0B No direct strikes; Partial lightning or
induced current; full magnetic field
LPZ1 No direct strikes; Partial lightning or
induced current; Damped magnetic field
LPZ2 No direct strikes; Induced current;
Further damped magnetic field
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Classification of SPDs
• Classification of SPDs are depend on the LPZ
(Lightning Protection Zone) under consideration.
Classes of SPDs as per IEC 61643 are:
• Class-A, Class-B, Class-C and Class-D
Class - A
Class-A protection is essentially external lightning
conductor on top of building connected to ground
through a down conductor.
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• Surroundings outside the building fall in LPZ0 (Lightning
Protection Zone-0).
• In this zone, 50% of lightning energy is transferred to
ground through class-A SPD.
• Balance 50% enters the building through power service
cables, Telecom conductors, etc.
• Lightning conductor can be single spike / multiple spike /
dome.
• Lightning conductor shall not touch the structure.
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• Down conductor shall be a cable rated for HV (50 sq.mm)
for structures of 80m or high.
• For other places, 40 mm X 6 mm MS flat strip insulated
from building is to be used.
• Availability of class-A protection for the buildings is to be
ensured by coordinating with electrical counter parts.
• It cannot be assumed to be present.
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Class - A
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Class - B
Class-B protection is the first stage protection, i.e.
before the equipment, at mains distribution panel.
These SPDs operate on arc chopping principle.
Lightning currents handled are:
• 10/350 micro sec. pulses
• 100 kA amplitude between N & E
• 50 kA amplitude between R/Y/B & N
Class-B SPDs are to be provided between each phase &
neutral and neutral & earth.
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Parameter Value / Limit
Nominal Voltage (U0) 230 V
Maximum Continuous Operating Voltage (Uc) >253 V
Temporary Over Voltage (Ut) 300V
Lightning Impulse Current Between R/Y/B & N > 50 kA, 10/350 micro sec.
for each phase
Lightning Impulse Current Between N & E > 100 kA, 10/350 micro sec.
Response Time (Tr) <200 n sec.
Voltage Protection Level (Up) <1.5 kV
Short Circuit Withstand & Follow Up Current
Extinguishing Capacity Without Back-Up Fuse (Isc)
>10 kA
Operating Temperature / RH 70 o C / 95% RH
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PARAMETERS OF SPDs
1. Nominal Voltage (Uo): It corresponds to the nominal
voltage of the system to be protected. The nominal
voltage is indicated in case of surge protective devices
for IT installations for type designation purposes. For
AC voltages it is indicated as RMS value.
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2. Rated Voltage (Uc): It is the RMS value of maximum
voltage which may be applied to the correspondingly
marked terminals of the surge protective device during
operation. It is the maximum voltage on the SPD in the
defined non-conductive state which ensures that this
state is regained after response and discharge.
3. Temporary Over Voltage (Ut): Voltage which can be
withstood by SPD (or safely disconnect) when applied for
a specific duration (5 m sec. or 200 m sec.)
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4. Voltage Protection Level (Up): The voltage protection
level of a surge protective device is the max.
Instantaneous value of the voltage on the terminals of a
surge protective device.
5. Voltage Withstand (Uw): Insulation withstand level (4
levels 1.5/2/4/6 KV)
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6. Lightning Impulse Current (Iimp): It is the standardized
impulse current curve, with a waveform 10/350 milli-sec.
its parameters (peak value, charge, specific power)
simulate the loads of natural lightning currents and
combined lightning current and surge protectors must
be capable of discharging such lightning impulse
currents several times without consequential damage to
the equipment.
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7. Nominal Discharge Current (In): It is the peak value of
an impulse current, waveform 8/20 milli-sec, which the
surge protective device rated for, according to a certain
test programme.
8. Follow-Up Current (If): Current delivered by the
distribution system which can be safely extinguished by
SPD
9. Response Time (Tr): Response times generally
characterize the response performance of the individual
protection elements used in surge protective devices.
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Class - C
Class-C protection comprises of fast acting MOV to
provide effective surge protection with low let through
voltage.
It is provided between phase and neutral.
Surge rating taken care is 50 kA 8/20 micro sec pulses.
Class-C SPD shall be a single compact device.
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Class-C SPD shall have following additional features:
• Indication (shows red) when device failed.
• Thermal disconnection of device when it starts
having heavy leakage current due to ageing/
handling several surges.
• Potential free contact for remote monitoring.
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Parameter Value / Limit
Nominal Voltage (U0) 230 V
Maximum Continuous
Operating Voltage (Uc)
300 V
Lightning Impulse Current
Between R/Y/B & N
> 10 kA, 8/20 micro sec.
for each phase
Response Time (Tr) <25 n sec.
Voltage Protection Level (Up) <1.6 kV
Operating Temperature / RH 70 o C / 95% RH
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Class B SPD
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Class C SPD
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Class (BC) SPD
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Class - D
All external data/power supply (AC/DC) lines connected
to electronic equipment are to be provided with class-D
SPDs at both ends of the conductors.
Class-D SPDs comprise of MOVs and GDs and
combinations. Their ratings are given voltage (supply)
wise.
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Class D SPD
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These SPDs shall have following additional features:
• Indication (shows red) when device failed.
• Thermal disconnection of device when it starts
having heavy leakage current due to ageing/
handling several surges.
• Potential free contact for remote monitoring.
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Some of the Class D surge protection devices
• Spark gaps (air gaps)
• Gas discharge tubes (GDTs)
• Zener diodes (avalanche diodes)
• Metal oxide varistors (MOVs)
• Transobers
• Relays
• Fuses
• PTCR (Positive Temperature Coefficient Resistor)
• TBU (Transient Blocking Unit)
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Gas Discharge Tubes
Two electrodes, close together, enclosed in a
tube filled with gas.
When the voltage rises, a low impedance arc is
formed between the two electrodes.
Symbol:
Also called gas-filled surge arrester.
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Gas Discharge Tube
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Silicon Avalanche Diodes
Semiconductor devices with similar characteristics
to varistors.
Symbol:
Also called “transorbs” and “clamping diodes”.
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Silicon Avalanche Diodes
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SAD Properties
Surge rating depends on size.
Surge ratings are much lower than MOVs, 1A 1kA
SADs lifespan also depends on:
Magnitude of surges
Duration of surges
Surge rating of SAD
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A prolonged over voltage will cause a SAD to fail.
Generally fail short-circuit, so they must be fused.
Capacitance is relatively low, less than 200pF.
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Variable resistor – resistance depends on voltage.
Symbol:
The most common type of varistor is the Metal
Oxide Varistor, or MOV.
Metal Oxide Varistors (MOVs)
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Metal Oxide Varistors (MOVs)
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Surge rating depends on disc size:
5mm MOV rating 100A
40mm MOV rating 100kA
MOVs lifespan depends on:
Number of surges
Magnitude of surges
Duration of surges
Surge rating of varistor
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A prolonged over voltage will cause a varistor to
fail.
They may become extremely hot and can catch
fire or explode!
Generally fail short-circuit, so they must be fused.
Capacitance is relatively high, between 1nF and
10nF.
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Lightning Protection Systems
A lightning protection system is a system designed to
protect a structure from damage due to lightning strikes by
intercepting such strikes and safely passing their extremely
high voltage currents to "ground".
Most lightning protection systems include a network of
lightning rods, metal conductors, and ground electrodes
designed to provide a low resistance path to ground for
potential strikes.
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Principle of Power Protection
• A protector performs like a switch controlled by
voltage.
• If the voltage is higher than the rated voltage of the
electrical line to be protected, then the protector
changes its state to low impedance and derives current
to earth.
• The usual state of the protector is being in high
impedance, so that the protector is transparent for the
installation.
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IEC-62305 STANDARD VER 1.0 55
TC-5
(Part-2)
IEC 62305 STANDARD FOR
LIGHTNING PROTECTION
JAYARAJAN.D
INSTRUCTOR (TELE)
IEC-62305 STANDARD VER 1.0 56
Structure of BS EN/IEC 62305
The BS EN/IEC 62305 series consists of four parts, all of
which need to be taken into consideration. These four
parts are:
Part 1: General principles,
Part 2: Risk management,
Part 3: Physical damage to structures and life hazard, and
Part 4: Electrical and electronic systems within structures
IEC-62305 STANDARD VER 1.0 57
Part-1: General principles
• This part serves as an introduction to the further parts of
the standard. It classifies the sources and types of
damage to be evaluated and introduces the risks or types
of loss to be anticipated as a result of lightning.
• It defines the relationships between damage and loss that
form the basis for the risk assessment calculations in part
2 of the standard.
IEC-62305 STANDARD VER 1.0 58
• Lightning current parameters defined in this part are
used as the basis for the selection and implementation
of the appropriate protection measures detailed in
parts 3 and 4 of the standard.
• This part also introduces new concepts for
consideration when preparing a lightning protection
scheme, such as Lightning Protection Zones (LPZs) and
separation distance.
IEC-62305 STANDARD VER 1.0 59
BS EN/IEC 62305 identifies four main sources of
damage:
S1 Flashes to the structure
S2 Flashes near to the structure
S3 Flashes to a service
S4 Flashes near to a service
IEC-62305 STANDARD VER 1.0 60
Each source of damage may result in one or more of
three types of damage:
D1 Injury of living beings due to step and touch
voltages
D2 Physical damage (fire, explosion, mechanical
destruction, chemical release) due to lightning
current effects including sparking
D3 Failure of internal systems due to Lightning
Electromagnetic Impulse (LEMP)
IEC-62305 STANDARD VER 1.0 61
The following types of loss may result from damage due
to lightning:
L1 Loss of human life
L2 Loss of service to the public
L3 Loss of cultural heritage
L4 Loss of economic value
IEC-62305 STANDARD VER 1.0 62
Point of strikeSource of
damage
Type of
damageType of loss
Structure S1
D1
D2
D3
L1, L4
L1, L2, L3, L4
L1, L2, L4
Near a structure S2 D3 L1, L2, L4
Service connected to
the structureS3
D1
D2
D3
L1, L4
L1, L2, L3, L4
L1, L2, L4
Near a service S4 D3 L1, L2, L4
IEC-62305 STANDARD VER 1.0 63
Lightning Protection Levels (LPL)
• Four protection levels have been determined and
each level has a fixed set of maximum and minimum
lightning current parameters.
• Lightning current for each LPL based on 10/350 µs
waveform:
LPL I II III IV
Maximum Current (kA) 200 150 100 100
Minimum Current (kA) 3 5 10 16
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The types of damage and loss
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Air termination networkRolling sphere radius
LPZ 0A
Direct flash, full lightning current, full
magnetic fieldSPD 0B/1
LPZ 0B
No direct flash, partial
lightning
or induced current, full
magnetic field
LPZ 1No direct flash, partial lightning or
induced current, damped SPD 0B/1
magnetic fieldLPZ 2No direct flash, induced
currents, further damped
magnetic field
LPZ 3SPD 0B/1
Down conductor network
SPD 0B/1LPZ 0B
Equipotential bonding by means
of SPD
Earth termination network
The LPZ Concept
IEC-62305 STANDARD VER 1.0 66
Part-2: Risk management
• This part specifically deals with making a risk assessment,
the results of which define the level of Lightning
Protection System (LPS) required.
• The first stage of the risk assessment is to identify which of
the four types of loss as identified in part-1, the structure
and its contents can incur.
• The ultimate aim of the risk assessment is to quantify and
if necessary reduce the relevant primary risks according to
the level of damages; i.e.
IEC-62305 STANDARD VER 1.0 67
R1 risk of loss of human life
R2 risk of loss of service to the public
R3 risk of loss of cultural heritage
R4 risk of loss of economic value
• Each primary risk (Rn) is determined through a long series
of calculations as defined within the standard.
• If the actual risk (Rn) is less than or equal to the tolerable
risk (RT), then no protection measures are needed. If the
actual risk (Rn) is greater than its corresponding tolerable
risk (RT), then protection measures must be instigated.
IEC-62305 STANDARD VER 1.0 68
Lightning Protection System (LPS)
IEC 62305-1 has defined four Lightning Protection Levels
(LPLs) based on probable minimum and maximum lightning
currents. These LPLs equate directly to classes of Lightning
Protection System (LPS).
The correlation between the four levels of LPL and LPS is
identified in Table 7. In essence, the greater the LPL, the
higher class of LPS is required.
IEC-62305 STANDARD VER 1.0 69
LPL Class of LPS
I I
II II
III III
IV IV
Relation between Lightning Protection Level (LPL) and Class of LPS
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6 Point protection Plan
Point 1: Capture the lightning strike.
Point 2: Conduct the lightning current to ground safely
Point 3: Dissipate the energy into the ground.
Point 4: Eliminate ground loops and differentials.
Point 5: Protect equipment from surges and transients
on power lines.
Point 6: Protect equipment from surges and transients
on communication lines.
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• Depending upon the consequences the designer may
choose either of the following types of external LPS:
1. Isolated
2. Non-isolated
• An Isolated LPS is typically chosen when the structure
is constructed of combustible materials or presents a
risk of explosion.
• Conversely a non-isolated system may be fitted where
no such danger exists.
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An external LPS consists of:
1. Air termination system
2. Down conductor system
3. Earth termination system
• These individual elements of an LPS should be
connected together using appropriate lightning
protection components (LPC).
• This will ensure that in the event of a lightning current
discharge to the structure, the correct design and
choice of components will minimize any potential
damage.
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1. Air Termination System
• The role of an air termination system is to capture the
lightning discharge current and dissipate it harmlessly
to earth via the down conductor and earth
termination system.
• Therefore it is vitally important to use a correctly
designed air termination system.
• IEC 62305-3 advocates the following, in any
combination, for the design of the air termination:
IEC-62305 STANDARD VER 1.0 75
• Air rods (or finials) whether they are free standing
masts or linked with conductors to form a mesh on the
roof.
• Catenary (or suspended) conductors, whether they are
supported by free standing masts or linked with
conductors to form a mesh on the roof.
• Meshed conductor network that may lie in direct
contact with the roof or be suspended above it (in the
event that it is of paramount importance that the roof
is not exposed to a direct lightning discharge)
IEC-62305 STANDARD VER 1.0 76
• It highlights that the air termination components
should be installed on corners, exposed points and
edges of the structure.
• The three basic methods recommended for
determining the position of the air termination
systems are:
1. The rolling sphere method
2. The protective angle method
3. The mesh method
IEC-62305 STANDARD VER 1.0 77
The rolling sphere method
• The rolling sphere method is a simple means of
identifying areas of a structure that need protection,
taking into account the possibility of side strikes to the
structure.
• The basic concept of applying the rolling sphere to a
structure is illustrated in Figure.
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• The rolling sphere method was used in BS 6651, the
only difference being that in IEC 62305 there are
different radii of the rolling sphere that correspond to
the relevant class of LPS.
Class of LPSRolling sphere
radius (m)I 20II 30III 45IV 60
IEC-62305 STANDARD VER 1.0 80
The protective angle method
• The protective angle is determined by the radius of the
rolling sphere.
• The protective angle, which is comparable with the
radius of the rolling sphere, is given when a slope
intersects the rolling sphere in such a way that the
resulting areas have the same size.
• This method must be used for buildings with
symmetrical dimensions (e.g. steep roof) or roof-
mounted structures (e.g. antennas, ventilation pipes).
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• The protective angle (α) is the angle created between
the tip (A) of the vertical rod and a line projected down
to the surface on which the rod sits.
• The protective angle differs with varying height of the air
rod and class of LPS. The protective angle afforded by an
air rod is given in the table.
• The protective angle depends on the class of LPS and the
height of the air-termination system above the reference
plane.
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Air-termination systems withprotective angle α
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• Air-termination conductors, air-termination rods,
masts and wires should be arranged in such a way that
all parts of the structure to be protected are situated
within the protected volume of the air-termination
system.
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Mesh method
• A “meshed” air-termination system can be used
universally regardless of the height of the building and
shape of the roof. A meshed air-termination network
with a mesh size according to the class of LPS is arranged
on the roofing.
• By using the ridge and the outer edges of the building as
well as the metal natural parts of the building serving as
an air-termination system, the individual meshes can be
positioned as desired.
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• The air-termination conductors on the outer edges of
the structure must be laid as close to the edges as
possible.
• The metal capping of the roof parapet can serve as an
air-termination conductor and / or a down conductor if
the required minimum dimensions for natural
components of the air-termination system are
complied with
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Meshed air-termination system
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Class of LPS Mesh size
I 5 x 5 m
II 10 x 10 m
III 15 x 15 m
IV 20 x 20 m
MESH SIZE
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Air terminations at roof edges, ridges, roof overhangs.
No metal installation protrudes above Air termination system.
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Down Conductor System
• The down conductor is the electrically conductive
connection between the air-termination system and the
earth-termination system.
• The function of a down conductor is to conduct the
intercepted lightning current to the earth-termination
system without damaging the building e.g. due to
intolerable temperature rises.
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Down Conductor System
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• The number of down conductors depends on the
perimeter of the external edges of the roof (perimeter
of the projection onto the ground surface).
• The down conductors must be arranged to ensure that,
starting at the corners of the structure, they are
distributed as uniformly as possible to the perimeter.
• Depending on the structural conditions (e.g. gates,
precast components), the distances between the
various down conductors can be different.
IEC-62305 STANDARD VER 1.0 95
• By interconnecting the down conductors at ground
level and using ring conductors for higher structures,
it is possible to balance the lightning current
distribution which reduces the separation distance ‘s’.
• There should always be a minimum of two down
conductors distributed around the perimeter of the
structure. Down conductors should wherever possible
be installed at each exposed corner of the structure
as research has shown these to carry the major part
of the lightning current.
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Class of LPS Typical distance
I 10 m
II 10 m
III 15 m
IV 20 m
Distance between down conductors
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• When internal reinforcing bars are required to be
connected to external down conductors or earthing
network either of the arrangements shown in the figure
is suitable.
• If the reinforcing bars (or structural steel frames) are to
be used as down conductors then electrical continuity
should be ascertained from the air termination system
to the earthing system.
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• For new build structures this can be decided at the
early construction stage by using dedicated reinforcing
bars or alternatively to run a dedicated copper
conductor from the top of the structure to the
foundation prior to the pouring of the concrete.
• This dedicated copper conductor should be bonded to
the adjoining/adjacent reinforcing bars periodically.
IEC-62305 STANDARD VER 1.0 100
Earth termination system
• The earth-termination system is the continuation of the
air-termination systems and down conductors to
discharge the lightning current to the earth.
• Other functions of the earth-termination system are to
establish equipotential bonding between the down
conductors and to control the potential in the vicinity of
the building walls.
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• It must be observed that a common earth-termination
system is to be preferred for the different electrical
systems (lightning protection systems, low-voltage
systems and telecommunication systems).
• This earth-termination system must be connected to the
equipotential bonding system via the main earthing
busbar (MEB).
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The standard distinguishes two types of earth electrode
arrangements, type A and type B.
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Type A arrangement
• Type A earth electrode arrangements describe
individually arranged vertical earth electrodes or
horizontal radial earth electrodes, which must be
connected to a down conductor.
• A type A earth electrode arrangement require at least
two earth electrodes. A single earth electrode is
sufficient for individually positioned air-termination rods
or masts.
• A minimum earth electrode length of 5 m is required for
class of LPS III and IV.
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• For class of LPS I and II the length of the earth
electrode is defined as a function of the soil resistivity.
• If different earth electrodes (vertical and horizontal) are
combined, the equivalent total length should be taken
into account.
• Single earth electrodes of type A must be
interconnected to ensure that the current is evenly
split. This is important for calculating the separation
distance s.
IEC-62305 STANDARD VER 1.0 105
Type B arrangement
• Type B earth electrodes are ring earth electrodes
encircling the object to be protected or foundation earth
electrodes. In Germany, the requirements for earth-
termination systems of new buildings are described in
DIN 18014.
• If it is not possible to encircle the structure by means of a
closed ring, the ring must be complemented by means of
conductors inside the structure. Pipework or other
permanently conductive metal components can also be
used for this purpose.
IEC-62305 STANDARD VER 1.0 106
• The earth electrode must be in contact with the soil
for at least 80 % of its total length to ensure that a
type B earth electrode can be used as a base for
calculating the separation distance.
• The minimum lengths of type B earth electrodes
depend on the class of LPS. In case of classes of
LPS I and II, the minimum earth electrode length
also depends on the soil resistivity.
IEC-62305 STANDARD VER 1.0 107
Foundation earth electrodes
• This is essentially a type B earthing arrangement. It
comprises conductors that are installed in the concrete
foundation of the structure.
• If any additional lengths of electrodes are required
they need to meet the same criteria as those for type
B arrangement.
• Foundation earth electrodes can be used to augment
the steel reinforcing foundation mesh.
IEC-62305 STANDARD VER 1.0 108
Step and touch voltage
IEC 62305-3 points out that, in special cases, touch or
step voltage outside a building in the vicinity of the down
conductors can present a life hazard even though the
lightning protection system was designed according to
the latest standards.
IEC-62305 STANDARD VER 1.0 109
Touch voltage
• Touch voltage is a voltage acting on a person between its
standing surface on earth and when touching the down
conductor. The current path leads from the hand via the
body to the feet.
• A downpipe, even if it is not defined as a down
conductor, can present a risk for persons touching it. In
this a case, the metal pipe must be replaced by a PVC
pipe.
IEC-62305 STANDARD VER 1.0 110
Step Voltage
• Step voltage is a part of the earth potential which can
be bridged by a person taking a step of 1 m. The current
path runs via the human body from one foot to the
other.
• The step voltage depends on the shape of the potential
gradient area. As shown in the figure, the step voltage
decreases as the distance from the building increases.
The risk to persons is therefore reduced the further they
are from the structure.
IEC-62305 STANDARD VER 1.0 111
STEP VOLTAGE AND TOUCH VOLTAGE
IEC-62305 STANDARD VER 1.0 112
Internal LPS design considerations
• The fundamental role of the internal LPS is to ensure the
avoidance of dangerous sparking occurring within the
structure to be protected.
• This could be due to lightning current flowing in the
external LPS or indeed other conductive parts of the
structure and attempting to flash or spark over to
internal metallic installations.
IEC-62305 STANDARD VER 1.0 113
• Carrying out appropriate equipotential bonding
measures or ensuring there is a sufficient electrical
insulation distance between the metallic parts can
avoid dangerous sparking between different metallic
parts.
IEC-62305 STANDARD VER 1.0 114
Lightning equipotential bonding
• Equipotential bonding is simply the electrical
interconnection of all appropriate metallic
installations/parts, such that in the event of lightning
currents flowing, no metallic part is at a different
voltage potential with respect to one another.
• If the metallic parts are essentially at the same potential
then the risk of sparking or flashover is nullified.
IEC-62305 STANDARD VER 1.0 115
Equipotential Bonding Arrangement
IEC-62305 STANDARD VER 1.0 116
• The gas, water and central heating system are all bonded
directly to the equipotential bonding bar located inside
but close to an outer wall near ground level.
• The power cable is bonded via a suitable SPD, upstream
from the electric meter, to the equipotential bonding bar.
• This bonding bar should be located close to the main
distribution board (MDB) and also closely connected to
the earth termination system with short length
conductors.
IEC-62305 STANDARD VER 1.0 117
• In larger or extended structures several bonding bars
may be required but they should all be interconnected
with each other.
• The screen of any antenna cable along with any shielded
power supply to electronic appliances being routed into
the structure should also be bonded at the equipotential
bar.
IEC-62305 STANDARD VER 1.0 118
• Indeed, as per IEC 62305-3, an LPS system can no longer
be fitted without lightning current or equipotential
bonding SPDs to incoming metallic services that have
live power and telecoms cables, which cannot be directly
bonded to earth.
• Such SPDs are required to protect against the risk of loss
of human life by preventing dangerous sparking that
could present fire or electric shock hazards.
IEC-62305 STANDARD VER 1.0 119
• The use of these SPDs alone “provides no effective
protection against failure of sensitive electrical or
electronic systems”, to quote IEC 62305 part 4, which
is specifically dedicated to the protection of electrical
and electronic systems within structures.
IEC-62305 STANDARD VER 1.0 120
Lightning equipotential bonding requires that all metal
conductive parts such as cable cores and shields at the
entrance point into the building be integrated in the
equipotential bonding system so as to cause as little
impedance as possible.
Examples of such parts include antenna lines,
telecommunication lines with metal conductors and also
optical fibre installations with metal elements.
IEC-62305 STANDARD VER 1.0 121
• The lines are connected with the help of lightning
current carrying elements (arresters and shield
terminals).
• An adequate place of installation is the point where
the cabling extending beyond the building transfers
to cabling inside the building.
• Both the arresters and the shield terminals must be
chosen according to the lightning current parameters
to be expected.
IEC-62305 STANDARD VER 1.0 122
Earthing and Earth Measurement
VER 1.0 123
TC-5
(Part-3)
EARTHING AND
EARTH MEASUREMENT
JAYARAJAN.D
INSTRUCTOR (TELE)
Earthing and Earth Measurement
VER 1.0 124
Earthing is required for the following purposes
1. To provide protection against lightning strikes & power
surges.
2. To protect equipment from surge voltages & currents and
human beings from electric shocks.
3. As one conductor in earth-return circuits where the
earth is part of the circuit.
4. Discharging induced voltages, in communication systems.
5. To minimize RF radiation and EM interference.
Earthing and Earth Measurement
VER 1.0 125
Properties of Earth
• Earth is not a good conductor.
• But it is an ideal Equipotential surface.
• It can SINK any amount of charge without any
appreciable rise in its potential.
• Effect of injected currents due to a fault or lightning is
negligible, if not felt only locally.
Earthing and Earth Measurement
VER 1.0 126
What is System Earth and Equipment Earth?
System Earth:
System earth means the connection of earth to the
neutral points of current carrying conductors such as the
neutral point of a circuit; a transformer, rotating
machinery, or a system, either solidly or with a current-
limiting device.
Earthing and Earth Measurement
VER 1.0 127
Earthing and Earth Measurement
VER 1.0 128
Equipment Earth:
Equipment Earth means the connection of earth to non-
current carrying conductive materials such as conduit,
cable trays, junction boxes, enclosures, and motor
frames.
Earthing and Earth Measurement
VER 1.0 129
Equipment earthing based on IS:3043-1987
Standards
Earthing and Earth Measurement
VER 1.0 130
There are two differences
1. In terms of connection
1. In system earthing, earth is connected to the current
carrying parts.
2. In equipment earthing, earth is connected to non
current carrying part or the chassis (the external body
of the equipment).
2. In terms of purpose
1. System earthing is used to protect the equipment.
2. Equipment earthing is used to protect the
personnel.
Earthing and Earth Measurement
VER 1.0 131
Characteristics of Good Earthing System
(Equipment Earth)
A good earthing systems will feature the following 7
characteristics:
1. Good electrical conductivity
2. Conductors capable of withstanding high fault currents
3. Long life – at least 40 years
4. Low ground resistance and impedance
5. Equipotential Bonding
6. Good corrosion Resistance
7. Electrically and mechanically robust and reliable
Earthing and Earth Measurement
VER 1.0 132
The following locations are suitable for locating earth
pit:
• Low lying areas close to the building or equipment
are good for locating Earth Electrodes.
• The location can be close to any existing water
bodies or water points but not naturally well-
drained.
Earthing and Earth Measurement
VER 1.0 133
The following locations are not suitable for locating earth
pit:
1. Dry sand, gravel chalk, lime stone, granite and any
stony ground locations should be avoided.
2. All locations where virgin rock is very close to the
surface.
3. Earthing electrode should not be installed on high
bank or made-up soil.
Earthing and Earth Measurement
VER 1.0 134
Soils suitable for making earth pit in the order of
preference:
1. Wet marshy ground
2. Clayey soil, loamy soil, arable land, clay or loam
mixed with small quantities of sand
3. Clay and loam mixed with varying proportions of
sand, gravel and stones
4. Damp & wet sand or peat.
Earthing and Earth Measurement
VER 1.0 135
Earth Resistance
• It is the resistance of soil offered to the passage of
electric current.
• Earth Resistance value of an earth pit depends solely
on soil resistivity at the location.
• Hence, it varies from soil to soil.
Earthing and Earth Measurement
VER 1.0 136
Soil Resistivity
Soil Resistivity mainly depends on:
• Soil composition
• Moisture content
• Dissolved salts
• Grain size and its distribution
• Temperature and
• Magnitude of current that flows.
Earthing and Earth Measurement
VER 1.0 137
Material Resistivity
Copper 1.7 X 10 -8 Ohm. Meter
GI 10 -7 Ohm. Meter
Wet soil 10 Ohm. Meter
Moist soil 100 Ohm. Meter
Dry soil 1000 Ohm. Meter
Bed rock 10000 Ohm. Meter
Earthing and Earth Measurement
VER 1.0 138
Required Earth Resistance Value
• Ideally, earth resistance has to be 0 Ω.
• But practically it should be 1 or 2 ohms
• Value of Earth Resistance depends on two factors:
1. Soil Resistance ( Electrode to Soil Resistance)
and
2. Electrode Resistance
Earthing and Earth Measurement
VER 1.0 139
Soil Resistance
The resistance offered by soil to the spread of electric
current/field is called Earth/Soil Resistance.
1. It depends mainly on Soil Resistivity .
And also on
2. Shape and size of earth electrode.
Earthing and Earth Measurement
VER 1.0 140
Electrode Resistance
• Electrode Resistance is the resistance of the
material of which the electrode is made of. It
also depends on shape and size of the electrode.
• Electrodes of GI, Copper and copper welded steel
are used to keep electrode resistance negligible
or less than 1 Ω.
Earthing and Earth Measurement
VER 1.0 141
Earth Electrode Types
1. Rod Electrodes
2. Strip Electrodes
3. Pipe Electrodes
4. Plate Electrodes
• A pipe, rod or strip has a much lower resistance than
a plate of equal surface area.
• The resistance is not inversely proportional to the
surface area of the electrode.
Earthing and Earth Measurement
VER 1.0 142
Plate Electrode: The approximate resistance to earth
of a plate can be calculated from:
R = ρ x L/A
Where,
ρ = resistivity of the soil (in Ω.m)
A = cross-sectional area of plate (in m2)
L = the length of the conducting path
Earthing and Earth Measurement
VER 1.0 143
Strip Electrode: In case of strip electrode earth
resistance is given by :
R = ρ/2πL [ln (8L/T) + ln (L/h)-2+(2h/L)-(h/L)2]
L is length,
h is depth of laying,
T is thickness
L has major influence than T
Earthing and Earth Measurement
VER 1.0 144
Pipe Electrode: In case of pipe electrode earth
resistance is given by:
R = (ρ / 2πL) [ln 8L / 2.7183 D]
Where
L is length of electrode,
D is diameter,
ρ is soil resistivity
Earthing and Earth Measurement
VER 1.0 145
Better earth Resistance can be achieved by:
1. Increasing the surface area of the electrode in contact
with the earth,
2. Increasing the depth to which it is driven,
3. Using several connected ground rods,
4. Increasing the moisture of the soil,
5. Improving the conductive mineral content of the soil,
and
6. Increasing the land area covered by the ground
system
Earthing and Earth Measurement
VER 1.0 146
Types of earthing
1. Conventional earth
2. Maintenance free earth
1. Conventional Earthing Practice
Earth Electrode:
Earthing and Earth Measurement
VER 1.0 147
Procedure of Installation
Earth pit of 600 mm dia and 2.5 m depth is made by
manual trenching or by using “Earth Auger”.
The electrode is placed at the center.
Top of the electrode is kept 30 cm above the ground.
After inserting the electrode, the hole is filled with
earth properly and water is spread to ensure good
contact between electrode and filling.
Earthing and Earth Measurement
VER 1.0 148
Earth Auger
Earthing and Earth Measurement
VER 1.0 149
If the soil is of high resistivity, it is treated with salt and
charcoal in appropriate proportion.
The pit is filled alternately with layers of common salt
and charcoal, each layer of about 2.5 cm thick up to a
depth of about 150 cm from the bottom of the
electrode.
A brick wall of 400 mm height below the ground level
is constructed in rectangular fashion and walls are
plastered and then filled with sand.
Earthing and Earth Measurement
VER 1.0 150
The surroundings of the earth electrode are kept
moist by periodically pouring water through the
pipe in order to keep the resistance below specified
value.
Coke treated electrodes shall not be situated within
6 meters of other metal structure. (This results in
rapid corrosion)
Earthing and Earth Measurement
VER 1.0 151
Problems of Conventional Earthing
• The salt poured in pit causes severe corrosion of GI
pipe and makes the earthing ineffective.
• The earth resistance value depends on “soil
resistivity” which depends on strata so the effect of
earthing is dependent on property of soil, season.
• The earth resistance value is high, fluctuating &
increases with time.
Earthing and Earth Measurement
VER 1.0 152
Earthing and Earth Measurement
VER 1.0 153
Conventional / Traditional Earthing
Earthing and Earth Measurement
VER 1.0 154
• Bolt nut mechanism is not a permanent solution
• It is corrosive
• It can not offer consistent resistance
• It is not Maintenance Free
Earthing and Earth Measurement
VER 1.0 155
Maintenance of conventional Earth
1) Check earth and its connections periodically at
interval of not more than one month, to ensure that
all connections are intact and soldered joints are in
proper condition.
2) Measure the earth resistance once in a year. Enter
the value, date of last test on the inspection pit
cover and in a register.
Earthing and Earth Measurement
VER 1.0 156
3) Water to be added every day to the earth electrode in
summer and once in two days in other seasons.
4) If earth resistance is more than the nominal value,
either renew the old earth or provide a new earth.
Earthing and Earth Measurement
VER 1.0 157
Maintenance free Earth
Earth Electrode:
Steel circular rods, bonded with copper on outer surface
to meet the requirements of Underwriters Laboratories
(UL) latest specifications.
(strength, corrosion resistance, low resistance path to
earth and cost effectiveness).
Earthing and Earth Measurement
VER 1.0 158
Copper Bonded Rods:
• Steel rods (minimum 17 mm dia, 3 m
long)
• Copper bonded
• Coating thickness - 250 microns min.
• UL marking
Service Life - 40 years in most soil types
Earthing and Earth Measurement
VER 1.0 159
Earth Enhancement material
• A superior conductive material that improves
conductivity of the electrode and ground contact area.
It improves earth’s absorbing power and humidity
retention capability.
• Non-corrosive, low water solubility, highly hygroscopic.
• Resistivity of less than 0.2 ohms-meter.
• Suitable for installation in dry/slurry form.
Earthing and Earth Measurement
VER 1.0 160
• Does not depend on continuous presence of water
to maintain its conductivity.
• Permanent & maintenance free and maintain
constant earth resistance.
Earthing and Earth Measurement
VER 1.0 161
Construction of Earth Pit
• A hole of 100mm to 125mm dia is augured/dug to a
depth of about 2.8 meters.
• Earth electrode is placed into this hole.
• It is penetrated into the soil by gently driving on the
top of the rod. Here natural soil is assumed to be
available at the bottom of the electrode so that
min. 150 mm of the electrode shall be inserted in
the natural soil.
Earthing and Earth Measurement
VER 1.0 162
Earth enhancement material (30-35 kg) is filled into
the hole in slurry form and allowed to set.
Remaining portion of the hole is covered by backfill
soil, which is taken out during digging.
A copper strip of 150mmX25mmX6mm is welded to
earth electrode for taking connection to the main
equipotential earth bus-bar in the equipment room
and to other earth pits, if any.
Earthing and Earth Measurement
VER 1.0 163
• Exothermic weld material should be UL listed.
• The main earth pit is located as near to the MEEB
in the equipment room as possible.
Earthing and Earth Measurement
VER 1.0 164
Earth enhancement powder with backfill soil
Exothermic weld
25X2 mm copper tape
2800
mm
mm
PVC insulated multistrand 35 Sqmm Cu cable as per IS: 694 (Duplicated)
Augured Hole 100 mm to 125 mm Dia
50 50 200
100 mm
500 m
m
Copper bonded steel earth electrode, minimum 3.0 mtr. long & minimum 17.0 mm dia.
Concrete lid cover, approx. 50 mm thick with pulling hook
Cement concrete
Inspection Chamber
MEBB 6X25X300 mm Cu bus bar
150
mm
To other earth pit
500
mm
6X25X150 mm Cu bus bar
150 mm (Minimum) Natural soil/backfill compound
Earthing and Earth Measurement
VER 1.0 165
Comparison with Conventional System
Corrosion Proof
Space Savings
Maintenance Free
Steady Performance
Very Long Life
All over Fit and forget Technology
Earthing and Earth Measurement
VER 1.0 166
Construction of Loop/ring Earth by
Providing Multiple Earth Pits
• At certain locations, it may not be possible to
achieve earth resistance of ≤1 ohm with one earth
electrode/pit due to higher soil resistivity. In such
cases, provision of loop/ring earth consisting of
more than one earth pit is done.
• The distance between two successive earth
electrodes should be between 3 to 6 m.
Earthing and Earth Measurement
VER 1.0 167
• At certain locations, it may not be possible to
achieve earth resistance of ≤1 ohm with one earth
electrode/pit due to higher soil resistivity. In such
cases, provision of loop/ring earth consisting of
more than one earth pit is done.
• The distance between two successive earth
electrodes should be between 3 to 6 m.
Earthing and Earth Measurement
VER 1.0 168
Horizontal Earthing and Ring Earthing
Earthing and Earth Measurement
VER 1.0 169
Earthing and Earth Measurement
VER 1.0 170
Earthing and Earth Measurement
VER 1.0 171
Earthing and Earth Measurement
VER 1.0 172
Equipotential Earth Busbar
• There is one Sub-Equipotential Earth Bus-bar for each of
the equipment room (SEEB).
• The equipotential earth bus-bar located in the IPS /Battery
charger room and directly connected to Class ‘B’ SPDs and
main earth pit is termed as Main Equipotential Earth
Busbar (MEEB).
• The EEBs have pre-drilled holes of suitable size for
termination of bonding conductors.
Earthing and Earth Measurement
VER 1.0 173
• EEBs are insulated from the walls and are installed on
the wall with insulator spacers of height 60mm.
• EEBs are installed at the height of 0.5m from the
room floor surface for ease of installation &
maintenance. All terminations on the EEBs are done
using copper lugs with spring washers.
Earthing and Earth Measurement
VER 1.0 174
Bonding Connections
• Each of the SEEBs are directly connected to MEEB. Also,
equipment/racks in the room are directly connected to
its SEEB. The bonding conductors are bonded to the
lugs by exothermic welding.
• Routing of bonding conductors from equipments to
SEEB & from SEEBs to MEEB should be as short and
direct as possible with min. bends. Connection from
SPD to MEEB should be as short as possible and
preferably without bend.
Earthing and Earth Measurement
VER 1.0 175
Component/Bonding Material Size
Main equipotential earth busbar
(MEEB)
Copper 300X25X6 mm
(min.)
Sub equipotential earth busbar
(SEEB)
Copper 150X25X6 mm
(min.)
Individual equipments to SEEB
using copper lugs with stainless
steel nut and bolts.
Multi-strand single core
PVC insulated copper
cable
10 sq.mm
SEEB to MEEB using copper lugs
with stainless steel nut and bolts.
Multi-strand single core
PVC insulated copper
cable
16 sq.mm
Surge protection devices (SPD)
to MEEB using copper lugs with
stainless steel nut and bolts.
Multi-strand single core
PVC insulated copper
cable
16 sq.mm
MEEB to main earth electrode Multi-strand single core
PVC insulated copper
cable (Duplicated)
35 sq.mm
Main earth pit to other earth pit in
case of loop earth
Copper tape 25mm X 2mm
Earthing and Earth Measurement
VER 1.0 176
Code of Practice for Earthing and Bonding
System for Signalling and Telecom
Equipments
RDSO/SPN/197/2008
Earthing and Earth Measurement
VER 1.0 177
Earth enhancement powder with backfill soil
Exothermic weld
25X2 mm copper tape
2800
mm
mm
PVC insulated multistrand 35 Sqmm Cu cable as per IS: 694 (Duplicated)
Augured Hole 100 mm to 125 mm Dia
50 50 200
100 mm
500
mm
Copper bonded steel earth electrode, minimum 3.0 mtr. long & minimum 17.0 mm dia.
Concrete lid cover, approx. 50 mm thick with pulling hook
Cement concrete
Inspection Chamber
MEBB 6X25X300 mm Cu bus bar
150
mm
To other earth pit
500
mm
6X25X150 mm Cu bus bar
150 mm (Minimum) Natural soil/backfill compound
Earthing and Earth Measurement
VER 1.0 178
EARTHING COMPONENTS
Copper Clad Earth Electrodes
Inspection joint
Backfill Compound
Copper Strip
Earth Pit Cover
Earthing and Earth Measurement
VER 1.0 179
Earthing of Signalling Equipments
It covers earthing & bonding for signalling equipments
with solid state components which are more susceptible
to damage due to surges/over voltages.
Suitable for EI, IPS, Digital Axle counters, Data logger etc.
Acceptable Earth Resistance at earth busbar - 1 ohm or
less.
Earthing and Earth Measurement
VER 1.0 180
Components of Earthing & Bonding system
• Earth electrode,
• Earth enhancement material,
• Earth pit,
• Equipotential earth bus-bar,
• Connecting cable & tape/strip
• Other accessories.
Earthing and Earth Measurement
VER 1.0 181
Earth Electrode
• Steel circular rods, bonded with copper on outer
surface to meet the requirements of Underwriters
Laboratories (UL) latest specifications.
• (strength, corrosion resistance and cost effectiveness).
Earthing and Earth Measurement
VER 1.0 182
• Steel circular rods, bonded with copper on
outer surface to meet the requirements of
Underwriters Laboratories (UL) latest
specifications.
• Steel rods (minimum 17 mm dia, 3 m long)
• Molecularly bonded copper
• Coating thickness - 250 microns
• UL marking
• CPRI tested – 23 kA for 1 second
• Service Life - 40 years in most soil types
Earthing and Earth Measurement
VER 1.0 183
Earth Enhancement Material
A superior conductive material that improves conductivity
of the electrode and ground contact area. It improves
earth’s absorbing power and humidity retention capability.
Mainly consist of Graphite , Portland cement with less
Bentonite content
Suitable for installation in dry/slurry form, does not
depend on continuous presence of water to maintain its
conductivity.
Permanent & maintenance free and in its “set form”,
maintain constant earth resistance.
Earthing and Earth Measurement
VER 1.0 184
• Highly Conductive cementious
compound
• Homogeneous and stable
• Doesn't leach into the ground
• Enhances the life of copper bonding
• Environmentally Friendly
• Low Resistivity – 0.2 Ω-m
• Suitable for all terrain
• Can be used in dry or slurry form
• Conducrete 10 kg – 3 bags
Earthing and Earth Measurement
VER 1.0 185
• A hole of 100mm to 125mm dia is augured/dug to a
depth of about 2.8 meters.
• Earth electrode is placed into this hole.
• It is penetrated into the soil by gently driving on the top
of the rod. Here natural soil is available at the bottom of
the electrode so that min. 150 mm of the electrode is
inserted in the natural soil.
Earth enhancement material (30-35 kg) is filled into the
hole in slurry form and allowed to set.
Earthing and Earth Measurement
VER 1.0 186
• Remaining portion of the hole is covered by backfill soil,
which is taken out during digging.
• A copper strip of 150mm X 25mm X 6mm is welded to
earth electrode for taking connection to the main
equipotential earth bus-bar in the equipment room and
to other earth pits, if any.
• Exothermic weld material is UL listed and as per RDSO
Spec.
Earthing and Earth Measurement
VER 1.0 187
• The main earth pit is located as near to the MEEB in
the equipment room as possible.
- At certain locations, it may not be possible to achieve
earth resistance of ≤1ohm with one earth
electrode/pit due to higher soil resistivity.
- Solution : Ring earth by provision of loop earth
consisting of more than one earth pit.
• The distance between two successive earth
electrodes should be between 3 to 6 m.
Earthing and Earth Measurement
VER 1.0 188
- These earth pits are inter-linked using 25mm X 2mm
copper tape to form a loop using exothermic welding.
- The interconnecting tape is buried at depth of 50cm
below ground level. This tape is also covered with
earth enhancing compound.
Earthing and Earth Measurement
VER 1.0 189
Measurement of Earth Electrode resistance
Earthing and Earth Measurement
VER 1.0 190
The earth tester generates an a.c. signal which is fed into the
system under test. Having checked that the conditions for
the test are met, the instrument automatically steps through
its measurement ranges to find the optimum signal to apply.
Measuring the current flowing and the voltage generated by
the instrument calculates and displays the system resistance
in the range of 0.001 to 20,000 ohms, depending on the
model chosen.
Earthing and Earth Measurement
VER 1.0 191
There are three basic test methods for measuring the earth
resistance:
1. Fall-of-potential method, or three-terminal test.
2. Dead Earth method (two-point test).
3. Clamp-on test method
Out of the above three methods, the first one is most
commonly used method of measuring the earth resistance
of an earth electrode.
Earthing and Earth Measurement
VER 1.0 192
1. Fall of potential method:
• For this type of test, special instruments such as Megger earth
testers are recommended for making tests to avoid the effect of
back e.m.f. and stray currents.
• In this method two auxiliary earth electrodes named as “Current
Electrode” (C) and “Potential Electrode (P) are placed at suitable
distance from the Test Electrode (E).
• All the electrodes shall be so placed that they are independent of
the resistance area of each other.
Earthing and Earth Measurement
VER 1.0 193
Earthing and Earth Measurement
VER 1.0 194
Set up for Measuring Earth Resistance
Earthing and Earth Measurement
VER 1.0 195
• An alternating current (I) is passed through the outer electrode C
and the voltage is measured, by means of an inner electrode P, at
some intermediary point between them.
• When performing a measurement, the aim is to position the
auxiliary test electrode C far enough away from the earth
electrode under test so that the auxiliary test electrode P will lie
outside the effective resistance areas of both the earth system and
the other test electrode.
• The electrode C shall be placed at least 30 meters away from the
test electrode E and the auxiliary potential electrode P shall be at
62% away from the test electrode.
Earthing and Earth Measurement
VER 1.0 196
• All the three must be in same straight line.
• A measured current is passed between the electrode E to
be tested and the auxiliary current electrode C.
• The potential difference between the electrode E and the
auxiliary potential electrode P is measured.
• The resistance of the test electrode E is then given by
R=V/I where;
R= resistance of test electrode in ohms,
V= reading of the voltmeter in volts,
I = reading of the ammeter in ampere.
Earthing and Earth Measurement
VER 1.0 197
• If the test is made at power frequency that is 50 Hz, the resistance
of the voltmeter should be high compared to that of the auxiliary
potential electrode P and in no case should be less than 20,000
ohms.
• The sources of current shall be isolated from the supply by a
double wound transformer.
• At the time of test, when possible, the test electrode shall be
separated from the earthing system.
• The auxiliary electrodes usually consist of 12.5 mm diameter mild
steel rod driven up to 1 metre in the ground.
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• Unless three consecutive readings of test electrode resistance
with different spacing of electrode agree the test shall be
repeated by increasing the distance between the electrodes E
and C upto 50 metres and each time placing the electrode P
between them.
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2. Dead Earth Method: This technique is quite popular
because of its simplicity. Only two leads are used, one
hooked to the test ground and one to a reference
ground.
• With this method, the resistance of two electrodes in a
series is measured by connecting the P1 and
C1 terminals to the ground electrode under test; P2 and
C2 connect to a separate all-metallic grounding
point (like a water pipe or building steel).
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• The dead earth method is the simplest way to obtain a
ground resistance reading but is not as accurate as the
three-point method and should only be used as a last
resort, it is most effective for quickly testing the
connections and conductors between connection points.
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3. Clamp on Method: The clamp on method is unique in
that it offers the ability to measure resistance without
disconnecting the ground system.
• It is quick, easy, and also includes the bond to ground
and overall grounding connection resistances in its
measurement.
• Measurements are made by "clamping" the tester
around the grounding electrode under test, similar to
how you would measure current with a multi-meter
current clamp.
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• The tester applies a known voltage without a direct
electrical connection via a transmit coil and measures
the current via a receive coil. The test is carried out at a
high frequency to enable the transformers to be as small
and practical as possible.
• For the clamp-on method to be effective, there must be
a complete grounding circuit in place. The tester
measures the complete resistance path (loop) that the
signal is taking. All elements of the loop are measured in
series.
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Some limitations of the clamp-on method include:
1.effective only in situations with multiple grounds in
parallel.
2.cannot be used on isolated grounds, not applicable
for installation checks or commissioning new sites.
3.cannot be used if an alternate lower resistance
return exists not involving the soil, such as with
cellular towers or substations.
4.results must be accepted on "faith."
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Measurement of Earth resistivity
• Earth tester normally used for these tests comprise the current
sources and metres in a single instrument and directly read the
resistance.
• The most frequently used earth tester is the four terminal
megger as shown in the figure.
• Four earth ground stakes are positioned in the soil in a straight
line, equidistant from one another.
• The distance between earth ground stakes should be at least
three times greater than the stake depth.
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• The earth tester generates a known current through the two
outer ground stakes and the drop in voltage potential is
measured between the two inner ground stakes.
• The tester automatically calculates the soil resistance using
Ohm’s Law (V=IR).
• The depth of electrodes in the ground shall be of the order of 10
to 15 cm.
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