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8/10/2019 2391- Earthing Arrangements
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Earthing ArrangementsEarthing Arrangements
ElectaCourse.com
2005 Last Updated August 2006
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Definitions from Part 2 BS 7671Definitions from Part 2 BS 7671
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Bonding conductor
A protective conductor providingequipotential bonding
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Where protection against electric shock does not rely
solely on basic insulation alone. Exposed-conductiveparts being connected to a protective conductor within
the fixed wiring of the installation.
Class I equipment
Class I insulation
Single-layer insulation
Live part
Exposed conductive
part
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Class II equipment
Where protection against electric shock relies on the
application of additional or supplementary insulation.
There is no provision for the connection of aprotective
conductorto exposed metalwork.
Class II insulation
live part
two layers of
insulation
exposed metalwork
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Double insulation
Double insulation (Class II) - Insulation
comprising both basic insulation andsupplementary insulation
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Earth fault loop impedance
The impedance of the earth fault current
loop starting and ending at the point of
earth fault.
Symbol ZSymbol Z
UnitUnit
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The earth fault loop
The earth fault loop starting at the point of fault
consists of:
the circuit protective conductor (c.p.c.)
consumers earthing terminal and earthing conductor
for TN systems, the metallic sheath of supply cable for TT systems, the general mass of earth
the path through the earthed neutral point of the
transformer
the transformer winding and phase conductor to point
of fault
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Earth leakage current
A current which flows to earth, or to
extraneous conductive parts, in a circuitwhich is electrically sound. This current
may have a capacitive effect including
that from the deliberate use of capacitors.
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Leakage current
Electric current in an unwanted conductive
path under normal operating conditions
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Electric current which flows in a protective conductor
under normal operating conditions
Protective conductor current
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Earthed equipotential zone
A zone within which exposed conductive parts and
extraneous conductive parts are maintained atsubstantially the same potential by bonding, such
that under fault conditions, the differences in potential
simultaneously accessible exposed and extraneous-
conductive parts will not cause electric shock.
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Direct contact (shock)
Results from
Making contact with parts of a circuit orMaking contact with parts of a circuit or
system which are live under normal conditionssystem which are live under normal conditions
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Earthing
Connection of the exposed conductive
parts of an installation to the main earthingterminal of that installation
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Extraneous conductive part
A conductive part liable to introduce a
potential, generally earth potential, and notforming part of the electrical installation.
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Functional earthing
Connection to Earth necessary for proper
functioning of electrical equipment
Table 51ATable 51AFunctional earthing conductors to be
coloured cream
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Contact of persons or livestock with
exposed-conductive parts which havebecome live under fault conditions.
Indirect contact
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Protective conductors
A conductor used for some measure of protection against
electric shock and intended for connecting together anyof the following parts
(i) exposed conductive parts
(ii) extraneous-conductive parts
(iii) the main earthing terminal
(iv) earth electrode(s)
(ii) the earthed point of the source, or an artificial neutral
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Protective conductors
earthing conductor
main bonding
conductor
circuit protective
conductor
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Shock conditionsShock conditions
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Direct contact
Contact of persons or livestock
with live parts
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Uo = 230V
Direct contact (shock)
ouchouch
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Direct ContactDirect Contact
Maximum shock voltageMaximum shock voltage
No disconnectionNo disconnection
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Protection against Direct contact (shock)
InsulationInsulation
BarriersBarriers
EnclosuresEnclosures
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Protection against Direct contact (shock)
Placing out of reachPlacing out of reach
ObstaclesObstacles
Protection of a specialist natureProtection of a specialist nature
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Uo = 230V
Indirect contact (shock)
Automatic operation of protective deviceAutomatic operation of protective device
Reduced shock riskReduced shock risk
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Protection against indirect contact (shock)
EE
EE
BB
AA
DD
OOSS
arthedarthed
quipotentialquipotential
ondingonding
nd automaticnd automatic
isconnectionisconnection
ffupplyupply
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Earthing ArrangementsEarthing Arrangements
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TT Earthing ArrangementTT Earthing Arrangement
PES
cut out
P.E.S
metering
earthing
conductor
from overhead
supply
to earth electrode
consumer unit
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1st Letter - Method of earthing for suppliers network
2nd Letter - Method of earthing at consumers
installation
T = Direct connection to earth at one or more points
T = Direct connection to earth
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Frequently used types of earth electrode
plate
lattice
rod
Regulation 542Regulation 542--0202--0101
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Earth electrodes recognised by BS 7671Earth electrodes recognised by BS 7671
The following types of earth electrode are recognised by
the Regulations:
(i) earth rods or pipes
Regulation 542Regulation 542--0202--0101
(ii) earth tapes or wires
(iii) earth plates
(iv) underground structural metalwork embedded in foundations
(v) welded metal reinforcement of concrete
(except pre-stressed concrete) embedded in earth
(vi) lead sheaths and other and other metal coverings of cables, where not
precluded by Regulation 542-02-05
(vii) other suitable underground metalwork
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Installation of earth electrodes
When installing earth electrodes the
following precautions should be observed
Regulation 542Regulation 542--0202--0202
Remember climatic conditions could affect
electrode resistance
The type and embedded depth of an earth
electrode shall be sufficient to avoid soildrying and freezing
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Earth electrode resistance
arth electrode resistance
The graph illustrates the relationship between electroderesistance and buried depth for a rod type electrode. Thedeeper the rod, the closer to the water table it becomes,
resulting in lower resistance
Typical value of resistance of
rod type electrode buried to
a depth of 1 metre (60
approx.)
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Reducing earth electrode resistance
educing earth electrode resistance
Under certain circumstances the value of
electrode resistance may be excessivelyhigh and steps must be taken to reduce
its value. The following methods may be
adopted:
use of extendable rods
use of additional rods
soil conditioning agents
(temporary measure
electrodes buried to a greater depth
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Use of additional rods to reduce resistance
se of additional rods to reduce resistance
distance (m)
depth (m)
As a rule of thumb, the distance between adjacentearth rods should not be less than the buried depth.
earthing
conductor
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Problems associated with the TT systemProblems associated with the TT system
Vulnerable to mechanical damage
Vulnerable to corrosion
High resistance as compared toHigh resistance as compared to
TN systemsTN systems
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Providing automatic disconnection for TT systems
roviding automatic disconnection for TT systems
Under phase to earth fault conditions overcurrentdevice should cut of supply rapidly.
Increase earth path resistance may be
sufficiently high so as to prevent automatic
disc connection resulting in:
ShockShock
FireFireand, or
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Use of the residual current device
se of the residual current device
Regulation 413Regulation 413--0202--1919
Preferred method of protection against indirect
contact, by means of residual current device.
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Calculating touch voltage
alculating touch voltage
Maximum permitted touch voltage = 50V unless
special location. (max 25V)
Regulation 413
egulation 413
02
2
20
0
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The following condition must be fulfilled:
RRA.A.IInn 50V.50V.Where:
RRAA is the sum of the earth electrode and protective
conductors connecting it to the exposed-conductive parts
I.I.
nn is the current causing automatic operation
of the r.c.d.
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LoadLoad
ExposedExposed
metalworkmetalwork
Test resistorTest resistor
Test buttonTest button
Search coilSearch coil
ToroidToroid
Operating coilOperating coil
The residual current device
he residual current device
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The r c d under healthy circuit conditions
he r c d under healthy circuit conditions
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The r c d under earth fault conditions
he r c d under earth fault conditions
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Protection and the TT system
rotection and the TT system
Remember! The earth fault loop impedance for a TT system
may be too high to allow circuit breakers and fuses to
operate under phase to earth fault conditions.
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TNTN--S Earthing ArrangementS Earthing Arrangement
Separate neutral and earth conductors
El t C
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1st Letter - Method of earthing for suppliers network
2nd Letter - Method of earthing at consumers installation
T = Direct connection to earth at one or more points
N = Consumers exposed metalwork directly connected to the
earthed neutral point of the supply
3rd Letter - Relationship between earth & neutral conductors
on suppliers network
S = Separate neutral and earth conductors at consumersinstallation
El t C
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Fig. 7
PES consumer
The circuit arrangement for the TN
he circuit arrangement for the TN
S system
system
El t C
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PES consumer
The TN
he TN
S system under fault conditions
system under fault conditions
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TNTN--CC--S Earthing ArrangementS Earthing Arrangement
combined neutral and earth conductors
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1st Letter - Method of earthing for suppliers network
2nd Letter - Method of earthing at consumers installation
T = Direct connection to earth at one or more points
N = Consumers exposed metalwork directly connected to the
earthed neutral point of the supply
3rd Letter - Relationship between earth & neutral conductors
on suppliers network
C = Combined neutral and earth on suppliers side
4th Letter - Arrangement of earth and neutral conductors
at consumers installation
S = Separate neutral and earth conductors at consumersinstallation
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PES consumer
PEN conductor
The circuit arrangement for the TN
he circuit arrangement for the TN
C S system
system
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The TN
he TN
C S system under fault conditions
system under fault conditions