2391- earthing arrangements

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


(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


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


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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2nd Letter - Method of earthing at consumers installation


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

www ElectaCourse com


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TNTN--CC--S Earthing ArrangementS Earthing Arrangement

combined neutral and earth conductors



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2nd Letter - Method of earthing at consumers installation


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

2391- earthing arrangements

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