2391 week 7

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Earthing

Arrangeme

nts

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Where protection against electric shock does not rely

solely on basic insulation alone. Exposed-conductive

parts 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 a

protective

conductor

to 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 and

supplementary insulation

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Earth

The conductive mass of Earth, whose

electric potential at any point is

conventionally taken as zero

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Earth Electrode

A conductor or group of conductors

in intimate contact with, and providing

an electrical connection to earth

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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 Z

Unit



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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 return path

• For TT and IT systems the earth return path

• 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

extraneous conductive parts, in a

circuit

which is electrically sound. This

current

may have a capacitive including

that from

the deliberate use of capacitors.

Deleted by BS
671:2001

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Leakage

current

Electric current in an unwanted conductive
path under normal operating conditions

NEW

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Earthed equipotential

zone

A zone within which exposed conductive parts and

extraneous conductive parts are maintained at

substantially 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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Earthing

Connection of the exposed conductive

parts

of an installation to the main earthing

terminal of that installation

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Direct contact (shock)

Results from

Making contact with parts of a circuit

system which are live under normal

conditions

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Earthing

Connection of the exposed conductive

parts of an installation to the main earthing

terminal of that

installation

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Extraneous conductive

part

A conductive part liable to introduce a

potential, generally earth potential, and not

forming part of the electrical installation.

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Fault

A circuit condition in which current flows through an

abnormal or unintended path. This may result from an

insulation failure or a bridging of insulation. Conventionally

the impedance between live conductors or between live

conductors and exposed or extraneous conductive parts

at the fault position is considered negligible.

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Functional earthing

Connection to Earth necessary for proper

functioning of electrical equipment

Table 51A

Functional earthing conductors

to be

coloured cream

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Contact of persons or livestock with

exposed-conductive parts which have

become 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 any

of the following parts

• Exposed conductive parts

• Extraneous-conductive parts

• The main earthing terminal

• Earth electrode(s)

• The earthed point of the source, or an artificial neutral

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Uo = 230V

Direct

contact

(shock)

ouch

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Direct Contact

Maximum shock voltage

No disconnection

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Protection against Direct

contact (shock)

Insulation

Barriers

Enclosures

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Protection against Direct contact (shock)

Placing out of

reach

Obstacles

Protection of a specialist nature

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Uo = 230V

Indirect contact (shock)

Automatic operation of protective device

Reduced shock risk

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Protection against indirect

contact (shock)

arthed

quipotential

onding

nd automatic

isconnection

upply

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TT 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 542-

02-01

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Earth electrodes recognised by BS 7671

The following types of earth electrode are recognised by the

Regulations:

Regulation 542-02-01



Earth rods or pipes



Earth tapes or wires



Earth plates



Underground structural metalwork embedded in

foundations



Welded metal reinforcement of concrete (except

pre-stressed concrete) embedded in earth



Lead sheaths and other metal coverings of cables

where not precluded by regulation 542-02-05



Other suitable underground metalwork

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Installation of earth electrodes

When installing earth electrodes the following

precautions should be observed

Regulation 542-02-02

Remember climatic conditions could affect

electrode resistance

The type and embedded depth of an earth

electrode shall be sufficient to avoid soil

drying and freezing

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Earth electrode

resistance

The graph illustrates the relationship between

electrode resistance and buried depth for a ‘rod

type’ electrode. The deeper 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

Under certain circumstances the value of

electrode resistance may be excessively high

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

Distance (m)

Depth (m)

As a ‘rule of thumb’, the distance between

adjacent earth rods should not be less than the

buried depth.

Earthing

conductor

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Problems associated with the TT

system

Vulnerable to mechanical damage

Vulnerable to corrosion

High resistance as compared

to TN systems

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Use of the residual current device

Regulation 413-02-19

Preferred method of protection against

indirect contact, by means of residual

current device.

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Calculating touch voltage

Maximum permitted touch voltage = 50V unless

special location. (max 25V)

Regulation 413-02-20

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The following condition must be

fulfilled:





50V.

Where:

is the sum of the earth electrode and protective

conductors connecting it to the exposed-conductive parts



is the current causing automatic operation

of the r.c.d.

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REMEMBER

50V

max, or

25V

max

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Load

Exposed

metalwork

Test resistor

Test button

Search coil

Toroid

Operating

coil

The residual

current

device

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Protection 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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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 phase &

neutral conductors on suppliers network

S = Separate neutral and earth conductors

at consumers

installation

30.11.2005

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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 phase &

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 consumers installation

Document Outline