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CHAPTER 6EARTHING SYSTEMCHAPTER 6: EARTHING SYSTEMThis is the
passage of current through the body of such magnitude as to have
significant harmful effects. Table 1 and Figure 1 illustrate the
generally accepted effects of current passing through the human
body. How, then, are we at risk of electric shock and how do we
protect against it?ELECTRIC SHOCKCHAPTER 6: EARTHING SYSTEMELECTRIC
SHOCK
Table 1: Effects of Current Passing Through the Human
BodyCHAPTER 6: EARTHING SYSTEMELECTRIC SHOCK
CHAPTER 6: EARTHING SYSTEMELECTRIC SHOCKTouching live parts of
equipment or systems that are intended to be live.Touching
conductive parts which are not meant to be live, but which have
become live due to a fault.There are two ways in which we can be at
risk:CHAPTER 6: EARTHING SYSTEMThree earthing systems such as
defined in IEC 364 are:1) exposed-conductive parts connected to
neutral TN;2) earthed neutral TT;3) unearthed (or
impedance-earthed) neutral IT EARTHING SYSTEMCHAPTER 6: EARTHING
SYSTEMPURPOSE OF EARTHING SYSTEMregards protection of persons and
property: mastery of insulation fault effects. They are considered
to be equivalent with respect to safety of persons against indirect
contacts CHAPTER 6: EARTHING SYSTEMCLASSISFICATIONSThe extent of
the earth fault and the consequences deriving from the touching of
live exposed-conductive-parts are specifically related to the
neutral condition of the power system and to the types of system
earthing.As a consequence, to select the proper device for the
protection against earth faults, it is necessary to know the
installation distribution system. The International Standard IEC
60364-3 classifies the electrical systems with the combination of
two letters.CHAPTER 6: EARTHING SYSTEMCLASSISFICATIONSThe first
letter indicates the relationship of the power system to
earth:>> T = direct connection to earth of one point, usually
the neutral, in ac systems;>> I = all live parts isolated
from earth or one point, usually the neutral, connected to earth
through an impedance.
The second letter indicates the relationship of the
exposed-conductive- parts of the installation to earth:>> T =
direct electrical connection of exposed-conductive parts to
earth;>> N = direct electrical connection of the
exposed-conductive parts to the earthed point of the power
system.
CHAPTER 6: EARTHING SYSTEMCLASSISFICATIONSSubsequent letters, if
any, indicates the arrangement of neutral and protective
conductors:S = neutral and protective functions provided by
separate conductorsC = neutral and protective functions combined in
a single conductor (PEN conductor).
For TN-S, TN-C-S and TT systems the following explanations
should aid a full understanding of the earthing arrangements and
their scope of application. The nomenclature of these system types
is as follows: T =Earth (from the French word Terre) N=Neutral
S=Separate C=Combined
CHAPTER 6: EARTHING SYSTEMTT SYSTEMSThis arrangement covers
installations not provided with an earth terminal by the
Electricity Supply Company. Thus it is the method employed by most
(usually rural) installations fed by an overhead supply. Neutral
and earth (protective) conductors must be kept quite separate
throughout the installation, with the final earth terminal
connected to an earth electrode by means of an earthing
conductor.
CHAPTER 6: EARTHING SYSTEMThe earth fault current returns to the
power supply node through the soil. In this type of electrical
installations the neutral is usually distributed and its function
is making the phase voltage (e.g. 240 V) available for the supply
of the single-phase loads of civil installations.Effective earth
connection is sometimes difficult. Because of this, socket outlet
circuits must be protected by a residual current device (RCD) with
an operating current of 30 mA. TT SYSTEMS
CHAPTER 6: EARTHING SYSTEM
TT SYSTEMSCHAPTER 6: EARTHING SYSTEMTN SYSTEMSIn TN systems, the
neutral is directly earthed, whereas the exposed-conductive-parts
are connected to the same earthing arrangement of the neutral.TN
electrical systems can be divided into three types based on the
fact that the neutral and protective conductors are separate or
not: 1) TN-S 2) TN-C 3) TN-C-S
CHAPTER 6: EARTHING SYSTEMTN SYSTEMS: TN-SThe neutral conductor
N and the protective conductor PE are separated
CHAPTER 6: EARTHING SYSTEM
TN SYSTEMS: TN-SCHAPTER 6: EARTHING SYSTEMTN SYSTEMS: TN-CTN-C:
the neutral and protective functions are combined into a single
conductor, called PEN.
CHAPTER 6: EARTHING SYSTEMTN SYSTEMS: TN-C-STN-C-S: the neutral
and protective functions are partially combined into a single PEN
conductor and partially separated PE + N
CHAPTER 6: EARTHING SYSTEMTN SYSTEMS: TN-C-S
CHAPTER 6: EARTHING SYSTEMTN SYSTEMSIn TN systems the earth
fault current returns to the power supply node through a direct
metal connection (PE or PEN conductor) without practically
affecting the earth electrode
CHAPTER 6: EARTHING SYSTEMIT SYSTEMSThe installation
arrangements in the IT system are the same for those of the TT
system. However, the supply earthing is totally different. The IT
system can have an unearthed supply, or one which is not solidly
earthed but is connected to earth through a current limiting
impedance.
CHAPTER 6: EARTHING SYSTEMIT SYSTEMSThe total lack of earth in
some cases, or the introduction of current limiting into the earth
path, means that the usual methods of protection will not be
effective. For this reason, IT systems are not allowed in the
public supply system. An exception is in medical situations such as
hospitals. Here it is recommended that an IT system is used for
circuits supplying medical equipment that is intended to be used
for life-support of patients. The method is also sometimes used
where a supply for special purposes is taken from a private
generator.CHAPTER 6: EARTHING SYSTEMThe earth fault current returns
to the power supply node through the earthing arrangement of the
exposed conductive parts and the capacities to earth of the line
conductors. IT SYSTEMS
CHAPTER 6: EARTHING SYSTEMEARTH FAULT LOOP IMPEDANCE
CHAPTER 6: EARTHING SYSTEM
Where:Zs = total loop impedanceZe = external loop impedanceR1 =
resistance of the circuit line conductorR2 = resistance of the
circuit cpcEARTH FAULT LOOP IMPEDANCESimplified VersionCHAPTER 6:
EARTHING SYSTEMIT SYSTEMS
CHAPTER 6: EARTHING SYSTEMEARTH ELECTRODE
