POWER SYSYTEM EARTHING
Eng. Mahmoud Kamel Mahmoud Kamel
E-Mail: eng_mahmoudkamel@yahoo.com
Pho: 01013344427
POWER SYSYTEM EARTHING
References
BS-EN 7430:2011: Code of Practice for Protective Earthing.
BS-EN 7671:2008: Requirements for Electrical Installation.
IEE Wiring Regulations 17th Edition
SCHNEIDER- ELECTRICAL INSTALLATION GUIDE 2013
IMPORTANCE OF EARTHING
IMPORTANCE OF A SUITABLE EARTHING :
A well designed earthing system is basic for any electrical
installation to avoid the danger associated with fault
currents.
When an earth fault occurs within an installation, dangerous
voltages can appear between conductive parts, and if these
parts are simultaneously accessible they might constitute a
shock hazard. The severity of any shock experienced by a
person at risk depends on
a. The type of current (i.e. a.c. or d.c.),
b. The magnitude of the voltages.
c. The amount of time which the earth fault is allowed
to persist.
FOR THE ELECTRIC POWER SYSTEM:
• In case of fault conditions, the fault may lead to consequential
damage or failure in equipment of the system.
• The system may be exposed to transient over voltages of
several times normal values which appear from line to ground
during normal switching of a circuit having a line to ground fault.
FOR HUMAN BEING:
• During these fault conditions, a high potential difference
between the meta parts of the system ( that should not be
carrying currentl in the normal Operation) and earth will be
produced, if a man touches these parts, he will be exposed to
an electric shock and it can be lethal.
The purpose of earthing the non-current carrying metalwork of
plant and equipment of a low-voltage installation is to:
a) Provide for a low earth fault loop impedance to facilitate
automatic disconnection of supply in the event of a fault to
exposed-conductive-parts.
b) Limit the rise of potential under earth-fault conditions of non-
current carrying metalwork with respect to earth to increase
the safety of persons and animals in proximity to such
metalwork.
THE WORD “ EARTH”:
Earth: is the conductive mass of earth, whose electric potential at
any point is conventionally taken as zero.
In fact, earthing is a word commonly used in electric power
systems work to cover both “system earthing “and “safety
earthing”
System Earthing:
is a connection to ground from one of the current-carrying
conductors of a distribution system.
Safety/Equipment Earthing:
is a connection to ground from one or more of the non-current
carrying metal parts of the wiring system or of apparatus
connected to the system.
EARTHING SCHEMES
Earthing
Arrangement
Earthed Neutral Independent
Systems (TN)
Isolated (IT)
Earths (TT)
T - Terre (French for earth) and meaning
a direct connection to earth.
I - No point is connected with earth
TNC TNS TNCS (isolation), except perhaps via a high
impedance.
N - Neutral C - Combined S - Separate.
EARTHING SCHEMES
The system earthing arrangements are depicted by a of
code letters.
1st letter 2nd letter 3rd letter
for TN
source consumer
T- Earthed N- Earthed Via Neutral C- Combined N & E
I- Isolated T- Independently Earthed S- Separate N & E
EARTHING SCHEMES
Earthed Neutral Systems (TN)
TN-C system (4 wires)
- one point at the supply source is connected directly to earth.
- The neutral conductor is also used as a protective conductor and
is referred to as a PEN (Protective Earth and Neutral) conductor.
EARTHING SCHEMES
Earthed Neutral Systems (TN)
TN-S system (5 wires)
- one point at the supply source is connected directly to earth.
- The protective conductor and the neutral conductor are separate.
EARTHING SCHEMES
Earthed Neutral Systems (TN)
TN-C-S system
- one point at the supply source is connected directly to earth.
- Part of the system uses a combined PEN conductor, which is at
some point split up into separate PE and N lines.
EARTHING SCHEMES
Independent Earths (TT)
- one point at the supply source is connected directly to earth.
- The protective earth connection for the consumer is provided by a
local earth electrode.
EARTHING SCHEMES
Isolated (IT)
- The electrical distribution system has no connection to earth at
all, or it has only a high impedance connection.
- The protective earth connection for the consumer is provided by a
local earth electrode.
EARTHING COMPONENTS
EARTHING COMPONENTS
Protective Conductors
It should be remembered that the following are all
types of
protective conductor:
• Circuit Protective Conductors (CPC)
• Protective Bonding Conductors
• Earthing Conductors.
EARTHING COMPONENTS
Circuit Protective Conductors (CPC) :
EARTHING COMPONENTS
Protective Bonding Conductors:
Or bonding conductor: protective conductor providing equipotential
bonding connections between conductive parts and the earth bar or
main earthing terminal (MET)
EARTHING COMPONENTS
Main Earthing Terminal (MET)
It is connected to the protective conductor of the source and via this to
earth electrodes installed in the electricity distribution supply system.
EARTHING COMPONENTS
Earth Electrode (Rod)
Quality earth rods are commonly made from either
solid copper, stainless steel or copper bonded steel.
Solid copper and stainless steel rods offer a very high
level of corrosion resistance at the expense of lower
strength and higher cost.
EARTHING COMPONENTS
Earthing Conductors:
The earthing conductor connects the system earth bar or main
earthing terminal (MET) to the system means of earthing.
The cross-sectional area of the earthing conductors shall be not
less than the value determined by the following formula
NOTE This equation is an adiabatic equation and is applicable for disconnection times
not exceeding 5 s.
EARTHING COMPONENTS
Earthing Conductors:
where:
S is the nominal cross-sectional area of the conductor in mm2
I is the value in amperes (rms for a.c.) of fault current for a fault of
negligible impedance.
t is the operating time of the disconnecting device in seconds
corresponding to the fault current I amperes
k is a factor taking account of the resistivity, temperature coefficient
and heat capacity of the conductor material, and the appropriate
Initial and final temperatures.
EARTHING COMPONENTS
Earthing Conductors:
where
T1 is the initial temperature, in °C;
T2 is the final temperature, in °C;
K and β have the values given in the following Table
EARTHING DESIGN
2- Earth conductors and their resistance calculation:
ρ is the resistivity of soil, in ohm metres (Ωm).
L is the length of the conductor, in metres (m).
d is the diameter of the round conductor, in metres (m). is
h the depth of the electrode, in metres (m).
EARTHING DESIGN
For System Earthing Design (Rsystem is required)
1- Earth electrodes and their resistance calculation:
For single rod electrode
For parallel connection of aligned rods
ρ is the resistivity of soil, in ohm metres (Ωm);
L is the length of the electrode, in metres (m);
d is the diameter of the rod, in metres (m).
n is the number of rods;
s is the spacing between the rods, in metres (m).
EARTHING DESIGN
3-Overall system resistance calculation:
Note
For Lightning Earth Network R system must be < 10 Ω
For Power Earth Network R system must be < 5 Ω
For Clean Earth Network R system must be < 0.5 Ω
