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System Earthing
&
Protective Earthing
Low Voltage Earthing System
All points that generate electricity or changes
system voltage must be earthed.
Type of Earthing Systems:-
Protective Earthing for persons and equipments
against electric shock
Technical earthing (Functional )
Protective earthing system must be bounded at some
points to the technical earthing system.
Technical earthing systems must not designed only to
clear earth fault current but also, it bust be to provide a
high integrity, low impedance path to earth for high
frequency leakage (up to 30 MHz ) currents and noise
caused by switching and lightning.
( due to high frequency current components, we must
use several parallel paths to earth)
Technical Earthing Systems
T N
• TN-C
• TN-S
TT
IT
Grounding systemsIEC 364
TN system
TN-C systems3 pole CB
•The transformer neutral is earthed;
•The frames of the electrical loads are
connected to the neutral.
The insulation fault turns into a short-circuit
and the faulty part is disconnected by Short-
Circuit protection Devices (SCPD).
The fault voltage (deep earth/frame), known as
«indirect contact» is » Uo/2 if the impedance of
the «outgoing» circuit is equal to that of the
«return» one.
When it exceeds the safety limit voltage, which
is normally 50 V, it requires disconnection.
• provides a return path for faults in the
LV grid.
• This ensures a distributed grounding
and reduces the risk of a customer nothaving a safe grounding.
• However faults in the MV network may
migrate into the LV grid grounding
causing touch voltages at LV clients.
• The utility is not only responsible for aproper grounding but also for the safety ofcustomers during disturbances in the powergrid.
• A fault in the LV network may cause touchvoltages at other LV clients.
• Most critical are faults at the ends of thebranches, where the circuit impedance is thehighest. In the design of LV-grids, this circuitimpedance should be limited.
• The maximum length of an outgoing cable istherefore limited. A practical length of acable was 300 m.
Advantages:
Provides a return path for faults in the LV grid.
Ensures a distributed grounding and reduces the risk of a
customer not having a safe grounding
Disadvantages:
Faults in the electrical network at a higher voltage level may
migrate into the LV grid
The utility is not only responsible for a proper grounding
A fault in the LV network may cause touch voltages at other LV
clients
Most critical are faults at the ends of the branches, where the circuit
impedance is the highest.
The maximum length of an outgoing cable is therefore limited
• Needs a very good earthing impedance of the network (about 2
Ω ) TN-C Inadequate for EMC problems
• The TN-C should be avoided since rank 3 harmonics and
multiples of it to flow in the PEN.
prevent the latter from being used as a potential
reference for communicating electronic systems, if
the PEN is connected to metal structures, both these
and the electric cables become sources of
electromagnetic disturbance
Multiple Earthing
Grounding systemsIEC 364
TN system
TN-S systems4 Pole CB
Load Load
Ud
Id
• LV cable with a grounded sheath is applied.
• Additional electrodes in the LV grid,
preferably at each user, divert external
induced (lightning) currents.
• In a TN-S system five conductors are
required.
• We prefer TN-S For:-
Very long network.
Loads with low natural insulation
(furnaces) or, with large HF filter (large
computers) and communication systems
Grounding systemsIEC 364
TN system
TNC-S system
Grounding systemIEC 364
TT system
• The transformer neutral is earthed;
• The frames of the electrical loads are also
connected to an earth connection.
• The insulation fault current is limited by the
impedance of the earth connections an the
faulty part is disconnected by a Residual
Current Device (RCD).
• Each customer needs to install and maintain it’s own ground
electrode.• The ground impedance at the customer should be low (Rc<30 Ω)
• RCD’s are required
Advantages
• Faults in the LV and MV grid do not migrate to other clients in the
LV grid
• A broken neutral conductor does not affect a single-phase
connection, but may cause damage to equipment using a three-
phase connection
• Good security condition : potential rise of the grounded
conductive part - limited at 50 V for a fault inside the installation
• No influence of the network evolution (fault loop impedance)
Disadvantages
• For large customers it is impossible to apply a TT system, since
the disconnecting time of the over-current protective device is
too long. A TN system always provides a low impedance return
path.
• In TT-systems high over-voltages may occur between all live
parts and PE conductor
The TT provides a good separation between the responsibilities
of the supplier and the customer and needs less control of the
transferred potentials for assessing safety in case of HV fault.
The same is valid in case of a phase to neutral fault in the LV
network.
• It is a very good for communication systems for very
low interferences
Grounding systemIEC 364
IT system
It is naturally earthed by the stray capacities of the network
cables. voluntarily by a high impedance of around 1,500 Ω
(impedance earthed neutral)
Ud
V1V2
V3
V13V23
We must avoid second fault by using very fast protection
(1st fault) : Id < 1 A
(2nd fault): Id ≈ 20 kA
AdvantagesThe voltage developed in the earth connection of the frames (a
few volts at the most) does not present a risk.
continuity of service
loads sensitive to high fault currents
Disadvantages
If a second fault occurs and the first one has not yet been
eliminated, a short-circuit appears and the SCPDs must provide
the necessary protection.
The frames of the relevant loads are brought to the potential
developed by the fault current in their protective conductor (PE).
Restrictions and precautions for using the IT earthing system
The restrictions for using the IT system are linked to loads and networks.
high earth capacitive coupling (presence of filters)
system for use in:
• Hospitals
• Airport take-off runways
• Arc Furnaces
• Plants with continuous manufacturing
processes
• Laboratories
• Cold storage units
• Welding Machines.
influence of MV earthing
systems
In MV, the neutral is not distributed and there is no
protective conductor (PE) between substations or
between the MV load and substation.
A phase/earth fault thus results in a single-phase
short-circuit current limited by earth connection
resistance and the presence of limitation
impedances.
IEC 364-4-442 states:
The earthing system in a MV/LV substation
must be such that the LV installation is not
subjected to an earthing voltage of:
Uo + 250 V : more than 5 s
Uo + 1,200 V: less than 5 s
This means that the
various devices connected to the LV
network must be able to withstand this
constraint.
The same standard states that if Rp > 1 Ω,
the voltage Rpx IhMT must be eliminated, for
example:
100 V under 500 ms;
500 V under 100 ms.
if Rp and RB are connected, the fault current
causes the potential of the LV network to rise
with respect to the earth.
If this is not so, Rp and RN must be
separate whatever the LV earthing
system.
If all the earth connections (substation
neutral- applications) have been grouped
into a single one, a rise in potential of LV
frames may be observed which can be
dangerous.
LV Grounding systems
• LV electrical network may supply several
types of applications.
• Only one type of Earthing System cannot
be suitable for all applications.
• It is advisable to ”mix” various
Grounding Systems in an electrical
installation.
A clash of technical cultures is inevitable:
• Electrical engineers have problems with the harmonicsgenerated by static
• converters. These harmonics cause temperature rises intransformers, destruction of capacitors and abnormalcurrents in the neutral;
• Electronic engineers place filters upstream of theirproducts, which do not always withstand over-voltagesand lower network insulation;
• lamp manufacturers are unaware of the problemscaused by energizing inrush currents, harmonics andhigh frequencies generated by certain electronic ballasts;
• Computer engineers (same applies to designers ofdistributed intelligence systems) are concerned withequipotentiality of frames and conducted and radiatedinterference.
Mixing of different Grounding systems
LV distribution
The most common systems are TT and TN
The TN-C system is particularly used but it needs
carefully designed SCPD.
It is not currently recommended in premises equipped
with communicating electronic systems as currents in
the neutral and thus in the PE cause potential
references to vary ( TT system)
RCDs are used for personnel protection (for very long
cables)
The TT system Is the easiest one to implement; insulation fault
currents are smaller than in TN or IT thus accounting for its value as
regards risk of fire, explosion, material damage and electromagnetic
disturbances.
The IT Systems (unearthed neutral) is used whenever continuity of
service is essential.
The TNC-S is increasingly chosen for large projects.
Actual grounding system
In residential areas
EDC
network
TN-C
Consumer
installation
HazardsIn residential
areas
F1) Direct contact fault loop
HazardsIn residential
areas
Effects of sinusoidal alternating current in
the range of 15 Hz to 100 Hz
Risk of electric shocks
Electric shock
• -It is caused by the current that flows through the human body.
• -The current depends mainly upon the skin contact resistance.
• -The contact resistance varies with ( thickness, wetness and
resistively of the skin ).
• -In general :
Current<5mA is not dangerous .
10mA< current <20mA
The current is dangerous because the victim loses muscular
control and so may not be able to let go .
Current>50mA the consequences can be fatal .
• Resistance of human body Rb:
Rb : between two hands or between one hand and a leg ranges
from 500 Ohms to 50 K Ohms
• if Rb = 50 K ohms
The momentary contact with 600 V may not be fatal .
I body = 600 V / 50 K Ohms = 12 mA
• but if Rb = 500 ohm and voltage is as low as 25V ac
I body = 25 V / 500 Ohms = 50 mA (may be fatal )
• the current is particularly dangerous when it flows in the region of the heart .
• statistical investigations have shown that a current may cause
death if it satisfies the following equation
Ib = 116/ square root ( t )
where :-
Ib : current flow through the body in mA
t : time of current flow second
116 : an empirical constant, expressing the probability of a
fatal out come .
[ IEEE transactions on industry and general application ]
vol. IGA - 4, No. 5 , pages 467 to 475.
• example:
A current of 116 mA for 1 s could be fatal .
Breaking time for RCDs 30mA (300mS), 60mA(150mS), 150mA(40mS)
Ventricular Fibrillation is considered to be the main cause of
Electrocution
CDetection
Measurement
Tripping
Operating principle of Earth
leakage protection
General Specifications
• Number of poles 2 or 4
• Rated voltage not exceeding 1000 v.
• Rated breaking load current
• Rated breakage earth leakage fault
current.
Installation of RCD
General Notes
Every installation which includes exposed
conductive parts should be protected by one or more
RCD
If an installation is protected by one RCD , This
device should be Located at the starting point of the
installation
The exposed conductive parts of the protected
appliance should be all connected to an earth
electrode of suitable resistance
Depending on the type of the installation and
the risks involved , it may be necessary to
provide RCD having different sensitivities in
order to protect different parts of the
installation
It is also necessary to arrange for a measure
of selectivity ( Coordination ) between the
operation of the RCD’s located at different
parts in the installation
Installation of RCD
RCD 300 mA
It is very important to use the current-operated residual
current devices (RCDs).
Current operated devices rated at up to 500mA have
been available for protecting installations and individual
sub-circuits for many years.
More recently sensitive RCDs (30mA and below) have
become available.
These are regarded as providing excellent protection
against electric shock, and can be fitted to sub-circuits
or socket outlets.
Protective Earthing
• Safety for persons.
• Proper operation and long life
time for equipments
Step & Touch Potential
• Earthing systems allow unwanted electrical
currents to flow harmlessly to earth.
• Their main function is to provide low
impedance (not only resistance) paths for high-
energy discharges and high frequency,
particularly lightning strikes, other transients
and fault currents.
The main markets for installing earthing systems:-
utility power generation, transmission and distribution.
lightning protection for buildings and high structures
Private electricity distribution networks in industrial and
commercial premises.
Protection of electronic equipment e.g. computer
installations, telecommunications.
Domestic housing and small commercial premises
Situations where a build-up of electrostatic potential
could
be dangerous, including oil refineries, petroleum filling
stations, grain storage, hospitals.
Earthing Installation
Typical earth electrodes include
simple surface earth electrodes rod (vertical) electrodes meshed electrodes,
cable with earth electrode effect foundation earth electrodes
Earth surface potential distributionVx* = f(x) around a vertical rod earth electrode
with length l = 3 m, diameter d = 0.04 m
Resistance and surface potential distribution
Step and Touch potentials
The most frequently used electrode materials are:
Steel
Galvanized steel
Steel covered by copper
High-alloy steel
Copper and copper alloys.
Earthing, Backfilling Materials
SOIL ENHANCEMENT OPTIONS
1. Conductive Concrete
30 to 90 ohm-meters
2. Bentonite
2.5 ohm-meters
3. Carbon-Based Backfill Materials
0.1 to 0.5 ohm-meters
4. Clay-Based Backfill Materials (GAF)
0.2 to 0.8 ohm-meters depending on moisture content
5- Marconite
0.1 ohm.meters
Grounding Grids
A common method for obtaining a low ground
resistance at high voltage substations is to use
interconnected ground grids. A typical grid system for a
substation would comprise solid copper conductors
buried at a depth of from 30 to 60 cm, spaced in a grid
pattern of about 3 to 10 m. At each junction, the
conductor: are securely bonded together.
