Earthing Systems by onx77558

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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.
         Technical Earthing Systems



    TN
•          TN-C
•          TN-S
    TT
    IT
    Grounding systems
         IEC 364
        TN system
               TN-C systems
3 pole CB
• The transformer neutral is earthed.
• The frames of the electrical loads are
  connected to the neutral.

 The faulty part is disconnected by Short-
 Circuit protection Devices (SCPD).
 The fault voltage (earth/frame), is » Uph/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
  not having 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 a
  proper grounding but also for the safety of
  customers during disturbances in the power
  grid.
• 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. In the design of LV-grids, this circuit
  impedance should be limited.
• The maximum length of an outgoing cable is
  therefore limited. A practical length of a
  cable 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 Medium 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 systems
          IEC 364
         TN system

4 Pole CB
                   TN-S systems




            Load      Load
     Id




Ud
• 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 system
    IEC 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 system
     IEC 364
    IT system
V2              V1




 V23          V13



         V3
                                            Ud




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)
We must avoid second fault by using very fast protection

                 (1st fault) : Id < 1 A

                 (2nd fault): Id ≈ 20 kA
Advantages
The 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.
      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.
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.
   Hazards
In residential
    areas
                                     Hazards
F2) Indirect contact fault loop
                                  In 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 .
•      example:
  A current of 116 mA for 1 s could be fatal .



 Break time for RCDs 30mA (300mS), 60mA(150mS),
 150mA(40mS)
Operating principle of Earth
   leakage protection
     Detection                 Tripping
                                          C




                 Measurement
                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
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
• 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
Vertical Rod

                                                   R




                     Earth surface potential distribution
               Vx* = f(x) around a vertical rod earth electrode
                  with length l = 3 m, diameter d = 0.04 m
Horizontal Rod

                 R
Horizontal Strip
                                       t
                                             b
                                L
   R
                                            c


       d = 2b / π


Different Horizontal Shapes



                              ℓ Σ : Sum of shape lengths
Variation of Earthing Resistance with Rod length
  Parallel Vertical Shapes

                                   Factor λ for parallel rods in a straight line
               1+λ a
Rn = R    ﴾    n       ﴿            Number of rods (n)               λ


     a=       ρ                               2                     1.0
              2πRs                            3                    1.66
                                              4                    2.15
                                              5                    2.54
n : number of rods                            6                    2.87
s : distance between rods in (m)
                                              7                    3.15
ρ : Soil resistivity ( Ω . m )
                                              8                    3.39
                                              9                    3.61
                                             10                    3.81


For 3 Rods in an equilateral triangle λ = 1.66
                                Number of rods   Factor λ
Factor for rods arranged in a   (n) along each
hollow square                     side of the
                                    square
                                      2            2.71
                                      3            4.51
                                      4            5.48
                                      5            6.14
                                      6            6.63
                                      7            7.03
                                      8            7.36
                                      9            7.65
                                     10            7.90
                                     12            8.32
                                     14            8.67
                                     16            8.96
                                     18            9.22
                                     20            9.40
Practical Rod material and Size
Design 0f Earthing cable


 S= I2 t / K

S: conductor cross sectional area (mm2)

I : rms value of the fault current (A)

t: time of short circuit current duration in Sec.( about 3 seconds)

K: Factor depends on the limiting temperature of earthing conductor
(conductor and insulation material)
 For copper and PVC cable K=115
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.
IMPULSE IMPEDANCE OF GROUNDING SYSTEMS

  The impulse impedance of a grounding system is necessary for
  determining its performance while discharging impulse currents to
  ground, as in the case of lightning and transient ground faults.
Step & Touch Potential
Step and Touch potentials
Permissible design values for Step and Touch Potentials



              165 + ρ s
                                                   165 + 0.25 ρ s
 E step =
                                     E touch =
                 t
                                                     t


ρs : Ground Surface resistivity beneath the feet
t : Fault duration



For safety, the design step and touch voltage must not exceed the
following;

Assume ρ s = 0.0 and the maximum fault duration 6 sec.
Therefore

E step and E touch must not exceed 67 volt
Measurements of the ground resistivity




          ρ= 2 π a R
Measuring of Earthing System Resistance

								
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