GROUNDING AND GROUNDING SYSTEM
by
Er. Y.S. JADAUN
1.0
GENERAL
1.1
Purpose

To provide means for passage of current to earth without adversely affecting
continuity of service:
- Under normal conditions
- Under fault conditions

To ensure safety of person:
- With in the grounded area
- In the vicinity of grounded area
1.2
Approach to Safe Grounding


1.3
Intentional ground: by use of electrodes/conductors.
Accidental ground: by controlling ground potential gradients.
Factors Responsible for Electric Shock Accidents





2.0
Soil resistivity and distribution of ground currents causing high potential
gradients.
High fault currents in relation to grounding system area and resistance to remote
earth.
Typical combination of time, position, posture & point of contact of person.
Inadequate contact resistance to limit body current.
Relatively large contact duration.
TOLERABLE BODY CURRENT




Effects of current passing through vital parts depend on:
- Magnitude
- Duration
- Frequency
Most dangerous effect: ventricular fabrillation causing immediate stoppage of
blood circulation.
Effect of frequency:
- Very high at 50/60 Hz.
- High at 25 Hz
- Low at 0 Hz
- Lower at higher Hz
Effects on body:
- 1 mA; tingling sensation.
-
2.1
1-6 mA (let go current): unpleasant but mussels controllable.
7-9 mA: threshold value, control possible.
9-25 mA: painful, difficult or impossible for hand to release the object,
breathing difficulty. Rescussitation works, all depends on duration of time.
26 mA on wards: uncertainty/death, all depends on duration of time.
Non-fabrillation current magnitude Ib is related to energy absorbed by the
body.
Tolerable Limit of Body Current
(Ib)2 t = EB where; Eb is energy constant
t
= duration of shock is seconds


2.2
Eb depends on time period and body weight. For time period of 0.03 to 3 sec. and:
- Body weight of 50 kg, Eb = 0.0135 and Ib = 0.116/√t
- Body weight of 70 kg, Eb = 0.0246 and Ib = 0.157/√t
- Body weight of 80 kg, Eb = 0.027 and Ib = 0.165/√t
- For higher weight of body, Eb may be somewhat higher.
- b may be generally taken as 0.0246
Care has to be taken if reclosure of C.B. is made as that may expose to second
shock immediately after first. Value of safe current may be taken a reduced one.
Resistance of Human Body



2.3
At 50/60 Hz: body may be treated as resistance (Rb).
Consider either one hand to both feet or foot to foot or one hand to another.
Resistance of body:
- Internal: about 300 Ω
- With skin: 500-3000 Ω
- Decreases with moisture/puncture of skin
- Average value may be taken as 1000 Ω, taking contact resistance zero
(shoes and gloves neglected).
Current Path Through Body and Exposure

Human foot: a circular metallic disc of 0.08 m radius (b). Then, ground resistance
in ohms, Rf = /4b
A corrective factor, Cs is required for effect of thin layer of material (is spread for
safety reasons).
Then,  is modified as Cs, s
Where,

δ
0.091  
 δs 
Cs 
2hs  0.09
hs = Surface layer thickness in m
s = Resistivity of surface layer.

Touch potential, Etouch = Ib (Rb + 1.5 Cs. s)
1000  1.5 C s .δ s 
t
With Sb = 0.0246 = (157 + 0.236 Cs. s)
Step Potential, Estep = Ib (Rb + 6 Cs. s)
0.157
Safe value of Estep
=
1000  6 C s .δ s 
t
= (157 + 0.942 Cs. s)
Safe value of Etouch =
3.0
0.157
DESIGN CONSIDERATIONS



3.1
Grounding Grid: A system of horizontal ground electrodes, inter connected bare
conductors buried in earth: in one location.
Grounding system: comprise all inter connection grounding facilities in one area.
Grounding mat.: A metallic plate of a system of closely spaced bare conductors
placed in shallow depths (0.3-0.5 m) above ground grid or elsewhere to obtain
extra protective measure minimizing danger of high Estep or Etouch.
Basic Concept








3.2
A continuous conductor loop surrounding the perimeter.
Conductors laid in parallel lines with in the loop.
Ground rods may be used at corners and near equipment & bonded with
conductors.
Depth of burial may be 0.6 m to 1.5 m.
Horizontal spacing of conductors may be 3-7 m.
Grid system be extended to entire area and beyond fencing or with in the fencing.
Spacing be reduced at the pheriphery sides to control potentials toward
peripheries.
Special care is required for Gas insulated Substations (GIS).
Conductors





3.3
Copper is good but it is costly; avoid use.
Galvanizing has low life against corrosion, hence is not good.
Black M.S. rods/flats may be used.
Use of round rods is better for uniform dispersal of currents and better life against
corrosion.
Flats are good for connecting metallic bodies and structures due to better
maneuverability.
Earth Resistance values of Grounding Grids



Generation stations
Major Grid Substations
Minor substations
< 0.5 
<1
<5
But basic requirement is checking for safe potentials.
3.4
Methods
(i)
(ii)
Grid of conductors
Electrodes Driven in the ground and tops connected
Advantages of (i) over (ii):
-
Where currents high, safety seldom possible by (ii)
Multiple path in (i) hence breakage on bad connection of not much
importance.
(ii) is also a grid but cost high. However it could be better if soil resistivity
is high at top surfaces.
4.
DESIGN OF GROUNDING SYSTEM
4.1
Critical Parameters






4.2
Maximum grid current (IG)
Fault duration (tf) and shock duration (ts)
Tf range from 0.25 s to 1.0 s and shock duration upto 3 s in general
Soil resistivity ()
Resistivity of surface layer (s)
Grid geometry
Procedure







Property map, resistivity of the area.
Conductor size: for maximum expected future fault current and maximum
possible fault clearing time.
Tolerable Etouch & Estep.
Calculate conductor length.
Preliminary design be made.
Check for safe values and modify if required.
Refine
4.2.1 Resistivity of Soil

By Wenner four-pin method.
4aR
 
2a
a
1

a 2  4h 2
a2  h2
Where,
=
apparent resistivity of soil in -m
R=
measured resistance in 
a=
distance between adjacent electrodes
h=
depth of electrodes in m
Since, h << a
Therefore,  = 2aR
4.2.2 Ground Fault Current (I0)
4.3

Zero sequence phase to phase to ground fault:
By simplified formula:
E.x 2
I0 
x 1 .x 0  x 2   x 2  x 0 
Where, x1, x2 and x0 are positive, negative and zero sequence reactance’s
respectively.

Zero simplified formula:
E
I0 
x1  x 2  x 0
Grounding Mat-Mesh Potential
It is a special case of touch potential when a man standing at the center of the mesh
touches a grounded metallic object under object.
The voltages Ex and Ey on the ground surface at a distance x metres horizontally from the
buried conductors and a point vertically above them on the ground surface are given by:
z  x z  D  x   PI  log 2z  x  * 3D  x  * etc. 
PI
log


2ππ
zz  D 
πL 
2D
3D

PI
z
Ey 
log
2ππ
d
where,
P=
Resistivity of soil (ohm-m)
I=
Current through conductors (Amp.)
L=
Total length of buried conductors (m)
Z=
Depth of burial of conductors (m)
D = Spacing between two parallel conductors (m)
d=
Diameter of buried conductors
Ex 
The touch potential is, then:
Etouch = Ex + Ey
= Sum of above expressions of Ex and Ey
Since, D is usually much greater than z, Etouch is approximately given by:
 z  x D  x  1
1
 2D  x  3D  x 
 PI

log 
 log 
*
* ....
2π
zdD
π
3D
 2D
 L

Substituting x = D/2 in the above expression, the mesh potential is:
D
!
1
 PI
3
5
7
Emesh   log
 log 4    6    8       
16zd π
 2π
 L
PI
 km
L
where, km = expression in the parenthesis.
To take care of the effect of irregularities in to account, a factor ki (“Non-uniformity
Factor”) is also mentioned. The mesh potential then becomes
Emesh  km ki
PL
L
The factor ki conform closely to an empirical curve and therefore, be expressed as:
ki = Number of parallel conductors in the basic grid excluding cross connections.
For tolerable value of Emesh:
Emesh ≤ Etouch (tolerable)
PL
 Etouch tolerable 
Or, km ki
L
Safe value of Etouch being known, the value of L can be calculated.
4.4
Transferred Potential
Under earth fault condition, the entire substation potential is raised above the true earth
by
E,
which is given by:
E=
I R, where:
I=
Fault current flowing from or in to the substation grounding mat to or from
earth
R=
Substation earthing resistance
Now, if a metallic object, which is connected to the substation grounding mat and is
coming out of the substation is touched by some one far away from the substation, he will
be subjected to the entire potential E.
Likewise, if a metallic object, which is earthed out side the substation, is touched by some
one standing in side the substation, then he will be subjected to the full potential E.
Investigation should be made to check for transferred potential/special points e.g.:
i. Communication circuits
ii. Pilot wires
iii. Rails
iv. Pipes
v. Low voltage neutral wires
vi. Building: which are close to the substation and especially those which are linked
directly with the substation by means of water pipes, cable sheaths, phone lines
etc.
vii. Reinforcements, cable racks etc.
viii. Operating handles
ix. Fences: Two methods are followed, viz:
a.
b.
inclusion of fence with in ground grid area,
placing the fence outside the ground grid area,
either with or without close electric coupling between fence and adjacent
earth along its length but no electric tie between the fence and the main
station grid. Which ever method is followed the pros and cons of the
methods should be considered carefully and then only the design be
finalized.
x. Lightning arrestor grounding
xi. Cable sheath grounding
5.0
RECOMMENDED DESIGN PROCEDURE
Following procedure is recommended;
i.
Investigation of soil characteristic, composition of soil.
ii.
Determination of maximum single phase to ground fault current: the value of
maximum fault current is to be determined for the following two cases:
a. Through the earthing mat itself
b. From earthing mat in to ground and vice-versa. The former is to be used to
determine the size of earthing strips to be used and details of joints etc.,
while the later is required to determine the maximum voltages which
might be produced in and out side the substation.
iii.
Calculate the length of the conductor required for grounding system.
iv.
Prepare lay out of the grounding mat system.
In preparing the design, following should be kept in mind.
a. The depth of laying electrodes is important as the resistance varies great
with the depth of electrodes.
b. Elemental lines which flow from an electrode to or from ground are
normal to the surface of the electrodes and depend up on the size, shape,
spacing and the number of electrodes connected in parallel.
c. Increase in size of electrode permits the spacing between the elemental
lines of current to be increased, which means reduction of earth resistance
value.
d. If the shape of the electrode is such as to cause reduction of the density of
elemental lines of current emanating from the electrodes, the earth
resistance will be lower.
e. Estimate the resistance which might be obtainable from the preliminary
design of the grounding mat system using the following formula:
P P
R

4r L
Where, P =
Resistivity of soil in ohm-metres
r=
Radius of a circular plate having the same area as that
occupied by the grid or mat
L=
Total length of buried conductor in metres
v.
Calculate the maximum grid potential rise.
vi.
Calculate step voltage(s) at the periphery using the formula:
PI
Estep periphery   ks ki
L
Where ks is the coefficient which takes in to account the effect of the number
n, the spacing D, and the depth of burial z of the grid conductors and is given
by:
11
1
1
1
1





 .................

π  2z D  z 2D 3D 4D

The value of Estep thus found should be less than Estep tolerable
Internal Step and Touch voltages
Investigation of transferred potential and special danger points
Ground conductor material
Ground conductor size: Each element of the ground system should be so
designed that it will:
i.
Resist fusing and deterioration of electric joints under most
adverse conditions of fault and its duration.
ii.
Be mechanically rugged to a high degree especially in the
locations exposed to physical damage.
iii.
Have sufficient conductivity so as not to contribute
substantially to dangerous local potential differences.
iv.
Connections to grid.
v.
Laying of earthing strips.
ks 
a.
b.
c.
d.
e.
6.0
REFINEMENT OF PRELIMINARY DESIGN
Lay the earthing system as designed, and keep a record of earthing resistance value
measured from time to time. If this value is found to be more, then the design should be
reviewed and some earthing strips/rods be added as required.
7.0
METHODS OF EARTHING
7.1
By Means of Earth plates
i.
ii.
7.2
Usually a copper earth plate of size 1 m x 1 m buried in the ground at about 2
m depth or so to which is connected the equipment to be earthed in case of low
earth resistivity.
By means of several earth plate laid in the ground at at-least 2 m depth in
parallel separated by some distance.
- The advantage is: low cost,
- The Draw backs are:
a. Good amount of excavation & back filling
b. Earth current is confined to small area – causing large amount of heating
of the surrounding earth thereby causing loss of moisture content and
therefore, increasing earth resistance.
This method is therefore used to small installations.
By Means of Rods Vertically Driven in to the Ground
i.
ii.
iii.
The method is most suitable due to large length of electrode allowing large
amount of earth current to pass through elemental lines of current spreading in
to large surrounding area of earth.
Very efficient method as the elemental lines of current quickly disperse and
concentration is diminished as the length of electrode is increased.
Some special features are:
a. Where permanent moisture level is low, larger rods can be used at lower
cost.
b.
c.
d.
e.
Seasonal variations in earthing resistance are low.
Cost is lower compared to Plate earthing and with same or better results.
Connections simple and inspection easy.
Behaviour for steep front surges better than that of other forms of
electrodes.
This method is not good where rocks are present at a depth of 1 m or more.
7.3
By Means of Horizontally Laid Electrodes
The earthing electrode takes the form of a wire or strip laid horizontally and buried in the
ground at suitable depth.
7.3.1 Advantages and Disadvantages
i.
ii.
7.4
Size of electrode reduces but the electrode must be mechanically strong.
There is no restriction regarding length of electrode but large excavation is
required.
By Means of Grounding Mat
This is a special case of method of horizontally laid electrodes and is very good for power
houses and substations. It provides multiple paths and therefore, defect at one or two
places or break etc. is inconsequential.
8.0
EARTHING OF OUT DOOR SUBSTATION ETC.
Structures and equipment is spread over a large area. Two point earthing is essential.
Hence ring form is required. This can be done by use of a number of rod electrodes and
joining them in parallel. The electrodes are to be placed at distance so that their areas are
not influenced or else the cost will be increased unnecessarily.
9.0
EARTHING/GROUNDING RESISTANCE
It is the ohmic resistance between the earth electrode/earth system and the general mass
of the earth. It depends on;
i.
Composition of soil.
ii.
Temperature and moisture content of soil.
iii.
Depth of burial.
iv.
Size, shape, spacing and number of electrodes/earth system.