substation grounding

1. SUBSTATION GROUNDING

2. Reasons for Substation Grounding System
The substation grounding system is an essential part of the overall
electrical system. The proper grounding of a substation is important for
the following two reasons:
1. It provides a means of dissipating electric current into the earth
without exceeding the operating limits of the equipment
2. It provides a safe environment to protect personnel in the vicinity of
grounded facilities from the dangers of electric shock under fault
conditions
The grounding system includes all of the interconnected grounding
facilities in the substation area, including the ground grid, overhead
ground wires, neutral conductors, underground cables, foundations,
deep well, etc.
The ground grid consists of horizontal interconnected bare conductors
(mat) and ground rods. The design of the ground grid to control
voltage levels to safe values should consider the total grounding
system to provide a safe system at an economical cost.

3. The following information is mainly concerned with personnel safety. The
information regarding the grounding system resistance, grid current, and
ground potential rise can also be used to determine if the operating
limits of the equipment will be exceeded.
 Safe grounding requires the interaction of two grounding systems:
1. The intentional ground, consisting of grounding systems buried at
some depth below the earth’s surface.
2. The accidental ground, temporarily established by a person exposed
to a potential gradient in the vicinity of a grounded facility.
It is often assumed that any grounded object can be safely touched. A
low substation ground resistance is not, in itself, a guarantee of safety.
There is no simple relation between the resistance of the grounding
system as a whole and the maximum shock current to which a person
might be exposed.
A substation with relatively low ground resistance might be dangerous,
while another substation with very high ground resistance might be
safe or could be made safe by careful design.

4. The circumstances that make human electric shock accidents possible are:
 Relatively high fault current to ground in relation to the area of the
grounding system and its resistance to remote earth.
 Soil resistivity and distribution of ground currents such that high
potential gradients may occur at points at the earth surface.
 Presence of a person at such a point, time, and position that the body
is bridging two points of high potential difference.
 Absence of sufficient contact resistance or other series resistance to
limit current through the body to a safe value under the above
circumstances.
 Duration of the fault and body contact and, hence, of the flow of
current through a human body for a sufficient time to cause harm at
the given current intensity.

5. Accidental Ground Circuit
There are two conditions that a person within or around the substation
can experience that can cause them to become part of the ground
circuit.
1. Touch Voltage
The potential difference between the ground potential rise (GPR) and
the surface potential at the point where a person is standing while
at the same time having a hand in contact with a grounded
structure.
2. Step Voltage
The difference in surface potential experienced by a person bridging a
distance of 1 m with the feet without contacting any other
grounded object.

6. Figure shows the fault current being discharged to the earth by
the substation grounding system and a person touching a
grounded metallic structure, H.
Exposure to Touch Voltage

7. Figure shows the Thevenin equivalent for the person’s feet in
parallel, Zth , in series with the body resistance, RB,, Vth is the
voltage between terminal H and F when the person is not
present. IB is the body current. When Zth is equal to the
resistance of two feet in parallel, the touch voltage is

8. Exposure to Step Voltage
Above figure show the conditions for step voltage.
Zth is the Thevenin equivalent impedance for the person’s feet in series
and in series with the body. Based on the Thevenin equivalent
impedance, the step voltage is
 
th
B
b
step Z
R
I
E 


9. The resistance of the foot in ohms is represented by a metal circular
plate of radius b in meters on the surface of homogeneous earth of
resistivity and is equal to:
)
( m



 
th
B
b
step Z
R
I
E 

b
Rf
4



10. 



6
2
,
,
2
5
.
1
2
,
,
2
3
08
.
0
min
4







f
Th
step
f
Th
touch
f
f
R
Z
is
equation
E
voltage
step
the
in
series
in
feet
for
impedance
equivalent
Thevenin
The
R
Z
is
equation
E
voltage
touch
the
in
parallel
in
feet
for
impedance
equivalent
Thevenin
The
R
b
g
Assu
b
R
The above equation assume uniform soil resistivity. In a substation, a
thin layer of high – resistivity material is often spread over the earth
surface to introduce a high – resistance contact between the soil and
the feet, reducing the body current.

11. Permissible Body Current Limits
The duration, magnitude, and frequency of the current affect the human
body as the current passes through it. The most dangerous impact on
the body is a heart condition known as ventricular fibrillation, a stoppage
of the heart resulting in immediate loss of blood circulation.
Humans are very susceptible to the effects of electric currents at 50 and
60 Hz. The most common physiological effects as the current increases
are perception, muscular contraction, unconsciousness, fibrillation,
respiratory nerve blockage, and burning.
 The threshold of perception, the detection of a slight tingling
sensation, is generally recognized as 1 mA.
 The let-go current, the ability to control the muscles and release the
source of current, is recognized as between 1 and 6 mA.
 The loss of muscular control may be caused by 9 to 25 mA, making it
impossible to release the source of current.
 At slightly higher currents, breathing may become very difficult, caused
by the muscular contractions of the chest muscles. Although very painful,
these levels of current do not cause permanent damage to the body.

12.  In a range of 60 to 100 mA, ventricular fibrillation occurs. Ventricular
fibrillation can be a fatal electric shock.
The only way to restore the normal heartbeat is through another
controlled electric shock, called defibrillation. Larger currents will inflict
nerve damage and burning, causing other life-threatening conditions.
NOTE
The substation grounding system design should limit the electric current
flow through the body to a value below the fibrillation current.

13. Dalziel [5] published a paper introducing an equation relating the flow of
current through the body for a specific time that statistically 99.5% of the
population could survive before the onset of fibrillation. This equation
determines the allowable body current.
.
tan
sec
,
exp
,
)
(
population
given
a
of
percent
certain
a
by
tolerated
energy
shock
electric
the
to
related
t
cons
empirical
S
S
k
osure
current
the
of
duration
t
A
body
the
through
current
the
of
magnitude
rms
I
Where
a
t
k
I
B
B
s
B
s
B





Dalziel found the value of k = 0.116 for persons weighing approximately
50 kg (110 lb) or k = 0.157 for a body weight of 70 kg (154 lb) [6]. Based
on a 50-kg weight, the tolerable body current is
)
(
116
.
0
b
t
I
s
B 
Above equation is based on tests limited to values of time in the range of 0.03 to 3.0 sec.
It is not valid for other values of time. Other researchers have suggested other limits [7].
Their results have been similar to Dalziel’s for the range of 0.03 to 3.0 sec.

14. Importance of High-Speed Fault Clearing
Considering the significance of fault duration both in terms of Equation a
and implicitly as an accident-exposure factor, high-speed clearing of
ground faults is advantageous for two reasons:
1. The probability of exposure to electric shock is greatly reduced by
fast fault clearing time, in contrast to situations in which fault
currents could persist for several minutes or possibly hours.
2. Both tests and experience show that the chance of severe injury
or death is greatly reduced if the duration of a current flow
through the body is very brief.
The allowed current value may therefore be based on the clearing time of
primary protective devices, or that of the backup protection.
It is more conservative to choose the backup relay clearing times in
equation a, because it assures a greater safety margin.
An additional incentive to use switching times less than 0.5 sec results from the research done by Biegelmeier
and Lee [7]. Their research provides evidence that a human heart becomes increasingly susceptible to
ventricular fibrillation when the time of exposure to current is approaching the heartbeat period, but that the
danger is much smaller if the time of exposure to current is in the region of 0.06 to 0.3 sec.

15. Tolerable Voltages
Basic Shock Situations

16. Ground potential rise (GPR):
The maximum electrical potential that a substation grounding grid may
attain relative to a distant grounding point assumed to be at the potential
of remote earth. GPR is the product of the magnitude of the grid current,
the portion of the fault current conducted to earth by the grounding
system, and the ground grid resistance.
Mesh voltage:
The maximum touch voltage within a mesh of a ground grid.
Metal-to-metal touch voltage:
The difference in potential between metallic objects or structures within
the substation site that can be bridged by direct hand-to-hand or hand-
to-feet contact.
Note: The metal-to-metal touch voltage between metallic objects or structures
bonded to the ground grid is assumed to be negligible in conventional
substations. However, the metal-to-metal touch voltage between metallic objects
or structures bonded to the ground grid and metallic objects inside the substation
site but not bonded to the ground grid, such as an isolated fence, may be
substantial. In the case of gas-insulated substations, the metal-to-metal touch
voltage between metallic objects or structures bonded to the ground grid may be
substantial because of internal faults or induced currents in the enclosures.

17. Step voltage:
The difference in surface potential experienced by a person bridging a
distance of 1 m with the feet without contacting any other grounded
object.
Touch voltage:
The potential difference between the ground potential rise (GPR) and
the surface potential at the point where a person is standing while at the
same time having a hand in contact with a grounded structure.
Transferred voltage:
A special case of the touch voltage where a voltage is transferred into or
out of the substation, from or to a remote point external to the
substation site. The maximum voltage of any accidental circuit must not
exceed the limit that would produce a current flow through the body
that could cause fibrillation.

18. Assuming the more conservative body weight of 50 kg to determine the
permissible body current and a body resistance of 1000 Ohms , the
tolerable touch voltage is
 
S
S
S
touch
t
C
E
116
.
0
.
5
.
1
1000
50 


And the tolerable step voltage is
 
S
S
S
step
t
C
E
116
.
0
.
6
1000
50 


sec
,
,
Re
,
,
current
shock
of
Duration
t
m
material
surface
the
of
sistivity
factor
derating
layer
Surface
C
V
voltage
touch
E
V
voltage
step
E
Where
S
S
S
touch
step








s
touch
mm
t
E
116
50 

Since the only body resistance for the metal to metal touch voltage is the
body resistance, the voltage limit is

19. S. No GROUND TYPE RESISTIVITY Ω - m
1 Sea Water 0.01 – 1.0
2 Wet Water 10
3 Average Soil 100
4 Dry Soil 1000
5 Bed Rock 104
6 Pure Slate 107
7 Sand Stone 109
8 Crush Rock 1.5 x 108