Station grounding
1. What is grounding?
It is an electrical connection with the general mass of earth through an earth
electrode.
2. What is difference between earthing and grounding?
Both have same meaning. The term earthing is used in U.K. and grounding in U.S.A.
ground means earth.
3. What are types of grounding?
There are two types
a. System grounding.
b. Equipment grounding.
4. What does mean by system?
Grounding of neutral point of equipment is called system grounding. For instance
grounding of generator neutral, transformer neutral etc.
5. What does mean by equipment grounding?
Grounding of non-current carrying metallic parts is called equipment grounding. For
instance no-current carrying parts include the following:
a. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.
b. Support structures, tower, poles etc. in the neighborhood of electrical circuits.
c. Sheath of cables.
d. Body of portable equipment such as iron, oven, etc.
6. What is the important of system grounding?
It is important because:
a. Earth fault protection is based on the method of neutral earthing.
b. System voltage during earth fault depends on neutral earthing.
c. It is a protection against arcing grounds, unbalanced voltages with respect to earth
and lighting.
7. What is the important of equipment grounding?
Equipment earthing ensures safety.
8. How safety could be ensured by equipment grounding?
In order to enumerate this, let us first find out the effects of current and voltage
developed during fault condition.
9. What is the permissible body current limit?
The magnitude and duration of current conducted through a human body at 50 Hz
should be less than those did that cause ventricular fibrillation.
(Ventricular fibrillation is considered to be the main cause of death due to electrical
shock). These below given data are also applicable for current limits to human body.
Current magnitude Physiological effect Description
1 mA Threshold of
perception
A current at which a person is just able to
detect a slight tingling in his hand or finger
1 – 6 mA Unpleasant to sustain This is often termed as let go currents. Do not
impair the ability of a person holding an
energised object to control his muscles and
release it.
6 – 9 mA Threshold of muscular
contraction.
These are threshold values, since 10.5 mA
current and 16 mA current are the let go values
for women and man respectively.
9 – 25 mA Muscular contraction May be painful and can make it hard or
impossible to release energised objects grasped
by the hand.
25 – 60 mA Muscular contraction Make breathing difficult.
60 – 100 mA Ventricular fibrillation Ventricular fibrillation, stoppage of heart or
inhibition of respiration might occur and cause
injury or death if time is more than 1 sec.
Hence the grounding equipment shock current can be kept below the value sufficient
to cause injury or death by lowering the step and touch potential.
10. How fibrillation current functions?
Fibrillation current is actually function of individual body weight.
For 50 kgs body weight: fibrillation current (IB) = 0.116/ยช ts (Limited to 0.03 – 3
sec. Range)
Where ts = duration of current exposure in sec.
Note = Above equation results = 116 mA for 1 sec. and 367 mA for 100 sec.
For 70 kgs body weight: fibrillation current (IB) = 0.157/ยช ts
Note = Above equation results = 157 mA for 1 sec. and 496 mA for 100 sec.
Above times are very - very important from the point view of clearing the fault.
Above limit dictates that grounding should e such that current magnitude through
human body should not increase the specified values.
In order to ensure above following have been done.
1. Current conductor have been burried in ground
a. At the depth of 600 mm in switchyard. Depth 600 mm is normally selected
because of freezing or drying out, the Resistivity of upper layers could vary
with seasons, while the Resistivity of lower soil layers remains nearly
constant.
b. Horizontal grid conductors are more effective in reducing the danger of high
step and touch voltages on the earth surface by creating equipotential surface
during fault conditions.
c. At the depth of 800 mm else where. Here depth is kept more because to care
for under grounding services. Example laying of power cables, drainage etc.
2. 25-mm dia copper rod electrodes have been driven in soil.
a. Upto 5 meters depth in 220 kV switchyard.
b. Upto 3 meters elsewhere.
Why only 5 meters and 3 meters depths have been selected is that the
resistance is diminishes rapidly with the first few feet of driving, but less so at
depths greater than 2 to 3 meters in soil of uniform resistivity.
These lengths are adopted in selecting the ground electrodes.
3. 4-inch layer of gravel in 220 kV switchyard has been used. Purpose of using
gravel is by doing steps 1,2 above tough and step potential are computed and
compared with tolerable potential and found as given below.
Potential Computed value Tolerable value
Tough 550 V 665V
Step 2.a switchyard with crushed rock surface 230V 2165 V
Step 2.b elsewhere with natural soil 166V 168.5 V
11. Why grounding is necessary?
The purpose of grounding is to maintain the surface under and around a station ate as
nearly zero potential as possible with reference to absolute earth so that operating
staff who walk in the station yard and tough equipments are ate earth potential and
when faults occur there is safety to staff and equipments.
12. What are the harms of under grounded system?
a. Step and tough potential will increase more than maximum tolerable value.
b. Under single line to ground fault the voltage to earth of the two healthy phases
rises from their normal phase to neutral voltage to full line voltage, which may
result in insulation break down.
c. The capacitive current in two healthy phases increases ยช3 times the normal value.
d. The capacitive current in the faulty phase is 3 times its normal value.
e. Experience shows that capacitive current in excess of 4 amps may be sufficient to
maintain an arc in the ionized path of the fault and this persistent arc phenomenon
is called ARCING GROUND, which ultimately cause high voltage build up.
Some time these voltage builds up to 5 to 6 times its normal value, which results
in break down of insulation.
f. Being fault current low, it is difficult to isolate fault.
13. How system grounding and equipment grounding achieved?
System grounding is obtained by grounding the neutral through resistance, through
transformer and through effective or solidly grounding.
Equipment grounding is obtained by Grounding of non-current carrying metallic
parts equipment. For instance no-current carrying parts include the following:
a. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.
b. Support structures, tower, poles etc. in the neighborhood of electrical circuits.
c. Sheath of cables.
d. Body of portable equipment such as iron, oven, etc.
14. What does mean by grounding electrode, grounding system, and grounding
resistance?
Grounding electrode: A conductor driven in the earth and used for collecting ground
current from or dissipating ground current into the earth.
Grounding system: Comprises all interconnected grounding facilities in a specific
area.
Grounding resistance: The resistance offered by the ground when power frequency
current is discharged to the ground through a particular grounding electrode or
grounding system.
15. How grounding resistance could be measured?
There are few methods, which can give approximately true value. These are
described below.
a. Fall of potential method: This method is applicable for small grid or sub station
where induction effect of voltage is less.
b. Measurement of earth resistance by 61.8% distance rule:
c. Alternate – 1 of fall of potential method: This method is influenced by induction
effect.
d. Alternate – 2 of fall of potential method:
16. How value of grounding resistance could be kept constant?
While measuring of grounding resistance is more than computed design value 0.11ฮฉ,
then following are recommended to reduce it. Add in water the following highly
conductive substances and pour into treated pit.
a. Sodium chloride (Nacl), known as common salt.
b. Calcium chloride (Ca CL2)
c. Sodium carbonate (Na2 CO3)
d. Copper sulphate (Cu SO4)
e. Soft choke and
f. Salt and charcoal in suitable proportions.
17. What is the effect of moisture content on earth resistivity?
The moisture content is expressed in percent by weight of dry soil. Dry earth weights
about 1440 kg per cubic meter and thus 10% moisture content is equivalent to 144 kg
of water per cubic meter of dry soil. So about 20% moisture, the resistivity is very
little affected. Below 20%, the resistivity increases very abruptly with the decrease in
moisture.
18. What is the effect of salt content in moisture on resistivity?
The resistivity decreases and the salt content is expressed in percent by weight of the
contained moisture. It will be noted that the curve flattens off at about 5% salt
content and a further increase in salt content gives little decrease in the soil
resistivity.
19. What is the effect of temperature on earth resistivity?
The temperature co-efficient of resistivity for soil is negative, but it is negligible for
temperature above freezing point. Below 0°C the water in the soil begins to freeze
and introduces a tremendous increase in the temperature co-efficient, so that as the
temperature becomes lower the resistivity rises enormously.
20. What does mean by neutral floating or neutral displacement?
When a ground fault occurs, there is a tendency of neutral shift with consequent
change in voltage on the un-faulted phases. This phenomenon is called neutral
floating or neutral displacement.
21. Why grounding of power cable is needed? How it should be done?
a. The magnetic fluxes produced by the three phases in a multi core power cable
almost cancel put each other, since the vector sum of these currents at any instant
is zero and practically there is no residual magnetic flux around the cable.
In case of single core cable, the magnetic flux induces the voltage in the metallic
sheath.
b. When the cable conductor is carrying alternating current, for safe and reliable
operation, the metallic sheath must be grounded. If the metallic sheath is at one
end the potential of the unearthed end could be much above the earth potential. If
both ends are grounded, a circulating current is induced in the metallic sheath.
c. The maximum acceptable induced voltage under normal load current operation is
limited by corrosion and safety considerations.
d. Code of practice of earthing (IS 3043) as well as electricity council London
recommended permissible induced voltage level of 65 Volts.
Hence keeping above all points in mind metallic sheath and armour of all multi core
power cables shall be earthed at both end equipment and switchgear end. Sheath and
armour of single core power cable shall be earthed ate switchgear end only. The
sheaths of shielded control cables should be grounded at both ends to eliminate
induced potentials.
22. In 220 kV switchyard why lightning arrestor should be properly grounded?
a. During lightning, surges should be discharged to ground, otherwise it will
puncture the equipment insulation and it is possible only when lightning arrestor
is grounded properly.
b. In order to make it effective, the ground terminal of lighting arrestor should be
connected direct to the tank of transformer. This will eliminate voltage build up
due to earth resistance. For example for each ohm of earth resistance the voltage
build up for 5000 Amps discharge current is 5 kV. Soil resistivity a should be
minimum and may be it is 3.5 ohm per meter.
23. Why grounding mat is important near ground switch operating handle and
disconnecting switch operating handle?
Equipment operating handles deserve special attention because of the higher
probability for co-incidence of adverse factors. For example,
a. Hand operation equipment such as grounding switches and disconnecting
switches requires the presence of operator near a grounded structure at a point
where opening of an energised circuit can some times result in an arc to the
structure or perhaps mechanical failure and electrical break down of a switch
insulator. A large percentage of fatal accidents from voltage gradients are in fact
associated with operating handles. Hence in order to avoid above problems
following should be an additional safety factors:
1. Use closer mesh in the vicinity of operating handle area (150-mm approx.) and
operating handle shall be directly connected to the earthing mat.
2. Use higher resistance surfacing such as crushed rock or both in order to bring
down the values of touch potential and step potential.
24. Why fences grounding are important?
Because the most dangerous touch contacts involves and outside the fence are
usually accessible to the general public. In order to minimise the effect of step
potential and touch potential following two philosophies could be adopted.
a. Inclusion of the fence within the ground grid area and
b. Placement of fence outside the ground grid area – not safe to use.
With this effective area is increased and reduces ground grid resistance substantially
and maximum ground – grid voltage rise as well. In this case the perimeter conductor
of grid normally either follow the fence line, or parallel to it at a short distance about
0.5 m – 1.5 m outside. In either case, the perimeter ground conductor and fence are
bonded electrically at frequent intervals.
25. What are the specifications for procurement of grounding conductor and grounding
rods?
Grounding conductor, pad, rods etc. should have following specifications:
a. Copper : 91.8 to 94.9%
b. Zinc : 2.0 to 3.0%
c. Tin : 0.8 to 1.5%
d. Lead : 2.0 to 2.5%
e. Iron : 0.5 to 1.0%
Impurities must be limited to the percentage specified below:
a. Nickel : 0.3% maximum.
b. Antimony : 0.3% maximum.
c. Manganese : 0.04% maximum.
d. Phosphorous : 0.04% maximum.
26. Why copper is only preferred as material for grounding?
An advantage of use of copper is in addition to their high conductivity, has the
advantage of being resistant to underground corrosion. Copper is cathodic with
respect to other metals that are likely to be burried in the vicinity.
Disadvantages of use of copper are,
a. Grid of copper forms a galvanic cell with burried steel structures, pipes and any
of the lead based alloys that might be present in cable sheaths, it is likely be
hasten the corrosion of the latter.
b. Use of tinned copper conductor accelerates and concentrates the natural corrosion
of metal in small area however cell potential with respect to steel and zinc
reduces by about 50% and practically eliminates this potential with respect to
lead.
27. What should be the frequency of measurement of earth resistivity?
As per IS: 3043, 1987, measurement of earth resistivity should be carried out
annually or biannually and value should be recorded.
28. What should the statutory provision of earthing?
a. Earthing shall generally be carried out in accordance with the requirement of
India electricity rule 1956, as amended from time to time and the relevant
regulations of the electricity supply authority concerned.
b. All medium voltage equipment shall be earthed by two separate and distinct
connections with earth. In the case of high and extra high voltages, the neutral
points shall be earthed by not less than two separate and distinct connections with
earth, each having its own electrodes at the generating station or substation and
may be earthed at any other point provided no interference is caused by such
earthing. If necessary, the neutral may be earthed through suitable impedance.
c. As for as possible all earth connections shall be visible for inspection.
d. All connections shall be carefully made. If they are poorly made or inadequate for
the purpose for which they are intended, loss of life or serious personal injury
may result.
e. Each earth system shall be so devised that the testing of individual earth electrode
is possible. It is recommended that the value of any earth system resistance shall
be such as to confirm with the degree of shock protection desired.
f. It is recommended that a drawing showing the main earth connection and earth
electrodes be prepared for each installation.
g. No addition to the current carrying system, either temporary or permanent shall
be made which will increase the maximum available earth fault or its duration
until it has been ascertained that the existing arrangement of earth electrodes,
earth bus-bar etc. are capable of carrying the new value of earth fault current
which may be obtained by this addition.
h. No cut-out link or switch other than a linked switch arranged to operate
simultaneously on the earthed or earthed neutral conductor and the live
conductors, shall be inserted on any supply system. This however, does not
include the case of a switch for use in controlling a generator or a transformer or a
link for test purposes.
i. All materials fittings, etc. used in earthing shall conform to Indian standard
specifications, wherever these exist.
29. What maintenance of earth electrodes should be done?
The neighbouring soil to the earth electrode shall be kept moist where necessary, by
periodically pouring water through a pipe where fitted along with it or by pouring
water in the immediate vicinity of the earth electrode.
Periodical visual inspection of all earth electrodes connection wherever available,
shall be carried out to ensure their rigidity and other signs of deterioration.
30. In case new installation is to be done, what basic guidelines should be followed for
grounding?
a. Earthing conductors in outdoor areas shall be burried 500 mm below finished
grade level unless stated otherwise.
b. Minimum 6000 mm spacing between rod pipe electrode shall be provided unless
stipulated otherwise.
c. Earthing conductor around the building shall be burried in earth at a minimum
distance of 1500 mm from the outer boundary of building.
d. Earthing conductors embedded in the concrete floor of the building shall have
approximately 100-mm concrete cover.
e. Earthing conductors along their run on columns, beams, walls etc. shall be
supported by suitable cleats at intervals of 750 mm.
f. Earthing conductors crossing the road shall be either installed in hume pipes or
laid at greater depth to suit the site conditions.
g. Whenever earthing conductors cross underground service ducts, pipes, trenches,
under ground service ducts, pipes, trenches, tunnels, railway track etc. it shall be
laid 800 mm below them.
h. Earthing conductor shall be burried 1000 mm outside the switchyard fence. Every
alternate post of the fence and gates shall be connected to earthing loop by one
lead.
i. Each earthing lead from the neutral of the power transformer shall be directly
connected to a rod or pipe or plate electrode treated earth pit, which in turn shall
be connected to station earthing.
31. How much resistance human body has?
Resistance of internal body tissues (Not including skin) : 300 ฮฉ.
Resistance of body including skin : 500 to 3000 ฮฉ.
32. What is the effect of voltage frequency and current on resistance of the human body?
a. For touch voltages upto approximately 50V the value of impedance of the skin
varies widely with surface area of contact, temperature, respiration etc. even for
one person.
b. For higher touch voltages in order of approximately 50V to 100V the skin
impedance decreases considerably and becomes negligible when the skin breaks
down.
c. Wet hand contact resistance becomes very low at any voltage.
d. With increase in frequency, impedance of skin decreases.
33. What are the paths of current through the body?
A value of 1000 ฮฉ is selected for the calculations that follows as representing the
resistance of a human body from hand to both feet and also from hand to hand or
from one foot to other foot.
Above paths includes vital organs such as heart.
a. Path from hand to foot is much more dangerous than foot to foot, since current
flow through heart during foot to foot current flow will be much less than the
current flow from hand to foot approximate ratio is 25:1
b. However deaths have occurred during foot to foot current flow. Hence can not be
ignored.
34. What are the effects of re-closure shock?
During re-closure, when fault is persisting a person might be subjected to the first
shock which would not permanently injure him, but would upset and disturb him
temporarily.
Next, a single fast automatic re-closure could in a second shock initiated within less
than 500 ms from the start of first. It is this second shock, occurring after a relatively
short interval of time before the person has recovered, that might cause a serious
accident. With manual re-closure the possibility of exposure to a second shock is
reduced since the time interval may be substantially greater.
35. State DC/AC equivalent factor (K).
Ratio of direct current (DC) to its equivalent rms value of alternating current (AC)
having the same probability of inducting ventricular fibrillation.
K = I DC fibrillation / I AC fibrillation (rms).
K = 3000 mA / 100 mA
K = 30 mA
Threshold of let-go is unlike AC there is no definable threshold of let-go for DC for
current magnitude below approximately 300 mA. Only the making and breaking of
current leads to painful and cramp like contractions of muscles.
Above approximately 300 mA, let-go may be impossible or only possible after
several seconds or minutes of shock duration. Below approximately 300 mA a
sensation of warmth is felt in the extremities during the flow of current. Above 300
mA unconsciousness frequently occurs.
36. Why AC is more dangerous than DC?
Because the excitatory action of current (stimulation of nerves and muscle, induction
of cardiac atrial or ventricular fibrillation) are linked to the changes of current
magnitude especially when making and breaking of the current. To produce the same
excitatory effects the magnitude of direct current flow of constant strength in 2 to 4
times greater than that of alternating current.
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• What are the design guidelines for electrical system?
