2. The objective of a grounding system are:
1. To provide safety to personnel during normal and
fault conditions by limiting step and touch potential.
2. To assure correct operation of electrical/electronic
devices.
3. To prevent damage to electrical/electronic
apparatus.
4. To dissipate lightning strokes.
5. To stabilize voltage during transient conditions and
to minimize the probability of flashover during
transients.
6. To divert stray RF energy from sensitive audio,
video, control, and computer equipment.
3. A safe grounding design has two objectives:
1. To provide means to carry electric currents
into the earth under normal and fault
conditions without exceeding any operating
and equipment limits or adversely affecting
continuity of service.
2. To assure that a person in the vicinity of
grounded facilities is not exposed to the
danger of critical electric shock.
4. INTRODUCTION TO GROUNDING
The primary goal of the grounding system throughout any
facility is SAFETY.
Grounding is implemented to ensure rapid clearing of faults
and to prevent hazardous voltage, which in turn reduce the
risks of fires and personnel injuries. Grounding serves the
primary functions of referencing the AC systems and
providing a means to ensure fault clearing.
99.5% survival threshold –
116 mA for one (1) second.
367 mA for zero point one (0.1) second.
5. SIX (6) GROUNDING SYSTEMS IN USE
1. Equipment grounds,
2. Static grounds,
3. Systems grounds,
4. Maintenance grounds,
5. Electronic grounds,
6. Lightning grounds.
6. EQUIPMENT GROUNDS:
Equipment grounds: An equipment ground is the
physical connection to earth of non-current carrying
metal parts. This type grounding is done so that all
metal part of equipment that personnel can come into
contact with are always at or near zero (0) volts with
respect to ground. All metal parts must be
interconnected and grounded by a conductor in such
away as to ensure a path of lowest impedance for flow
of ground fault current. Typical items (equipment) to be
grounded are; electrical motor frames, outlet boxes,
breaker panels, metal conduit, support structures, cable
tray, to name a few.
7. STATIC GROUNDS:
A static ground is a connection made between a
piece of equipment and the earth for the purpose of
draining off static electricity charges before a flash
over potential is reached. This type grounding
system is utilized in dry materials handling,
flammable liquid pumps and delivery equipment,
plastic piping, and explosive storage facilities.
8. SYSTEM GROUNDS:
A system ground refers to the point in an
electrical circuit that is connected to earth.
This connection point is typically at the
electrical neutral. The sole purpose of the
system ground is to protect equipment. This
type ground also provides a low impedance
path for fault currents improving ground fault
coordination. This ensures longer insulation
life of motors, transformers and other system
components
10. ELECTRONIC AND COMPUTER GROUNDS:
Grounding for electronic equipment is a special
case in which the equipment ground and the
system ground are combined and applied in unity.
Electronic equipment grounding systems must not
only provide a means of stabilizing input voltage
levels, but also act as the zero (0) voltage
reference point. Grounding systems for the modern
electronics installation must be able to provide
effective grounding and bonding functions well into
the high frequency megahertz range.
11. LIGHTNING PROTECTION:
Lightning protection grounding requirements are
dependent upon the structure, equipment to be
protected, and the level of lightning protection
required of desired.
12. FACTORS TO BE CONSIDERED
The area available for installation of the grounding system. This
could lead to the requirement and utilization of chemical rods, or
wells.
Water table and seasonal changes to it.
Soil condition and resistivity, Also elevation above sea level and
hard rocky soil are concerns that would need to be addressed.
Available fault currents (i.e., three (3) phase, line-to-ground, and
line-to-line-to ground, etc.).
NEC and ANSI/IEEE requirements. Also include here the
requirements of the process equipment to be installed.
Consideration to the number of lightning strikes and thunder
storm days per year.
Utility ties and/or service entrance voltage levels.
Utilization of area were ground system is to be installed, (i.e., do
not install under paved parking lot).
13. DEFINATION OF PROTECTIVE EARTH
(A protective earth( PE connection ensures
that all exposed conductive surfaces are at
the same electrical potential as the surface of
the Earth, to avoid the risk of electrical shock
if a person touches a device in which an
insulation fault has occurred. It also ensures
that in the case of an insulation fault, a high
fault current flows, which will trigger an
overcurrent protection device (fuse, MCB)
that disconnects the power supply .
14. IEC NOMENCLATURE
The first letter indicates the connection between earth and
the power-supply equipment (generator or transformer):
T : direct connection of a point with earth
I : no point is connected with earth (isolation), except
perhaps via a high impedance
.The second letter indicates the connection between earth
and the electrical device being supplied:
T : direct connection with earth, independent of any other
earth connection in the supply system
N : connection to earth via the supply network
15. TN NETWORK
In a TN earthing system, one of the points in
the generator or transformer is connected
with earth, usually the star point in a three-
phase system. The body of the electrical
device is connected with earth via this earth
connection at the transformer
17. The conductor that connects the exposed
metallic parts of the consumer is called
protective earth PE
.The conductor that connects to the star
point in a three-phase system, or that
carries the return current in a single-
phase system is called neutral N
.Three variants of TN systems are
distinguished:
18. TN-S : PE and N are separate conductors
that are only connected near the power
source
.TN-C : A combined PEN conductor fulfills
the functions of both a PE and an N
conductor
19. TN-C-S : Part of the system uses a
combined PEN conductor, which is at
some point split up into separate PE and N
lines. The combined PEN conductor
typically occurs between the substation
and the entry point into the building,
whereas within the building separate PE
and N conductors are used.
20. TN-S :separate protective earth (PE) and
neutral (N) conductors from transformer to
consuming device, which are not
connected at any point after the building
distribution point.
21. TN-C :combined PE and N conductor all
the way from the transformer to the
consuming device .
.
22. TN-C-S earthing system :combined PEN
conductor from transformer to building
distribution point, but separate PE and N
conductors in fixed indoor wiring and
flexible power cords .
23. TT NETWORK
In a TT earthing system, the protective
earth connection of the consumer is
provided by a local connection to earth,
independent of any earth connection at the
generator.
.
24. IT NETWORK
In an IT network, the distribution system
has no connection to earth at all, or it has
only a high impedance connection.
25. PROPERTIES
TN networks save the cost of a low-impedance
earth connection at the site of each consumer.
Such a connection (a buried metal structure) is
required to provide protective earth in IT and TT
systems .
TN-C networks save the cost of an additional
conductor needed for separate N and PE
connections. However to mitigate the risk of
broken neutrals, special cable types and lots of
connections to earth are needed .
TT networks require RCD protection and often
an expensive time delay type is needed to
provide discrimination with an RCD downstream
26. SAFETY
In TN an insulation fault is very likely to lead to
a high short-circuit current that will trigger an
over current circuit-breaker or fuse and
disconnect the L conductors.
In the majority of TT systems the earth fault
loop impedance will be too high to do this and
so an RCD must be employed
27. In TN-S and TT systems (and in TN-C-S
beyond the point of the split), a residual-
current device can be used as an
additional protection. In the absence of
any insulation fault in the consumer
device, the equation IL1+IL2+IL3+IN =0
holds, and an RCD can disconnect the
supply as soon as this sum reaches a
threshold (typically 10-500 mA). An
insulation fault between either L or N and
PE will trigger an RCD with high
probability
28. In IT and TN-C networks, residual current
devices are far less likely to detect an
insulation fault.
In a TN-C system they would also be very
vulnerable to unwanted triggering from
contact between earths of circuits on
different RCDs or with real ground thus
making their use impractical. Also RCDs
usually isolate the neutral core which is
dangerous in a TN-C system .
29. In TN-C and TN-C-S systems any connection
between the combined neutral and earth core
and the body of the earth could end up carrying
significant current under normal conditions and
could carry even more under a broken neutral
situation.
In TN-C and TN-C-S systems any break in the
combined neutral and earth core which didn't
also affect the live conductor could theoretically
result in exposed metalwork rising to near "live"
potential
30. SUBSTATION EARTHING CALCULATION
METHODOLOGY
Calculations for earth impedances and touch
and step potentials are based on site
measurements of ground resistivity and
system fault levels. A grid layout with
particular conductors is then analysed to
determine the effective substation earthing
resistance, from which the earthing voltage is
calculated.
31. In practice, it is normal to take the highest fault
level for substation earth grid calculation
purposes. Additionally, it is necessary to
ensure a sufficient margin such that expansion
of the system is catered for.
To determine the earth resistivity, probe tests
are carried out on the site. These tests are best
performed in dry weather such that
conservative resistivity readings are obtained
32. EARTHING MATERIALS
1. Conductors: Bare copper conductor is usually
used for the substation earthing grid. The copper
bars themselves usually have a cross-sectional
area of 95 square millimetres, and they are laid at
a shallow depth of 0.25-0.5m, in 3-7m squares. In
addition to the buried potential earth grid, a
separate above ground earthing ring is usually
provided, to which all metallic substation plant is
bonded.
33. 2. Connections: Connections to the grid and
other earthing joints should not be soldered
because the heat generated during fault
conditions could cause a soldered joint to fail.
Joints are usually bolted, and in this case, the
face of the joints should be tinned.
3. Earthing Rods: The earthing grid must be
supplemented by earthing rods to assist in the
dissipation of earth fault currents and further
reduce the overall substation earthing resistance.
These rods are usually made of solid copper, or
copper clad steel.
34. 4. Switchyard Fence Earthing:
The switchyard fence earthing practices are
possible and are used by different utilities.
These are:
(i) Extend the substation earth grid 0.5m-1.5m
beyond the fence perimeter. The fence is then
bonded to the grid at regular intervals.
(ii) Place the fence beyond the perimeter of the
switchyard earthing grid and bond the fence to its
own earthing rod system. This earthing rod
system is not coupled to the main substation
earthing grid.
35. Apparatus Parts to be Earthed Method Of Connection
Power Transformer Transformer tank
Connect the earthing bolt on transformer tank to
station earth. Connect the neutral to earthing
system
High Voltage Circuit
Breakers
Operating mechanism, frame
Connect the earthing bolt on the frame and the
operating mechanism of Circuit Breaker to
earthing system
Surge Arrester Lower Earth Point To be directly connected to the earth mat
Support of bushing
insulators, lightning
arresters, fuse, etc..
Device Flange or Base Plate
Connect the earthing bolt of the device to the
station earthing system
Potential Transformer
Potential transformer tank, LV
neutral.
Connect the transformer earthing bolt to
earthing system Connect LV neutral of phase
lead to case with flexible copper conductor
Isolator
Isolator frame, operating
mechanism, bedplate
Weld the isolator base frame, connects it to the
bolt on operating mechanism base plate and
station earth.
Current Transformer
Secondary winding and metal
case
Connect secondary winding to earthing bolt on
transformer case with a flexible copper
conductor.
Different Equipments and Ground Connections
36. HOW TO DETERMINE CORRECT NUMBER OF
EARTHING ELECTRODES
Number of Earthing Electrode and Earthing Resistance
depends on the resistivity of soil and time for fault current to
pass through (1 sec or 3 sec). If we divide the area for
earthing required by the area of one earth plate gives the
number of earth pits required.
There is no general rule to calculate the exact number of
earth pits and size of earthing strip, but discharging of
leakage current is certainly dependent on the cross section
area of the material so for any equipment the earth strip size
is calculated on the current to be carried by that strip.
37. HOW TO DETERMINE CORRECT NUMBER OF EARTHING
ELECTRODES
First the leakage current to be carried is calculated and
then size of the strip is determined.
For most of the electrical equipment like transformer, diesel
generator set etc., the general concept is to have 4 number
of earth pits. 2 no’s for body earthing with 2 separate strips
with the pits shorted and 2 nos for Neutral with 2 separate
strips with the pits shorted.
The Size of Neutral Earthing Strip should be capable to
carry neutral current of that equipment.
The Size of Body Earthing should be capable to carry half
of neutral Current
38. HOW TO DETERMINE CORRECT NUMBER OF EARTHING
ELECTRODES
For example for 100kVA transformer, the full load current is
around 140A.
The strip connected should be capable to carry at least 70A
(neutral current) which means a strip of GI 25x3mm should
be enough to carry the current and for body a strip of 25×3
will do the needful. Normally we consider the strip size that
is generally used as standards.
However a strip with lesser size which can carry a current of
35A can be used for body earthing. The reason for using 2
earth pits for each body and neutral and then shorting them
is to serve as back up. If one strip gets corroded and cuts
the continuity is broken and the other leakage current flows
through the other run thery by completing the circuit.
39. HOW TO DETERMINE CORRECT NUMBER OF EARTHING
ELECTRODES
Similarly for panels the no of pits should be 2 nos. The size
can be decided on the main incomer circuit breaker.
For example if main incomer to breaker is 400A, then body
earthing for panel can have a strip size of 25×6 mm which
can easily carry 100A.
Number of earth pits is decided by considering the total fault
current to be dissipated to the ground in case of fault and
the current that can be dissipated by each earth pit.
Normally the density of current for GI strip can be roughly
200 amps per square cam. Based on the length and dia of
the pipe used the number of earthing pits can be finalized.
40. RECOMMENDED PRACTICES FOR GROUNDING
Grounding and bonding are the basis upon which safety and power
quality are built. The grounding system provides a low-impedance path
for fault current and limits the voltage rise on the normally non-
current-carrying metallic components of the electrical distribution
system.
During fault conditions, low impedance results in high fault current
flow, causing overcurrent protective devices to operate, clearing the
fault quickly and safely. The grounding system also allows transients
such as lightning to be safely diverted to earth.
Bonding is the intentional joining of normally non-current-carrying
metallic components to form an electrically conductive path. This helps
ensure that these metallic components are at the same potential, limiting
potentially dangerous voltage differences.
Careful consideration should be given to installing a grounding
system that exceeds the minimum NEC requirements for improved
safety and power quality.
41. RECOMMENDED PRACTICES FOR GROUNDING
Equipment Grounding Conductors
Isolated Grounding System
Branch–Circuit Grounding
Ground Resistance
Ground Rods
Ground Ring
Grounding Electrode System
Lightning Protection System
Surge Protection Devices (SPD) (formerly called TVSS)
42. 1. EQUIPMENT GROUNDING CONDUCTORS
The IEEE Emerald Book recommends the use of equipment-grounding
conductors in all circuits, not relying on a raceway system alone for
equipment grounding. Use equipment grounding conductors sized equal
to the phase conductors to decrease circuit impedance and improve the
clearing time of overcurrent protective devices.
Bond all metal enclosures, raceways, boxes, and equipment grounding
conductors into one electrically continuous system. Consider the
installation of an equipment grounding conductor of the wire type as a
supplement to a conduit-only equipment grounding conductor for
especially sensitive equipment.
The minimum size the equipment grounding conductor for safety is
provided in NEC 250.122, but a full-size grounding conductor is
recommended for power quality considerations.
44. 2. ISOLATED GROUNDING SYSTEM
As permitted by NEC 250.146(D) and NEC 408.40 Exception, consider
installing an isolated grounding system to provide a clean signal
reference for the proper operation of sensitive electronic equipment.
Isolated grounding is a technique that attempts to reduce the chances
of “noise” entering the sensitive equipment through the equipment
grounding conductor. The grounding pin is not electrically connected to
the device yoke, and, so, not connected to the metallic outlet box. It is
therefore “isolated” from the green wire ground.
A separate conductor, green with a yellow stripe, is run to the
panelboard with the rest of the circuit conductors, but it is usually not
connected to the metallic enclosure. Instead it is insulated from the
enclosure, and run all the way through to the ground bus of the service
equipment or the ground connection of a separately derived system.
Isolated grounding systems sometimes eliminate ground loop circulating
currents.
Note that the NEC prefers the term isolated ground, while the IEEE
prefers the term insulated ground.
46. 3. BRANCH-CIRCUIT GROUNDING
Replace branch circuits that do not contain an equipment ground with
branch circuits with an equipment ground. Sensitive electronic
equipment, such as computers and computer-controlled equipment,
require the reference to ground provided by an equipment grounding
conductor for proper operation and for protection from static electricity
and power surges.
Failure to utilize an equipment grounding conductor may cause
current flow through low-voltage control or communication circuits, which
are susceptible to malfunction and damage, or the earth.
Surge Protection Devices (SPDs) must have connection to an
equipment grounding conductor.
47. 4. GROUND RESISTANCE
Measure the resistance of the grounding electrode system to
ground.
Take reasonable measures to ensure that the resistance to ground is 25
ohms or less for typical loads. In many industrial cases, particularly
where electronic loads are present, there are requirements which need
values as low as 5 ohms or less many times as low as 1 ohm.
Measuring earth resistance with fall of potential method (photo credit:
eblogbd.com)
For these special cases, establish a maintenance program for sensitive
electronic loads to measure ground resistance semi-annually, initially,
using a ground resistance meter. Ground resistance should be
measured at least annually thereafter.
When conducting these measurements, appropriate safety precautions
should be taken to reduce the risk of electrical shock.
Record the results for future reference. Investigate significant changes in
ground resistance measurements compared with historical data, and
correct deficiencies with the grounding system. Consult an electrical
design professional for recommendations to reduce ground resistance
49. 5. GROUND RODS
The NEC permits ground rods to be spaced as little as 6 feet apart,
but spheres-of-influence of the rods verlar.
Recommended practice is to space multiple ground rods a minimum of
twice the length of the rod apart. Install deep-driven or chemically-
enhanced ground rods in mountainous or rocky terrain, and where soil
conditions are poor. Detailed design of grounding systems are beyond
the scope of this document.
51. 6. GROUND RING
In some cases, it may be advisable to install a copper ground ring,
supplemented by driven ground rods, for new commercial and
industrial construction in addition to metal water piping, structural
building steel, and concrete-encased electrodes, as required by Code.
Grounding rings provide a convenient place to bond multiple
electrodes of a grounding system, such as multiple Ufer grounds,
lightning down-conductors, multiple vertical electrodes, etc.
Install ground rings completely around buildings and structures and
below the frost line in a trench offset a few feet from the footprint of the
building or structure. Where low, ground impedance is essential,
supplement the ground ring with driven ground rods in a triplex
configuration at each corner of the building or structure, and at the mid-
point of each side.
The emergency generator connected to the ring-ground, and additionally
grounded to reinforcing rods in its concrete pad (photo credit:
psihq.com)
The NEC-minimum conductor size for a ground ring is 2 AWG, but
sizes as large as 500 kcmil are more frequently used. The larger the
53. 7. GROUNDING ELECTRODE SYSTEM
Bond all grounding electrodes that are present, including metal
underground water piping, structural building steel, concrete-encased
electrodes, pipe and rod electrodes, plate electrodes, and the ground
ring and all underground metal piping systems that cross the ground
ring, to the grounding electrode system.
Bond the grounding electrodes of separate buildings in a campus
environment together to create one grounding electrode system.
Bond all electrical systems, such as power, cable television, satellite
television, and telephone systems, to the grounding electrode system.
Bond outdoor metallic structures, such as antennas, radio towers, etc. to
the grounding electrode system. Bond lightning protection down-
conductors to the grounding electrode system.
55. 8. LIGHTNING PROTECTION SYSTEM
Copper lightning protection systems may be superior to
other metals in both corrosion and maintenance factors.
NFPA 780 (Standard for the Installation of Lightning
Protection Systems) should be considered as a minimum
design standard.
A lightning protection system should only be connected to a
high quality, low impedance, and robust grounding
electrode system.
57. 9. SURGE PROTECTION DEVICES (SPD) (FORMERLY CALLED TVSS)
The use of surge protection devices is highly recommended.
Consult IEEE Standard 1100 (The Emerald Book) for design
considerations. A surge protection system should only be
connected to a high quality, low impedance, and robust
grounding electrode system.
Generally, a surge protection device should not be installed
downstream from an uninterruptible power supply (UPS).
Consult manufacturers’ guidelines.
59. EARTHING DEFINITIONS- AS PER BNBC-
EARTH CONDUCTORS IN PANEL AND TO CIRCUITS MINIMUM CROSS-SECTIONAL AREA
OF COPPER EARTH CONDUCTORS
—IN RELATION TO THE AREA OF ASSOCIATED PHASE CONDUCTORS
Cross-sectional Area of Phase
Conductor(s) (mm²)
Minimum Cross-sectional Area of the
Corresponding
Earth Conductor (mm²)
Less than 16
Same as cross-sectional area of phase
conductor but not
less than 14 SWG (3.243 mm²)
16 or greater but less then 35 16
35 or greater
Half the cross-sectional area of phase
conductor
60. EARTHING DEFINITIONS- AS PER BNBC-
NB- In case of copper
wire being used as
earth conductors, the
size of the wire shall
not be less than half
the area of the largest
current carrying
conductor supplying
the circuit.
Restriction-
Aluminium or copper
clad aluminium (clad
(PP of clothe) – sojjito)
conductors shall not be
used for final
connections to earth
electrodes
61. EARTHING DEFINITIONS- AS PER BNBC-
Earth Lead
Earth lead is the link which provides connection between the
earth conductor(s) and the earth
[alvaa]electrode(s). Earthing leads shall be run in duplicate
down to the earth electrode so as to increase the safety
factor of the installation. Copper wire used as earthing lead
must not be smaller than 8 SWG (12 mm²).
In normal provision the earthing lead connects the earth
electrode to a bus bar called earth bus bar. earth conductor
is connected to Earth bus bar
62. EARTHING DEFINITIONS- AS PER BNBC-
Earth Electrodes
The earth electrode shall, as far as practicable, penetrate
into permanently moist soil preferably below ground water
table. The resistance of earth electrodes shall not be more
than one ohm. The following types earth electrodes are
recognized for the purpose of Code -Copper rods, Copper
plates, Galvanized iron pipes. Copper rods shall have a
minimum diameter of 12.7 mm. Copper plates shall not be
less than 600 mm x 600 mm in size, with 6 mm thickness.GI
pipes shall have a minimum diameter of 50 mm. Earthing
cable sizes should be adequate with respect to the phase.
63. Phase cable size Earthing cable size
Below 4 rm Earthing not less than 3.243 mm2 or 14
SWG.But for the unavailability of this size it
can be considered as 4 rm. So it should not be
less than 4 rm
4 rm to 16 rm Same size of the phase
25 rm and 35 rm 16 rm
35 above Half of the phase cable size
In case of multiplication at LT panel As it is power cable like 240/300/400/500 rm.
Then earthing cable size should be half of the
total size. Say,if the phase cable is 6X300 rm
then earthing cable will be 6X150 rm or 3X300
rm
EARTHING DEFINITIONS- AS PER BNBC-
69. EARTHING DEFINITIONS- AS PER BS 7671-
Requirements of using RCD
RCDs are designed to disconnect the circuit if there is a leakage current.
By detecting small leakage currents (typically 5–30 mA) and
disconnecting quickly enough (<300 ms).
RCDs should operate within 25–40 milliseconds with any leakage
currents (through a person) of greater than 30 mA,
RCDs operate by measuring the current balance between two
conductors using a differential current transformer. This measures the
difference between current flowing through the live conductor and that
returning through the neutral conductor. If these do not sum to zero,
there is a leakage of current to somewhere else (to earth/ground or to
another circuit), and the device will open its contacts.
