Earthing system for substations
1. scope
This
specification covers the design, manufacturing, testing, delivery,
commissioning, and guarantee of the Grounding system used in 500 kV & 220 kV, and
66 kV substations.
2.
definitions
2.1 ground grid – substation
The
complete mesh of copper conductors is laid in the earth to provide a common ground
for electrical devices or metallic structures.
2.2 ground
rods - substations
Copper
rods and extensions are driven into the earth and connected to the ground grid to
lower its resistance and also to dissipate ground current into the earth.
2.3 connectors
Copper
or a copper alloy including bimetallic devices, of suitable electric conductance
and mechanical strength for connecting conductor to conductor, conductor to the ground rod, and conductor to metalwork.
2.4 ground resistance
The
ground resistance is the ohmic resistance of the grounding system to remote
earth.
2.5 ground
A
conducting connection by which any steelwork, equipment, and the electric circuit are
connected to the earth.
2.6 ground potential rise (gpr):-
The
maximum voltage that a substation grounding grid may attain to a distant point is assumed to be at the potential of remote earth.
2.7 exothermal weld
This
is a process whereby a suitable graphite mold is placed around conducting
pieces to be connected (i.e. ground electrodes and grounding leads).
2.8 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.
2.9 step voltage
The
difference in surface potential experienced by a person bridging
a distance of 1 m with the feet without contacting any grounded object.
2.10 ground mat
A
solid metallic plate or a system of closely spaced bare conductors that are connected
to and often placed in shallow depths above a ground grid or elsewhere at the
earth's surface, in order to obtain an extra protective measure minimizing the
danger of the exposure to high step or touch voltages in a critical operating
area or places that are frequently used by people.
3. Classification
The
grounding materials covered by this specification are:-
3.1 copper conductor
Stranded
copper conductor used to form ground grids and provide the connection from
equipment to ground grid.
3.2 flexible ground leads
Multi-stranded
insulated copper conductor used for temporary grounding of HV equipment.
3.3 ground rods, extensions and the
connections for attachement of copper conductor.
3.4 bimetallic connectors
3.5 shield wire.
Conductors
consisting of galvanized steel wire, hard/soft drawn copper clad steel wire,
or soft annealed copper clad steel wire and installed in counterpoise or
crowfoot arrangement.
3.6 connector cable
Connector
cables consist of hard-drawn stranded copper conductors. These connectors
shall be used for connections between all above-ground conductive metal ports.
4. requirements
4.1 environmental Conditions:
4.1.1 The Environmental conditions listed in the table
(1) shall apply.
Table (1) Standard Environmental Conditions
|
Clause |
Description
|
Value |
|
|
1-
|
Pressure
MB-annual means. |
1013 |
|
|
2- |
Atmospheric
Temperature °C:- Maximum
mean daily Minimum
mean daily Yearly
mean
|
47* -5 30 |
|
|
3- |
Relative
humidity % Maximum
relative humidity Minimum
relative humidity Average
relative humidity Daily
mean
|
100 20 75 95 |
|
|
4- |
Rainfall
mean-annual total (mm)
|
65 |
|
|
5- |
Maximum
wind speed (m/sec.) at 10m above ground level.
|
35 |
|
|
6- |
Soil
temperature at a depth of 1.5 m. |
25°C
|
|
|
7- |
Solar
energy radiation W/m² |
>1100
|
|
|
8- |
Wind
pressure N/m² |
766
|
|
|
9- |
Absolute
black bulb °C max. recorded |
75.7
|
|
|
10- |
Thunderstorms |
Occasional
|
|
|
11- |
Sand
storms |
Occur
occasionally, especially in the desert.
|
|
|
12- |
Pollution
|
Heavy
pollution >50 mS
|
|
|
13- |
Seismic
load
|
According
to the Egyptian Code and the attached map. |
|
Note: This table is valid
altitude (0-1000m)
The maximum temperature shall be in accordance with the substation location in
* For
Cairo Zone
4.1.2 All
outdoor locations are subject to sand storms. Consequently, all equipment shall
be proof against ingress of sand.
4.2 System
Parameters
4.2.1 The system parameters listed in tables
(2,3,4) shall apply.
|
Table (2) 500 kV System Parameters |
||
|
|
|
|
|
Item |
Parameter |
Value |
|
1- |
Frequency
Hz |
50 |
|
2- |
Configuration |
3phase |
|
3- |
Neutral arrangement |
solidly earthed |
|
4- |
Service system voltage
kV r.m.s |
500 |
|
5- |
Rated system voltage for equipment kV r.m.s |
550 |
|
6- |
Duration of max. temporary over voltage. sec. |
0.5 |
|
7- |
Rated short-time withstands current. KA r.m.s |
40/50 |
|
8- |
Rated duration of short circuit . sec. |
1 |
|
9- |
Rated peak withstand current. KA |
100/125 |
|
10- |
Lightning
impulse withstand voltage 1.2/50 ms
(LIWL) kV
peak |
1550 |
|
11- |
Switching impulse withstand voltage 250/2500 |
1175 |
|
|
ms to earth (SIWL).
kV peak |
|
|
12- |
One minute
power frequency withstand voltage |
680 |
|
|
to earth. kV
r.m.s |
|
|
13- |
Min. specific leakage path cm/kV service voltage |
|
|
|
For
equipment
cm/KV |
3.5 |
|
|
For surge
arrester cm/KV |
4.5 |
|
14- |
Allowable Minimum Clearance (mm): |
|
|
|
For the receiving steel structure : |
|
|
|
- Between
live conductors and earth. mm |
4500 |
|
|
- Between
phases in air phase to phase mm |
6000 |
|
|
For Equipment : |
|
|
|
- Between
live conductors and earthed parts
mm |
3700 |
|
|
- Between
phases in air phase to phase mm |
4500 |
|
15- |
Radio interference level.
micro. volt. |
Max. 2500 |
|
Table (3) 220 kV System Parameters |
||
|
|
|
|
|
Item |
Parameter |
Value |
|
1- |
Frequency
Hz |
50 |
|
2- |
Configuration |
3phase |
|
3- |
Neutral arrangement |
solidly earthed |
|
4- |
Service system voltage kV
r.m.s |
220 |
|
5- |
Rated system voltage for equipment kV r.m.s |
245 |
|
6- |
Duration of max. temporary over voltage. sec. |
0.5 |
|
7- |
Rated short-time withstands current. KA r.m.s |
40/50 |
|
8- |
Rated duration of short circuit. sec. |
1 |
|
9- |
Rated peak withstand current. kA |
100/125 |
|
10- |
Lightning
impulse withstand voltage 1.2/50 ms
(LIWL) kV
peak |
1050 |
|
11- |
One minute
power frequency withstands voltage |
460 |
|
|
to earth.
kV r.m.s |
|
|
12- |
Min. specific leakage path cm/kV service voltage for
: |
|
|
|
Equipment cm/KV |
3.5 |
|
|
For surge arrester
cm/KV |
4.5 |
|
13- |
Allowable Minimum Clearance (mm): |
|
|
|
For the receiving
steel structure |
|
|
|
- Between live conductors and earthed parts mm |
2500 |
|
|
- Between phases in air
mm |
3750 |
|
|
For the equipment
|
|
|
|
- Between live conductors and earthed parts mm |
2100 |
|
|
- Between phases in air
mm |
2400 |
|
14- |
Radio interference level.
micro. volt. |
Max. 2500 |
|
Table (4) 66 kV System Parameters |
||
|
|
|
|
|
Item |
Parameter |
Value |
|
1- |
Frequency Hz |
50 |
|
2- |
Configuration |
3phase |
|
3- |
Neutral arrangement |
solidly earthed |
|
4- |
Service system voltage KV r.m.s |
66 |
|
5- |
Rate system voltage for equipment kV r.m.s |
72.5 |
|
6- |
Duration of max. temporary over voltage. Sec. |
0.5 |
|
7- |
Rated short-time withstands current. KV r.m.s |
31.5 |
|
8- |
Rated duration of short circuit. sec. |
1 |
|
9- |
Rated peak withstand current. KA |
80 |
|
10- |
Lightning
impulse withstand voltage 1.2/50 ms
(LIWL) KV |
325 |
|
11- |
One minute
power frequency withstand voltage |
140 |
|
|
to earth.
KV r.m.s |
|
|
12- |
Min. specific leakage path cm/kV service voltage |
|
|
|
For surge arrester |
4.5cm/KV |
|
|
For equipment: outdoor |
3.5 cm/KV |
|
|
: indoor |
2.5 cm/KV |
|
13- |
Allowable Minimum Clearance (mm) |
|
|
|
For the receiving steel structure |
|
|
|
- Between live conductors and earthed parts |
900 |
|
|
- Between phases in air |
1120 |
|
|
For the equipment (indoor/outdoor) |
|
|
|
- Between live conductors and earthed parts |
800/900 |
|
|
- Between phases in air |
800/1120 |
|
|
|
|
4.3 Quality
Assurance:
4.3.1 Applicable
Codes and Standards
The earthing system for substations provided under
this specification shall conform to the applicable codes and standards of:
|
EUS –
Electricity Utility Specifications: American
Society For Testing And Material |
|
|
ASTM
B227-70 |
Hard-drawn cold steel wire |
|
International
Electrotechnical Commission |
|
|
IEC 60364
|
Electrical
installation of buildings |
|
IEC 61089
|
Round
wire concentric lay overhead electrical stranded conductors. |
|
IEEE –
Institute of Electrical And Electronics Engineers |
|
|
IEEE
80-2000 |
Guide for
safety in substation grounding. |
|
IEEE
81-1983 |
Guide for
measuring earth resistivity, ground impedance, and earth surface potentials of
a ground system. |
|
IEEE
837-2002 |
Standard
for qualifying permanent connections used in substation grounding. |
4.3.2 Alternative Codes and Standards
The
tenderer may propose alternative codes and standards provided it is
demonstrated that they give an equivalent or better degree of quality than the
referenced codes and standards. Acceptability of any alternative code or
standard is at the discretion of the purchaser.
4.4.3 Precedence of Codes and Standards
In
case of conflict between this specification and any of the referenced codes and
standards, the following order of precedence shall apply:
-
This specification.
-
EUS specification.
-
IEC recommendation and publications.
-
Other referenced codes and standards.
-
Acceptable alternative codes and standards.
4.4 DESIGN
4.4.1 General
The
grounding system shall be designed to meet the parameters stipulated for
operation on the system specified.
The
first step in the practical design of a grid or mat consists of inspecting the
layout plan of equipment and structures. A continuous grounding conductor shall
surround the grid perimeter to avoid current concentration and hence high
gradients at projecting ground conductor ends. Within the grid, conductors shall
be laid in parallel lines, and at reasonably uniform spacing. They shall be
located, where practical, along rows of structures or equipment to facilitate
the making of ground connections. The preliminary design shall be adjusted so
that the total length of the buried conductor, including cross-connections and
rods, is at least equal to that required to keep local potential differences
within acceptable limits.
4.4.2 Soil
Investigation
The
resistivity is extremely variable from one spot to another according to the
nature of the soil, degree of humidity, and temperature. If the resistivity varies
appreciably with depth, a greater number of readings should be taken. It is
often desirable to use in increased range or probe spacing for a reasonably
accurate estimate of the soil resistivity.
For
substation design, soil resistivity readings shall normally be taken under dry
conditions, during the summer months, provided it does not cause delays to the project
schedule.
Backfilled
material shall have the same soil resistivity or better than that of the
original soil.
A
number of measuring techniques are described in detail in ANSI/IEEE 80-2000.
The Winner’s four-pin method is the most commonly used technique. Winner’s
method is based on the assumption that soil resistivity is uniform.
In case the soil resistivity readings are within +10%
of mean value, manual calculations shall be done by taking the mean value.
In
case of wide variations in field readings, computer software shall be used to
simulate multilayers soil conditions consistent with the varied field readings
for determining the applicable value of soil resistivity for calculation
purposes.
4.4.3 Grounding
system Layout/Arrangement
1. The
grounding grid shall consist of a network of interconnected horizontally buried
conductors and vertically buried ground rods into the ground to provide for
grounding connections to grounded neutrals, equipment ground terminals,
equipment housings, and structures, and to limit the maximum possible shock
voltage to safe values during ground fault conditions.
2. The
grounding grid shall encompass all of the areas within the fence, and shall
extend at least 1.5 meters outside the substation fence on all sides (if space
permits), including all gates in any position (open or closed) to enclose as
much ground as practicable and to avoid current concentration and hence high
gradients at the grid periphery. A perimeter grid conductor shall also surround
the substation control building, at a distance of 0.5-1.5 meters.
The
grounding grid shall be installed beneath the 66 KV building (if applicable).
3. Grounding
grid shall be buried at a depth ranging from 0.5 to 1.5 m below the final earth
grade (excluding asphalt covering).
4. The grounding grid conductors shall
preferably be laid at reasonable uniform spacing. Depending upon site
conditions, the typical spacing of the main conductors generally ranges between 3
meters to 15 meters. In congested areas, reduced intervals may be desirable.
Grid spacing should be halved around the perimeter of the grid to reduce periphery
voltage gradients. It may also be desirable to subdivide the corner meshes into
quarter areas to reduce normally higher mesh voltages at such locations.
5. Main
conductors and secondary conductors shall be bonded at points of crossover by
thermo weld process.
6. The
ground rods shall have minimum dimensions of 16f mm x 3.0 meter However, for other space-limited
installations extra long rods may be considered.
It is
recommended that rods be installed in the tops 50 cm minimum below grade and
bonded to the grounding grid by thermo weld process.
The
rods shall, in general, be installed at all points in the grid as defined
above, in particular near surge arrester connections and transformer neutral
where large ground currents may be expected. The rods installed mainly along
the grid perimeter will considerably moderate the steep increase of the surface
gradient near the peripheral meshes.
7. Specified
values of some parameters having a substantial impact on the grid design are summarized
in table 5 for ready reference.
8. Test
links arrangement must be done so that the opening of one link does not
interface with ground connections other than the one under test at least four
links test shall be provided.
9. A
(0.15 m) layer of high resistivity surface material such as crushed stone shall
be spread on the earth's surface above the ground grid in the switchyard are to
provide a high resistance surface treatment to reduce the hazard from
step/touch potential to persons during a severe fault.
In
some location such as in indoor substation it is not reasonable to be able to
use crushed stone, so other means to increase the safety of person may be
required by providing an insulated floor covering indoors, such as plastic
tiles, rubber mats, etc… .
Table 5 : Grounding Grid Design
Parameters to be Considered
|
Description |
Specified Values |
|
Grid
current that flows between ground grid and surrounding earth (Ig). |
Up to
design calculations. |
|
Surface
layer resistivity gravel. |
3000, ohm
meter (min) |
|
Current
projection factor for future system growth. |
1.0 |
|
Maximum
possible fault clearing time. |
1.0 sec. |
|
Duration
of fault current for determining allowable body current. |
1.0 sec.
Or back up clearing time. |
|
Depth of
ground grid conductors. |
0.5 to 1.5
meters. |
|
Typical
spacing between parallel conductors. |
3 to 15
meters |
|
Resistance
of ground system during summer. |
< 0.5 W |
|
Standard
stranded copper conductor sizes for grounding. |
95 mm2,
120 mm2 150 mm2,
185 mm2 and 240
mm2. |
4.4.4 Selection
of Grounding Conductor Material, Size And Joints
Soft/
hard drawn, stranded copper shall be used for the ground grid conductors and
grounding leads. The conductor shall be round shaped for maximum
cross-sectional contact with the ground. Copper shall be used for ground rods.
Each
element of the ground system (including grid proper, connecting ground leads,
and electrodes) shall be so designed that it will:-
1. Resist
fusing and deterioration of electric joints under the most adverse combination
of fault-current magnitude and fault duration to which it might be subjected.
2. Minimum
size of the grounding conductor
The
following equation shall be used to evaluate the minimum conductor size (in mm2)
as a function of conductor current.
Where:
I is
the r.m.s current in kA
Amm2 is
the conductor cross section in mm2
Tm is the maximum allowable temperature in
°C.
Ta is the ambient temperature in °C.
Tr
is the reference temperature for
material constants in °C.
a0 is the thermal coefficient of resistivety
at 0°C in 1/C°.
ar is the thermal coefficient of
resistivity at reference temperature Tr in 1/C°,
rr
is the resistivity of the ground
conductor at referenced temperature Tr in micro ohms-cm.
K0 =
tc
is the duration of fault current
in sec.
TCAP is the thermal capacity factor per unit volume
(from material
constant table 6) in J/cm3/°C.
Note
that ar and rr are both
to be found for the same reference temperature.
Table
6 provides the material constants for stranded, annealed, soft copper wire at
20° C.
Table 6: Material constants for stranded,
Annealed, Soft Copper Wire
|
Material
conductivity (%) |
100.00 |
|
ar @ 20° |
0.00393 |
|
K0 = (1/a0) @ 0° C |
234 |
|
Fusing
Temperature (°C) |
1083 |
|
rr @ 20° C (mW/cm) |
1.7241 |
|
TCAP
Factor Effective Value (J/cm3/°C) |
3.422 |
Table 7: Recommended
Ground conductor Sizes
|
Conductor size for
equipment grounding leads mm2 |
|
Conductor size for main
ground grid, embedded or exposed grounding system mm2 |
|
95 |
|
120 |
|
120 |
|
150 |
|
185 |
|
185 |
|
240 |
|
30x6/40x5 |
|
2x240 |
|
2 x 240 |
The
recommended standard grounding copper conductor sizes are shown in table 7. The
conductor size shall be calculated per formula indicated in item 4.4.4 and then
the final choice of the conductor size shall be made from the nearest higher
sizes shown in table 7.
The
designer should take precautions to ensure that the temperature of any
conductor will not exceed the maximum allowable temperature of the lowest rated
component.
The
following design parameter shall be taken into account while calculating the
conductor size.
a. Maximum temperature
(Melting/fusing) of the grid copper conductor is 1083° C , taking into account
the exclusive use of exothermic connectors on the buried ground grid.
b. Maximum temperature for the
above-grade joints for equipment grounds, system neutral and ground buses shall
be within a 250- 350° C.
c. The fault clearing time (tc)
shall be taken as one second.
d. The fault current shall be
regarded as highest specified switchgear symmetrical short circuit rating in a
particular substation.
e. Maximum allowable conductor
temperature during short circuit 300° C.
3. Selection of joints
All
joints should be evaluated in terms of conductivity, thermal capacity,
mechanical strength and reliability.
Exothermic
welded joints shall be used on buried ground grid (cross-over points, etc.)
which make the connections and internal part of the homogenous conductor.
Exothermic
welding connection kits will also be used for connecting, all earthing material.
Unless otherwise supplied by contractor, the kit shall consist of:
(a) Graphite moulds for right angle,
tee, and straight through connections.
(b) Handle clamp for the moulds.
(c) Welding metal powder.
(d) Starting flint gun.
(e) Mould scraper.
(f) Mould cleaning brush.
The kit shall contain all the required equipment to
form the welded connection. The mould shall be re-usable after cleaning.
The
above grade joints of pigtails with the respective connectors shall be
compression type, whereas the connector itself shall be bolted to the
respective equipment, structures, etc. Equipment installed inside the
substation control building shall be connected to the exposed grounding system
conductor by means of compression joints, and no bolted connections shall use.
All other connections, such as the ground bar routed in indoor substation
buildings, shall be exothermic. All bolted and compression joints shall
withstand a maximum temperature of 250°C.
4.4.5 Ground Rods
1. Substation Ground Rods
The
earth rods shall be minimum 3 m long of solid hardened copper rod of 16 mm
diameter two parts each 1.5 m length with a screwed connection at each end to
allow the fitting of a hardened steel tip on the driven end and the fitting of
further lengths of rod and a steel driving cap on the upper end. Each screwed
connection shall have a shoulder so that the driving force is transmitted
through the body of the rod and not through the threads.
It is
intended that the rod/rods would be driven to the required depth then cut at
ground grid level and joined to the grid by means of a short length of
conductor and a special connector supplied with the rod.
2. Ground
Rod Couplings
These
shall be of aluminum bronze, counter bored to enclose fully the threads on the
rods. The design shall ensure that, when assembled, there will be direct rod to
rod contact when the rod driving force is applied.
3. Ground
Rod Driving Head
The
driving head shall be of high strength steel, threaded to fit the ground rod
couplings. The design shall ensure that there is direct driving head to rod
contact when the rod driving force is applied. The driving head shall be
suitable for re-use.
4.4.6 Ground
Electrode Copper Mat
This
shall be of copper bars. The overall dimension shall be 1000 mm x 1000 mm.
4.4.7 Structure And Equipment Grounding
Requirements
1. General
The
grounding connections provided to substation equipment and structures fall
under two categories, namely.
a. Safety Grounds. (equipment
grounds)
b. System Grounds.
The safety grounds is the connection to earth of non
current carrying metal parts to protect personnel from hazards whereas the
system grounds is to protect the equipment.
Certain categories of the substation equipment may
require both the “safety” as well as the “system” grounds. As a matter of
principle, all non-current carrying conductive parts and all neutrals shall be
connected to the grounding grid.
2. Equipment Requiring Only Safety Grounds
2.1 Steel Structures and Switch Racks
Switch
racks and every steel structure that supports insulators or electrical
equipment shall be grounded by means of bolted connection at two (2) diagonally
opposite legs. Equipment mounted on steel supporting structures shall have
separate grounding conductors. The ground conductor shall be supported on the
structure at 1.0-meter intervals by clamps.
2.2 Fences
/ Gates
If
space permits a perimeter ground conductor shall be laid which follows the
fence line and the gate in any position (open or close) at a distance of 0.5 –
1.5 m beyond (outside) the fencing. The perimeter ground conductor and the
fence then shall be bonded electrically at corner posts, gate posts and every
alternate line post. The gates shall be bonded to the gate posts with a
flexible copper cable or braid.
The
barbed wire the top of the SSD (Safety and Security Directive) type fence, if
applicable, shall be bonded to the grounding grid.
2.3 Cables
Metallic
cable sheaths shall be effectively grounded by connecting a flexible braid to
the sheath to eliminate dangerous induced voltage to earth.
a. Control
Shield of control cables shall be grounded at both
ends to the grounding grid. In some cases a separate conductor shall be run in
parallel with the control cable and connected to the two sheath ground points.
b. Power Cables
The sheath of single conductor power cables within
substation area shall be grounded at one end, preferably at the source end
only, in order to reduce the sheath current.
For long cables. The sheath should be grounding at
both ends and at each splice.
Power cable potheads shall be case-grounded via one of
the mounting bolts.
If ring type CTs are installed on power cables, the
grounding of sheath shall be done such that the sheath current to ground will
not influence CT secondary current.
c. Instrument
Cables
Instrument cables carrying milliamps, analog or
digital signals shall have their metallic screening grounded at one point by
means of PVC insulated grounding wire connected to separate instrument ground
bar which is insulated from cubicle ground.
d. signal
Cables
All signal cables used in telemetering and
communications shall have their shield grounded at one end only to reduce
interference from stray sources.
2.4 Cable Tray System
Cable
tray system shall be grounded with bare copper conductor of
50 mm2 size at both ends and shall be bonded across gaps including
expansions gaps.
2.5 Control Buildings
Control
building(s) shall be treated as a part of the substation, and shall be grounded
using the same safety criteria as the substation. The control building shall be
encircled by a grounding conductor. Reinforcement of the control building shall
not be connected to the main grounding grid. If may therefore be necessary, lay
ground grid beneath the control building.
All electrical apparatus and metallic doors shall be
bonded via two independent connections.
All air conditioning ducts inside the control building(s) shall be grounded at both ends and cross bonded at all joints. Further, angle irons installed on indoor trenches to support the metallic covers shall also be grounded at both ends.
2.6 Control
Cabinets, Operating Mechanism Housing, Box , etc.
All
the metallic enclosures of these boxes/cabinets shall be connected to the
grounding grid through the grounding terminals.
The
door(s) of all cabin, junction boxes, etc., shall be bonded to the respective
housing with a flexible copper conductor.
A
copper ground bus of a minimum of 25 mm x 6 mm size shall be provided inside these
cabinets. All grounding connections from individual items including motor
frames shall be connected directly, but separately, to this grounding bus.
2.7 Metallic
Conduits
All
metallic conduits shall be connected to the grounding grid at each manhole or
at terminating points by using a conductor size of 50 mm2. Conduits terminating
in metal junction boxes shall be grounded by means of earthing studs or brazed
connections.
2.8 circuit
Breakers and Disconnect Switches
All
circuit breakers and disconnect switches shall be connected to the grounding
grid from two opposite points. A good electrical connection shall be maintained
between the steel structure and any bolted accessories mounted on it.
2.9 Operating
Handles for Outdoor Switches
A
large percentage of fatal accidents from voltage gradients are in fact
associated with manual operating handles of disconnect switches, etc.
A
metal grounding plate or mat (operating platform), shall be placed where the
operator must stand to operate the device. The operating handles shall be
connected by a ground conductor (preferably flexible wire, braid strap) from
the vertical operating pipe to the ground mat and the ground mat is connected
directly to the ground grid.
2.10
Terminal
transmission towers located adjacent to the substation shall be connected to
the substation grounding grid at two diagonally opposite points. The shield
wire shall be connected to the grounding grid.
2.11 Reclosers
The
tank of recloser(s) shall be safety grounded at two locations. The respective
control cabinets shall also be connected to the grounding grid.
2.12 Oil Tanks
and Oil/Water Pipings
All
oil tanks shall be grounded at two points with bolted cable connections from
two different points to the grounding grid. Oil piping shall be grounded at
intervals of 12 m. Runs shorter than 12 m shall be grounded at least at two
points. Water piping shall be connected to the grounding system at all service
points. In addition, copper conductors of adequate size shall be connected
to the main water pipe from two separate points of the grounding grid.
2.13 Metal-enclosed Switchgear
Metal
enclosed switchgear shall have three safety grounds connected to the switchgear
grounding bus. Withdrawable circuit breakers and PTs shall be provided with a reliable connection to the ground bus.
2.14 Grounding
of Lighting Equipment
Grounding of the lighting fixtures, lamp holders,
lamps, receptacles and metal poles supporting lighting fixtures shall be per
IEC standard.
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