GROUNDING SYSTEMS
earthing system for
substations
This specification covers the design, manufacturing, testing,
delivery,
commissioning and guarantee of Grounding system used in
500 kV
& 220 kV
and
66 kV substations.
definitions
ground grid – substation
The complete mesh of copper conductors laid in the earth to provide a common ground
for electrical devices or metallic structures.
ground
rods - substations
Copper
rods and extensions are driven into the earth and connected to the ground grid to
lower its resistance and also dissipating ground current into the earth.
connectors
Copper
or a copper alloy including bimetallic devices, of suitable electric conductance
and mechanical strength for connecting conductor to conductor, conductor to
ground rod, and conductor to metalwork.
ground resistance
The
ground resistance is the ohmic resistance of the grounding system to remote
earth.
ground
A conducting connection by which any steelwork, equipment, and electric circuit is connected to the earth.
ground potential rise (GPR):-
The maximum voltage that a substation grounding grid may attain to a distant point assumed to be at the potential of remote earth.
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).
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.
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.
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 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.
Classification
The
grounding materials covered by this specification are:-
copper conductor
Stranded the copper conductor used to form ground grids and provide the connection from
equipment to the ground grid.
flexible ground leads
Multi-stranded the insulated copper conductor used for temporary grounding of HV equipment.
ground rods
, extensions and the
connections for attachment of copper conductor.
bimetallic connectors shield wire.
Conductors consisting of galvanized steel wire, or hard/soft drawn copper clad steel wire, or soft annealed copper clad steel wire and installed in counterpoise or crowfoot arrangement.
connector cable
Connector cables consisting of hard drawn stranded copper conductor. These connectors shall be used for connections between all above-ground conductive metal ports.
requirements
environmental Conditions:
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 the 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- |
Thunder
storms |
Occasional
|
|
|
11- |
Sand
storms |
Occur
occasionally especially in the desert.
|
|
|
12- |
Pollution
|
Heavy
pollution >50 mS
|
|
|
13- |
Seismic
load
|
According
to Egyptian Code and attached map. |
|
Note: This table is valid
altitude (0-1000m)
Maximum
temperature shall be in accordance with substation location in
For
Cairo Zone
All outdoor locations are subject to sand storms. Consequently, all equipment shall be proof against the ingress of sand.
System
Parameters
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 overvoltage. 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
(LIL) 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 overvoltage. 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
() kV
peak |
1050 |
|
11- |
One minute
power frequency withstand 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 overvoltage. 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 |
|
|
|
|
Quality Assurance:
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. |
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.
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.
DESIGN
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.
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 summer months, provided it does not cause delays to 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.
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 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 range 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 to 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 meters 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) a 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 locations such as in indoor substations, it is not reasonable to be able to
use crushed stone, so other means to increase the safety of a 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 the 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 currently. |
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 the 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. |
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
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 are 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 250- 350° C.
c. The fault clearing time (tc) shall be taken as one second.
d. The fault current shall be regarded as the highest specified switchgear symmetrical short circuit rating in a particular substation.
e. Maximum allowable conductor the temperature during short circuit 300° C.
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 the 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 the contractor, the kit shall consist of:
(a) Graphite molds for the right angle, tee, and straight through connections.
(b) Handle clamp for the molds.
(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 mold 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.
Ground Rods
Substation Ground Rods
The earth rods shall be minimum of 3 m length of the 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.
Ground Rod Couplings
These shall be of aluminum bronze, counterbored to enclose fully the threads on the rods. The design shall ensure that, when assembled, there will be a direct rod to rod contact when the rod driving force is applied.
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.
Ground Electrode Copper Mat
This shall be of copper bars. The overall dimension shall be 1000 mm x 1000 mm.
Structure And Equipment Grounding Requirements
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.
Equipment Requiring Only Safety Grounds
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.
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.
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 the 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 the 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.
Cable Tray System
Cable tray system shall be grounded with the bare copper conductor of
50 mm2 size at both ends and shall be bonded across gaps including
expansions gaps.
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 the 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.
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 minimum 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.
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.
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.
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.
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.
Reclosers
The tank of recloser(s) shall be safely grounded at two locations. The respective control cabinets shall also be connected to the grounding grid.
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.
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.
Grounding of Lighting Equipment
Grounding of the lighting fixtures, lamp holders, lamps, receptacles, and metal poles supporting lighting fixtures shall be per IEC standard.
Table 8: Application Guide of Conductor Sizes for Equipment Safety Grounding
|
No. |
Description |
Conductors size (mm2) |
Earthing
locations |
|
|
Station Fault Level |
||||
|
40 kA |
31.5 kA |
|||
|
1. |
Steel Structures |
240 |
185 |
At two (2) locations diagonally opposite |
|
2. |
Power Transformers Tank |
240 |
185 |
At two (2) locations diagonally opposite |
|
3. |
Circuit Breakers and disconnect switches |
240 |
185 |
At two (2) locations diagonally opposite |
|
4. |
Operating handles for outdoor disconnect switches |
240 |
185 |
|
|
5. |
Surge arresters |
240 |
185 |
|
|
6. |
Coupling capacitor Voltage transformers |
240 |
185 |
|
|
7. |
Power Cables (11 kV and above) |
240 |
185 |
At one end only |
|
8. |
Station Services Transformer |
120 |
185 |
At two (2) locations diagonally opposite |
|
9. |
Metal enclosed switchgear |
240 |
185 |
At three (3) locations one at each end and middle |
|
10. |
Control Cables |
95 |
95 |
At both ends |
|
11. |
Instrument Cables/ Signal Cables |
50 |
50 |
|
|
12. |
Instrument Transformer |
240 |
185 |
|
|
13. |
Shunt Capacitors |
120 |
|
|
|
14. |
Lighting Masts |
240 |
185 |
|
|
15. |
AC-DC Main Distribution Panels |
120 |
120 |
|
|
16. |
AC-DC Sub-Distribution Panels |
95 |
95 |
|
|
17. |
Control and Relay Panels and Local Control Panels
(risers) |
240 |
185 |
|
|
18. |
Metal Fence/Gate |
240 |
185 |
|
|
19. |
Cable Tray/Metallic Conduits |
50 |
50 |
|
|
20. |
Oil Tanks/Pipes, etc. |
50 |
50 |
|
Equipment
Requiring both Safety and System Grounds
Power Transformer Tanks
Power transformer tanks shall be safely grounded at two points diagonally opposite to each other. These connections shall be made from two different points of the grounding grid.
A separate system ground shall be provided for the neutral of the transformer by means of two stranded copper wires as the ground grid conductor size.
The neutral grounding wires shall be insulated from the transformer tank by support insulators mounted on the tank wall and shall be connected to the grounding grid directly.
Independently mounted radiator bank and XLPE cable termination boxes shall be separately grounded at two diagonally opposite locations.
Instrument Transformers
Potential and current transformers shall have their metal case grounded.
The grounding terminal of the potential and current transformers shall be connected to the grounding grid. The neutral point of the secondary connection of potential and current transformer shall be grounded to the ground grid in the control/relay
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