STRINGING TENSIONS :

The insulators, bus bars and connections should not be stressed to more than one

3.4 CLEARANCES

The following are the minimum clearances for out-door equipment and rigid conductors in air.

Minimum Clearance

Phase to phase spacing in isolators and switches

3.4.2 Normally adopted phase spacings for strung bus are indicated below :

11 KV - 1300 mm

3.4.3 a) The minimum clearance of the live parts to ground in an attended outdoor sub-station and the sectional clearance to be maintained between live parts in adjacent sections for safety of persons while working with adjacent sections alive are given below

Voltage rating

(KV) Minimum Clearance to

ground (mm.) Sectional Clearance (mm.) 3.4.3 b) The bottom most portion of any insulator or bushing in service should be at a

minimum height of 2500 mm above ground level.

3.4.4 The earthing practice adopted at generating stations, sub-stations and lines should be in such a manner as to provide :

a) Safety to personnel

b) Minimum damage to equipment as a result of flow of heavy fault currents c) Improve reliability of power supply

3.4.5 The primary requirements are :

The impedance to ground (Resistance of the earthing system) should be as low as possible and should not exceed 1 ohm in large stations and 2 ohms in small sub-stations and 0.5 ohms in power sub-stations. In distribution transformer sub-stations, it could be 5 ohms

The step and touch potentials should be within safe tolerable limits 3.4.6.1 Resistance of the earthing system

R = P + P 4r L

where P = Soil resistivity in ohm meter

L = Length of the conductor buried in meters

R = Radius in meters of a circle having the same area as that occupied by earth mat.

The value of ‘R’ should be within limits as specified in para 2.4.5 (a)

3.4.6.2 The size of earth mat conductor (steel strip) shall be:

A (Steel) = 26 x 1 √t circular mills

(Or 0.0013 x I√t sq.mm) for bolted joints = 21.6 x I √t circular mills

(Or 0.011 x I√t sq.mm) for welded joints Where A = Cross section

I = Fault current in amps., at the station

= Fault MVA x 10³

√3 x system kv

and t = Time in seconds during which current is applied

3.4.6.3 In order to take care of corrosion etc. some extra margin may be provided in the size of earthing strip.

3.4.7.1 The permissible values of step and touch potentials can be calculated as follows : i) Safe tolerable E step (Volts) = 165 + Ps

√t

where Ps is the resistivity of the ground in ohm meter just beneath the feet of a person (3000 for crushed rock). ‘t’ is the time in seconds to clear earth (fault by the concerned breaker (0.5 may be assumed)

ii) Safe tolerable E touch (Volts) = 165 + 0.25 Ps

3.4.7.2 The maximum attainable step and touch potentials can be calculated as follows.

(These potentials shall be less than the tolerable value, calculated under clause 2.4.7.1)

a) Maximum attainable E step = Ks.Ki.Pl L

Where Ks = 7 x ( 1 + 1 + 1 + 1 + n terms) 22 2h D+h 2D 3D

n = Number of parallel conductors in the grid D = Spacing of conductors in grid in meters h = Depth of burial in meters

Ki = 0.655 + (0.172 x n)

P = Average resistivity of the ground in ohm meters

l = Maximum total RMS current flowing between the ground grid and

Earth in amps

L = Length of buried conductor in meters

b) Maximum attainable E touch = Km. Ki. P.l L

Where Km = 7 x logs D x D + V x log 3 x 5 x 7 x (n-2) term 44 16 hd 22 4 6 8

(Approximately) Km = 7/44 x loge(d^r / 16 hd) + 7/22 Loge 0.25 n = Number of parallel conductors in the grid d = Equivalent dia of conductor used in metres D = Spacing of conductors Ingrid in metres h = Depth of burial in metres

Ki = 0.655 + (0.172 x n)

P = Average resistivity of the ground in ohm metres

l = Maximum RMs current following between the ground grid and

earth in amps

L = Length of buried conductor in metres

REFERENCES

1) LAURENT , P.G “general fundamentals of electric grounding technique,” Bulletin De La Societe Franciaise Des Electriciens 1 (July 1951) 368-402.

2) AIEE committee report “Voltage gradient through the ground under fault conditions

“, AIEE Trans, 77 pt.III (1958 ) 669-692.

3.4.7.3 All exposed steel earthing conductors should be protected with bituminous paint.

PLATE EARTHS

i)EHT Substation - 1.3 M x 13 M.Ms cast iron plates 25mm thick Plates are to be buried vertically in pits and surrounded by finely divided coke, crushed coal or char coal atleast 155 mm all round the plates. Plates should not be less than 15 m apart and

s

should be buried to sufficient depth to ensure that they are always surrounded by moist earth.

3.4.7.5 PIPE EARTH ING

a) EHT substations Cast iron pipes 125 mm in diameter 2.75 m long and not less than 9.5 mm thick. Pipes 50.8 mm in dia and 3.05m long.

Pipes are to be placed vertically at intervals of not less than 12.2 m in large stations surrounded by finely broken coke crushed coal and charcoal atleast 150 mm around the pipe on the extra depth.

a) Peripheral or main earth mat 100 x 16 m MS flat

b) Internal earth mat 50 x 8m MS flat to be placed at 5m apart

c) Branch connections Cross section not less than 64.5 sq.mm i) Joints are to be kept down to the minimum number

ii) All joints and connections in earth grid are to be brazed, riveted, sweated, bolted or welded.

For rust protection the welds should be treated with barium chromate. Welded surfaces should be painted with red lead and aluminium paint in turn and afterwards coated with bitumen. Joints in the earthing conductor between the switchgear units and the cable sheaths, which may require to be subsequently broken should be bolted and the joint faces tinned. All joints in steel earthing system should be made by welding except the points for separating the earthing mat for testing purposes which should be bolted. These points should be accessible and frequently supervised.

3.4.7.6 In all sub-stations there shall be provision for earthing the following :

a) The neutral point of each separate system should have an independent earth, which in turn should be interconnected with the station grounding mat

b) Equipment frame work and other non-current carrying parts (two connections) c) All extraneous metallic frame work not associated with equipment (two connections)

d) Lightning arrestors should have independent earths which should in turn be connected to the station grounding grid.

e) Over head lightning screen shall also be connected to the main ground mat.

3.4.7.7 The earth conductor of the mat could be buried under earth to economical depth of burial of the mat 0.5 meters.

3.4.7.8 MEASUREMENT OF RESISTANCE :

3.4.7.8.1 The earth resistance can be measured by any of the following methods : i) Three point method

ii) Four point method

3.4.7.8.2 THREE POINT METHOD : In this method, two temporary electrodes or spikes are driven, one for current and the other for voltage, at a distance of 150 ft. and 75 ft. respectively from earth electrode under test and ohmic values of earth electrode resistances are obtained from the earth megger. The earth resistance is calculated from the following formula :

R = P log 10 (4 L/D) where, 2 Π L

R = Electrode resistance in ohms

L = Length in centimeters of the Rod driven under ground D = Diameter in cms. of the rod

P = Earth resistivity in ohm-cm

3.4.7.8.3 FOUR POINT METHOD : In this method, four spikes are driven into the ground at equal intervals of ‘a’ cms. The depth of insertion of spikes into the ground must not exceed one twentieth of ‘a’. The two outer spikes are connected to the current terminals of earth megger while the two inner spikes are connected to potential terminals. The megger handle is rotated till a steady reading ‘R’ ohms is recorded. Assuming that the soil is homogeneous the earth resistivity ‘P’ in ohms/

cubic centimeter (ohm/cm) is calculated from the following formula :

P = 2 Π a R

Increased spacing of spikes gives high values for ‘P’ and this effect is believed to be due to different strata in the earth’s depth having different resistivities.

3.5 INSULATORS

3.5.1 Technical particulars of the tension and suspension type porcelain insulators shall normally be as that of line insulators. The post solid core type insulator shall be capable of being mounted either up-right or under hung.