Tasa de subocupación en el mercado laboral generado

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The potential difference between a step and the tip of the raised hand touching a substation structure during the flow of the earth fault current through the latter is known as touch potential.

The step potential and touch potential depend upon the following aspects:

 Earth fault current  Duration of earth fault

 Whether short time (less than 3 sec.)  Whether sustained (more than 3 sec.)  Fault current flowing through body  Values of body resistance in the path

The design of grounding system should be such that the voltage gradient in volts/metre on the surface of the ground should be less than the permitted value.

3.4 Soil Resistivity Measurement

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The earth resistivity is measured by driving 4 electrodes in ground in a straight line at equal spacing of 20-25m (Fig. 1). These electrodes are connected to terminals of an earth tester. A typical earth tester has 4 terminals C1, C2, P1 and P2. The electodes are connected to the tester in the order of C1, P1 and P2, C2. The handle of the tester is rotated in case of manual one or the button is pressed (in case of motorised tester) and the reading of the resistance is read on the megger scale. The reading of meggar is used in calculating the soil resistivity in Ohm-meters.

If R is the resistance measured then the Specific Resistivity = 2 aR

where a= Distance between the electrodes

The design of earth mat is based on the results of earth resistivity measured in the switchyard area. The earth resistivity is taken at about 15 places in switchyard and more particularly near shunt reactor, transformer, Circuit Breaker and control room locations.

Prior to the testing of soil resistivity and earth resistance, site should refer to the guidelines issued by the Operation Services Deptt. at Corporate Centre. The operation manual of the testing instrument available at site should also be referred.

3.5 Earthing Material

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Following Table gives the typical size and materials required for different earthing items in the substation:

Table -1: Typical sizes of materials used for Switchyard Earthing

Sl. No. Item Size Material

1. Main Earthing Conductor to be buried in ground (for earthed mat & earth pipes)

40 mm dia Mild Steel rod

2. Conductor above ground & earthing leads (for equipment)

75 x 12 mm G.S. Flat

Galvanised Steel

3. Conductor above ground & earthing leads (for columns & aux. Structures)

75x12 mm G.S. Flat

Galvanised Steel

4. Earthing of indoor LT panels, Control panels and out door marshalling boxes, MOM boxes, junction boxes & lighting panels etc.

50 x 6 mm G.S. Flat

Galvanised Steel

5. Rod Earth Electrode 40 mm dia

3000 mm long

Mild Steel

6. Pipe Earth Electrode (in treated earth pit) as per IS.

40 mm dia 3000 mm long

7. Earthing for motors 25 x 3 mm GS flat

Galvanised Steel

8. Earthing conductor along outdoor cable trenches

50 x 6 mm MS flat

Mild Steel

3.6 Earthing Conductor Layout

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i) Earthing conductor in outdoor areas is buried at least 600 mm below finished ground level or as specified.

ii) Wherever earthing conductor crosses cable trenches, underground service ducts, pipes, tunnels, railway tracks etc. it is laid at minimum 300 mm below these and be re-routed in case it fouls with equipment/structure foundations etc.

iii) Tap-connections from the earthing grid to the equipment/structure to be earthed are terminated on the earthing terminals of the equipment/structure as per earthing details.

iv) Earthing conductors crossing the road is laid 300 mm below the road or at greater depth to suit the site conditions.

v) Earthing conductors embedded in the concrete should have approximately 50 mm concrete cover.

vi) Earth grid should be extended beyond 2000 mm from the switchyard fencing towards out side.

vii) A minimum clearance of 1500 mm is maintained between the earthing conductor and the control room building.

3.7 Equipment and Structure Earthing in Substation

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i) Earthing pads are provided for the apparatus/equipments at accessible position. The connection between earthing pads and the earthing grid is made by two short earthing leads (one direct and another through the support structure) free from kinks and splices by 75 mm x 12 mm GS earth flat. The GS earth flat is welded to a MS Rod riser which is connected to the earth mat in ground.

ii) All steel/RCC columns, metallic stairs etc. are connected to the nearby earthing grid conductor by two earthing leads. Electrical

continuity is ensured by bonding different sections of rails and metallic stairs.

iii) Metallic pipes, conduits and cable tray sections for cable installation are bonded to ensure electrical continuity and connected to earthing conductors at regular interval. Apart from intermediate connections, beginning points are also connected to earthing system.

iv) A separate earthing conductor should be provided for earthing the lighting fixtures, receptacles, switches, junction boxes, lighting conduits etc.

v) A continuous ground conductor of 16 SWG GI wire is run all along each conduit run and bonded at every 600 mm by not less than two turns of the same size of wires. The conductor is connected to each panel ground bus, all junction boxes, receptacles, lighting fixtures etc.

vi) Railway tracks within switchyard are earthed at a spacing of 30 m and also at both ends.

vii) 50 mm x 6 mm MS (or of specified size) flat runs on the top tier and all along the cable trenches and the same is welded to each of the racks. Further this flat is earthed at both ends at an interval of 30 mtrs. The M.S. flat is finally painted with two coats or Red oxide primer and two coats of Post Office red enamel paint or of specified material.

viii) In isolator the base frame is connected to the earth mat.

The following Table-2 gives the various parts required to be earthed alongwith their method of connection

Table:2 Details of Apparatus /Structures to be earthed in Switchyard Sl. No. Apparatus Parts to be

Earthed

Method of connection

1. Support of bushing insulators, Lightning Arrester, fuse, etc.

Device flange or base plate Earth terminal of each pole of 3 phase Surge Arrester

Connect the earthing bolt of the device to station earthing system. In the absence of earthing bolt or in case of connection to non- conducting structures, connect device fastening bolts to earth When the device is mounted on a steel structure, weld the structure, mounting the device flange; each supporting structure of apparatus to earthing mesh via separate conductor

2. Cabinets of control and relay panels

Frameworks of switchgear and cabinets

Weld the framework of each separately mounted board and cabinet minimum at two points to the earth conductor of earthing system. 3. High-voltage Circuit Breakers Operating mechanism, frame

Connect the earthing bolt on the frame and to operating mechanism of CB to earthing system

4. Isolator Isolator base

(frame), operating mechanism bedplate.

Weld the isolator base frame, connect it to the bolt on operating mechanism base plate and station earth.

Provide an auxiliary earth mat of 600 mm x 600 mm of earth conductors in the ground near the earth switch and connect the both.

5. Surge Arrester Lower earth

point

To be directly connected to the earth mat.

6. Potential Transformer/CVT CVT tank. LV neutral, LV winding phase lead (if stipulated by the designers)

Connect the transformer earthing bolt to earthing system.

Connect LV neutral of phase lead to case with flexible copper conductor.

7. Current Transformer Secondary winding and me tal case

Connect secondary winding to earthing bolt on transformer case with a flexible copper conductor, the case being earthed in the same way as support insulators.

8. Power transformer Transformer tank

Connect the earthing bolt on transformer tank to station earth. Connect the Neutral directly to two dedicated earth pits.

9. Fencing Alternate

Fencing portions

GS earth flat connects the fencing to earth mat.

10. Water tanks Lightning rods

provided over the top of water tank

GS earth flat connects the lightning rods to earth mat.

11. Cable trays & supports Cable trays and support

GS flat running near trays is welded at a spacing of 750 mm and connected to earth mat at about 30 m distance.

12. Shunt Reactor Tank Same as in transformer. NGR is also connected to two earth pits.

3.8 Jointing

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i)Earthing connections with equipment earthing pads are bolted type. Two bolts are provided for making each connection. Equipment bolted connections, after being checked and tested are painted with anti-corrosive paint/compound of specified material.

ii)Resistance of Joint should not be more than the resistance of the equivalent length of the conductor.

iii)All ground connections are made by electric arc welding. All welded joints are allowed to cool down gradually to atmospheric temperature before putting any load on it. Artificial cooling is not allowed.

iv)Each earthing lead from the neutral of the power transformer/reactor is directly connected to two pipe electrodes in treated earth pit (as per IS) which in turn, are buried in Cement Concrete pit with a cast iron cover hinged to a cast iron frame to have an access to the joints. All accessories associated with transformer/reactor like cooling banks, radiators etc. are connected to the earthing grid at minimum two points.

v)Earthing terminals of each lightning arrester & Capacitor Voltage Transformer is directly connected to rod earth electrode which in turn is connected to station earthing grid.

3.9 Measurement of Earth Resistance

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Three electrode methods is used for measuring the earth resistance in switchyard (Fig. 14). To measure the earth resistance both C1 and P1 terminals of megger could be connected to a spike that is driven in ground and connected to earth mat whereas terminals P2 and C2 are connected to the equidistant spikes driven in ground (not connected to earth mat). The value of of R could be read in the scale with the rotation of the handle of megger or press of a button. This will give the value of earth resistance. The value as far as possible should be below 1 ohm. In case this value is high water should be sprinkled in the earthing pits for improvement of earth resistance.

DO’S DON’TS