Brief Description of the Set-Up and Activities of the Power and Telecommunication Coordination Committee (ptcc)




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6.0 Precautions
While conducting the LF induction test following precautions may be taken for ensuring accuracy in test results.
(i) It may be ensured that the distance between the power line earth point and telecom line earth points are atleast at 3 kms.



  1. Length of conductors connecting the telecom line from earth up to the paralleling length should run perpendicular to the direction of parallelism.




  1. The voltmeter used for measurement of induced voltages should have Zin= 10 Mega Ohm or more.




  1. The test should be arranged only during the dry season.

Appendix I to Chapter VII

(Refer Para 4.3)
Statistical Tables – Value of ‘t’


Degree

of

Freedom


Value of ‘t’ in Percentage for the Confidence Interval






70

80

90

95

98

99

1

2

3



4

5

6



7

8

9



10

11

12



13

14

15



16

17

18



19

20

21



22

23

24



25

26

27



28

29

30



40

50

120



200

1.963

1.386


1.250

1.190


1.156

1.134


1.119

1.108


1.100

1.093


1.088

1.083


1.079

1.076


1.074

1.071


1.069

1.067


1.066

1.064


1.063

1.061


1.060

1.059


1.058

1.058


1.057

1.056


1.055

1.055


1.050

1.046


1.041

1.036


3.078

1.886


2.638

1.533


1.476

1.440


1.415

1.397


1.383

1.372


1.363

1.356


1.350

1.345


1.341

1.337


1.333

1.330


1.328

1.325


1.323

1.321


1.319

1.318


1.316

1.315


1.314

1.313


1.311

1.310


1.303

1.296


1.289

1.282


6.314

2.920


2.353

2.132


2.015

1.943


1.895

1.860


1.833

1.812


1.796

1.782


1.771

1.761


1.753

1.746


1.740

1.734


1.729

1.725


1.721

1.717


1.714

1.711


1.708

1.706


1.703

1.701


1.699

1.697


1.684

1.671


1.658

1.645


12.706

4.303


3.182

2.776


2.571

2.447


2.365

2.306


2.262

2.228


2.201

2.179


2.160

2.145


2.131

2.120


2.110

2.101


2.093

2.086


2.080

2.074


2.069

2.064


2.060

2.056


2.052

2.048


2.045

2.042


2.021

2.000


1.980

1.960


31.881

6.965


4.541

3.747


3.365

3.143


2.998

2.896


2.821

2.764


2.718

2.681


2.650

2.624


2.602

2.583


2.567

2.552


2.539

2.528


2.518

2.508


2.500

2.492


2.485

2.479


2.473

2.467


2.462

2.457


2.423

2.390


2.358

2.326




63.657

9.925


5.841

4.604


4.032

3.707


3.499

3.355


3.250

3.169


3.106

3.055


3.012

2.977


2.947

2.921


2.898

2.878


2.861

2.845


2.831

2.819


2.807

2.797


2.787

2.779


2.771

2.763


2.756

2.750


2.704

2.660


2.617

2.576




Appendix II to Chapter VII

(Refer Para 2.1.4)
Measurement of Soil Resistivity


  1. Introduction

The study of the problem of low frequency electromagnetic induction between power and telecommunication lines involves knowledge of the resistivity of the earth. The mutual coupling between two earth return circuits depends upon the dispersion and distribution of the ground currents circulating in the circuits. This in turn is largely determined by the earth resistivity. Almost invariably the resistivity of the earth is far from uniform even within a local area and further all known methods of measurement of earth resistivity suffer from various limitations.


A general idea of the order of soil resistivity in any region may be had from the type of soil or geological structure in that area. A number of factors affect the soil resistivity such as moisture, nature and concentration of salts in the soil, temperature, seasonal variations etc. It is not possible to predict to a precise degree the resistivity to be expected in a given area or the electrode resistance at given site. In every case, it is necessary to carry out an actual measurement to determine the earth resistivity in a given area.


  1. Method of Measurement

Several methods of measurement are known but each one suffers from one limitation or other, due to which the different methods are found to give sometimes widely divergent results.


A low frequency test is the most desirable method for determining the earth resistivity, since it gives an overall average figure for the area concerned. Experiments were conducted by the PTCC in the 50s to determine the suitability of different methods and electrode spacing to be used while collecting data of soil resistivity for low frequency induction calculations. The experiments established that the four-electrode method is the most suitable with large electrode spacing of 150 feet. The PTCC practice is therefore to collect soil resistivity data by using this method keeping an inter-electrode spacing of 50 meters.


  1. Four-Electrode Method

Other methods include the use of a search coil but the general preference is for the ‘four electrode method’ explained below using the meggar earth tester.


The familiar earth testing meggar is an instrument, which can be conveniently used for carrying out the four-electrode method of measuring resistivity. It has a generator producing DC at 500 or 1000 volts with the current coils connected in series. The potential coil is mounted on the same shaft as the current coil and has a definite inclination to the latter. The terminals of the current coil and the potential coil are brought out to four independent terminals C1, C2 and P1, P2. Both the potential and current coils tend to rotate in the field of permanent magnet, when the generator is worked, the direction of the movement being opposite to each other. Therefore, when the instrument is connected to a test earth and the generator is worked, the position taken by the combined current-potential coil will be proportional to the ratio of V/I or the mutual resistance between the current and potential circuits.
The meggar is known to give fairly accurate results of earth resistivity. Where the average resistivity over a large area and depth are required, the electrode spacing has to be kept correspondingly high. While there should be no objection to keep such wide spacing with the meggar, the skin effect of the ground currents places a limitation on the spacing that can be used. The results with the meggar are reasonably reliable for electrode spacing up to about 150 to 200 feet.


As indicated in Figure 5, the meggar earth tester has four terminals marked P1, P2, C1, C2 and four similar electrodes are driven into the ground at equal distances of 50 meters in the region where the soil resistivity is to be determined (should be driven about 1 meter depth). If these electrodes are designated as A,B,C, and D, the extreme electrodes A and D should be connected to C1 and C2 of the meggar. The electrodes B and C should be connected to P1 and P2. By operating the meggar handle continuously at uniform speed, we can read the electrode resistance ‘R’ on the meggar scale.


The soil resistivity is given by the equation
ρ = 2 π a R
Where

ρ = Soil resistivity in Ohms/cm³.

R = Meggar reading on Ohms.

a = Distance between two electrodes in Meters.


In one type of instrument currently available, the connections are made as in Figure 6 and the value of soil resistivity is evaluated from the formula.
ohm meters

Where


R = Resistance of ground as measured.

2l = Distance between electrodes A and D in the meters.

2a = Distance between electrodes B and C in meters.
The value in Ohms/cm³ can be obtained by multiplying the result by 100.
In all measurements of soil resistivity, it is necessary to see that these are carried out in the driest part of the year.

CHAPTER VIII
Code of Practice for Protection from Earth Potential Rise


  1. Introduction

When an earth fault occurs in a power system, some of the fault current returns via the earth, through the earthing system (e.g., earthing of towers and power sub-stations etc). This current raises the potential of the earthen system with respect to a remote earth for the duration of the fault. This is known as ‘Earth Potential Rise’. During such a fault, due to the transfer of potential between the EPR areas and outside points, by conductors of telecom circuits and other metallic structures etc, serious hazard may result to telecom installations, telecom personnel and customers.


1.1 Locations Prone to EPR
The following are the locations where Earth Potential Rise may occur:


  1. Areas near power sub-stations earthing system.

  2. Areas near pole mounted sub-station (transformer) on low voltage system.

  3. Areas near power line towers having earth electrodes.




    1. Factors Affecting EPR

Following factors are to be considered for assessment of hazard due to EPR.




  • Type of power network

  • Fault current level

  • Power grounding system (Earth resistance)

  • Soil resistivity

  • Local conditions

Power networks are classified according to the technique used in connecting the neutral point to ground. The grounding network affects both the level and duration of the fault current.


According to CCITT directives.
(i) In case of networks of which the neutral point is earthed directly or through low impedance, the rise in earth potential must be considered and its value must be calculated or measured for the highest value of the current flowing to earth through the earthing system of the electrical installation.


  1. In case of network of which the neutral point is earthed through an arc suppression coil without additional devices for clearing faults, it is not necessary to consider the rise in earth potential as the current flowing through the earthing system is limited to a very low value by the arc suppression coil.




  1. When the neutral point is earthed through an arc suppression coil with additional device for clearing the faults, the earth potential needs to be considered only in those generating stations or sub-stations, where the device is fitted and should be determined as in (i) above.

It can be assumed that the additional device operates only rarely and does not justify the whole of a system protected in this way being regarded as if all the neutral points were permanently earthed.




  1. In case of network of which the neutral point is isolated the rise in earth potential is to be considered only in case of network of large extent, which may give rise to a very large capacity current in the fault (more than several hundred amperes); further the rise in potential occurs only in neighborhood of the fault and in consequence, there is no need to consider it in the case of generating stations or sub-stations, except when the fault occurs within the station.




  1. EPR Limits and hazard zone

The EPR contours which define hazard zone for the telecom plants are as given below:




Sl. No.

Type of Telecommunication Plant

Type of Power System

High Reliability Lines

Other Lines

1.

2.



3.

Terminal apparatus, joints, cabinets, pillars, manholes, pits, poles
Telephone exchanges

Cables


  1. Metal Sheathed




  1. Plastic insulated and

plastic sheathed, PIJF

cable

650V

430V



650V

7 KV

430V

430V



430V

7 KV




2.1 Zone of EPR
The zone of EPR near an earthing system varies from some tens to some thousands of metres, depending on soil resistivity, layout of the earth electrode, power network, fault current levels and other local conditions. The zone of EPR in urban areas is small compared to rural areas. Only EPR zones having potential higher than the values given above in 2.0 are considered as dangerous.
The power supply authority will in all cases be able to provide the value of the earthing system fault current at any location, the resistance of the earthing system and the soil resistivity of the area. It can also supply the data regarding the magnitude of EPR and the extent of hazard zone occurring at these fault locations.
Measurements and calculations of EPR zone are normally done by Power authorities. In order to ensure the safety of telecom assets and personnel Telecom authorities should ascertain the EPR hazard zone from the concerned Power authorities.


  1. Measurement of EPR Zone


3.1 Theoretical Formula for Measurement of EPR zone
The distance X in meters at which the ERP may rise to a value Ex can be determined from the formula.
metres
Where,

ρ = Soil resistivity in ohm meter.

L = Fault current in Amps.

Ex = EPR voltage (limit as specified in 2.0 above for a particular

telecom installation).
Above formula gives good results only in case of simple earthing systems such as single electrode earthing systems. As the area of earthing system increases the error increases rapidly.
For larger earthing systems the more practical formula would be

Where Ex = The potential at radial distance ‘d’ from the perimeter of the earth mat in volts.
l = the maximum fault current through the earth mat in Amps.

R = the measured resistance of the earth mat in ohms.

D = Half the diagonal distance of the mat in meters.

d = Distance in meters from the perimeter of the earth mat.


In those cases, when the earth mat is not essentially rectangular in shape, the value of ‘D’ should be half length of the diagonal of a square, having the same area as that enclosed by the irregular earth mat.
Substituting the EPR voltage limit for Ex the distance ‘d’ of that voltage contour from the perimeter of the earth mat in meters can be calculated from the above formula.
Example
As given by the Power authorities for a power sub-station.

R = 1 ohm

Note: The value of earth resistance of a sub-station is normally less than 1 Ohms; however this may vary depending on the size of the sub-station. The exact value of earth resistance can be obtained from Power authorities.
l = 5 KA (after considering necessary screening factors).
Dimensions of earth mat = 20x25 meters.
Solution
Half the diagonal distance of earth mat


Ex = 430 V ( for a telephone exchange as per Para 2.0)

d + 16 =


= 186.046 (say 187 meters)
d = 187 – 16 = 171 meters
Conclusion

The effective EPR zone is 171 meters from the perimeter of the earth mat of the given power sub-station.




    1. Practical Measurement of EPR Zone

In theoretical formula uniform earth resistivity is assumed. In practice however the earth resistivity is rarely uniform and equi-potential contours around an earthing system are often distorted away from the postulated circular shape for the hemispherical electrode because of the rectangular shape of the station earthing system and other earthed conductors such as metallic sheathed cables, water or gas pipes etc buried in the earth in the vicinity. For an accurate determination of a particular EPR hazard zone, a practical test is therefore necessary, which will have to be conducted by Power authorities and co-ordinated by Telecom authorities.


Where the calculations by above formula indicate that there is a border line problem or if there is a doubt about the reliability of the parameter values used in the calculation, the hazard zone may be determined by practical measurements.
Because of the costs involved, practical measurements cannot be made in each and every case under consideration and it may be more economical instead to institute protective measures suggested in the following paras.


  1. Protection Measures


4.1 Co-ordination with Power authorities
(i) By proper co-ordination with power authorities at various levels it may be ensured that the earth resistance of the power earthing systems are maintained within limits and they are installed as far away as possible from the telecom installations and cables, so that the effect of EPR is not there on the telecom circuits. It should also be ensured by Power authorities that the protective systems operate properly and fault tripping timings are maintained within limits.
(ii) In case of new installations the effective zone of influence of EPR may be ascertained from concerned Power authorities so as to ensure safe separation at the initial stage itself.


    1. Minimum Separation for Telecom Cables in the Soil

In the absence of other experiences, local measurement or calculated values of EPR, the following minimum separations in soil between telecom cable with a metal sheath in direct contact with the soil and a high voltage power earthing system should be observed.


Separation in soil (in meters) between telecom cables and high voltage earthing systems beyond which no calculation or measurement is necessary:


Earth Resistivity in

Ohm Meters



Power Network System with




Isolated Neutral or Arc Suppression Coil

Directly Earthed Neutral

Location


< 50

2

5

Urban

5

10

Rural

50 - 500

5

10

Urban

10

20

Rural

500 - 5000

10

50

Urban

20

100

Rural




10

50

Urban




20

100 – 200*

Rural

*200 meters in areas with extremely severe soil conditions i.e. 10,000 ohm meters

In the case of tower earthing, half the above distances can be used, if the power lines include earth wires.


Where the local situations do not permit such separation, the telecom cables should be provided with insulation, for example by placing the cables in insulated plastic tubes in the hazard zone or using plastic sheathed cable. When the magnitude of hazard is extremely high, optical fiber cables or radio relay systems may be used instead of metallic cables.

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