|Part I Gives graphs for determining separation necessary between power and telecommunication lines, to keep the induction within the prescribed limit (430V). This separation is referred to as minimum safe separation.
Part II Gives graphs showing S/Sm Vs Induced Voltages (in units of 430V), for estimating induced voltages where the minimum safe separations prescribed in Part I cannot be achieved in practice. This will enable provision of necessary protection on telecommunication circuits.
Part III Gives details about use of protection measures to be adopted in cases of voltages exceeding the prescribed limits.
Part IV Gives method of estimation of induced voltage where tap lines, more than one power line and extensions to existing power lines are involved.
Part V Explains the step-by-step procedure for examination of individual cases of parallelism.
4. In the preparation of this procedure, it has been assumed that the power lines would be essentially radial ones i.e. the sources of supply would be only at one end. This assumption is justified, as the bulk of the power lines in this voltage category would be operated in this manner. Infinite bus has been assumed at nearest extra high voltage grid sub-station at which the voltage level of 11, 22 or 33 KV is created. This assumption is also justified as the error creeping in by neglecting the system impedances behind the extra high voltage bus is insignificant. Further, the error, if any, would be on the safe side. In the preparation of the graphs for estimation of fault currents a fault resistance of 20 ohms in conformity with the recommendation of the latest CCITT directives has been taken into account. For estimating mutual coupling between power and telecommunication lines, Carson curves have been used.
To Determine Minimum Safe Separation Necessary Between
Power and Telecommunication Lines to Keep the Low Frequency Induction within the Prescribed Limit of 430 Volts
1. Plate 1 (a and b) in Appendix III to Chapter IV gives a set of graphs for determining minimum safe separation i.e. the separation necessary to keep the low frequency induction within the prescribed limit of 430 volts. For these curves the abscissa represents the product of fault current in amps causing the induction and length of parallelism in Kms, while the ordinate represents the minimum safe separation in meters. The soil resistivity is a parameter and separate curves have been given for soil resistivity of:
(i) 1,000 Ohms-cm. (ii) 3,000 Ohms-cm
(iii) 5,000 Ohms-cm (iv) 7,500 Ohms-cm
(v) 10,000 Ohms-cm (vi) 15,000 Ohms-cm
(vii) 20,000 Ohms-cm (viii) 50,000 Ohms-cm
2. For using the curves in Plate 1, knowledge of fault current on the power circuit causing the induction is necessary. This can be determined for any situation from the graphs given in Plates 2(a) to 2(c) in Appendix III to Chapter IV. For using the graphs given in Plates 2 (a) to 2(c), the following basic information of the power circuit is necessary:
Voltage class of the power line - 11, 22, or 33 KV.
Size of conductor used for the power line.
Total capacity of the step-down transformer in MVA at the main grid sub-station creating the voltage of the power line.
Distance of the point of fault (end of parallelism) from the main grid sub-station in Kms.
3. For 11 KV lines the effect of the capacity of the main step-down transformer is insignificant and hence only the conductor size is the parameter in the graphs given in Plate 2(c). Conductor size of 0.03 sq.in.cu.eq. ACSR, 0.04 sq.in.cu.eq. ACSR and 0.06 sq.in.cu.eq. ACSR only have been covered, being popular sizes.
The capacity of the main step-down transformer has some effect in the case of 22 KV and 33 KV lines. Hence, in Plates 2 (a) & 2 (b) graphs have been drawn for the capacities of step-down transformer, viz.5 MVA and 30 MVA for each of the sizes of conductor. The curve for
K = 30 MVA
can be used for all cases where the capacity of the step-down transformer exceeds 30 MVA. Similarly the curve for
K = 5 MVA
can be used for transformer capacities of 5 MVA and below. For values of transformer capacities between 5 MVA and 30 MVA, suitable interpolation should be made.
4. In practice in any parallelism section the separating distance between the power line and the telecommunication line will not be same throughout. In such cases it is necessary to determine the average separation. The method to be adopted for determining the average separation is given in Appendix I to Chapter IV.
5. A knowledge of the soil resistivity for determining safe separation in any situation is very essential. The soil resistivity at any place can be measured by means of the Evershed Earth Tester. The method of determining soil resistivity by means of Ever shed Earth Tester is explained in Appendix II to Chapter IV.
The method of determining safe separation for any situation has been demonstrated by means of the following examples:
Figures 1 shows the schematic of a 33 KV S/C line using 0.06 sq.in.cu.eq. ACSR. At the main grid sub-station, at ‘A’, there are two numbers 4 MVA, 132/33 KV transformers. In the route of this power line, parallelism with communication circuits occurs in the section BC. Determine the safe separation. The soil resistivity of the place is of the order of 10,000 ohms/cm³.
18.9KM 40KM 13KM 10KM
A B C D E
2 x 4 MVA
Voltage class of the power line = 33 KV
Size of conductor =0.06 sq.in.cu.eq. ACSR
Step-down transformer capacity = 2x4 = 8 MVA
Distance of Point ‘C (end of parallelism) from
main step-down ‘A’ = 58.9 kms
Referring to Plate 2(a), corresponding to the above conditions on the power circuit,
Fault current causing induction = 230 Amps
Length of Parallelism = 40 Kms
Product of fault current and length Of parallelism = 230 x 40
= 9200 Amp Kms
Referring to Plate 1 (a), the minimum safe separation, corresponding to soil resistivity of 10,000 ohm/cm³ for Amp Km of 9200, Sm = 720 meters
Figure 2 shows the schematic diagram of an 11 KV S/C line using 0.04 sq.in.cu.eq. ACSR. At the main grid sub-station there are two numbers 7.5 MVA, 132 / 11 KV transformers. In the route of this 11 KV power line, parallelism with communication circuits occurs in the section CD. Determine safe separation, if the soil resistivity of the region is of the order of 5,000 ohms/cm³.
5 KM 10KM 18KM
A B C E
Voltage class of the power line = 11 KV
Size of conductor = 0.04 sq.in.cu.eq. ACSR
Step-down transformer capacity = 2x7.5 = 15 MVA
Distance of point of fault (end of parallelism)
from main step-down transformer = 30 kms
Referring to Plate 2(c), corresponding to the above,
Fault current causing induction = 122 Amps
Length of Parallelism = 15 Kms
Product of fault current and length of
parallelism = 122x15
= 1830 Amp Kms
From Plate 1(b), the minimum safe separation,
corresponding to 1830 Amps Kms and soil
resistivity of 5000 ohms/Cm³, Sm = 21.5 meters
NOTE: When separations required are very small, it is necessary to ensure that it is more than 1 and ½ times the height of the power supports.
Figure 3 shows the schematic diagram of an electrical system. For the 11 KV S/C line, 0.03 sq.in.cu.eq. ACSR is used. At the grid sub-station creating 11 KV there are two numbers 1 MVA, 33/11 KV transformers. For power parallelism in section CE, determine safe separation. The soil resistivity in the region is about 20,000 ohms/cm³.
2 x 4 MVA 2 x 1 MVA
A B C E
132KV 33KV 11KV
Voltage class of the power line = 11 KV
Size of conductor = 0.03 sq.in.cu.eq. ACSR
Step-down transformer capacity = 2x1 = 2 MVA
Distance of point of fault (end of parallelism)
from main grid sub-station = 20 kms
Referring to Plate 2(c), corresponding to the above,
Fault current causing induction = 142 Amps
Product of fault current and length of parallelism= 142x20
= 2840 Amp Kms
From Plate 1(a), the minimum safe separation,
corresponding to 2840 Amps Kms and soil
resistivity of 20,000 ohms/Cm³, Sm = 130 meters
To estimate induced voltage on Telecommunication Circuits in situation where the average separation that can be maintained under field conditions, is less than minimum safe separation obtain from Plate1
Very often situations arise where the minimum safe separation as determined by Plate-1, cannot be maintained due to difficult terrain conditions or problems of maintenance. In such situations, the power and telecommunication engineers should investigate the maximum possible separation that can be maintained under the field conditions and determine the possible induction on the telecommunication circuit under that condition. This is required to determine the protective measures necessary for the telecommunication circuits. Plates 3 (a) to 3 (i) in Appendix III to Chapter IV; give graphs for determining induction on telecommunication circuits for the separations other than the minimum values. In these graphs the abscissa of X-axis represents the ratio of the actual average separation that can be maintained under field conditions (S) to minimum safe separation (Sm) determined from Plate 1, while the ordinate or Y-axis represents the actual induced voltage in units of 430 volts. The Plates 3 (a) to 3 (i) cover graphs for various values of soil resistivity. In each of these graphs, the minimum safe separation (Sm), is a parameter.
The use of these graphs has been demonstrated by the following examples:
In the case of Example I, if the field conditions limit the actual separation to 450 meters, determine the induction on the paralleling telecommunication circuit.
From Example I
= Minimum safe separation
= 720 meters
S = Average separation that can be actually maintained
= 450 meters
From Plate 3 (e), (for soil resistivity = 10,000ohms/cm³) corresponding to = 720 meters and ,
Induced Voltage = 1.53 x 430
= 656 Volts.
= Minimum safe separation
= 130 meters
S = Actual separation that can be maintained
= 95 meters
From Plate 3 (g), (for soil resistivity of 20,000ohms/cm³) corresponding to =130 meters and = 0.73,
Induced Voltage = 1.11 x 430
= 477 Volts.
Protection of Telecommunication Circuits in Cases of
Voltages Exceeding the Prescribed Limits
In cases where the induced voltage on the telecom circuits, as determined from Plates 3(a) to 3(i) exceeds the prescribed safe limit of 430 volts, it is necessary to consider various protection measures so that the equipment installed and the personnel working on these circuits are not subjected to the influence of hazardous potentials.
One of the measures commonly adopted for protection is the use of three electrode Gas Discharge (GD) tubes. Two electrodes of the tube are connected to the wires of a telephone pair and the third electrode to the earth, through the earth cap. Under normal conditions, the telecom line is kept insulated. The gap breaks-down and the telecom line is virtually earthed, when induced voltage excess the pre-determined value (250V). Through the discharge path, the earth connection to the tube should be of very low resistance so that the voltage across the tube is restricted to safer values.
The following example illustrates the use of GD tubes to restrict the induction to safe values.
In order to reduce the induced voltage of 656 volts to safe value in Example IV, determine the number of GD tubes to be installed on telecom line. The number of GD tubes is calculated as below:
` = 2.2
= 3 (rounded off to the next higher digit).
Two GD tubes should be installed at the ends of telecom lines in the paralleling section and one in the middle of these two.
In case the number of GD tubes worked out are more than three, two of these should be fitted at each end of the paralleling section of the telecom lines and the rest of the GD tubes be fitted in between space at equal intervals.
GD tubes worked out by the above method should be fitted on all the telecom wires in the paralleling section.
Estimation of Induced Voltage where
(a) Tap lines are involved;
(b) More than one power line in parallelism with DOT line; and
(c) Extensions arise to existing lines.
The procedure for estimation of induced voltages on telecom lines paralleling tap lines is illustrated by Example II given under part I. Here, CD can be considered as a tap line branching off from the main line AE at C (Figure 2). Parallelism occurs in section CD and estimation of the induced voltage in this case is clarified by Example-II.
More than one Power Line in Parallelism with DOT line
In all calculations of induced voltage, it is assumed that the faults on all the paralleling power lines do not occur simultaneously. With this assumption, it is possible to estimate the induction on a given section of telecom line due to different paralleling power lines individually. The final value of induced voltage to be considered is the severest of the estimated value of induced voltage due to the different power lines.
Extension Arise to Existing Lines
The estimation of induced voltage in cases where extensions arise to an existing line is illustrated by Example VII below.
Let us consider that in Example I, in Part I, the line is extended to E for a distance of 10 Kms using the same conductor and that parallelism occurs in the section DE (Figure 1).
Distance of Point ‘E’
From step-down station ‘A’ = 81.9 Kms
From Plate 2 (a), fault current = 176 Amps
Length of parallelism = 10 Kms
Product of fault current and
length of parallelism = 1760 Amp Kms
From Plate 1(b), for soil resistivity of 10,000 ohms/cm³
Safe separating distance = 24 meters
Average separation = S (say)
Average separation S can be calculated with the help of Appendix I to Chapter IV. After calculating average separation the ratio of average separation to safe separation should be calculated and from this ratio induced voltage can be found out from the Plate 3(e) (for soil resistivity of 10,000 ohms/cm³).
Step by Step Procedure for Examination of Individual Cases
Procedure to be followed for New Power Line Constructions
The Executive Engineer or the SDO in-charge of the power line construction should carry out the following:
Survey the route for investigating the maximum separations that field conditions permit. Once knowledge of separations that can be maintained in different portions is available, the average separation for the entire parallelism stretch can be estimated as per the procedure given in Appendix I to Chapter IV.
Measure soil resistivity at a few places by means of Evershed Earth Tester. The method has been explained in Appendix II to Chapter IV. As many measurements as possible along the route should be taken for soil resistivity. In any case, it should not be less than three. There will be variation in the value of soil resistivity from point to point. The value to be chosen depends on the general trend. This has been demonstrated by means of an example below. It is important to measure the soil resistivity during the dry season.
The soil resistivity survey based on 160 feet or 50 meters inter electrode spacing, reveals the following information.
Location of Measurement
Meggar Reading (R)
Soil Resistivity (2πaR) Ohms/cm³
From the above survey, it can be concluded that this region is of a soil resistivity of 50,000 ohms/cm³.
Calculate the value of fault current for a single line to ground fault on the power line at a point corresponding to the end of parallelism. For this purpose, use Plate 2(c) for 11 KV, Plate 2(b) for 22 KV and Plate 2(a) for 33 KV lines. The basic data of the power system required for calculation of fault current are:
Operating voltage of the power line.
Conductor size of the power line.
Capacity of main step-down transformer.
The distance of the point of fault (end of parallelism) from the main step-down station.
Determine the minimum safe separation necessary corresponding to:
Product of fault current and length of parallelism.
If the minimum safe separation obtained in (d) above is less than the average separation determined in (a) above, the route as surveyed can be adopted.
If the minimum safe separation in (d) above is more than the average separation in (a) above, then estimate the induced voltage on telecommunication circuits from Plates 3 (a) to 3(i) corresponding to:
Refer the proposals for the approval of the route of the power line to the State Level Power & Telecommunication Co-ordination Committee (SLPTCC). While referring the case, forward:
(i) A route map of the power line, drawn to a scale 1˝ = 1 mile or 1 cm = 0.5 Kms, showing separations between the power line and telecommunication line also to the same scale.
(ii) Results of the calculations in (a) to (f) above.
Procedure for New Constructions in respect of Telecommunication Lines
The construction officer in-charge of telecommunication line construction should do the following:
Survey the route for investigating the maximum separations that field conditions permit and prepare a route map showing the separations.
Measure soil resistivity at new places by means of Evershed Earth Tester.
Refer to Section C of Chapter III for further detailed procedure in respect of telecom lines.
Appendix I to Chapter IV
(Refer Para 4 in Part I)
Procedure for Estimating Average Separation Between
Power and Telecommunication Lines in Any Parallelism Section
In practice the routes of the power line and the paralleling telecommunication line will be such that the exposure in the entire stretch of parallelism will not be uniform. Consider any parallelism section, as for instance, the one shown in Figure 4. The power line can be deemed to be made up of a large number of small sub-sections. Let d1, d2….. dn be the lengths of the various sub-sections. With respect to these small sub-sections, the exposure between power and communication circuits can be reasonably assumed to be uniform. In other words, the separating distance between the two systems can be assumed to be uniform with respect to this sub-section. Let S1, S2….. Sn be the separating distances corresponding to the sub-section d1, d2….. dn respectively.
S = Equipment weighted average separation for the entire parallelism section.
= Total length of parallelism.
Then we can write
The method of determining equivalent average separation for any parallelism section is explained in Example IX. In Figure 5 values of for various values of have been given to facilitate quick calculation.