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FOCL projects

EXPEIENCE IN DESIGNING THE MECHANICAL PART OF FOCL

Additionally, see LineS User Guide. Description of application in projects.
This page of the site contains examples of using different programs for different solutions in mounting fiber optic communication lines’ cables and recommendations on source data input.

 

Attention!
In case a fiber optic communication line(FOCL) is mounted on an existing overhead line (FOCL OHL) the first program to use for calculation should be LineMount (LineMountCad), provided the cable is selected correctly. At the same time, the calculation takes into account the existing wire of the overhead power transmission line and the mounted fiber optic cable, self-supporting optical cable of optical ground-wire cable.

Recommendations on source data input

Attention!
According to 2.5.185 of EIR, the mechanical calculation of optical ground wire cable, optical phase wire cable, optical self-supporting cable should be performed for reference loads, based on the permissible tension method, following the rest of the requirements for wires and cords of overhead lines, see the section of EIR in “Fiber optic communication lines on overhead lines”.

1. Basic information for mounting a fiber optic cable to existing overhead lines (FOCL OHL)

When mounting a fiber optic cable to existing overhead power transmission lines, it is necessary to know the following (the first 3 points are a must):

1. Typical supports projects of the overhead lines, correspondingly, the permissible tension affecting the supports, especially dead-end supports and strain-angle supports.

2. Types and sectional area of wires mounted on the overhead lines.

3. Numbers of supports, type of supports according to the type of wire attachment – strain, intermediate supports.

4. Measurements of sag of the lowest wire on the existing lines for the selected spans, accessible for taking the measurements, for each strain sector with the temperature of the measurement (the measure that is desirable and useful for providing reliability and better quality of the project).

To get the data on existing overhead lines, the data from the owners of the existing electric networks should be used, the organizations exploiting and maintaining the overhead lines. The measurements are taken with the help of instruments.

Typical projects of supports of an overhead power transmission lines usually contain information of the permissible tension (stress) for the phase of the power transmission line. Depending on the line being one-chain or two-chains (3 or 6 wires of phases), the overall stress affecting the supports is the combination of loads to a phase, for overhead lines of 35kV and more, the ground wires are also taken into consideration.

Mounting a fiber optic cable (FOCL) always creates additional load on supports, as compared to the data of typical supports’ projects, this is why the stress (tension) in the fiber optic cable should be minimal and well-justified. In case of mounting a fiber optic cable to supports of 0.4 kV when the typical project of supports took into account possible mounting of radio lines, but the option was not used, the fiber optic cable could replace the loads of the radio line.

The thing to remember is not to load the supports with additional unjustified loads produced by the strass from a fiber optic cable. It is necessary to consider that, if portals of substations are used for mounting the fiber optic cable, the portals does not allow for any serious loads. Usually, the portals are designed for the stress of, depending on the peaks' constructions, 70-150 – 450 kg (daN) to a phase, rarely for larger loads. Correspondingly, the descents from supports to portals are loaded not according to the loads on a support, but according to the loads on a portal. The same situation is observed  with the load-bearing capacity of constructions in walls in case the cable enters a building.
(Let there be a cable with the sectional area of 140mm2, a portal with vibrated reinforced-concrete peaks with the permissible load for a phase of 150kg. The maximum permissible tension is 150kg (daN)/140mm2=1.07 daN/mm2. At average annual temperature, the tension could change from 0.6 to 1.0daN/mm2. It is desirable to make the load on the portal even less than that. The mounting (operating) sag of the fiber optic cable should not be less than 0.3-0.5m).

The permissible stress for the strength of the fiber optic cable is usually denoted in the type of the wire in kN. The permissible stress (tension) affecting the fiber optic cable according to the manufacturer’s data is just a reference for rough calculation. There is no sense in mounting the cable to its full-extent stress. This calls for an optimal solution – decreasing the tension in the cable to such a degree that the sag of the self-supporting optical cable and the wire of the overhead line are compatible (the sag of cable is roughly equal or a bit less than that of the wire). In case of optical ground-wire cable it is necessary to take into account the permissible distance between the wire and the cord in the span, according to table 2.5.16 of EIR (the sag of optical ground-wire cable will be less than that of the wire, but not more, to provide the distance between the wire and the cord in the span according to EIR).
The wires of the overhead lines could be mounted with the tension which should not exceed that denoted in 2.5.7 table of EIR. However, these are the values for supports that allow for such loads. For supports of an overhead line of, say, 10 and 0.4 kV, as well as for some overhead lines of 35kV, the wires could be mounted with the tension denoted in 2.5.7 table of EIR. In the same way, the wires of more capacity should not be mounted with the tensions denoted in 2.5.7 table of EIR if they have larger sectional areas than those denoted in the typical project of supports. In this case, the tension should be decreased not to exceed the permissible stress on supports. The stresses (tension) are defined in this case depending on the loads permissible for the supports of overhead lines and portals, which are taken according to the typical projects of supports, portals, and they are lower that those denoted in 2.5.7 table of EIR.

Conclusion. To define the permissible tension in a fiber optic cable (self-supporting optical cable and optical ground-wire cable) it is required to perform simultaneous calculation of mounting stress and sag of the wire (“Wire” field in the program) comparing their sag.
At the same time, the permissible tension in the wire are taken according to EIR or lower and those denoted in typical projects of supports азк overhead lines (tension according to EIR – for strong supports of OHL, for weak constructions of supports this tension must not be used, it should be taken from typical projects of supports). An ideal way is to take measurements (additional expenditures in the project) of the wire’s sag at particular temperatures for well-known manufacturers and sectional areas of wires in the known spans of the existing overhead lines and sort out the permissible tensions in LineMount program based on mounting temperatures, and used the found tension in calculations. If it is impossible to take the measurements, the tension is taken according to EIR and typical projects.

The permissible tension in a fiber optic cable is sorted out by decreasing the value denoted in the type of the cable by its manufacturer.

(For example, the permissible operating tension of the cable is 10.0kN (denoted in the type of the cable). This would be 1000daN, which is approximately 1000kg (the difference in daN and kg is not essential for the calculation). Let the cross section area of the solid part of the cable be 140mm2 (the area of hydrophobic filler and space are not taken into account) the sectional area could be calculated according to the cable diameter, but there will be some, permitted deviation from actual calculation). Given the permissible tension and sectional area, we define the maximum permissible tension for the cable 100daN/140mm2=7.14daN/mm2. The tension at average annual temperature is taken as 60% form the maximum – 4.3 daN/mm2, if the manufacturer's data do not give a definite velue for stress according to which the tension is defined.
So, the maximum mechanical tension in the cable is 7.1 daN/mm2, at average annual temperature – 4.3 daN/mm2. It is not necessary to make this more precise that two figures after the decimal point. These values for the cable are input into the program and the calculation is performed both for the wire and the cable.

If the cable is selected correctly according to the permissible tension and possible span lengths, the first calculation will show that the sag of the cable is much less than that of the wire, provided the span lengths are the same. This is not required for an optical self-supporting cable. For an optical self-supporting cable it is enough to have roughly equal sag with the wires of the overhead line. That is – it is required to decrease the permissible tension in the cable for this project, making it a lot less than it is allowed by the cable construction. As a result, the tension affecting the supports will decrease from 1000kg to an optimal value, which will make the sag of the wire and the cable compatible (this could be achieved by decreasing the maximum stress and tension at average annual temperature for a particular project, sorted out).
If the sag of an optical self-supporting cable in large spans is bigger than that of the wire at the maximum stress (manufacturer’s data), it is necessary to take the cable designed for a greater stress and sort the tension out once again. The stronger is the cable, for example, not for 10.0kN, but for 15kN, etc., the easier it is to provide smaller sag of the optical self-supporting cable for the same conditions and lower the loads on supports. But the cable will cost more.

Bigger values for stress are taken for an optical self-supporting cable, but for an optical ground-wire cable the insufficient sag of the cable as compared to that of the wire is not necessary as well. It is necessary only to provide the distance between the wire and the cord in the span according to table 2.5.16 of EIR, considering the distance between the wire and the cord on supports.

After sorting out the maximum permissible tension in the fiber optic cable at average annual temperature for the project,  the final note on accepted tension is made to the project:

For optical self-supporting cable: “To decrease the loads affecting the supports, the tension in the fiber optic cable (optical self-supporting cable) is taken by comparing the sag of the fiber optic cable and that of the wires of the overhead line, verified in loading modes by the program for mechanical calculation for sector… maximum permissible tension - … daN/mm2, at average annual temperature - …daN/mm2; for sector… etc.”

For optical ground-wire cable: “With the purpose of decreasing the loads affecting the supports and peaks, the tension in fiber optic cable (optical ground-wire cable) are taken considering the distance between the cord and the wire in the span according to table 2.5.16 of EIR, verified in loading modes and for minimal distance between the wire and the cord (in case of ice accumulation on the cord and absence of ice on the wires) in mechanical calculation program for the sector… maximum permissible tension - … daN/mm2, at average annual temperature - …daN/mm2; for sector… etc.”

The basic justifying calculation is done by LineMount(Cad, damp), the results are verified by LineMech(Cad). The tension sorted out and accepted for mounting the fiber optic cable in LineMount(Cad) is provided for calculation. Sometimes, after verifying the data in loading modes (clearances in case of ice accumulation, increase in the distance between the wire and cord because of unacceptable distance between the wire and earth parts, especially the cord in case of ice accumulation on the cord (optical ground-wire cable) and the absence of ice in the wire of the overhead line), it is required some justified increase of the tension in fiber optic cable to provide the clearances and distances. All this should be calculated.
It is possible upon the requests of the owners of crossed objects, for example, of federal highways, the calculation of clearances between the fiber optic cable and the crossed object at different climatic conditions could be required by LineCross(Cad) program. The calculation of the crossing is performed (for the fiber optic cable it is harder to provide the clearance in case of ice accumulation without wind), the draft of the crossing at a particular scale is prepared (by the program, ported to AutoCad) and attached to the project.

For important FOCL it could be necessary to calculate the tearing forces for intermediate attachment fitting of the wire by LineLoad program. After calculating the tearing forces some changes could be required for the FOCL project in all the programs. See the User Guide for LineLoad for more details.
The tension accepted for the project in fiber optic cable for different sectors of FOCL could differ depending on the span lengths of the overhead line, in sectors of descents from supports to portals and other constructions (walls of buildings, etc.).

The basic thing is that, when comparing the sag of the wire in the program for calculating mounting stresses and sag, the stresses and sag for mounting the fiber optic cable is received as well. The simultaneous calculation of the wire and cable in LineMount(Cad, damp) is justifying material provided for resolution and project expertize. Additionally, the program prepares the Register for mounting the FOCL on the overhead line, the vibration dampers are calculated, the support-by-support scheme of FOCL OHL is ported to AutoCad.

Several overhead power transmission lines could come along the route of the fiber optic cable, having different nominal tensions of the overhead lines and different supports. Correspondingly, there will be different sectors of fiber optic cable with different mechanical tension. The justification of the tension in the sectors and mounting tables and sag, at the same time, could be received within one project, one calculation.

The programs allow minimizing the loads on constructions when mounting the fiber optic cable. The loads themselves (maximum stress, the tension is given, sorted out for the project) are present in the calculation, loads per unit length, reference loads from own weight of the wire, ice, wind, temperature, stress are calculated by LineMech(Cad).

The influence of the loads affecting the load-bearing constructions (supports, portals, wall attachments, etc.) minimized by the programs are evaluated by a specialist, professional constructor.

Separate data and some details on FOCL OHL are given in sections of the UserGuide.

2. “Stand-alone” mounting of FOCL cable

When a fiber optic cable is mounted separately, not on supports of an overhead power transmission line, the approach to its design is the same as for mounting an optical self-supporting cable (see point 2 of this page), with some of the limitations removed, including those of the cable construction. In this case, it is necessary to respect the clearances between the cable itself and the ground and a crossed object. The calculation is the same. If it is impossible to mount the self-supporting cable in some strain spans due to its length and clearance, it is necessary to mount the cable on a steel strand. The calculation for such sectors should be done separately, not according to the data on the cable, but on the data of the steel strand with the cable. The cable itself will not bear any load, the calculation will be done based on the load-bearing capacity of the strand (it is necessary to consider the sectional area, modulus of elasticity and linear thermal expansion coefficient for the strand only, and the equivalent diameter and weight per unit length  both of the strand and the cable). It won’t be necessary to switch to a different cable with a greater tensile strength.

Example. Mounting FOCL cable across the street.

1. Selects the cable recommended based on mechanical strength for the span and you’ll have the permissible stress for the cable. To receive the mechanical characteristics of the cable it is necessary to inquire them from the manufacturer. It is better to ask for the data on the cables of different strength.

2. Ask the designers-constructors for the permissible operating load (not breaking load) for mounting the strain attachment to the walls of apparatus floors – the lowest value will be the permissible stress on load-bearing constructions.

3. Perform calculation in LineMech for the lowest stress permissible for load-bearing constructions and for the cable. The value for stress was divided by the sectional area of the cable and got the maximum permissible tension input in the source data of the program. To perform calculation, input, for example, the minimum span that is a bit less than the actual span length in whole numbers, the maximum is little bit larger and the pitch of 1 meter. The program performs the calculation for groups of spans on even terrain with the same height of wire attachment – see the equivalent spans over the first building.

The theory of mechanical calculation and an example of “manual” calculation are given on “Theory and Practice” page.

3.1. Based on the calculation results for the span that is closer to a real one, we get the sag at different modes. The maximum sag could be either in case of ice accumulation without wind (mode 2) or at maximum temperature (mode 7). For self-supporting communication cables the maximum sag most often occurs in case of ice accumulation (mode 2). Base on this we regard the clearance with the rim of the roof on the first building.

3.2. If it is obvious that the clearances are not provided (sagging curve 3), there are two possible variants:
a) the permissible tension was taken based on tensile strength of the attachment – give the task to the builders to strengthen the attachment of the cable in the wall of the apparatus floor. Given the new tension, go back to step 3.

b) the permissible tension was taken based on the tensile strength of the cable – use the cable of a higher mechanical strength, go back to step 3.

3.3. It is obvious that there is a great reserve of the clearance (sagging curve 1).
To lower the loads on the attachment fittings of the cable, decrease the permissible tension in the cable. Perform one more calculation in LineMech program. The data are the same, but the permissible tension is a little bit less. Verify the sag once again to see that the clearances are provided (sagging curve 2).

3a. The sagging curves of the cable could be acquired with the help of new modification of LineMech – LineMechCad, which creates the supports-spotting patterns of the line route profile and the sagging curves of wires, cords and cables in AutoCad (not earlier than 2007 version). This allows to visually check out the clearance on the line route profile at different temperatures. Recently, it has become customary thing to see not only the supports spotting on the profile, but the sagging curves of the wires, cords, self-supporting  cables for the entire route.

4. Perform calculation of the clearance between the FOCL and the rim of the roof of the right building in  LineCross program. At the same time, the climatic conditions should be the same as in LineMech calculation, and the permissible tensions should be the ones accepted for the previous calculation (see 3.3.). The exact values should be given for the span length, height of attachment, marks, and the distance to the crossing from the left point of attachment of the cable. As well as the operating temperature of the crossing with the rim of the roof and the temperature of ice accumulation and the maximum temperature (for the maximum sagging curve form LineMech calculation). Perform calculation.

The theory of crossings’ “manual” calculation and some explanations for the program are given on “Theory and Practice” page.

4.1. It is necessary that the cable does not touch the rim of the roof at any conditions (modes 2 and 7 of LineMech). The clearance of at least 5-7cm should be provided.
Variants:

a) If there is some reserve of the clearance, it should be decreased to lower the permissible tension in the cable;
b) If there is no clearance – increase the permissible tension in the cable.

4.2. In case the clearance is provided, leave it at that.
Note for the project:
“To decrease the loads on load-bearing constructions, the project accepts for the selected cable (type of the cable), given the necessary clearances and permissible loads on the maximum tension of …daN/mm2 (stress – daN or kN), the permissible tension at annual temperature of … daN/mm2”. Supply the project with the corresponding calculation result by LineMech and the draft of the crossing with the reference clearance, the respective sag and the calculation result by LineCross.

4а. Exept for the tables’ data of the calculation results for the clearance in LineCrossCad modification in AutoCad (not earlier than 2007 version), the draft of the crossing is prepared with the sagging curve at the required temperature at a particular scale with all the data necessary for the crossing’s draft.

5. To make the calculation results realize in practice, it is necessary to mount the cable with the tension corresponding to the calculation results in the program for calculating mounting stresses and sag – LineMount. The program uses the last justified values for permissible tension and the same climatic conditions. The actual span length should be identified, in this case, the strain attachment fitting for the cable should be chosen, and the direction to attest the span should be given. As the result of the calculation, we get the mounting stresses and sag at different temperatures of mounting.

When mounting the cable at the measured air temperature, it is necessary to provide either the stress with the help of dynamometer or the sag from the calculation. The program gives the values for a set of temperatures of -30, -20, -10, 0, +10, +20, +30, +40C (the temperatures allowed for mounting the cable should be taken into consideration, its casing could be damaged due to low temperatures). For some intermediate mounting temperature the mounting stresses and sag are interpolated. Some overdraw should be provided in the process of mounting. The mounters are aware of this. The calculations (by default) do not take into account the further creep elongation of the wires, cords and self-supporting cables (in maintenance). This is why when designing it is necessary to give recommendations to decrease the mounting sag by 3-5-7-10%. Taking into account the further creep elongation, mounting stresses and sag are calculated in % from the elongation indicated for the wires, cords in reference literature, for cables – in Rules on Mounting Self-Supporting Optical Cables.
To avoid misreadings, it is better to make a note under the table with mounting stresses and sag:
“The table of mounting stresses and sag of the wire and cord are given regardless of further creep elongation. When mounting the wire and cord, decrease the sag by 5-10%”,
or
“The tables of mounting stresses and sag are comprised considering the further creep elongation provided the time of mounting before fixating the cable clips”.

We do recommend to supply the project with both calculation, regarding (settled stress mode) and regardless of the elongation, so that the mounting organization could control the elongation.

The theory of “manual” calculation for stresses and sag and some explanations for the program are given on “Theory and Practice” page.

Note:
In real life it happens not the way it should: the cable was mounted without the calculation, in summer (not even the maximum temperature) it came to touch the rim of the roof and beat against it when there is wind. The results are sad.

3. Optical self-supporting cable with the option of mounting a fiber optic cable

It is necessary to know the type and the sectional area of the wire and the load-bearing capacity of the constructions (supports, portals) of the existing power transmission line.
These data could be fount in the passports of the overhead lines. It is necessary to inquire the owner of the line, find the typical projects of the supports of the line and, if possible, the project of the overhead line.

Warning: Remember that in small strain spans (up to 50-60 meters) you could mount the cable “manually”. If the weight is 100kg this would be the 100daN of the mounting stress (1kN), which constitutes to the tension of 1daN/mm2 provided the sectional area of the cable is 100mm2. At the same time the permissible stress (tension) in case of ice accumulation and wind, input in the source data of the programs, could constitute 2kN (2daN/mm2) for the same data. For short strain spans the mounting sag of several centimeters is not necessary. When using a cable of 15, 20, 25kN, do not draw it with the same permissible stress (tension). The cables with the same tensile strength are designed for larger spans, in short spans they are mounted with lower strength.

1. The type of optical self-supporting backbone cable with the necessary characteristics to be put under the cross-arms of the overhead line is selected. The spans of the overhead line form small (descends from supports to portals up to 30m and between portals of substations) up to 300m.

2. The inquiry to the manufacturer of the optical self-supporting backbone cable is made with the request for mechanical characteristics for the line of the cables of different mechanical capacity with the same optical characteristics.
The answer is received:

Cable  ОКМС-А-2/4(2,4)Сп

Name kN
15 19,5 20 25
Diameter, mm 13,6 14,2 14,2 14,6
Modulus of elasticity, gPa 14,9 17,7 18,4 21,7
Sectional area, mm2 117 127 127 135
Linear thermal expansion coefficient, 1/К 1,91х10-6 1,08х10-6 9,58х10-7 3,68х10-7
Weight per unit length, kg/km 148 161 161 171


Warning: The constructions of the cables are constantly updated, this is why the inquiry of the characteristics should be made for each project. Do not take the values given here, they are obviously outdated.

3. Perform the calculation in LineMech inputting the permissible tension based on the maximum stress indicated in the passport of the cable. The permissible tension at average annual conditions (average operating) should be taken as 60-70% from the maximum, if not indicate otherwise by the manufacturer. The minimum and maximum spans, divisible by the pitch, should be taken based on the spans of the existing power transmission overhead line. The pitch for initial calculation could be 10-25m, to clearly see the changes in the sag when the span length is change.

The theory of mechanical calculation and an example of “manual” calculation are given on “Theory and Practice” page.

For cables different in their tensile strength at given climatic conditions, we get the picture of changes in sag in reference modes (the first one in calculation) – different combinations of loads according to EIR and in mounting modes (the last one in calculation). The calculation for mounting modes in LineMech are not the data to rely on when mounting. For the mounting process it is necessary to have the calculation of equivalent spans of the strain sectors, tension of the equivalent span for calculating mounting sag according to a separate formula, see the page “Calculation of mounting stresses and sag (“manually” and explanations for the program)”) of  “Theory and Practice” section  , However, it is better to use the following programs and prove all.

4. By analyzing the calculation results, we get that for a cable with a greater tensile strength the span length increases as well. The maximum sag of the cable will most often be acquired in mode 2 of the calculation – ice, no wind; sometimes the maximum sag could happen at the maximum temperature (mode 7). On an overhead line the maximum sag most often occurs at maximum temperature.
So, the cable has been preliminary selected according to its mechanical characteristics based on span lengths, but the most important thing is to compare the sag of the cable with that of the wire of the existing overhead line. It is required to define the maximum tension (maximum and at average annual temperature) for the selected cable for this particular span.
Digression. The right thing to do would be to measure the sag of the overhead line’s wires in each strain sector, at some particular spans of the existing overhead power transmission line at attested temperatures. Then, given the sag and the temperature of the measurements, perform calculations for the known wires of the overhead line, sort out the permissible tension in the wires of the line and make a more exact comparison between the sag of existing wires and that of the mounted cable.

4а. Attention. From the above said it is clear that in case of mounting FOCL on an existing overhead line, the first calculation will be the calculation in LineMount (LineMountad)&damp, where for the sectors description it is necessary to input the wire of the existing line in “Wire” field, and the selected cable in the “Cord” field. At the same time, it is necessary to indicate the maximum permissible tension and the tension at average annual temperature for the wire of the overhead line, which is prescribed both by old and new EIR (in case there are no measurements, depending on the year of the construction). It is possible that the tension was taken based on the load-bearing capacity of the supports. This variant should also be checked. It should be clear that the wire was not overdrawn to compare the sag of the wire and cable. Usually, if the cable is selected correctly, the sag for maximum tension in the cable is less than that of the wire, which is unnecessary. As well as it is unnecessary to overload the supports of the overhead lin. This is why we decrease the tension in the cable to such values of sag that they are compatible with that of the wire of the overhead line. Look on the data of the large spans and make the final conclusions on the accepted tension in the cable, the maximum of at average annual temperature. Verify the sag of the wire in loading, not mounting modes in LineMech and LineCross (considering the equivalent span in this case).

5. It is better to divide the existing overhead line into sectors according to different groups of spans with the purpose of using cables of different mechanical capacity (permissible tensile strength) or decreasing the maximum operating stress in the cable when using one, more solid cable for all the sectors of the line. There is no doubt, that two sectors of the overhead line require the decrease in the operating tensile strength to 1.0-2 or 3 daN/mm2 (based on the stress, tensile stress of 100 - 200, 300 or a little more daN) – these would be the descends from the supports to portals, passages from the supports to the buildings from different sides of the route. In small strain spans, where necessary, the operating maximum stress (tension) could be lowered as well.

6. Perform calculation in LineMech for the cables using the tensile stress, stress acquires with the help of LineMount for each sector. Analyze the sag. If necessary, decrease or increase the permissible stresses. Select an acceptable, optimal variant, when there is no excessive load on supports and the necessary clearances are provided.

Digression. If you are “given” the cable of 30kN, do not use it for the maximum stress (this load will go to the supports and, in case of ice accumulation with wind, it will be bad), when it is necessary to load it for 15 (which is 1.5 ton already), better not more than 10kN on the lines of 100kV and more (depending on the comparison of the sag of the wire and the cable) and even 1-3kN for descends. On the lines of 0.4kV – usually 160daN for the wire, on lines of 10kV – 450-650daN for the phase. It always better to keep the tension low. Anyway, FOCL on an existing overhead line always creates additional load.

6а. Where necessary, on important sectors of the route, one could show the sagging curves of the communication cable on the line profile, using the modification on LineMech program LineMechCad The sag of self-supporting cables at a particular scale could be done by the program for different temperatures and for ice load.

7. Where it is necessary to make the draft of the crossing with engineering objects (highways, railways, etc.), the detailed calculation of clearances is made in LineCross. The calculation is done for the accepted maximum stress (tension) from the previous calculation for the sector of the route and for the same climatic conditions. The customer is given the draft of the crossing with the calculation results by the program. Based on the calculation results, it is possible to change the permissible stress (tension) and go back to previous calculation. As the cable is not mounted on insulator strings – the calculation for breakdown mode and accounting the weight of the strings is not necessary.

The theory of crossings’"manual"calculation and some explanations for the program are given on Theory and Practice page.

7а. Except for the table results of the crossing’s clearance calculation, the draft of the crossing is made in the modification of the program LineCrossCad with the sagging curve at the necessary temperature at a particular scale with all the data necessary for preparing the layout plan of the crossing.

Yet, a personal request: Self-supporting dielectric communication cables, as a rule, have the biggest sag in case of ice accumulation. As this is a rare phenomenon during a year, heavy sagging of the cable is observed as rarely. When crossing highways, possible at other conditions, I would suspend a short light-reflective stripe in the center of the crossing warning about the cable for highly loaded transport.

8. If all the clearances are provided, the loads on supports are the lowest possible, the following note could be made for the project:
“To decrease the loads on supports, providing the permissible crossings’ clearances and sag, the maximum permissible tension (stress) accepted for the project / at average annual temperature in the optical water-resistant cable … 20 kN on sectors:
1-2, 5-6 - 2/1.5 daN/mm2;
2-3 - 5/4 daN/mm2;
3-4 - 7/5 daN/mm2;
4-5 - 8/6 daN/mm2
in cable... etc."

9. The line designer has selected the cables and permissible operating stresses for sectors of the route, provided the necessary sag of FOCL cables, compared with that of the wires of the overhead line, clearances with the crossed objects. The mounters should be given the tables of the mounting stresses and sag at particular mounting temperatures. The basis for mounting are the final calculation of the fiber optic cable in LineMount, acquired by comparing the sag with the tensions adjusted (in necessary) after the calculations in LineMech and LineCross.
The calculation is given in the same way as when comparing the sag for the settled stress mode.

10. The technical length of the cable in incompatible with the straight sectors of overhead power transmission line, so there is a chance of placing the splicing box of the cable of FOCL on an intermediate support of the OHL. Besides the actual need of temporary bracing of such intermediate supports in the course of mounting, it is necessary to indicate the strain attachment for a FOCL cable on this intermediate support for the calculation in LineMount. The equivalent span will be different.

11. The presence of ballasts on the wires of the OHL should warn the designers of FOCL. This means that there are vertical forces directed upward here affecting the wire, dangerous as the wire approaches the cross-arms and the body of the support, especially at the lowest temperatures. For a fiber optic cable this means, that given the same height of attachment as on the neighboring supports, there will be the same force directed upwards, raising the attachment fitting of the cable. To account for such situations, use LineLoad program (see the site). It is possible that replacing the intermediate suspension attachment of the cable to a strain one, or increasing the height of the cable attachment on the support could be necessary.

12. In case of considerable stresses in FOCL cables and large spans, the FOCL OHL cable are equipped with vibration dampers according to the Organization Standard 34.20.265-2005, calculated in LineMount with the additional “damp” module.

13. The calculations comparing the sag are performed for a settled stress mode.
When mounting, it is necessary to provide some overdraw.
The calculations (by default) are done regardless of the further creep elongation of the wires, cords, and self-supporting cables (in maintenance). This is why, when designing, it is necessary to give recommendations to decrease the mounting sag by 3-5-7-10%. Taking into account the further creep elongation, mounting stresses and sag are calculated in % from the elongation indicated for the wires, cords in reference literature, for cables – in Rules on Mounting Self-Supporting Optical Cables.

To avoid misreadings, it is better to make a note under the table with mounting stresses and sag:
“The table of mounting stresses and sag of the wire and cord are given regardless of further creep elongation. When mounting the wire and cord, decrease the sag by 5-10%”,
Or
“The tables of mounting stresses and sag are comprised considering the further creep elongation provided the time of mounting before fixating the cable clips”.
We do recommend to supply the project with both calculation, regarding (settled stress mode) and regardless of the elongation, so that the mounting organization could control the elongation.
The mounting tables regarding the elongation are given only for fiber the optic cable (the right part of the calculations, the “Cord” part), the comparison for the wires of the overhead line is not necessary.

14. In modification of LineMount program LineMountCad there is an option of receiving the graphical picture of support-by-support scheme of OHL with the FOCL in AutoCad with the notes on characteristics of the OHL and FOCL, where the placing of the splicing boxes.
The support-by-support scheme of FOCL on OHL is much more informative than the tables, it contains the data of the supports-spotting register, as well as mounting stresses and sag.
 

4. Replacing the ground wire of an OHL with an optical ground-wire cable

Replace the steel ground wire of an OHL of 110kV with the wires АС150/19 to an optical ground-wire cable. The types of supports and span lengths are know.

1. Optical ground-wire cable ОКГТ-ц-1-24-(G.652)-12,6/56 24 fibers is selected. The manufacturer has provided its characteristics (only mechanical characteristics are left here):

Sectional area of steel 34,74 mm2
Sectional area of aluminum 55,02 mm2
Reference sectional area 89,76 mm2
Diameter 12,6 mm
Weight of the cable 390 kg/km
Tensile strength 5779 kg
Maximum permissible load 4072 kg
Average operating load 1734 kg
Final modulus of elasticity 99,11 kN/ mm2
Linear thermal expansion coefficient 16,61х10-6 1/К
Range of operating temperatures -60...+80С
Minimal mounting temperature -30С

2. We get the following data on the cable to input into the catalog:

Diameter, mm 12,6
Sectional area, mm2 89,76
Weight per unit length, kg/m 0,39
Modulus of elasticity, daN/mm 2 9911
Linear thermal expansion coefficient, 1/К 0,0000166
Maximum permissible tension, daN/mm2 4072kg=3995daN/89,76mm2=44,5 daN/mm2 (see the digression below)
Average operating permissible tension 1734kg=1701daN/89,76mm2=19,0 daN/mm2

Digression. The maximum permissible tension here according to the manufacturer’s data is 70% from the tensile strength, which does not entirely correspond to the customary values of EIR requirements, but is stated for optical ground-wire cables in “Rules of designing, building and maintaining fiber optic communication lines on overhead lines with the voltage exceeding 100kV and higher”, there is also a link in the “Rules … 0.4-35kV.

3. The main task is to provide the required vertical distance between the wire and the cord in the middle of the span. The distance depends on the length of the overall span of the OHL and is indicated in 2.5.16 table of EIR. An overall span – the span the length of which is defined by nominal vertical distance from the wires to the ground, provided the supports are set up on an ideally even terrain. The distance between the wire and the cord is defined based on conditions of lightning-proofness at +15C.
На данном сайте некоторые сведения об этом приведены на странице “Ground wire. Optical ground-wire cable. Defining the tension (manually and explanations for the programs)" of “Theory and Practice” section.

4. The second task is to decrease the load on the supports. The existing cord was mounted with the decreased permissible tension, sufficient to provide the required distance between the wire and the cord in the span. It is undesirable to exceed the stress.

5. According to old EIR, the maximum permissible tension for the wire АС150/19 was 12.2 daN/mm2, at average annual temperature – 8.1daN/mm2. Perform the mechanical calculation for the wire at the known climatic conditions in LineMech program. The calculation should be done with the pitch of 1m for the range of average spans in different sectors of the OHL. For overall (equivalent) span of the selected sector in mode 5 (temperature +15C, no wind, no ice) find the sag of the wire АС150/19.

6. Find out, what sag could be permitted for an optical ground wire cable. It is clear, that to provide the distance between the wire and the cord according to table 2.5.16 of EIR, the sag of the optical ground wire cable should be less than that of the wire. Define the sag of the optical ground-wire cable in advance using the formula on page “Ground wire. Optical ground-wire cable. Defining the tension (manually and explanations for the programs)" of  “Theory and Practice” section.

7. Perform the calculation for the optical ground-wire cable in LineMech for the permissible tension defined earlier (44,5; 19,0) for the same conditions as for calculation of the wire АС150/19. We discover that the sag is small and could be increased to the required one by lowering the tension in the optical ground-wire cable. At the same time, the loads on supports will be decreased. By decreasing the permissible tensions, we sort out that the required sag of the optical ground-wire cable at different tension is:
maximum - 21,0 daN/mm2, at average annual temperature - 19,0 daN/mm2.
There is the solution. The value could be different for a different section of the OHL.

The theory of mechanical calculation and an example of “manual” calculation are given on “Theory and Practice” page.

Make a note to the project:
“When providing the necessary distance between the wire and the cord in the span according to 2.5.16 of EIR, alongside the decrease in the loads on supports, the maximum permissible tension in the optical ground-wire cable -ц-1-24-(G.652)-12,6/56 24 accepted for the project is 21.0 daN/mm2, at average operating conditions – 19.0 daN/mm2”. If the values for some sectors of the route are different, give the data for them as well.

Attention. To exclude the overlapping between the wire and the cord it is recommended to verify the distance between the wire and the cord in case of ice accumulation for the variant when there is ice on the cord (optical ground wire cable) and no ice on the wire.

8. If there is an undercrossing of the OHL of 110kV with the cord under the existing OHL of 500kV, it is necessary to perform the detailed calculation in LineCross, taking the same climatic conditions and the permissible tension in the optical ground-wire cable accepted in the previous step.

The theory of crossing’s calculations "manually" and explanations for the program are given of “Theory and Practice” page.

8а. Except for the table data of the crossing’s clearance calculation result, the modification of LineCrossCad prepares the draft of the crossing with the sagging curve at the required temperature at particular scale with all the data necessary for the layout plan of the crossing.

9. The basis for the mounting are the calculations done in LineMount.
The calculation is performed for the sectors of the route with the accepted climatic conditions and operating maximum permissible stress (tension) justified in previous calculations.

Digression. If the measurements of the stress and sag of the wires were taken, it is convenient to perform simultaneous calculation for the wire and the optical ground-wire cable for additional justification by placing the cable in the “Cord” field. In this case the co-relation between the sag of the wire and the cable at different temperatures will be obvious, as well the distance between the wire and the cord in the span. The sag at the mounting temperatures reflect the normal behavior of the wire and the cable uninfluenced by wind or ice.

9а. Attention. As well as in step 4a of mounting the optical self-supporting cable, in case of mounting an optical ground-wire cable to an OHL the first calculation could be in LineMount (LineMountCad), where in “Sectors” section the wire of the existing overhead power transmission line should be denoted as “Wire”, while the selected cable should be denoted as “Cord”. At the same time, the maximum permissible tension at average annual temperature should be indicated for the wire of the OHL, which is prescribed both by modern and old EIR (depending on the year of the OHL construction). It is possible that the tension in the wire was taken based on the load-bearing capacity of the supports. It should be obvious that the wire was not overdrawn, so that the sag of the wire and the cable would be compatible. Usually, when the wire is selected successfully, the sag at maximum tensions for the cable are less than that of the wire. As well as it is unnecessary to overload the ground wire peaks of supports of the overhead lin. This is why we decrease the tension in the optical ground-wire cable to such values of sag that the distance between the wire and the cord in the span according to 2.5.16 of EIR-7 is provided.

Look thorough the data on the large spans and make the final decisions on the accepted tensions in the cable, maximum and at average annual temperature. The calculation of the mounting stresses and sag is taken as the final one. Verify the sag of the cable in loading, not mounting modes in LineMech and LineCross (considering the equivalent span in this case).
Attention. Verify the distance between the wire and the cord for cases of ice accumulation on the cord and the absence of ice on the wires of OHL.

10. It is necessary to provide some overdraw when mounting.
The calculations (by default) do not take into account the further creep elongation of the wires, cords and self-supporting cables (in maintenance). This is why when designing it is necessary to give recommendations to decrease the mounting sag by 3-5-7-10%. Taking into account the further creep elongation, mounting stresses and sag are calculated in % from the elongation indicated for the wires, cords in reference literature, for cables – in Rules on Mounting Self-Supporting Optical Cables.
To avoid misreadings, it is better to make a note under the table with mounting stresses and sag:
“The table of mounting stresses and sag of the wire and cord are given regardless of further creep elongation. When mounting the wire and cord, decrease the sag by 5-10%”,
Or
“The tables of mounting stresses and sag are comprised considering the further creep elongation provided the time of mounting before fixating the cable clips”.
We do recommend to supply the project with both calculation, regarding (settled stress mode) and regardless of the elongation, so that the mounting organization could control the elongation.

11. LineMount (lineMountCad) program with the “damp” module calculates vibration dampers for the cord according to the Organization Standard 34.20.264-2005.

In modification of LineMount program LineMountCad there is an option of receiving the graphical picture of support-by-support scheme of OHL with the FOCL in AutoCad with the notes on characteristics of the OHL and FOCL, where the placing of the splicing boxes.

The support-by-support scheme of FOCL on OHL is much more informative than the tables, it contains the data of the supports-spotting register, as well as mounting stresses and sag.

Requirements for cables’ characteristics for the reference catalog of the programs (the cables’ characteristics should be asked from their manufacturer)

1. Diameter, mm – the outer diameter of the wire, cord, self-supporting insulated cable including the insulation. Experiences wind and ice loads.
2. Sectional area, mm2 – sectional area of the solid part of the self-supporting cable – (reference sectional area).  For calculation sakes the air between the lashing wires, greasing and other similar materials are not taken into account. Experiences stress applied to the wire, cord, the supporting part of self-supporting insulated wire, cable due to its own weight, tension, wind and ice loads, temperature changes.
3. Weight per unit length, kg/m – the weight of 1 meter of the wire, cord, all the self-supporting insulated wire, cable. 1 kg=0,981 daN=9,81 N (approximating these to 1 daN, 10 N practically does not affect the calculation results).
4. Modulus of elasticity of the cable, daN/mm2 - 1,0 hPa=1000000000 Pa=1,0 kN/mm2=100 daN/mm2. For self-supporting cables with solid load-bearing elements of aramid fibers, fiberglass plastic, the modulus of elasticity is low as compared to the metal solid elements.
5. Linear thermal expansion coefficient, 1/K – the change in the length of cable at temperature change by 1 degree. For self-supporting cables with solid load-bearing elements of aramid fibers, fiberglass plastic, the coefficient is low as compared to the metal solid elements.
For inputting to the program reference catalog 0,000002 corresponds to 2х10-6 (2.0Е-6).

Requirements to climatic conditions data asked for at weather stations.

1. Ice, mm – according to weather stations data, if the data are unavailable – based on the charts and recommendations of EIR. Its return period is once in 25 years.
2. Maximum, minimum (not the temperature of the coldest 5 days), average annual temperatures according to weather stations data, those are absolute temperatures, their return period is once in 25 years.
3. Ice-with-wind and ice-without-wind temperatures are taken according to EIR.
4. Maximum wind velocity, m/s – return period is once in 25 years and wind velocity in cases of ice accumulation (wind pressure calculation given in EIR) are taken according to weather stations data, if no such data are available, than according to EIR.

Requirements for Maximum Setting Tension, Stresses

1. Maximum permissible tension, daN/mm2 – tension in the material of the cable, input by the line designer for project calculations and accepted for the project.
Tension – stress, divided by the load-bearing sectional area, mm2, of the cable. The permissible tension is usually not more than 50% of tensile stress of mechanical capacity.
2. Permissible tension at average annual (average operating) conditions, daN/mm2 – the tension permissible at average annual temperature. Usually, it is 60 - 75% from the maximum permissible tension if the manufacturer’ s data do not indicate otherwise.

The reliability coefficients are taken as equal 1 if there are no data according to 2.5.11.
If the customer did not give them in the task, they should not be considered.