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Ali Hirji

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									USE OF AERIAL BUNDLED CABLES
FOR LOW VOLTAGE DISTRIBUTION
Ali Hirji, Raycap Corporation, Athens


Summary: Compared with conventional bare conductor overhead systems, worldwide experience proves that the use of Low
Voltage Aerial Bundled Cables (LV ABC) is cheaper, safer, more reliable, requires less tree clearing and pruning and is more
aesthetic, less labour intensive, requires lesser maintenance and eliminates supply interruptions and forest fires being initiated by
conductor clashing.
A Brief Description of LV ABC System:
A LV ABC Mains Cable has four insulated conductors, each comprising of an aluminium conductor, insulated with
cross linked polyethylene, twisted together to form a bundle. Electrical connections are made by the use of
insulation piercing connectors and the bundled cores are supported at intermediate poles and strain poles with easy
to install mechanical fittings. Ganged insulated switchgear to which fuses can be added is used for electrical
protection and isolation.

1. Useage: LV ABC is used worldwide in a total of approximately 80 countries, and the various motives for
     adopting LV ABC include:
            Prevention of conductor clashing and initiation of fires.
            Reduction of tree cutting and improvement of the environmental impact of overhead distribution.
            Improvement of safety and reliability.
            Achievement of overall economy.
      LV ABC is used for new extensions as well as for replacing existing bare overhead lines and has found
      application in
            Urban areas on poles
            Urban areas on facades of buildings
            Rural areas on poles
2.   Choice of System:
      There are at least five designs of LV ABC in common use thoughout the world. These follow either of the two
      basic forms:
            Fully supported (Refer Fig I) – where all the equal-sized phase and neutral cores share the mechanical
               load. This type is used in Norway, Sweden, Austria, Germany, Ireland, U.K., New Zealand and
               Australia.
            Neutral supported – the three phase cores are laid up around an aluminium alloy neutral which acts as
               a catenary to support the whole bundle. In the Middle East, India and Finland the neutral is bare,
               whereas in France the neutral is insulated. In Italy, a second protective outer sheath is added to the
               three phases with the neutral having only single insulation – (Refer Figures 2 & 3).

           Self-supporting LV-ABC              LV-ABC Lines with insulated            LV-ABC Lines with bare
           Lines                               neutral messenger                      neutral messenger




           Fig 1                               Fig 2                                  Fig 3


      For a 4-wire, low voltage system with a solidly earthed neutral, the fully supported cable system provides the
      following benefits:

              The fully sized neutral is important in a multiple earthed neutral system and to avoid extensive losses
               due to unbalanced loading.
             The neutral is at less mechanical risk
             Longer spans are achievable – an important factor when existing poles are retained for reconstruction
              work and for rural applications.
             Easier erection because all cores, rather than just the neutral are supported at suspension and anchor
              clamps.
             The cable is cheaper to manufacture

3. Cable: Fully supported and neutral supported cables are manufactured to Harmonisation Document HD 626 by
  CENELEC and Australian Standard AS3560.
   The compacted, stranded circular aluminium conductor (material alloy 1350), is insulated with cross-linked
   polyethylene whose resistance to ultra-violet light degradation is provided with an appropriate percentage of
   carbon black pigment dispersed in it. The combined thermal and mechanical stress (method 6 –self supporting
   cables) to determine the cable behavior is performed in accordance with 2.3.6 of HD626 and an onerous
   adhesion Test of the compound to the conductor is included in AS3560.
   Because the cable has to be compatible with Insulation Piercing Connectors, the maximum thickness of
   insulation at any point on the circumference of the conductor and the maximum and minimum conductor
   diameter are important criteria in the cable design.
                              The physical size and spacing of the rib profiles on both the neutral and phase cores is
                              very important to enable linesmen to clearly identify the neutral and to differentiate
                              between all phase cores. The neutral is fully ribbed to make it similar to aerial service
                              cables with the heights of the ribs on the phase cores noticeably greater than those on
                              the neutral core. In addiction the phase cores are required to have an embossed or
                              indented number. Please see Fig 4.

    Fig 4                      The system can consist of additional 1 or 2 insulated aluminium conductors with cross
                               sections of 16sq mm or 25sq mm as pilot wire for street lighting.

    The service lines of all LV ABC systems are usually also of the self supporting type, composed of 2 to 4 factory
    bundled insulated aluminium conductors with cross sections of 16sq mm, 25sq mm or 35sq mm.

4. POLE HARDWARE: Pole hardware consists of Open Eye Bolts and Hook Brackets. These fitting are designed
  to have:
           Adequate strength
           A close range of opening forces in two directions – vertically downward and horizontally away from
            the pole. The open eye bolt as shown in Fig 5 is suitable for fixing to either a timber pole or a concrete
            pole with a preformed hole.
    The hook bracket as shown in Fig 6 is suitable for clamping or banding to a concrete pole where a preformed
    hole does not exist.




             Fig 5                                      Fig 6

5. Mechanical Fittings: Mechanical fittings for supporting and anchoring LV-ABC cables consist of :
         - Suspension clamps
         - Yokes
         - Strain clamps
    Suspension Clamps : These support the LVABC cables at intermediate and angle poles. The traditional J-
    Clamp with a rubber bush and wing bolt can be used for self supporting as well as cables with bare and
    insulated neutral messenger. The clamp as shown in Fig 7 can be used without any special tools since it is
    equipped with a wing nut and has no loosable parts. The rubber bush is based on a weather and UV resistant
    elastomer and the clamp is made from hot galvanized steel. The line can be in a straight direction or a deviation
    angle up to 300.
       Standard Type         Twisted Type        Type with Bolts




    Fig 7                                                                   Fig 8

    Yokes : The Yoke, made from hot dip galvanized steel is used when the cable has a deviation angle from 30 0 to
    600. Fig. 8 shows a typical yoke with two standard suspension clamps. For large line deviations i.e. greater than
    600, use of two strain or anchor clamps becomes necessary.
    Strain/anchor clamps : A typical clamp designed to anchor a self-supporting LV-ABC cable is shown in Fig 9.
    The strap of the clamp can be made of hot dip galvanized steel or extruded aluminium for improved corrosion
    resistance and lighter weight. The straps are shaped to provide a sufficiently large window area so that the cable
    tails have adequate space to exit the clamp without contact with the straps. The risk of abrading the insulation of
    the cores is further reduced by the use of straps with rounded edges.




    Fig 9

    A single M12 bolt and self locking nut allows the strain clamp to be fixed to pole brackets with closed eyes or
    also screws with closed eyes. The clamp is made of weather and UV resistant glass fibre reinforced polymer.
    Figure 10 shows an example of a pole structure using both suspension and strain clamps for making a T –
    connection and Fig 11 shows an LV-ABC anchoring while connecting to an underground cable . Fig 12 shows
    the connection of LV-ABC to a bare overhead line.
    Similarly anchor clamps and angled clamps for service cables are also available.




      Fig 10                                 Fig 11                                 Fig 12

6. Insulation Piercing Connectors: The technology for insulation Piercing Connectors used for joining mains
  cables and main cables to branch has evolved over twenty five years in Europe, passing through three main
  steps :
    1. The connector had two separate bolts; one for the main and one for the tap. Insulation piercing occurred on
         the main cable only and the service line was stripped. Bolts were in direct electrical contact with the
         conductor and the insulation over the connector was a rigid cover and did not provide any resistance to
         water ingress.
         The disadvantages of this included the risk of failure due to nicking of the strands of the service conductor
         due to the stripping operation carried out by the linesmen with a knife, risk of accident because of the
         energized bolt during live line installation, corrosion and oxidation due to lack of water and air tightness to
         the connection and the possibilities of over and under tightening of the bolts.
    2.   The connector had two separate bolts with shear off heads to provide the correct torque during tightening.
         The bolts were in direct electrical contact with the conductors and the connector had a flexible cover. The
         risk of accidents due to energized bolts and corrosion/oxidation problems still remained.
         This led to the development of a one bolt connector with simultaneous connection to the main and tap
         cores. The bolt was insulated from the main and tap cores and the piercing was achieved through insulation
         piercing blades, with the bolt being integrated into the plastic body of the connector.
         However, the risk of accident due to the exposed blades and corrosion still remained.
    3.   The connector has one bolt with simultaneous tightening of the main and tap cores. The bolt was insulated
         from the insulation piercing blades and designed with a shear head device. The connectors were fully
         insulated and sealed using elastomer seals which firmly conform to the cable insulation and with silicone
         grease on the aluminium alloy/brass/copper contact blades, to prevent any moisture ingress.
         This design met with all customer needs and complied with safety requirements.
         The features of the Insulation Piercing Connectors evolved in this third stage of evolution include:
         a. Easy and safe to fit on energized cables.
         b. Have no life metal parts that are accessible.
         c. The plastic components are made from strong UV and weather resistant glass reinforced nylon.
         d. Elastomeric inserts seal around the insulation piercing contact blades.
         e. Cold applied integral caps seal the end of the branch cable, preventing corrosion and ensuring reliable
              electrical contact.
         f. Fastener, nut and washer are protected against corrosion.
         g. The bolts are fitted with shear heads.
         The range of Insulation Piercing connectors available today include.
         Insulation Piercing connectors for main to main connections.
         Insulation Piercing connectors for main to service connections.
         Insulation Piercing connectors for connecting bare aluminium/copper main conductors to insulated service
         conductors.
         Insulation Piercing connectors for connecting LV-ABC cores to underground cable lines.
         The fitting of a typical IPC is shown in Fig 13. Fully insulated hexagon socket wrench with ratchet is used
         for tightening and shearing the bolt heads.




    Fig 13

7. Water Proof Pre-Insulated Compression Lugs and Connectors : While bare compression lugs and connectors
  can be used along with adhesive coated heat shrinkable tubings for insulation and sealing on LV ABC, pre
  insulated connectors and lugs are preferred since they avoid the need for carrying a heat shrinking torch to the top
  of a pole and doing shrinking at a height.
  Three versions of the Pre-insulated and sealed connectors are available to meet with the different mechanical load
  requirements for self-supporting cables, for phase conductors of LV-ABC with neutral messenger and for the
  insulated neutral conductors of LV-ABC. Pre-insulated lugs are suitable for any insulated, stranded aluminium
  conductors.
  Stripped cables, using insulating stripping tool for LV-ABC as shown in Fig 14, are inserted in the connector up
  to the block in the connector or lug end as the case may be, and crimped according to the marks with appropriate
  crimping die over the insulation. The electrical contact and the sealing by the elastomer ring is achieved during
    the crimping process. The lugs are available with aluminium palm and also as bimetallic lugs with an aluminium
    tubular portion friction welded to a copper palm.
    The insulation material is made of weather and UV resistant polymer.
    A typical pre-insulated connector is shown in Fig 15 and a typical pre-insulated lug is shown in Fig 16.




      Fig 14                               Fig 15

                                                                                         Fig 16
  8. Switchgear : Insulated switchgear especially designed for LV ABC is available for mounting on :
         a concrete or wooden pole, no cross arm required (Fig 17) or
         the wall of a building


                                                    There is a preference for three-pole rather than four pole units to
                                                    avoid the possibility of incorrectly connecting the neutral core of
                                                    the main cable. Switchgear can be fitted with blades or fuses.
                                                    Fuses accommodated are the NH Type to DIN Specifications with
                                                    blade type contacts and sizes as shown in Fig 17, of 160A, 250A,
                                                    400A.
                                                    A typical disconnecting-fuse switch comprises a fully insulated
                                                    frame made of impact resistant weather-proof plastic, a pivoted
                                                    removable switching flap and cover hoods for the terminals. The
    Fig 17                                          frame holds the tap access conductor terminals which are
     connected to the fuse contacts. The switching flap has fuse holders suitable for the
     DIN spec fuses. The opening eye is integrated into the flap which can be operated
     by means of operating rods.

  9. Wall Mounted Saddles and Cable Ties for LV-ABC Lines : Wall mounted
    saddles as shown in Fig 18 are designed to install LV-ABC lines (self supporting
    and insulated neutral types) along – side walls and poles.
    The LV-ABC cable is fixed to the saddle by a cable tie. A second cable can be
    installed on the same support by hanging it to the bottom side with an additional
    cable tie. The body and the cable tie are made of weather and UV resistant polymer
    material. Usually, every 0.7m, a saddle is installed on a wall.                          Fig 18

10. Weak-Link Hooks : These are used as hangers between the pole support fitting
    and the strain/ anchor or suspension clamp in areas where damage to the LV ABC line could
    be expected from falling trees. Weak links withstand normal working loads but the
    controlled failure mechanism releases the cable in the event of overloads, enabling the cable
    to drop to the ground. A typical weak link is shown in Fig 19.                                    Fig 19

11. Construction of LV-ABC Lines: Labour costs involved in the construction of LV ABC lines can be reduced by
    using equipment and work practices that :
          - Suit all situations encountered
          - Are safe
          - Are easy to apply
          - Ensure reliable installations
    The main cause for failures is poor workmanship during installation or connecting. Training to all linesmen to
    ensure awareness of equipment, work practices and materials is essential. The tension stringing method, Fig 20,
    has been proven to be the best.
                      Fig 20



This method is:
     Safe because there is no lifting of the cable at any stage.
     Economical as a minimum cost equipment is used such as drumstand, running sheaves and hydraulic
         winch.
     Reliable. The cable insulation cannot be damaged by other aerial cables, trees, vehicles, rough ground
         and rocks.
     Low cost since it is quick and requires minimum size of crew.
     Flexible as setting up of rollers, of running ropes and stringing of cable can be carried out as separate
         tasks to suit available resources, vehicles, weather and available time.
     Suitable for running LV-ABC under bare live LV or HV lines.
Stocking: A nylon stocking as shown in Fig 21 is used. This has no splinters (unlike metal stocking) and
enables LV-ABC to be strung under live bare LV and HV lines.




Fig 21                                  Fig 22

Rope : A polypropylene rope is required of the appropriate diameter.
Swivel : A swivel as shown in Fig 22 is used with the pulling stocking to eliminate twist.
Rollers : The stringing block consists of a plastic coated roller, suspension assembly and a strap with clamping
device or a plastic roller, a steel loop and a swivelling suspension hook. Please refer to figs 23 & 24
respectively.




  Fig 23                           Fig 24
                                                        Fig 25


Running Cable: The step by step procedure is as follows :
1. Pull out rope through the rollers fixed to the poles.
2. Attach stocking and rope to the cable.
3. Pull through rope and cable.
4. Apply brake at drum end to keep cable from being abraded by trees, rough ground aerial services and
     telephone cables.
5. Get cable up to near sag with winch
6. Grab cable with Comealong. The tensioning device is attached to the pole and to the Comealong and the
     ratchet operation of the tensioning device gets cable up to sag
7. Attach one strain clamp
8. Grab cable at the other end with Comealong
9. Attach second strain clamp – cable is now up to sag
10. Fit suspension clamps
11. Remove rollers.
For an installation where a LV ABC is used for replacing an existing bare LV line the procedure will be as
follows:
a. Run LV ABC cable
b. Energize LV ABC cable
c. Change over service connections from bare to LV ABC
d. Remove cross arms
The tension used while stringing LV ABC and the maintenance of a defined sag are governed by pole spacing,
ground clearance, structure strengths, construction limits and fitting limits.
The maximum working tension of the bundled cores is the peak assigned tension allowable for various values of
temperature and wind, such as 50C and no wind or 150C and 500MPa wind. Taking into account that
mechanical stresses have to be transferred through the XLPE insulation, particularly at strain/anchor fittings, it
is considered to adopt 40MPa as the maximum working tensile stress of the conductor. On this basis, the
maximum working tension of a 4x95sqmm bundled cable is 4x95x40 = 15.2KN
However, the practicality of opening the bundle to fit service connectors usually needs to be taken into account
and so for a 4x95sqmm cable an Every Day Tension of approximately 6KN has been chosen as the most
appropriate for the majority of the situations.
The Straining Tension (kg) is related to the weight of the cable (kg) per metre, the Span length in m and the sag
in the middle of the span by the Formula :

     PxL2
T=
      8F
Where     T = Tension in kg
          P = Weight of cable per m in kg
          L = Span length in m
          F = Sag in the middle of the span in m
For example a 4x95sqmm cable has a weight per metre i.e. P = 1 • 35 kg/m.

If the span length is 40m and sag in the middle of the span (after taking into account ground clearance and
circuit to circuit clearances etc) is
0 • 51m at 25ºC,
                1,35 x 40 x 40
Therefore T =
                   8 x0,51

T = 529 kg
 In case the span between poles varies along the route length,
 then the “equivalent” span is calculated as :
                         ( span1) 3  ( span2) 3  ( span3) 3 ...
 Equivalent Span =
                             ( span1  span2  span3...)

 See Fig 26 for clearance recommendations.



                                                                           Fig 26

12. Possible Strategies to consider when changing from bare overhead lines to LV ABC : The possible strategies
      a Utility could consider include :
         Having a clear, committed company policy for usage
         Developing an implementation plan
         Communicating frequently
         Producing documentation
         Introducing specific sizes and types of cables
         Conducting training for:
             - All linesmen
             - Supervisors
             - Technical staff
         Co-ordinating purchase of:
             - Cable and fittings
             - Construction equipment
             - Tools
         Developing some champions within the organizations
         Save time and money – outsource work to organizations having the total integrated system knowledge
13.   Economic evaluation : An economic evaluation after taking into account the costs of :
         Material
         Labour
         Operating & maintenance expense
         Secondary available expense
         Set – up, inclusive of training, tools and construction equipment.
        An economic evaluation of a utility showed that LV ABC was cheaper than bare overhead lines for both new
        and retrofit work. Payback period for adopting LV ABC was only one year.

       ECONOMIC EVALUATION

            Operating & Maintenance Costs                        Bare                   LVABC
                                                              76 sqmm                   95sqmm
                                                              $/km/year                $/km/year

        Energy Losses                                               2507                    2006
        Breakdown – maintenance                                       46                       5
        Breakdown – foregone revenue                                 110                       2
        Routine maintenance                                         2670                    2014
        Tree clearing                                               7000                     250
        Fines                                                        264                      44
        Electrical accidents                                          16                       2

       Savings are due to lower costs in a) Tree clearing b) Maintenance c) Labour and these offset the higher material
       cost for the cable.

14. Conclusion: LV ABC System provides electricity supply authorities with tremendous opportunities to lower cost
      and to improve safety and reliability.
BIOGRAPHY

Ali Hirji was born in Mumbai (India) on 18th August 1946. He graduated in Electrical Engineering, from the
College of Engineering, Pune affiliated to the Pune University. His employment experience includes Mahindra
Engineering and Chemical Products Ltd., Raychem Corporation, Tyco Electronics and Raycap Corporation. His
special fields of interest include Power Cable Accessories, Connectors and Fittings for Cables and Overhead Lines.
Ali Hirji was a Member of the Bureau of Indian Standards Committee for the Drafting of the Indian Standard
IS13573 covering the Performance Requirements for Power Cable Accessories upto 36KV and also a Faculty
Member of IEEMA (Indian Electrical & Electronics Manufacturers Association) for Training Utility Engineers
under the Government of India’s Electrification Schemes.

								
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