CABLE
Objective for the Lesson :: To familiarize the necessity of protective devices on the telecommunication systems. Need to avoid the hazards of power induction due to electrical lines and lightning and railway traction. The necessity to prevent the equipment and personnel from the unexpected source of dangers.
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The power telecom co-ordination committee came into existence in 1949as per Resolution No. EL.11-151 (7) dated 30th May, 1949 from Govt of India by the Ministry of works, Mines and Power . Its earlier name was Central standing committee.
Various committees constituted are
Central PTCC :: Three meetings in a year. Power lines above 33 KV
State level PTCC :: Frequency is once a month. Power lines above 11 KV up to 33 KV.
1.0 IMPACT OF POWER PARLLELISM ON TELECOM NETWORK ::
Parallel running of Power and Telecom Underground cables / lines may cause electrostatic and electromagnetic induction and interference on the telecom circuits existing on the telecom lines / cables.
Development of hazardous longitudinal voltage on telecom circuits under single line to ground fault condition of the power line / power cable.
Circuits under worst fault condition of the power line / power cable can be limited to the safe value recommended by the CCITT / ITU.
In case this safe separation is not possible protective measures are to be taken so that no
electrocution hazard is caused to the operating and maintenance personnel and no damage is caused to the telecom equipment.
Crossing of Power and Telecom cables / lines require keeping adequate vertical and horizontal clearances between the power wires and telecom wires or cables.
1.1 EFFECT OF ELECTRIC TRACTION ON TELECOM CIRCUITS 1.10 ELECTRIC INDUCTION ::
The overhead category and contact wires of the traction system will be at a high voltage of 25 KV with respect to earth. This produces in the neighboring telecom wires , thereby
endangering the equipment and working personnel and also causing interference to circuits.
The extent of induction depends on a number of factors the most important of which will be separating distance between the two parallel networks. The CCITT mentioned that if current exceeding 15 milli amperes is traversed in the body of a person for a certain length of time endangers the life of the personnel.
1.11 Remedial Measures ::
Shifting of Telecom lines / cable to safe separating distance
Use of suitable type of cables. ( where long parallelism occurs, use Aluminum sheathed cables)
Use of booster Transformers and return conductors by the Railways.
Electrical Bonding of Rails to have higher screening effect of Rails.
Installation of Gas Discharge tubes at both ends of the telecom line / able involved in parallelism and at individual points along the line as may be necessary as per the induced voltage calculated theoretically. The fuses and heat coils may also be used.
1.12 PROTECTION FOR UNDER GROUND CABLES ::
The following general conditions to be observed during laying in view of safety of personnel and hazard to equipment and cable.
a. All the cable should be referred to PTCC for clearance so that the induced voltage under earth fault condition of neighboring power line and cables may be assessed theoretically and protections may be advised.
b. How ever as mentioned in EI G-2000, U/G cables can be laid even with out referring to PTCC for a length of 800 meters subject to minimum horizontal clearance of 2 ft from the nearest power cable. In such a case , care should be taken to have as much horizontal separation as possible from the power cable.
c. Cable passing near the power lines especially the power line supports are subjected to grave hazards of burning. To prevent this at least 50 meters separation will be necessary.
d. Laying of underground cables just below the alignment of HT & EHT power lines should be avoided. Cases of burning and damage to the telecom cables have been reported under earth fault condition of the power line.
e. While laying, safe separating distances should be observed.
f. When it is not possible to keep the safe separation, the cables may be laid in PVC piles /Ducts.
g. In case of poly-al cable, it is essential to maintain the continuity of the poly-Al and armour at every joint to provide proper screening effect.
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i. When the cable enters a power station area the earth potential rise can not be avoided. Top prevent the hazards due to this, it is necessary to break the armour and play screen
continuity at a point where the voltage due to earth potential rise reaches 430 volts.
Further, all cable conductors at this point and at the subscribers premises should be provided with GD tube protection. No Joints or Tees are allowed with in the zone of earth potential rise.
j. While crossing a power cable, a minimum vertical clearance of 2 ft should be observed and crossing should be as nearly right angles as possible. In case it may not be possible, laying of cables solid in bitumen for a distance of 3 ft on either side of the crossing point should be done.
k. For laying Telecom cable sin the vicinity of power cables, the EI G –2000 prescribes the following separation.
Minimum Horizontal and Vertical clearance – 2 ft. In no case less than 18 “for single core power cable & 12” for multi core power cable.
1.13 SAFETY MESURES FOR STAFF ::
While attending faults or laying cables the safety measures mentioned below must be adopted.
a. Cable routes subject to dangerous induction should be permanently marked by warning indicators along the route. This will warn the workers and remind them of the precautions they must take.
b. During storms the likely hood of power line faults are more and so workmen should be forbidden from working on the cables during storms in the vicinity.
c. Workmen must not be allowed to carry on single handed. They should always work in parties.
d. When using insulated clothing, gloves, shoes, etc or tools or electrical equipment which are insulated form earth, soldering irons and lamps and line man’s telephones sets, care must be taken before work begins to ensure that insulation is adequate and intact.
e. Before cutting a cable sheath a low resistance electrical connection has to be made between two parts of the sheath to be separated by the cut. This applies to the armour of the cable also.
f. The cable sheath should be earthed on both sides of the proposed cut.
g. All cable conductors should be handled by insulating gloves and tolls. Before any conductors is cut it must be earthed on both sides of the proposed cut. This earthing can be achieved by connecting to the cable sheath. In addition a spike earth prepared with 1
metre GI spike must be used. The earth connection should be removed only after completing all the work.
h. The cable sheath at all joints should be bonded
i. The GD tubes provided should be in proper functioning state.
j. Maintenance staff should handle the coaxial cable after disconnection at both the repeaters to restrict the induced voltage on one repeater.
2.0 LOW FREQUENCY INDUCTION :