Mirror: G K Bhat
When we design a system, each one of the component and the related works are important. Failure of the smallest component, minor error in designing and execution of the project will end up with major problems. The cable and cable laying are another area where we have to take utmost care. Even designing stage, while calculating the cable requirement we have to be very careful. The designing technique of a designer of the system and his technical skill will help us to reduce the cable and achieve the redundancy of the system, meanwhile a poor designer’s design will increase the requirement of the cable , still we may not able to achieve the redundancy . We have to take lot of care while laying the cable to avoid cable cut and power induction.
The cabling should be tamper proof at all the places , the termination of the cable also be in such a way , that there should not be any room to tamper/snapping of cable purposely or by accident. All the cables required to cross the roads should be through proper road crossing conduits. All outside cables either armed cables or should be provided with HDPE duct. Should be laid minimum at the depth of below two and half feet from the surface, buried as per the cable laying specification. (Brick and sand overlay on the cable). All the underground and over ground cable routes have to be marked with proper markers. Over ground cables (If armed cable) can be laid by using metal trays, or else should be through proper MS Conduits/metal race ways. All the cables and connectors should be of high quality.To avoid the use of low quality /unbranded cables, the system integrator/vendor while supplying the cable, should support the brand and quality bysubmitting the manufacturer’s certificate with batch number.
General Guide line of Laying of cable underground (OFC) in side campus:
- Generally 6/12 pair cables are suggested, however if need be higher capacity OFCs can be used, but the same has to be justified for its advantage.
- OFC cables are available with deferent length, therefore while selecting the length of the cable, and has to be chosen in such a way that joining/cutting of the cable should be avoided as far as possible.
- All out door OFC cable should be laid using HDP ducts, no naked cable without conduit should be laid underground.
- The suggested size of HDPE is 50 mm.
Cable Trench: All out door cable has to be laid underground, the specification of the trench is given below. The size of the trench should be, 2.5 feet depth and 1 feet wide.
Laying of cable:
- After digging the trench entire length of the trench has to be filled with sand of six inch thick from the bottom.
- HDPE duct has to be laid over the sand fill.
- While laying the duct it should be ensured that there is no twist/damage or holes in the HDPE.
- After laying, (wherever applicable) the pipe has to be tested by closing one end and pressurising from other end to ensure that the duct is laid properly.
- After laying the duct, the duct has to be covered with slab of curved shape of 150 mm in diameter with 30 mm thickness, in such a way that the HDPE duct is covered properly.
- After laying the cement slab over the duct, the trench to be filled with 6 inch thickness using sifted earth.
- Over the back filling of the earth, a marker tape has to be laid for entire length, which will be called second level identification marker.
- The trench has to be filled with loose earth for another 1 feet.
- Over this back filling second marker tape has to be laid in such a way that the marker tape has to be at the centre of the trench at equal distance from both side of the trench wall.
- The final back filling of the trench has to be completed using all the remaining earth.
Compacting: The back filling of the trench has to be compacted. If the work is done during non-rainy season, watering to be done for entire length of the trench, till the soil sinking possibility is over. This has to be done to avoid the back fill sinking at later stage.
Splicing Chambers: When OFC cable to cable has to be joined and protected by using proper chamber, located at all the splicing locations. The chamber can be either RCC or of brick lining with cement plastered. If it is brick lined, the thickness of the wall should be 9 inch. The inner dimension of the chamber should be 1x1x1 metre size. The chamber should be made in such a way that the edges of the chamber should be 6 inch above the ground level to avoid the water getting inside the chamber during rainy season. The chamber should be covered with appropriate chamber cover preferably made of cast iron. The cover should be proper with side frames and during rainy season the water should not get into the chamber.
Drainage crossing: The drainage crossing if required should be done using an additional conduit of GI pipe of 4 inch diameter or a Hume pipe of 6 inch diameter, the HDPE duct should pass through this conduit. Bothe side of the pipe should be embedded with wall of the drainage for a depth of one feet at both side of the wall of the drainage.
Road Crossing: Road crossings of the cable has to be through a Hume pipe of minimum 6 inch diameter. However wherever the road cutting is not permitted, road crossing will be made by horizontal drilling. In case if it is horizontal drilling, then the conduits to be provided by using the appropriate pipes according to the soil condition. This will be discussed during final discussion of the site before starting execution of work. The pipe ends at both sides of the road should be 1 meter away from the edge of the road. If there is a drainage/trench along the side of the road the pipe should be laid in such a way that the pipe should be crossing the drainage. Both ends of the road crossing should be provided with chambers of the similar specification of the splice joint chamber.
UTP Cables: All the above specification/procedure will be followed while laying UTP cable, under the ground, however there is no requirement of using HDPE duct for UTP cable but the cable should be armoured cable and soft cables are not recommended while laying the cables underground.
Cable laying over ground: Over ground laying of the cable (OFC /UTP), should also be through metal conduit/race way/ cable tray. The size of the metal raceway/conduit shell be according to the number of cable passing through the race way/conduit. If the cable tray is used to lay the cable over ground the cable should be armoured cable if the cable try is an open tray.
Will be continued……
G K Bhat