UNDERGROUND CABLE INSTALLATION IN GROUND DUCT AND TRAY
Cable Installations Methods In Ground Duct & Cable Tray
The arrangement and method of cable laying both in ground duct and cable tray is an important factor to current carrying capacity and working performance of cable in its life time. Cables rating factors varies for air/ground ambient temperature variation, laying condition is a factor as such as
i. Ground temperature;
ii. Ambient air temperature;
iii. Depth of cable laying;
iv. Thermal resistivity of soil.
To protect the laid cable in future it also important to care on the location and procedure of cable trench excavation, ground strutting shuttering and pumping laying in backfill, specified quality protective covers drawing in ducts, reinstatement of the original surface without variation for type of rock shattered rock sand backfill footpath and roadway.
Cable may be laid direct in the ground with protective covers drawn into pipes or ducts laid in troughs or on racks or supported on trays or cleats as may be required as site situations and requirements.
Somewhere it may necessary to bring the cable up above ground, must be protected mechanically and by sun shield.
Cable Laid Direct in Ground
To install cable in underground first step to find out the suitable route line considering the points- shortest distance, minimum bends, easy access to maintenance and operation.
The depth of underground cable trench depends on operating voltage ratings, exm- for 132kV cable it’s may 1.5 m & for 33kV cable it’s may 1.2 m depth and width of the trench depends on the number of circuits be laid, spacing should be 0.25 m to 0.5 m for reducing the mutual heating
The conventional methods for cable laying in is as follows:
I. Laying directly on the ground;
II. Drawing into ducts or pipes;
III. Laying in trough;
IV. Laying on racks and cleats.
It’s important to consider for carry out an underground cable informing the police or any other state or Municipal Authority of his intention to commence and complete excavation public road.
All public safety by providing temporary traffic lights, warning notices flagmen picket fences and ropes along the route of any open trench and providing and adequate number of and maintain approved red flasher warring lights during the hours of darkness.
Filling in of trenches should not be commenced in adequate Engineer inspection and approval of the cables accessories in situ and the backfill material.
There is an example for laying procedure of underground cable project where service provider should lay the cables 1.1m deep direct in the ground in the following manner.
At least 100mm of builders’ fine sand graded and washed should be placed and compacted to form a bed for the cables;
After the cables have been laid it should be covered with additional screened backfill well punned over and around the cables to a level of 250 mm above the top the uppermost cable. Mechanical runners should not be used for this work. Protective covers are to be laid so that they completely cover the full width of the excavation;
Where in the opinion of the assigned Engineer the soil on the site is unsuitable for backfilling the service provider should arrange for the importation of suitable material. All backfill should have a thermal resistively value of not greater the 1.5oC m/w;
Where cables are laid in surfaced roads the backfill should be wholly sand to 100 mm below grounds level. The final sub-layer should be have broken hardcore from selected spoil. Temporary reinstatement with bricks should be carried out by the service provider and should match the level of existing road surface, while permanent reinstatement will be done by City Corporation / owner of the road specification and procedure;
Where more than one horizontal layer of cables is laid the level of the upper layers of cable should be gauged from the level of the finished bottom of the trench and marked on the side of the trench at frequent intervals before the installation of the lower layers to ensure that the correct vertical spacing is maintained;
Unless otherwise specified all power cables should be laid as follows.
I. Spacing between inner phases of single core cables laid flat of adjacent three phase circuits 750 mm. Single core cable phase spacing 300 mm;II. Spacing between trefoil groups of adjacent circuits 700 mm. Trefoil grouping should be secured by plastic ties or approved means every one meter of route length;III. Spacing between three core cables 700 mm.
Cable Drawn Into Ducts
Ducts and pipes would be ensure before pulling the cables, the duct should be dared by drawing through a mandrel of slightly less diameter than the duct. Any lubricant sued should have no deleterious effect on the cables.
Cable ducts should be completely smooth internally and of approved material diameter and thickness. Ducts may be of high impact PVC glazed earthenware or pitch fiber. In all the above cases the ducts should be completely covered on top and sides with 150 mm of concrete 1.2.4 mix. All occupied ducts except those for pilot cables should be fitted with Bentonite.
Ducts should have an internal diameter appropriate to the cables to be installed but should not be less than 100mm internal diameter. The minimum internal diameter otherwise should be the sum of the cable diameter (s) plus 35 mm.
Cable duct should be only on straight rout and where the route should deviate, manhole should be used to pull in cable and re-direct it in to successive duct.
Plastic spacers should be sued between the layers of cables when encasing the cables in concrete.
Each spare duct should be provided with a galvanized draw wire left in situ.
Any ducts or pipes not used should be sealed by plugs supplied by the service provider before backfilling.
All ducts whether occupied or not should be water and vermin proof sealed by an approved method and materials.
Road And Duct Crossings
Cables should be drawn into underground ducts at all road crossings. The ducts extend at least one meter beyond the edge of the road. Separate ducts should be installed for each main power cable. Each duct cluster should contain 100% spare ways. Each spare duct should be provided with a galvanized draw wire loft in situ.
Where cables have to cross ditches culverts bridges and irrigation channels etc., its need to provide suitable supports for the cables details of which should be followed by local standard.
Cables Installed in Concrete Trenches
In substations concrete trenches and cable ducts will be provided and installed for the cabling limited to the substation area. These trenches will not be filled with sand. All cable duct entries to buildings must be bell mouthed and whether in use or not for cables should be sealed against entry of water oil and vermin with a suitable semi-plastic compound.
All cables routed in concrete trenches should be suitably supported by means of cleats or racks and raised from the trench floor by means of suitable spacers. All cables should be run in neat and orderly manner and the crossing of cables within the trench should be avoided as far as possible. It must replacing the trench covers during the execution of the work.
Cable Supported on Racks & Trays
All single core and multi core main power cables should be installed in cleats fixed at specified distances.
All control cables should be installed on trays and neatly tied to the tray work by means of plastic straps at least every 2 meters.
The whole of the cable supporting steelwork including all cleats clamps nuts bolts washers’ fixings and other items to complete the installation for cables. The cable supporting steel work should be earthed properly.
All cables are to be run with a particular regard to neatness of appearance. Multiple runs are to be arranged so that cables entering or leaving the run do so in a logical manner.
Three core cables must be secured using claw type or multi core unit cleats. It is better to not use wooden cleats.
Every non – flexible type cable should be securely supported at a point more than 1m from its terminal gland to prevent stressing the termination and on vertical runs passing through floors immediately above the floor.
For single core cables carrying three – phase alternating current non-magnetic trefoil cleats should be used. The cheating arrangements should be adequate to withstand the short circuit forces.