CARE IN LAYING & JOINTING OF UNDERGROUND CABLES

CARE IN LAYING & JOINTING OF UNDERGROUND CABLES

1. It has been observed that the construction staff, in laying and jointing of the cable, is not exercising the sufficient care. Instances have come to the notice of the Director General of major breakdowns of cables have been caused by bad workmanship during cable laying. It is, therefore, of utmost importance that all construction officers and staff take due precautions and personal care while laying and jointing cables. The maintenance divisions must also take proper acceptance tests before taking over the cable work and bringing them into commission. All instances of defective construction work and unsatisfactory test must be brought to the notice of the higher authorities in due time.

2. The following points must be strictly observed.

i) The cable drums must be carefully handled at the store yard as well as at the sites of the work. For ensuring careful loading in and unloading from railway wagons and trucks by the construction staff during primary and local distribution, operation must be carried out under the supervision of construction officers or engineering supervisors. In no circumstances shall the drums be dropped from any height even if it is small. The usual methods to be employed for loading or unloading of cable drums.

a)Use of a crane.
b)Use of slopping planks between loading board of the truck and the road surface.
c) aking temporary ramp at site.

ii)In almost all the towns it is necessary to lay the cable in the trench by pulling along the run of the trench. While slight dragging in the trench permissible in the case of armored heavy cables, it must be borne in mind that unarmored cable shall never be dragged, as otherwise while doing so the sheath is liable to be damaged. Sufficient labor force should be engaged for paying out the cable without much strain being felt by the sheath or the conductors inside. When cables of large sizes are laid pulling long pieces may not be feasible and therefore ‘flexing’ may be resorted to i.e. cables are laid out on the road in the form of large loops and then pulled along the trench eliminating the curves one by one. This process requires very close attention especially when the loops narrow down, otherwise the cable may get kink and unperceived fractures, which show up later in the form of incipient faults, may develop. Responsible staff or trained labor must therefore be stationed at the curves to ensure that the diameter of the loops is kept within limits. The radius of any bend less than 3 shall not be permitted. An arrangement is being made to procure ‘cable rollers’, which will obviate any possible necessity of dragging or flexing. Till such time the rollers are made available and special pre-cautions shall be taken to prevent rough handling during ‘flexing’ or pulling.

(iii) After a cable has been laid 6” of soft earth should be filled. Shingles or gritty soil should be avoided, as these tend to damage the cable ‘sheath’.

(iv) While refilling cable trench, the refilled soil must be rammed thoroughly in repeated layers of 6” each, till the surface of the road is reached. It usually takes one man to ram the soil of the trench refilled by extra six persons.

(v) While drawing cables in conduits the open end of the cable where cable grip is to be attached must be nicely dressed down so that the tension of the grip applies firmly to the rigid body comprising of the core and the sheathing. If the sheathing is loosely covering the core, as in the case of an undressed end, there is the possibility of the sheathing getting fractured or in worse cases even getting pulled off under force.

(vi) Ends of cable section on the drum in the laid section at jointing points and in manholes must be ‘sealed’ immediately after cutting the cable. The practice of leaving the open ends unsealed is greatly responsible for undermining the general insulation of the core.

(vii) All junction cables and main cables must be tested with pressure (dry carbon di-oxide gas) at a pressure of 10 pounds for a sustained period of 12-24 hours before being laid. The overall length of the cable must again be similarly tested immediately after the sections have been jointed. Any leakage discovered must be traced and rectified.

(viii) Insulation resistance of all junctions and main cables must be tested after jointing the complete lengths of the cable. In making insulation tests the limit of 5,000 Mega Ohm per mile for local cables and 10,000 Mega Ohm per mile for junction cables must be attained after applying the temperature correction. Temperature correction in terms of Multiplier constant is given below:

(ix) All cable joints must be tested for leakage by using “DESICCATOR PUMPS” for introducing dry air in the joints and testing for leakage by applying soapsuds over the surface of the cable.

(x) In the case of directly buried cables the joint must be laid ‘solid’ in bitumen.

(xi) In the case of protected cables laid through ducts the joints must rest properly over cable bearers so that the load is borne by cable tails and not by the jointing sleeve.

(xii) Under no circumstances must be plumbers metal as supplied, be adulterated with lead or tin, as these tends to weaken the metal, which subsequently develops cracks. “Obvious cases of supply of bad plumbers metal must be specially dealt with. The plumbers metal must be withdrawn from use and report submitted to the Director General for further necessary action.”