Powercable Jointing

Powercable Jointing Points : powercable jointing, preparation of the ends of the cable, making sleeve for the joint, jointing, insulating the joint, jointing of lead sleeve, soldered joint, amalgaline plumbing The cable jointing is the work of a highly skilled man, since the successful working of the cable depends upon the quality of the joint made. All joints in the cables must be mechanically and electrically perfect as imperfect joints are the source of weakness and danger. The cable jointing can be divided into following heads: (a) Preparation of the ends of the cable When all the materials required for cable jointing and their equipment is ready the seal of the cable should be broken. Remove the lead sheath only two-thirds of its thickness for a length of about 15 cm. (about 4 cm. greater than the half length of the sleeve of the joint) by means of a back knife and hammer. Care should be taken to make a slanting cut in the lead sheath so that the core insulation may not be damaged.

After this operation, bare the conductor for a length of about 0.625 cm. more than the half length of the sleeve and the bare stranded conductor should be cleaned with naptha. The remaining length of the cable from which 2/3” thickness of lead is removed is protected (i.e. 3 .cm approx.) with the help of a tape. Then the stranded end of the conductor is secured with the help of two turns of binding wire, also near the end of the of cut the cable is secured with a few turns of tinned copper binding wire. Now remove the first binding wire made solder it either with the help of a soldering iron or by pouring molten solder from one ladle to another. For such soldering, powered resin may be used as flux. (b) Making sleeve for the joint For making the sleeve for the joint, the conductors are made to butt each other in the sleeve and are soldered. The sleeve usually is made from a sheet of copper, which is cleaned thoroughly and is bent to form a split tube. Two holes of about 1.25 cm. diameter are drilled from each end of the tube. Then a layer of pure tallow is applied on the inside and outside of the tube. Now a layer of solder is applied on the inside and outside of the sleeve, first by rolling in it powdered resin and then dipping it in molten solder. The excess of the solder on the sleeve is usually removed by means, of pulling a piece of rag through the sleeve when hot. (c) Jointing Before the actual process of jointing starts, a lead pipe about 20 cm. long is taken to cover the jointing copper sleeve and a hole of 0.5 cm. dia. is made which is called the filling hole. Then beat up the lead so that a lip is formed at the top of the hole. Drill two holes more, one at each end so as to act as air vents. Clean the ends of the lead pipe and scrap outer edges slightly. If it is not done, the joint will not take solder. Now thoroughly clean the lead sheath on each end of the cable and pass this lead pipe on one end of the cable.

Now insert the prepared ends of the copper or aluminium conductors of the cable into the copper sleeve. It is necessary that this sleeve should be a close fit over the conductors, and conductors should be placed so as to butt each other. Pour molten solder, over the split copper sleeve from a small ladle and keep another ladle below the joint to catch the drops of the solder. The excess of the molten solder should he wiped off. Care must be taken that the joint is in a horizontal position. It should be remembered that the jointing should not take more than half a minute otherwise the insulation of the cable will be spoiled. (d) Insulating the joint The method of insulating rubber insulated cables and paper insulated cables is different. For insulated rubber cable, India rubber tape is used with the help of rubber solution which is available either in collapsible tubes or in tins. Before ‘the rubber tape is provided over the joint, it is necessary to check up the rubber insulation on the cable; if it is damaged during the soldering operation, it must be cut away. Also bare the insulation for about 3 cm. on, each side of the joint. Clean the insulation by scraping so that no dust or threads etc. are left out. The rubber insulation on the cable now should be tapered by means of curved scissors for about 1.5cm. on each side. Now just beyond the point of taper, start providing tape on the bare conductors and copper sleeve. Care should be taken not to use rubber solution for the first layer, i.e. rubber solution should not come in contact with copper conductor. After first layer the second layer is provided, so that this layer overlaps the previous one for halt’ of its width: over the first layer, apply a small quantity of rubber solution and the serving of this tape now should be continued (only after the conductor is covered with rubber tape) upto the tapered portion of the rubber insulation. The serving of the tape should be continued until the insulation provided is of so much thickness that it conies up to the level of the rubber insulation with continued use of rubber solution on both sides of the tape. The last two layers should be continued beyond the start of tapered rubber insulation for about 1.25cm. For insulating the joint, care should be taken that the work must be clean and the rubber solution should he sparingly used. The cable insulation should be kept tapered only for the minimum period and in no case dirt or dust should come in contact with it.

For insulation of paper insulated cable joints impregnated cotton ‘tape usually’ available in tins, is used. Before such tapes are used, they are kept in a hoi solution of sleeve compound at a temperature of about 230°F, but in no case the tape should come in contact with the container vessel. The impregnated tape should be removed from the compound only when it has sufficiently cooled. The applied tape should be drawn as tightly over the joint as possible in order to exclude air, and moreover each lap should overlap the preceding one by half its width. In case of high voltage cables, (not in case of low voltage ad medium voltage cables) after the insulation is provided over the conductor, the core insulation is removed in a tapered fashion for a length of 4 cm. to 75cm. depending upon the voltage. This is obtained thus: Give a turn on the cable with the help of a copper wire and remove the paper insulation, and now give the turn at a distance of about 0.6cm. from the previous joint (moving towards the end of the cable) and, again remove the paper insulation up to this point and so on. Now the joint should be covered with an impregnated cotton tape; the thickness of the cotton tape provided should vary from 1 ½ to 2 times that of the paper insulation. The tape should be tapered to tape 1 .25cm. beyond the lead sleeve. (e) Jointing of lead sleeve After the conductors have been soldered and an insulation is provided over them, it is necessary to protect this insulation against mechanical injury and from the effect of atmosphere. Such protection is usually provided by slipping a lead sleeve over the joint and filling it with a compound. This sleeve is the same one which has been mentioned earlier and slipped over the cable before the conductors are soldered. Assuming that the lead sleeve and the lead over the cable have been well cleaned or scraped,, the following are two methods of jointing the sleeve with the lead sheath on the cable:

i) Soldered joint
ii) Amalgaline plumbing i) Soldered Joint Slip the lead sleeve over the insulated joint and dress down its ends over the cable sheath. Insert cotton waste into the joints near the ends. Light up a blow lamp and adjust its flame about 5 cm. long Warm up the lead on the sleeve and on the cable, taking care that only the tip of the flame should touch the lead and to heat the cable only at one point. Now bring the solder near the cable joint and apply flame on the solder only, so that it is laid first roughly over the joint in the form of lumps. Now heat the joint by moving the flame round the cable. Care should be taken to solder the edges properly. In order to check whether any pin- holes are left or not while the joint is warm apply tallow or a solution of glycerin and soft soap solution. Thus any pin-hole left is soldered properly. Similarly the other end of the sleeve is soldered. (ii) Amalgline plumbing In this method the lead on the sleeve and the cable are properly cleaned and the ribbon of amalgaline wrapped round the lead sheath of the cable. Then the lead in the sleeve is dressed down so that it tightly fits over the ribbon. Now apply heat by means of a blow tamp until the ribbon melts and at the same time the lead sheath of the cable and sleeve, melts and at the same time the lead sheath of the cable and sleeve melts and run together to form a solid joint. Such a method is quicker and neater.

Filling Compound in the Sleeve The compound to be filled in is heated in a pan or a ladle, taking care not to overheat. If it is overheated the insulating properties of the compound will deteriorate. So the compound should be heated up to a particular temperatures which varies according to the composition of the compound and thermometer should be tied for noting the temperature. In general the compound should not be heated beyond 300° F.

When the compound is melted the lead sleeve is warmed up taking care not to overheat it, otherwise the solder at the ends will be melted and the joint will be weakened. The warming of it is necessary, otherwise the poured compound will solidify and thus prevent the complete compounding of the joint. Pour the molten compound into the lip hole of the sleeve until it comes out of the vent holes of the sleeve. Allow the sleeve to cool the filling by means of soldering a lead cover over it, while the air vents should be soldered with drops of solder.