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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Connection and termination of wires and cables. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric installation work In order to feel confident and safe in your apartment, you should check whether the cross-section of the wires of the electrical wiring corresponds to the maximum actual load, as well as the current of the protective fuses or the circuit breaker. Most often, contact failure occurs at the junction of the wires. The operability and durability of electrical wiring largely depend on how well the wires are connected to each other and the wires are connected to the contacts of electrical components. Therefore, it is advisable to get acquainted with the methods that ensure a reliable connection before starting work. The main purpose of each connection is a reliable and durable contact in an electrical circuit. When connecting wires, it should be borne in mind that the resistance of the connection should not exceed the resistance of the wire itself; in addition, it is necessary to ensure sufficient mechanical strength in the joints, especially in those parts of the chain where accidental stretching is not ruled out. According to the nature of the connection, they are classified into unearthly (welding, soldering, crimping) and detachable (on bolts, screw terminals, pins or stranding). As already mentioned, the most common wires for electrical wiring are with aluminum conductors, and they are relatively inexpensive. However, it is aluminum conductors that are the most difficult to connect, because an oxide film (hard and refractory) is always present on their surface, which is formed as a result of the oxidation reaction of aluminum with oxygen. The oxide film is a very poor conductor of electric current, so the detachable connections heat up noticeably. Of course, before connecting the wires, the film can be removed by stripping, but it instantly re-forms. In addition, aluminum wire has a low yield strength; this disadvantage is especially evident in screw connections (screw clamps): aluminum is simply is squeezed out from under the clamp, the contact is significantly weakened. The oxide film also greatly complicates the implementation of permanent connections: when soldering, it prevents the core from adhering to the solder, and when welding, it forms unwanted inclusions in the melt. In addition, aluminum oxide melts at a temperature of at least 2000 ° C (this is 3 times higher than the melting point of pure aluminum). Wires with copper conductors, as well as with conductors made of copper alloys (brass, bronze), are best connected by soldering. Let's consider each of the types of connections separately. Detachable connections The easiest way to connect wires to each other is a simple twist. In order to implement it, it is necessary to free the ends of the wire at a length of 3-5 cm from insulation and clean them to a shine with a small file or sandpaper. It is necessary to twist the cores very tightly, coil to coil. The ends remaining after twisting are carefully cut off with a file, and the extreme turns are pressed with pliers. Wire twisting can also be done using the bandage method: the stripped ends are clamped in a hand vise and wrapped with soft stripped wire (for the bandage, it is best to take copper wire with a diameter of 0,6-1,5 mm; in this case, the diameter of the bandage wire should not be larger than the diameter of the strands being twisted) . The middle part of the bandage should be made apart: if later it becomes necessary to solder this connection, the solder will better penetrate to the junction of the wires. After connecting, the ends of the wires are bent at a right angle, and another 8-10 turns of the bandage are applied on top. The ends of the veins remaining from the twisting are sawn off with a file. The method of simple or bandage twisting is applicable only for connecting wires to each other; it is impossible to connect the wire to the contacts of electrical components by twisting. The most convenient (and also quite reliable) way to connect wires to electrical components is to connect using contact clamps, which can be screw and spring. The technique for making connections with contact clamps is as follows. If single-wire aluminum and stranded copper conductors are involved in the connection, the screw terminals are equipped with a shaped washer or an asterisk washer, which prevents the core from being squeezed out from under the fastening; and for connecting wires with an aluminum core - also a split spring washer, which provides constant pressure on the core (Fig. 18).
Before connection, the wire is stripped in the usual manner in a section corresponding to three diameters of the screw terminal plus 2-3 mm. To ensure reliable contact, aluminum conductors can be cleaned with fine sandpaper lubricated with petroleum jelly. If the core is multi-wire, then at its end the individual wires are twisted into a tight flagellum. Then the end of the core is bent into a ring (with a diameter equal to the diameter of the clamp screw) using round-nose pliers or pliers. It is best to bend the ring clockwise, this will prevent it from unwinding when the screw is tightened. The clamping screw or nut is tightened until the spring washer is fully compressed, after which it is tightened by about another half turn. Currently, electrical components are equipped with screw fasteners of a clamp-butt type: when making such connections, the stripped and stripped end of the wire is not bent into a ring, and the straight end of the core is inserted into the clamp and pressed with a screw. Spring-type clamp-type connections are mainly used in luminaires with fluorescent lamps for connecting wires to lamp sockets. Their design is a springy plate made of high-quality bronze, which firmly presses the wire core to the clamp body. This connection design completely eliminates a spontaneous connector, and in order to release the wire if necessary, it is enough to insert a steel knitting needle (thin screwdriver tip) into the clamp, bend the spring plate and release the wire. All parts used for connection with aluminum wires must have an anti-corrosion galvanized coating. The same requirement applies to steel parts. An aluminum wire with a cross section of 2,5 mm2 is connected to copper reinforcing wires (for example, with chandelier wires), solid and stranded, using chandelier clamps. First, the wires to be connected are cleaned with sandpaper (copper in the usual way, and aluminum - under a layer of petroleum jelly) and lubricated with quartz-vaseline paste. After stripping, the wires are attached to the bar and pressed with screws with spring washers. The connection is inserted into the base of the chandelier clamp and closed with a lid. When purchasing electrical components with screw clamps, it is necessary to pay attention to the type of clamps, because some electrical installation devices (a number of threaded cartridges for incandescent lamps, cartridges for fluorescent lamps and starters, walk-through and built-in small-sized switches) are equipped with clamps that provide connections only with copper wires. One-piece connections All methods of detachable connections are convenient primarily because, if necessary, they can be easily disassembled and then restored again. However, connections of this type do not always provide high reliability and durability of the contact. Therefore, in cases where it is necessary to ensure the special strength of the connection, it is produced in an integral way: by soldering, welding or crimping. This method of connecting wires, like soldering, is widely used to connect electrical contacts - both in wires and in electrical household appliances for connecting the leads of electrical elements. Soldering is often used in electronic equipment. However, the use of soldering does not apply to contacts that are subjected to mechanical stress or heat. In the process of soldering, in addition to the strands of wires and the contact surfaces to which the wires are attached, solders and fluxes are also involved. Solder is a lead-tin alloy in the form of a wire or stick, which, when soldering, plays the role of a connecting material. Solders of two grades are produced for soldering ordinary wires: POS-30 or POS-40; they differ from each other in the content of tin in percent by weight (30 and 40%, respectively). The melting temperature of solders for POS-30 is 225 °C, and for POS-40 - 234 °C. For soldering semiconductor devices, solders with the addition of bismuth, gallium, cadmium are used; Additives give solders fusibility, their melting point does not exceed 150 °C. If cermet parts are involved in soldering, then a powder mixture is used as solder. Fluxes in the soldering process play the role of insulators of the soldered surfaces from the formation of an oxide film during heating; in addition, they reduce the surface tension of the solder. Fluxes must meet the following requirements: - in the range of solder melting temperatures, fluxes must maintain the stability of their chemical composition (not decompose into components) and activity; - they should not enter into a chemical reaction with metal and solder; - products of interaction of fluxes with an oxide film should be easily removed by washing or evaporation; - Fluxes must have a sufficiently high fluidity. Universal fluxes (suitable for soldering both aluminum and copper wires with parts made of various metals) are rosin and soldering acid. For soldering steel wires, a flux of the following composition would be more suitable: 3 parts of etched hydrochloric acid and 1 part of a saturated aqueous solution of ammonia. On sale there are alloys in the form of bars or wire, in which solder and flux are combined. Before soldering, the ends of the wires are freed from insulation and carefully cleaned with sandpaper until a shine appears. To get a stronger connection, it is preferable to tin the ends of the cores (cover with a layer of molten solder). Next, the ends are twisted together. There are many ways to twist the cores for soldering (Fig. 19-22). The use of one or another method depends on the material of the core, its cross section and the functional purpose of the connection.
Before soldering, copper conductors can be twisted in any way, and aluminum conductors are preferable with a groove (in this method, almost the entire surface of the contacted conductors is covered with solder, so it is more reliably protected from the formation of an oxide film. When soldering wires with a large cross section, it is best to use bandage twist or a combination of bandage twist and groove twist, since it is difficult to tightly and firmly twist large section wires together. The surface of the bandage wire should also be tinned with molten solder. If it is necessary to connect stranded wires by soldering, then after stripping the wires of each wire, they are twisted together into pigtails and only after that they are twisted (Fig. 22). The soldering technique is not particularly difficult. It consists of the following steps: 1. Heat up the soldering iron. The degree of heating can be checked by immersing the tip of the soldering iron in solid ammonia: if the ammonia hisses and blue smoke comes from it, then the soldering iron is warm enough and you can start soldering; do not overheat the soldering iron; 2. In the process of heating, scale usually forms on the tip of the soldering iron, so it must be cleaned with a file; 3. The working part of the soldering iron is first immersed in the flux, and then in the solder, so that droplets of molten solder remain on its tip. It is not necessary to take very large drops of solder, it is enough such an amount that the solder covers the wires from all sides during soldering and at the same time the turns of the bandage or twist are visible from under the layer; 4. The surfaces of the twisted cores are heated with a soldering iron, filling the gaps between them with molten solder; 6. When the soldering site has cooled down, a cotton swab dipped in acetone removes the flux residues and products of its reaction with the oxide film. If a twist of thick wires is soldered, then to obtain a strong connection, a large amount of solder is needed, which cannot be conveyed at a time on the tip of the soldering iron. In this case, soldering is easier to perform in a slightly different way: the twisted strands of wires are heated with a soldering iron, then a solder stick is brought directly to the tip of the soldering iron, the solder melts and flows into the twist itself. As we have already said, soldering aluminum conductors is quite difficult due to the oxide film that instantly forms on the aluminum surface even after it has been thoroughly stripped. To facilitate the soldering process, you can use one of two methods.: - firstly, aluminum conductors can be pre-cleaned with sandpaper heavily lubricated with petroleum jelly. Abrasive particles of sandpaper will remove the oxide film, and petroleum jelly will prevent its re-formation; - secondly, in order to avoid the formation of an oxide film, the tinning of the ends of the cores before twisting must be done under a layer of sewing oil or molten rosin, adding a little steel filings to them. A soldering iron tip rubs the core under pressure, while steel filings peel off the oxide film, and a layer of oil or rosin insulates the core from the interaction of aluminum with atmospheric oxygen. After soldering steel wires, the products of the interaction of the flux with the oxide film are removed with an oily rag and cooled. The layer of solder, as in other cases, should cover the entire twist. But the most reliable and durable connection of both aluminum and copper wires is provided by welding. Although this method is more complicated and more time-consuming compared to other types of connections, and also requires special equipment, it is still available at home (when laying and repairing electrical wiring and electrical appliances on your own). The essence of welding is the contact heating of the ends of the wires with a carbon electrode until a molten ball is formed, which is formed at the point of contact of the wires with the electrode. The use of the welding method when connecting aluminum or copper conductors is limited by their cross section: aluminum conductors can be welded if their cross section does not exceed 10 mm2, and copper - with a cross section of 4 mm2. For the production of welding work, a laboratory 9-ampere autotransformer (LATR) is used, somewhat modified to perform this operation. It is necessary to remove the slider that regulates the voltage from the transformer, and wind the secondary winding over the mains (primary) winding. The secondary winding must be isolated from the network with several layers of special transformer paper and several layers of insulating tape on a cotton basis or varnished fabric. After such re-equipment, the voltage at the output of the transformer must be at least 6-10 V and the power must be at least 0,5 kW. The electrode and the ends of the wires to be welded are connected to the ends of the secondary winding of the transformer. In the absence of a LATR device, it can be made (winded) independently. As the core of the transformer take W-shaped transformer iron; the cross section of the magnetic circuit must be at least 25 cm2. The number of turns for the primary and secondary windings is easy to calculate using the following formulas:
where W1 and W2 are the number of turns of the primary and secondary windings; U1 and U2 - voltage at the input and output of the transformer; S - section of the magnetic circuit of the transformer W-shaped iron. Consider a specific example: it is necessary to wind a transformer with a magnetic circuit cross section of 30 cm2, which can be used at a mains voltage of 220 V; the output voltage should be 10 V. Under these conditions W1 = 40 x 220/30 = 293,33, i.e. 293; W2 \u40d 10 x 30 / 13,33 \u13d 293, that is, 13. Thus, the primary winding of the transformer should consist of 0,8 turns, and the secondary - of 1. Provided that for the primary winding it is necessary to use a wire with a diameter of 15-20 mm, the total cross section of the wires of the secondary winding must be at least 2-3 mmXNUMX. It is most convenient to wind the winding simultaneously with three parallel wires with a diameter of XNUMX mm. For the manufacture of the electrode, you can use the carbon brush of the old commutator motor or the graphite liner of the trolley rod. A small hole is hollowed out in the brush or liner, into which the flux is placed and which contributes to the formation of a ball from the melt. The finished electrode with flux is securely fixed in the clamps. When welding on such equipment, an assistant is needed, since, by manipulating two contacts at the same time, it is impossible to turn the transformer on and off on its own. But if, in addition to the transformer, you make a clamp (Fig. 23), which will simultaneously fix both the carbon electrode and the connected wires, then the assistant will be superfluous.
The preparation of wires for the implementation of a permanent connection by welding is similar to the preparation for soldering, although it has one peculiarity: the twisting of wires for welding must be performed only in a parallel way, and the ends of the strands remaining from twisting must have the same length, providing both wires with reliable contact with the carbon electrode (Fig. 24).
Flux is also involved in the welding process. Its purpose is the same as for soldering - to protect the melt from atmospheric oxygen. The flux for welding consists of 5 parts of potassium chloride, 3 parts of sodium chloride and 2 parts of cryolite; can be used as a welding flux and ordinary borax (sodium tetraborate). The welding process is carried out in the following order: the carbon electrode is fired (it is safer to do this in the open air), flux is poured into the hole of the carbon electrode, the wire twist is lowered into the flux mass and pressed against the electrode, the transformer is turned on. Under the action of an electric current, the carbon electrode begins to heat up, the flux melts and envelops the wires to be welded, stopping the access of oxygen to them and thereby preventing the oxidation of the metal of the wires. When the melting point of the metal is reached, the veins melt and merge into a ball. The transformer is turned off. In order for the transformer to be turned off at any time, a pass-through switch is used in its design (these are usually used on the cords of floor lamps), which is diverted with a separate cord and held in hand. After the place of soldering has finally cooled down and hardened (it is impossible to open the contact of the cores and the electrode before this time, since you can get a serious burn with splashes of molten metal), it is cleaned of flux, varnished and insulated. Where should the welding transformer be installed during welding? Based on considerations of the quality of welding work, the transformer should be located in close proximity to the place of work, that is, the length of the wires connecting the transformer to the carbon electrode and the wires to be welded should be minimal. The farther the transformer is from the place where the welding is done, the greater the voltage loss due to the length of the electric line, and therefore the quality of the welded joint suffers. Without welding experience, if it is necessary to get a connection of wires (or wires with a part), it is by means of welding that one should not rush to do a responsible operation right away - at first it is better to master the welding technology on unnecessary wire cuts. The connection and termination of the wires by crimping is carried out as follows. Wires and cables are released from insulation in a section equal in length to the tubular part of the ferrule (half the length of the connecting sleeve) plus 2 mm for wires and 10 mm for cables. The end of the wire freed from insulation is covered with a layer of petroleum jelly or paste and cleaned with a metal brush to a shine. Then clean the end of the wire from contaminated Vaseline and cover it again with clean Vaseline. The stripped end of the core is inserted into the tip or connecting sleeve, cleaned and filled with zinc-vaseline or quartz-vaseline paste, so that the core enters the tip until it stops, and into the connecting sleeve - half its length. After that, they are pressed in two places, that is, they are crimped. For wires with a cross section of 16-50 mm2, pincers of the PK-1 type are used; for wires with a cross section of 16-240 mm2, a hydraulic press of the RGP-7M type is used; After removing the burrs and inspecting the pressed sleeves or lugs, the section of the wire or cable core between the lug and the insulation or the sleeve and the insulation is carefully cleaned of paste residues, coated with an air-drying moisture-proof varnish (for example, asphalt) to protect against corrosion and wrapped with insulating tape. From above, the insulating tape is covered with a layer of the same varnish. The connection and branching of pre-twisted single-wire aluminum conductors with a cross section of 2,5-10 mm2 can be performed by crimping (without sleeves and paste) using KSP-4 tongs. With this method, in order to obtain a good contact, it is necessary to carefully clean the connecting ends and maintain their cleanliness and the cleanliness of the crimping elements of the tongs during the crimping process. Aluminum single-wire wires are connected to copper clamps of electric motors and electrical apparatus in the same way as to installation products. This was the last of the methods of connecting wires (or wires and electrical components) that are used in the installation and repair of electrical wiring (electrical appliances). And now a few rules (or tips) common to all connection methods: - insulation from the ends of the wires for twisting should be removed in such a way that the twisting consists of at least five turns; - since insulation is removed at the junctions of cores and wires and metal is exposed, there is a possibility that over time the metal may corrode (interacting with air moisture), which will affect the strength and quality of the connection, therefore it is recommended to protect the twist and adjacent stripped sections of the wire coating of asphalt-bitumen varnish, bitumen or oil paint; - sections of wires stripped of insulation after making connections (by any method) must be reliably insulated, and various cores of two or more core wires are insulated separately; the insulation should not only cover the junction itself, but also capture the wire braid on both sides. In damp and damp rooms, it is better to use polyvinyl chloride instead of rubberized insulating tape to insulate wire junctions. - connections and branches of wires should be made only in the appropriate boxes with closing lid. By the way, in junction and junction boxes, wires can be tightened with screw connections; for this, nuts or screws are pressed into the base of the boxes (Fig. 25);
- regardless of the method of connection, they should be located in places where the impact on them of stretching and other mechanical loads would be excluded; - branching and junction boxes should be located in places easily accessible for repair work (for example, branching boxes should not be masked under ceramic tiles or a layer of plaster, they should be installed in such a way that the cover is flush with the wall); - since aluminum conductors are very unstable to fracture, it is recommended to connect them by soldering; - all parts and contacts connected to aluminum wires must have an anti-corrosion galvanic coating. Author: Korshevr N.G.
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