ELECTRIC CABLE MANUFACTURE
David A. O’Malley
Cables come in a variety of sizes for different uses, from supertension power cables which carry electrical power at more than 100 kilovolts, down to telecommunication cables. The latter in the past utilized copper conductors, but these have been superseded by fibre optic cables, which carry more information in a much smaller cable. In between there are the general cables used for house wiring purposes, other flexible cables and power cables at voltages below those of the supertension cables. In addition, there are more specialized cables such as mineral insulated cables (used where their inherent protection from burning in a fire is crucial-for example, in a factory, in a hotel or on board a ship), enamelled wires (used as electrical windings for motors), tinsel wire (used in the curly connection of a telephone handset), cooker cables (which historically used asbestos insulation but now use other materials) and so on.
Materials and Processes
The most common material used as the conductor in cables has always been copper, due to its electrical conductivity. Copper has to be refined to high purity before it can be made into a conductor. The refining of copper from ore or scrap is a two-stage process:
1. fire refining in a large furnace to remove unwanted impurities and cast a copper anode
2. electrolytic refining in an electrical cell containing sulphuric acid, from which very pure copper is deposited on to the cathode.
In modern plants, copper cathodes are melted in a shaft furnace and continuously cast and rolled into copper rod. This rod is drawn down to the required size on a wire-drawing machine by pulling the copper through a series of precise dies. Historically, the wire-drawing operation was conducted in one central location, with many machines producing wires of different sizes. More recently, smaller autonomous factories have their own, smaller wire-drawing operation. For some specialist applications the copper conductor is plated with a metal coating, such as tin, silver or zinc.
Aluminium conductors are used in overhead power cables where the lighter weight more than compensates for the inferior conductivity compared to copper. Aluminium conductors are made by squeezing a heated billet of aluminium through a die using an extrusion press.
More specialized metallic conductors utilize special alloys for a particular application. A cadmium-copper alloy has been used for overhead catenaries (the overhead conductor used on a railway) and for the tinsel wire used in a telephone handset. The cadmium increases the tensile strength compared to pure copper, and is used so that the catenary does not sag between supports. Beryllium-copper alloy is also used in certain applications.
Optical fibres, consisting of a continuous filament of high optical quality glass to transmit telecommunications, were developed in the early 1980s. This required a totally new manufacturing technology. Silicon tetrachloride is burnt inside a lathe to deposit silicon dioxide on a blank. The silicon dioxide is converted to glass by heating in a chlorine atmosphere; then it is drawn to size, and a protective coating is applied.
Many insulation materials have been used on different types of cables. The most common types are plastic materials, such as PVC, polyethylene, polytetrafluoroethylene (PTFE) and poly- amides. In each case, the plastic is formulated to meet a technical specification, and is applied to the outside of the conductor using an extrusion machine. In some instances, materials may be added to the plastic compound for a particular application. Some power cables, for example, incorporate a silane compound for cross-linking the plastic. In cases where the cable is going to be buried in the ground, a pesticide is added to prevent termites from eating the insulation.
Some flexible cables, particularly those in underground mines, use rubber insulation. Hundreds of different rubber compounds are needed to meet different specifications, and a specialist rubber compounding facility is required. The rubber is extruded on to the conductor. It must also be vulcanized by passing through either a bath of hot nitrite salt or a pressurized liquid. To prevent adjacent rubber-insulated conductors from sticking together, they are drawn through talc powder.
The conductor inside a cable may be wrapped with an insulator such as paper (which may have been soaked in a mineral or a synthetic oil) or mica. An outer sheath is then applied, typically by plastic extrusion.
Two methods of manufacturing mineral insulated (MI) cables have been developed. In the first, a copper tube has a number of solid copper conductors inserted into it, and the space between is packed with a magnesium oxide powder. The whole assembly is then drawn down through a series of dies to the required size. The other technique involves continuous welding of a copper spiral around conductors separated by powder. In use, the outer copper sheath of an MI cable is the earth connection, and the inner conductors carry the current. Although no outer layer is needed, some customers specify a PVC sheath for aesthetic reasons. This is counter-productive, since the main advantage of MI cable is that it does not burn, and a PVC sheath negates this advantage somewhat.
In recent years the behaviour of cables in fires has received increasing attention for two reasons:
1. Most rubbers and plastics, the traditional insulation materials, emit copious quantities of smoke and toxic gases in a fire, and in a number of high-profile fire incidents this has been the main cause of death.
2. Once a cable has burnt through, the conductors touch and fuse the circuit, and so electrical power is lost. This has led to the development of low smoke and fire (LSF) compounds, both for plastic and rubber materials. It should be realized, however, that the best performance in a fire will always be obtained from an MI cable.
A number of specialized materials are used for certain cables. Supertension cables are oil-filled both for insulation and cooling properties. Other cables use a hydrocarbon grease known as MIND, petroleum jelly or a lead sheath. Enamelled wires are typically made by coating them with a polyurethane enamel dissolved in cresol.
In many cables the individual, insulated conductors are twisted together to form a particular configuration. A number of reels containing the individual conductors revolve around a central axis as the cable is drawn through the machine, in operations known as stranding and lay-up.
Some cables need to be protected from mechanical damage. This is often done by braiding, where a material is interwoven around the outer insulation of a flexible cable such that each strand crosses each other one over and over again in a spiral. An example of such a braided cable (at least in the UK) is that used on electric irons, where textile thread is used as the braiding material. In other cases steel wire is used for the braiding, where the operation is referred to as armouring.
Larger cables are supplied on drums of up to a few metres in diameter. Traditionally, drums are wooden, but steel ones have been used. A wooden drum is made by nailing together sawn timber using either a machine or a pneumatic nailing gun. A copper-chrome-arsenic preservative is used to prevent the wood from rotting. Smaller cables are usually supplied on a cardboard reel.
The operation of connecting the two ends of cables together, known as jointing, may well have to be carried out in a remote location. The joint not only has to have a good electrical connection, but must also be able to withstand future environmental conditions. The jointing compounds used are commonly acrylic resins and incorporate both isocyanate compounds and silica powder.
Cable connectors are commonly made out of brass on automatic lathes which manufacture them from bar stock. The machines are cooled and lubricated using a water-oil emulsion. Cable clips are made by plastic injection machines.
Hazards and their Prevention
The most widespread health hazard throughout the cable industry is noise. The noisiest operations are:
· the copper fire refinery
· continuous casting of copper rods
· cable drum manufacture.
Noise levels in excess of 90 dBA are common in these areas. For wire-drawing and braiding the overall noise level depends upon the number and location of machines and the acoustic environment. The machine layout should be planned to minimize noise exposures. Carefully designed acoustic enclosures are the most effective means of controlling the noise, but are expensive. For the copper fire refinery and continuous casting of copper rods the main sources of noise are the burners, which should be designed for low noise emission. In the case of cable drum manufacture the pneumatically operated nail guns are the principal source of noise, which can be reduced by lowering the air-line pressure and installing exhaust silencers. The industry’s norm in most of the above cases, however, is to issue hearing protection to workers in the areas affected, but such protection will be more uncomfortable than usual due to the hot environments in the copper fire refinery and continuous casting of copper rods. Regular audiometry should also be conducted to monitor each individual’s hearing.
Many of the safety hazards and their prevention are the same as those in many other manufacturing industries. However, special hazards are presented by some cablemaking machines, in that they have numerous reels of conductors rotating around two axes at the same time. It is essential to ensure that machine guards are interlocked to prevent the machine from operating unless the guards are in position to prevent access to running nips and other rotating parts, such as large cable drums. During the initial threading of the machine, when it may well be necessary to permit the operator access inside the machine guard, the machine should be capable of moving only a few centimetres at a time. Interlock arrangements can be achieved by having a unique key which either opens the guard or has to be inserted into the control console to allow it to operate.
An assessment of the risk from flying particles-for example, if a wire breaks and whips out-should be made.
Guards should preferably be designed to physically prevent such particles from reaching the operator. Where this is not possible, suitable eye protection must be issued and worn. Wire-drawing operations are often designated as areas where eye protection must be used.
In any hot metal process, such as a copper fire refinery or casting copper rods, water must be prevented from coming into contact with molten metal to prevent an explosion. Loading the furnace can result in the escape of metal oxide fumes into the workplace. This should be controlled using effective local exhaust ventilation over the charging door. Similarly the launders down which the molten metal passes from the furnace to the casting machine and the casting machine itself need to be adequately controlled.
The principal hazard in the electrolytic refinery is the sulphuric acid mist evolved from each cell. Airborne concentrations must be kept below 1 by suitable ventilation to prevent irritation.
When casting copper rods, an additional hazard can be presented by the use of insulation boards or blankets to conserve heat around the casting wheel. Ceramic materials may have replaced asbestos in such applications, but ceramic fibres themselves must be handled with great care to prevent exposures. Such materials become more friable (i.e., easily broken up) after use when they have been affected by heat, and exposures to airborne respirable fibres have resulted from handling them.
An unusual hazard is presented in the manufacture of aluminium power cables. A suspension of graphite in a heavy oil is applied to the ram of the extrusion press to prevent the aluminium billet from sticking to the ram. As the ram is hot, some of this material is burnt off and rises into the roof space. Provided that there is no overhead crane operator in the vicinity and that roof fans are fitted and working, there should be no risk to the health of workers.
Making either cadmium-copper alloy or beryllium-copper alloy can present high risks to the employees involved. Since cadmium boils well below the melting point of copper, freshly generated cadmium oxide fumes will be generated in great quantities whenever cadmium is added to molten copper (which it must be to make the alloy). The process can be carried out safely only with very careful design of the local exhaust ventilation. Similarly the manufacture of beryllium-copper alloy requires great attention to detail, since beryllium is the most toxic of all the toxic metals and has the most stringent of exposure limits.
The manufacture of optical fibres is a highly specialized, high-technology operation. The chemicals used have their own special hazards, and control of the working environment requires the design, installation and maintenance of complex LEV and process ventilation systems. These systems must be controlled by computer-monitored control dampers. The main chemical hazards are from chlorine, hydrogen chloride and ozone. In addition, the solvents used to clean the dies must be handled in extracted fume cabinets, and skin contact with the acrylate-based resins used to coat the fibres must be avoided.
Both plastic compounding and rubber compounding operations present particular hazards which must be adequately controlled (see the chapter Rubber Industry ). Although the cable industry may use different compounds than other industries, the control techniques are the same.
When they are heated, plastic compounds will give off a complex mixture of thermal degradation products, the composition of which will depend upon the original plastic compound and the temperature to which it is subjected. At the normal processing temperature of plastic extruders, airborne contaminants are usually a relatively small problem, but it is prudent to install ventilation over the gap between the extruder head and the water trough used to cool the product down, mainly to control exposure to the phthalate plasticizers commonly used in PVC. The phase of the operation which may well warrant further investigation is during a changeover. The operator has to stand over the extruder head to remove the still-hot plastic compound, and then run the new compound through (and on to the floor) until only the new colour is coming through and the cable is centralized in the extruder head. It can be difficult to design effective LEV during this phase when the operator is so close to the extruder head.
Polytetrafluoroethylene (PTFE) has its own special hazard. It can cause polymer fume fever, which has symptoms resembling those of influenza. The condition is a temporary one, but should be prevented by adequately controlling exposures to the heated compound.
The use of rubber in making cables has presented a lower level of risk than other uses of rubber, such as in the tyre industry. In both industries the use of an antioxidant (Nonox S) containing b-naphthylamine, up to its withdrawal in 1949, resulted in cases of bladder cancer up to 30 years later in those who had been exposed prior to the withdrawal date, but none in those employed after 1949 only. The cable industry, however, has not experienced the increased incidence of other cancers, particularly of lung and stomach, seen in the tyre industry. The reason is almost certainly that in cable manufacture the extrusion and vulcanizing machines are enclosed, and employee exposures to rubber fumes and rubber dust were generally much lower than in the tyre industry. One exposure of potential concern in rubber cable factories is the use of talc. It is important to ensure that only the non-fibrous form of talc (i.e., one which does not contain any fibrous tremolite) is used and that the talc is applied in an enclosed box with local exhaust ventilation.
Many cables are printed with identification markings. Where modern video jet printers are used the risk to health is almost certainly negligible due to the very small quantities of solvent utilized. Other printing techniques, however, can result in significant solvent exposures, either during normal production, or more usually during cleaning operations. Suitable exhaust systems should therefore be used to control such exposures.
The main hazards from making MI cables are dust exposure, noise and vibration. The first two of these are controlled by standard techniques described elsewhere. Vibration exposure occurred in the past during swaging, when a point was formed at the end of the assembled tube by manual insertion into a machine with rotating hammers, so that the point could be inserted into the drawing machine. More recently this type of swaging machine has been replaced with pneumatic ones, and this has eliminated both the vibration and the noise generated by the older method.
Lead exposure during lead sheathing should be controlled by using adequate LEV and by prohibiting eating, drinking and cigarette smoking in areas liable to be contaminated with lead. Regular biological monitoring should be undertaken by analysing blood samples for lead content at a qualified laboratory.
The cresol used in the manufacture of enamelled wires is corrosive and has a distinctive odour at very low concentrations. Some of the polyurethane is thermally degraded in the enamelling ovens to release toluene di-isocyanate (TDI), a potent respiratory sensitizer. Good LEV is needed around the ovens with catalytic afterburners to ensure that the TDI does not pollute the surrounding area.
Jointing operations present hazards to two distinct groups of workers-those that make them and those that use them. Manufacture involves the handling of a fibrogenic dust (silica), a respiratory sensitizer (isocyanate) and a skin sensitizer (acrylic resin). Effective LEV must be used to adequately control employee exposures, and suitable gloves must be worn to prevent skin contact with the resin. The main hazard to users of the compounds is from skin sensitization to the resin. This can be difficult to control since the jointer may not be able to avoid skin contact altogether, and will often be in a remote location away from a source of water for cleaning purposes. A waterless hand cleanser is therefore essential.
Environmental hazards and their prevention
In the main, cable manufacture does not result in significant emissions outside the factory. There are three exceptions to this rule. The first is that exposure to the vapours of solvents used for printing and other purposes are controlled by the use of LEV systems which discharge the vapours to the atmosphere. Such emissions of volatile organic compounds (VOCs) are one of the components necessary to form photochemical smog, and so are coming under increasing pressure from regulatory authorities in a number of countries. The second exception is the potential release of TDI from enamelled wire manufacture. The third exception is that in a number of instances the manufacture of the raw materials used in cables can result in environmental emissions if control measures are not taken. Metal particulate emissions from a copper fire refinery, and from the manufacture of either cadmium-copper or beryllium-copper alloys, should each be ducted to suitable bag filter systems. Similarly any particulate emissions from rubber compounding should be ducted to a bag filter unit. Emissions of particulates, hydrogen chloride and chlorine from the manufacture of optical fibres should be ducted to a bag filter system followed by a caustic soda scrubber.