CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables
Electrical Cables Unit Aims By the end of the unit participants should be able to: State the component elements of electrical cables a) Conductor b) Insulation c) Sheathing d) materials used for conductors Syllabus Reference: 2.3.03
Electrical Cables Cable structure The three main components The job of a cable is to carry current. It must be able to do this safely and effectively without failing and without creating danger to persons, livestock or property in the process. In order to carry current a cable must have an electrical conductor. This conductor must be of sufficient size to carry its predicted load without overheating and thus causing other problems. The conductors have to be insulated from each other and from any surrounding metal surfaces. This ensures that the current flows along its predetermined path and also prevents the possibility of electrical shock if anyone should happen to touch the cable or its enclosure. The insulation will only remain effective if it remains undamaged or is protected against contamination. This means that in most cases, the insulated conductor has to be provided with some form of mechanical protection in the form of covering, sheathing or enclosure.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables
Cable used as a live conductor can be divided into three basic component parts, namely: the conductor the insulation the mechanical protection
The conductor First of all, the conductor needs to be a good conductor of electricity, or in other words, it needs to have a low resistivity. Silver could be chosen for this purpose as it is an excellent conductor at normal temperatures, but sheer economics rules it out as a possibility. Very closely behind silver comes copper. It is a very good conductor, it has good mechanical properties, it is flexible and it is easy to terminate. Copper may not be cheap, but it’s a lot cheaper than silver and years of experience have shown that it has a lot going for it.
If pure economics of cable manufacture is the main consideration then aluminium must receive some serious consideration. It may not be as good a conductor as copper, having a resistivity about 1.6 times greater (see right), but a look at the commodity prices in the morning paper will show that it is possible to afford to make a bigger cable and still save money.
Another reason why you might choose aluminium is the fact that it is much lighter than copper; the density of aluminium being less than a third of that for copper (see left). Those two facts alone must make aluminium a strong contender in the conductor stakes.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables There are plenty of other conductors about, such as iron, lead, tin, brass and zinc, but they all have their problems, so that when considering which cables to use, we keep coming back to the same two: copper or aluminium Before deciding on aluminium though, consider this: Aluminium is difficult to terminate as it oxidises easily and is prone to electrolytic corrosion. Bearing that in mind, the choice could be up to you!!!! The insulation Most people who have any experience of cables will probably think of insulation as PVC, and that’s it. Individuals would be quite justified in thinking that PVC (polyvinyl chloride) is by far the most common form of cable insulation in use today and that one would really need to hunt around to find anything different. However, XLPE (cross-linked polyethylene) and LSF (low smoke and fume) cables have grown in popularity for many types of installation because of their electrical, thermal and environmental properties. The insulation has to keep the current in the conductor, so it’s most important feature is that it must have a high electrical resistance. Air is a good insulator and this is the reason why overhead transmission is carried out using bare conductors separated from each other and from earth by the air, which insulates and cools them. Where they are supported by towers or poles these bare conductors are suspended from glass or porcelain insulators which are mechanically strong and have the very high resistivity necessary for insulation purposes. Being a good insulator is not the whole story though.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables A PVC insulator has to do its job effectively in some pretty arduous conditions. PVC may be cheap, flexible, easily coloured and impervious to water, but it tends to soften as the temperature increases above 70 °C, and at temperatures below 0 °C it may become brittle and crack when handled. Butyl rubber is a synthetic rubber which is frequently used for high-temperature terminations such as heating elements. It will operate continuously without deterioration in temperatures up to 85 C. Silicon rubber insulation can be used on cables in situations where temperatures are very high. It has an operating temperature of 150°C and even if the cable is completely burnt, the insulation forms a silicon ash which is in itself an insulator, so the cable may continue to function until the ash is disturbed. For even higher temperatures (up to 185°C) glass fibre insulation may be used, and you can frequently find examples of this type of insulation on the internal wiring of heating appliances.
Remember Conductors must have a low resistivity Insulators must have a high resistivity
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables MIMS (Mineral-insulated metal-sheathed)
One of the most remarkable insulating materials is magnesium oxide (or magnesia). This is a white powder not unlike powdered chalk to look at.
Magnesium oxide is virtually indestructible and keeps its insulating properties even though its temperature rises to well above 1000°C, after which the conductors will have melted and broken down. This is the insulating material which is to be found in Mineral-Insulated Cable, a type of cable renowned for being robust. Magnesium oxide is hygroscopic and must however be kept completely dry, and since it can absorb moisture from the atmosphere, all terminations must incorporate a seal to act as a moisture barrier. Paper Insulation In some cables, notably lead-sheathed armoured cable, paper is used as an insulator. For this purpose, paper which has been impregnated with mineral oil is applied to the conductor in the form of a multi-layer tape. Impregnated paper is an excellent insulator and is to be found in cables operating at very high voltages. As with magnesia, the paper insulation can be broken down if water penetrates the cable, and for this reason paper-insulated cables must have a watertight sheath and sealed terminations. Low toxicity Before leaving the topic of insulation we must consider the effects of smoke emission. General-purpose PVC cable, when it becomes very hot, can emit poisonous smoke, which can create a serious hazard. This danger is well known, and many contracts now carry a stipulation that low-toxicity (LSF) cables have to be used.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables The mechanical protection Bare conductors are supplied for overhead transmission, use air for insulation, and require no further mechanical protection. Insulated conductors are available to the installer who is going to make his or her own provision for the mechanical protection of the cables. Cables in conduit and trunking This type of cable is usually in the form of PVC-insulated singles. These have a conductor surrounded by a single layer of PVC insulation and no further covering. PVC singles are normally installed in conduit or trunking or as part of the internal wiring of apparatus. Being surrounded by a protective enclosure, they are not exposed to the risk of mechanical damage and it is normally quite unnecessary to make any further provision. Do, however, make sure that the cable is not damaged during installation by drawing too many through a small aperture or by leaving sharp metal edges unprotected. PVC-insulated and sheathed cable This is used a great deal for domestic and light commercial wiring. This has PVC-insulated conductors which are surrounded by a second overall sheath of PVC. This type of cable can be used where the risk of mechanical damage is slight. It is normally run above ceilings, under floors and within walls and often needs no further protection, although additional protection can be given where necessary.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables Armoured cable In addition to insulation and sheathing, incorporates a layer of metallic armouring which can protect the cable from impact. This type of cable is widely used in industrial installations, where it is to be seen clipped directly to the surface or run in ducts or trenches.
MIMS Mineral-insulated metal-sheathed (also known as ‘pyro’) cable has a seamless metallic (normally copper) sheath and solid conductors. The insulating medium is magnesium oxide, as previously described, and this is a very robust cable. MIMS cable can put up with some very rough treatment and can withstand temperatures far in excess of those which would destroy any other cable. Additional protection from the effects of electrolytic corrosion when in contact with other metals in damp surroundings can be given by means of a PVC outer sheath.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 16 – Electrical Cables PX and FP 200 These are of similar construction and can sometimes be an alternative to MIMS where conditions are less arduous. The insulated conductors are enclosed in an aluminium sheath and the whole cable is enclosed in an outer PVC covering. This type of cable can be concealed in the fabric of a building or clipped directly to the surface. They are resistant to a fair amount of mechanical damage and can also be used in situations where conditions are wet or dirty. When using PX and FP200 cable, manufacturers’ data must be consulted for current ratings and resistance. Quiz You have now looked at several words associated with electrical cables. The words in the list below are included in the square. Mark each word as you find it. They may be found in any direction. F
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CABLE SHEATH MIMS COPPER ALUMINIUM CONDUCTOR INSULATOR MAGNESIA
PVC ARMOURED PROTECTION PAPER AIR CORE WIRE COIL
R O C O M
For more information see pages 159 to 161 of ‘Electrical Installations Level 2 2330 Technical th Certificate’ – revised for the 17 Edition IEE Wiring Regulations (ISBN 978 0 435401 09 2).
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