WHAT IS A CABLE MADE OF?
Cables are intended to transport electric current. They must be able to fulfil this role safely with respect to equipment and personnel.
Cables generally consist of three main parts:
- Conductor
- Insulation
- Mechanical protection
The conductor transports electric energy. Several conductors sharing the same cable will be separated and isolated from each other.
Should the mechanical protection be damaged, the insulation (if the insulation sheath is not damaged) must not be affected. In this case, the insulation sheath is the only effective protection and it also provides mechanical protection.
The conductor
The conductor must have a low resistivity (low resistance), this characteristic is provided by certain metals.
The conductor must also have other physical properties. It must be ductile and flexible. A ductile metal may be pulled and drawn without breaking, this is a basic quality which is required when manufacturing cables. However, there are other factors that a manufacturer (or a user) requires of a cable,these concern the weight and cost. Silver is thus one of the best conducting metals but it is easy to understand why copper conductors are preferred.
Aluminium is also increasingly used as a metal conductor. Aluminium has a resistivity 1.6 times greater than copper, and the same current therefore requires a larger crosssectional area, however aluminium is (far) less expensive and much lighter, (you can clearly feel the difference when you pull the cables).
The insulation
Properties
The main function of the insulation on the conductive core of a cable is to “prevent the electricity from escaping” from the cable in question, and to prevent any external contact with a live part (i.e. a person touching the conductor).
The insulation is required to be flexible, to support differences in temperature and resist mechanical constraints and external attack. The main property required is however high electric (or dielectric) resistance (or resistivity).
The insulation of an electrical energy transport cable must have the following properties
- High electric resistance
- A certain amount of flexibility
- Resistance to temperature changes
- Mechanical resistance to impact and external aggression (chemical, atmospheric, etc.)
Insulation material
PVC is the most commonly used material. PVC is the abbreviation for Polyvinyl chloride. The principal advantages of PVC are:
- Good insulating quality (high resistance)
- Waterproof
- Low cost
- Easy to colour
- Good mechanical strength
The main disadvantages of PVC are:
- Softens above 70°C
- Becomes brittle below 0°C3
Some qualities of PVC can resist temperatures of up to 85°C, but most remain within the range indicated above (0 – 70°C). The other materials used as cable insulations are, among others:
- Vulcanized rubber
- Synthetic rubber
- Silicone (derived from silicon)
- Paper (treated with resin)
Paper insulation was used in HV cables but has now been replaced by synthetic rubber for almost all applications. It is however still used in some flexible cables where a high flexibility is required.
Synthetic rubbers are used for high or low temperatures.
Silicone is used for very high temperatures, i.e. approximately 150°C.
Mechanical protection
The main function of the mechanical protection is to prevent damage to the conductive core’s insulation, which could cause electrocution, sparks or start a fire.
Cable Sheath
The sheath is the cable’s mechanical protection.
The type of cable shown is used to connect bedside lamps or your washing machine, fridge, etc.
Most cable protective sheaths are made of PVC; rubber is used for cables requiring extra flexibility. This type of cable is mainly used for domestic distribution, and in tertiary applications and offices where there little risk of mechanical damage.
Different cable sheaths (mechanical protection)
Sheathed cables
These are the cables which you see “lying around” at home, in stores, in the office, or even on worksites (during construction). This is the cheapest and quickest means of laying cables. The cables are protected by their own sheaths only, thus providing minimal mechanical protection.
Most sheathed cables have an external PVC sheath which may also be made of:
- Synthetic or natural rubber
- Agglomerate PVC
- Braid (metal or other)
The 3 cable types shown correspond to:
a) a stranded core conductor
b) a rigid core conductor with ground (earth) wire
c) three rigid core conductors with a fourth ground conductor
Armoured cables
The cables are mechanically protected by a steel armour if there is a risk of attack (impact, compression, rodents, etc.).
This is the type of cable which is frequently installed in our industry due to the permanent risk of mechanical damage.
The conductors and their insulation are protected by a metal or even plastic sheath, armour or braid (term as applicable). A cable may have several sheaths/armoured coverings. However, this additional mechanical protection more generally consists of steel wire armour (SWA) as shown in the figure.
The armour may also consist of
- A helically-wound steel tape.
- An aluminimum tape (or sheath)
These armoured cables may be installed directly in contact with equipment and/or laid in cable trays, trenches, conduits, etc.
Mineral-insulated cables
These cables have a mechanical protection which consists of a metal sheath, which is why it is specified in their designation: mineral insulated and metal sheathed cables. These types of cables can be found in our industry with “fire resistant cables”, i.e. cables used to resist fire in safety circuits and areas where there is a risk. In the instrumentation field, we use them as thermocouple cables installed in various environments.
There are two types of metal sheaths:
Mineral Insulated Copper Sheath (MICS).
Mineral Insulated Aluminium Sheath (MIAS)
The conductors in this type of cable are insulated at high pressure using magnesium oxide powder.
These cables generally have an additional PVC sleeve over the metal sheath to protect
against corrosion and provide good resistance to damp atmospheres. These cables are
connected using special cable glands to avoid the moisture entering the cable.
