Posted 17 July 2019
This issue’s instalment of ‘What’s in a Cable’ covers insulation.
Insulation is a non-conductive material used to coat the conductor to keep the electricity flowing to its intended destination. In the case of a transformer or motor armature this may be a simple enamel varnish but in most cables and wires thermosetting or elastomeric compounds are normally used.
With the exception of overhead power lines, conductors are normally insulated within a cable to ensure the electricity, whether it is data, signal or power, only goes where you want it to and doesn’t jump from conductor to conductor.
Insulation types vary enormously in range and application. Here are the popular types.
Thermoplastic compounds
This includes PVC, Polyethylene and many Low Smoke Halogen Free (LSHF) materials. Thermoplastic compounds are defined as materials that can be melted in an extruder and, when cooled, reform with the original properties unaltered. The compounds are supplied in bulk in granules about the size of a match head. The advantage of these compounds is that they are relatively easy to work with and the equipment needed in manufacturing is generally fairly simple. It is also quite easy to change colours within a material type. However the cost of the compound varies significantly with high performance LSHF compounds costing up to five times that of basic PVC.
Cable insulation
Elastomeric or Curing compounds. This group includes rubbers and materials that are altered after extrusion by a catalyst, for example by cross-linking the molecules to improve the performance of the material. XLPE is commonly used for signal and power cables.In the case of silicone rubber, the silicone is squeezed out of the extruder cold, like a putty and then enters the curing process – normally a steam tunnel or through salts at very high temperatures.
Taping
Taping is not as popular as it used to be, although some products are still insulated by winding a tape around the conductor. Paper taping is used for some power cables and PTFE is used for heat resistant and high performance wires. One advantage of taping is the conductor is central or concentric within the insulation. In the case of PTFE the insulated wire is then subjected to extreme heat for a very short time (sintering) to fuse the edges of the tape together and stop it unwinding.
Mica tape is often used to ensure circuit integrity during a fire. Mica is a naturally occurring substance which is bonded to an inert substrate that is wrapped around the conductor prior to the insulation being applied.
Lapping or braiding. These are now mainly used for heat resistant cables working over 250°C. Glass or ceramic fibres are wound around the conductor, normally in two layers in opposite directions with a glass fibre braid overall to hold it all together. These cables are stable at high temperatures (up to 750°C) but are not suitable at normal ambient temperatures where there may be moisture. Most of the products are silicone varnished to ease handling during installation but the varnish burns off at a high temperature.
As you can see there are many different types of insulation each with a specific job. PVC and XLPE are the most common and offer great all round properties in terms of flexibility and cost. Other materials are crucial for high performance applications such as fire alarm cables where it is important a cable can carry on functioning even in the event of fire.
When choosing insulation it is important to establish what the cable or wire is expected to do. Temperature, voltage, electrical characteristics, flexibility, performance in the event of fire and other physical factors all need to be taken in to account when specifying. With literally hundreds of different grades of PVC, eighty-plus grades of silicon and practically every other compound being able to be split into subgroups, there are insulations to meet practically every need.