Suppose that we have to measure an oven temperature (800 °C) with a thermocouple and that the measuring equipment is located at a distance of dozens of meters from the measuring point. Usually a few meters are sufficient to bring the temperature down to 200 °C or less. From that point, in order to reduce the costs, it is possible to use either extension or compensating cable to connect the thermocouple to the measuring device.
Thermocouples can be made from bare wires but need an electrical insulation. While one can use ceramic beads or mineral filled sheaths for high temperature, there exists a variety of application which only require a thin insulation medium to perform the measurement (furnace tests, laboratory tests, combustion tests, autoclave validation, freezers tests, etc.).
Therefore thermocouple cables exist in a variety of types and insulations.
The most common are fluorinated resins, Kapton®, glass fibres and ceramic fibres.