This cable test typically applies to thermoplastic insulation and sheathing of cables. Thermoplastic materials are those which can be re-softened by heat. The standard for pressure testing at high temperatures for insulations and sheaths is BS EN 60811-508.
When current passes through an electric wire it generates heat as a result of the resistance to current flow. This heat, and heat from other cables and electrical equipment in close proximity, may result in the softening of the insulation and sheathing. The intention of the hot pressure cable test is to determine if the cable insulation and sheathing material can withstand a degree of pressure exerted on the cable at elevated temperatures without causing significant damage and a potential safety risk.
The Cable Lab conducts pressure testing for cables at high temperatures as part of our ongoing QA processes but this cable test is also available on third-party cables with independent report analysis.
To accurately conduct pressure testing you need an air oven with natural circulation and an indenting device to apply pressure to the material surface. This indenting device is a rectangular blade with a thickness of 0.7mm which can be pressed perpendicularly against the test piece under the influence of an applied force.
The temperature of the oven and the force to be applied are determined by the standard for that material type. Similarly, the length of time the cable is left under pressure inside the oven is also determined this way. At the end of the specified duration the test piece is rapidly cooled under load by spraying the test piece with cold water on the spot where the blade is pressing. The test piece is then removed from the apparatus and cooled further by immersion in water.
Once the hot set pressure test has been conducted, a narrow strip is cut from the cable sample test piece and a measuring microscope is used to measure the indent left in the cable by the blade, to be expressed as a percentage of the sheath or insulation thickness. The result is recorded as a pass or fail based on the degree of permanent indentation allowed in the standard.