Thermal ageing of PVC cables

Thermal ageing of polymer cables can lead to serious safety, performance and cost issues. Bob Merrick, Commercial Manager - Technical, Prysmian Cables, discusses thermal testing and operating temperature ranges for general-market PVC cables.

Australia experiences temperatures ranging from -10 to +40°C depending on location and season. Cables installed in roof and wall cavities risk encountering extreme heat for prolonged periods during their service life, this exposure can be exacerbated by installations where thermal insulation is encountered.

Modern cables are manufactured using polymers formulated to give the following characteristics: electrical protection, mechanical protection, ease of installation (cable preparation at termination), and long service life.

Thermal ageing of polymers over time may cause a loss of these characteristics such that a cable can be rendered unsafe for its intended daily use. Critical aspects of type testing for low-voltage consumer cables include thermal-based testing. These tests are designed to give a representation of the deleterious effects on cable components due to prolonged exposure to heat.

Insulation and sheath compounds

The most common insulation and sheath compounds used in ‘general building wires’, those being manufactured to AS/NZS 5000.1 and 5000.2, are  polyvinyl chloride (PVC). A typical formulation will consist of: PVC copolymer, mineral fillers, lubricants, stabilisers (thermal), plasticisers and colourants.

In a correctly formulated compound, these components are blended to provide excellent electrical, mechanical and processing capabilities. In relation to thermal performances of the various PVC compounds the following characteristics are evaluated:

Loss of mass

Loss of mass is the evaluation of loss of plasticisers and lubricants due to volatilisation during heating. The temperature and duration of ageing is dependent on the type of compound and its temperature rating, for example, a V-90 compound will be aged at 115 ± 2°C for 120 hours.  The test consists of comparison of weight of the compound before and after ageing.

Accelerated ageing

Accelerated ageing takes place in ovens using prescribed temperature, air flow and air disturbance. The temperature and duration of ageing is dependent on the type of compound and its temperature rating, for example, a V-90 compound will be aged at 115 ± 2°C for 504 hours.

The test consists of a comparison of the physical characteristics (tensile and elongation) of the compound based on before- and after-ageing values, effectively reporting loss in mechanical properties.

Pressure test (hot deformation)

Pressure testing is the measure of deformation of a compound due to force during a heat cycle. As with the previous two tests, the temperature is dependent on the type of compound and its temperature rating, for example, a V-90 compound will be tested at 90 ± 2°C. A load is applied to the specimen over a prescribed area during heating. The specimen is cooled under load and the resultant permanent deformation (indent) is measured.

Heat shock

Heat-shock testing is a measure of a compound’s ability to withstand heating under stress without cracking. In this test, specimens are wound onto a mandrel and heated at 150 ± 3°C for one hour. After cooling the specimens are examined for any cracking.

Plasticiser exudation

This test, although performed at 23 ± 2°C, is important in that it is designed to provide an indication of the stability of the plasticiser base of the PVC compound.

Failure mechanism

When exposed to heating, poorly formulated PVC compounds will rapidly demonstrate a loss of physical properties due to loss of plasticiser. Operation in elevated temperatures (such as found in roof cavities and cladded walls) is the primary concern for these cable types. The main failure reported is cracking of the insulation and sheaths, exposing the conductors and rendering the installation unsafe. Instances of green, oily fluid leaking at switches and power points should be investigated. While the oily substance itself is not electrically dangerous, it is a known indicator of thermal ageing in poorly formulated PVC compounds.

In Australia there is not a propensity to operate low-voltage PVC cables at current ratings high enough to cause thermal degradation due to self heating. Australian Standard 3008.1.1 considers current ratings for PVC insulated cables at a maximum continuous operating temperature of 90°C. This rating is qualified by note 3 of table 1 - Limiting temperature for insulated cables viz: “The normal operating temperature of thermoplastic cables, including flexible cords installed as installation wiring, are based on a conductor temperature of 75°C. This is due to the risk of thermal deformation of insulation if the cables are clipped, fixed or otherwise installed in a manner that exposes the cable to severe mechanical pressure at higher temperatures.

V-90 and V-90HT insulated cables may be operated up to the maximum permissible temperatures 90 and 105°C provided that the cable is installed in a manner that is not subject to, or is protected against, severe mechanical pressure at temperatures higher than 75°C. Such applications also allow for cables to be used in - (a) locations where the ambient temperatures exceeds the normal 40°C, eg, equipment wiring in luminaires and heating appliances, or in roof spaces affected by high summer temperatures; and (b) locations affected by bulk thermal insulation that restricts the dissipation of heat from the cable ...”.

How to determine if a cable is suitable for the intended operating temperature

In Australian Standards, cable insulation and sheaths compounds have designations to define their performance criteria and specific uses.

The AS/NZS 3808 - Insulating and sheathing materials for electric cables nominates V-75, V-90 insulation & 3V-75, 3V-90, 4V-75 & 5V-90 sheath designations - these compound designations are called up in the relevant cable build standards for these cable types. These designations are cross referenced in both AS/NZS 3008.1.1 - Electrical installations - Selection of cables - Cables for alternating voltages up to and including 0.6/1 kV - typical Australian installation conditions and AS/NZS 3000 - the wiring rules.

For all cables manufactured to AS/NZS 5000.1 or 5000.2 the insulation compound designation must be shown in the cable sheath markings, hence a product such as 2.5 mm² 2-core and earth flat building wire will have in the marking legend either the compound designation V-75 or V-90, dependent on the manufacturers choice of compounds. By reference to table 1 of AS/NZS 3008.1.1 it can be seen that V-75 designated compounds are suitable for a maximum permissible temperature of 75°C and a V-90 designated compound is suitable for a maximum permissible temperature of 90°C (subject to previously stated note 3).

Summary

Whichever marking is shown on the sheath, the primary responsibility for the cable supplier is to ensure all relevant testing has been performed to ensure the integrity of the compounds used in the cable and that such compounds are suitable for the operating temperatures indicated by the insulation designation. It is the responsibility of the installer to select and install wiring within the guidelines of AS/NZS 3000 Wiring Rules, to ensure thermal runaway conditions are mitigated.