Why black oversheaths are common with SWA cables

Categories: T&I  Traders and Installers 

Steel wire armoured (SWA) cables normally have a black oversheath – why is this the case? We delve into the details…

01/11/2023 - 01:00 AM

The oversheaths of armoured cables, (otherwise known as steel wire armoured SWA cables), tend to be designed and manufactured in one colour – black. The exception being Prysmian FP500 cable, which has a red othersheath to distinguish it as a fire resistant cable. Prysmian SWA cables have a carbon black filled PVC outer sheath sheath because they have been specially designed and formulated by us to not only meet the standards but to ensure the cables are robust and durable. SWA cables are commonly used in a variety of electrical and power distribution applications, and the choice of a black oversheath serves both functional and practical purposes.


Complying to British Standards

The use of a black oversheath has become common practice in the UK.

BS 7655-4.2 is the Specification for insulating and sheathing materials for cables and specifies the requirements for the general application ‘PVC sheathing’. The British Standards say that the oversheath should be an extruded layer of PVC material conforming to the requirements of Type 9 specified in BS 7655-4.2.

Oversheaths tend to be black when properly formulated and compounded because it provides good resistance to deterioration from normal daylight UV.

Other colours may be used providing there is an agreement between the manufacturer and the purchaser, however, to increase the strength of the polymers that make up the sheath, the sheath itself would require additional additives to protect the cable from UV radiation.


UV Resistance

Just like the human body, thermoplastics such as PVC can suffer from UV radiation when exposed to normal daylight. UV radiation can break down the chemical bonds in polymer chains that feature in thermoplastics, which creates free radicals that can damage the polymer chains even further. This ultimately weakens the mechanical properties and will eventually render materials useless over time.

Black materials tend to absorb and dissipate UV radiation more effectively than lighter colours. Even in the UK, where the weather can be very cloudy and changeable so that exposure to sunlight is inconsistent, it’s crucial to protect the cables from the potentially harmful effects of UV rays. A black oversheath helps to prevent the breakdown of the polymers in the cable's sheath due to prolonged exposure to daylight.



The oversheath acts as an extra layer of protection surrounding the layer of steel wire and makes the cable more robust. The density of a black oversheath not only makes the cables more UV resistant and durable in outdoor environments, but also helps protect the cable from mechanical damage – for example, if heavy excavation machinery came into contact with the cable already installed in the ground.


In conclusion, the black oversheath of SWA cables serves multiple crucial purposes, especially UV resistance, compliance to the British Standards and durability. The use of carbon black in sheaths helps safeguards the cables from environmental factors and mechanical impacts.


All Prysmian armoured cables are manufactured in the UK. Our commitment to quality ensures that our SWA and AWA cables meet or exceed industry standards for reliability and performance.