A Guide to Cable Sheaths and Jacket Types
Get to know the various cabble sheath types CST, LSF, PVC, SWA....
19/02/2018 Nathanael Owen 1372 0 Cable
The are a wide variety of different cable sheaths and jackets which all serve a different purpose. Understanding the difference helps you make an informed decision when it comes to selecting the right cable for your requirements.
This guide explains the differences between some of the most common types of sheath that are used for cable manufacture.
PVC (Polyvinyl Chloride) Cable
Polyvinyl Chloride (PVC) is one of the most widely used polymers in the world thanks to it's versatility, low manufacturing costs and ready availability. The raw materials used to make PVC are salt and oil which are used to produce ethylene and chlorine. Chlorine is one of the groups of elements called halogens which are highly reactive with other elements and can be used to form different compounds. The Ethylene and chlorine are combined and undergo polymerisation and this process produces PVC as we know it. PVC has excellent water and oil resistance and good tensile strength and can easily be combined with additional additives to change the materials properties for other purposes. PVC can be found in different forms in thousands of different building materials from pipes to window frames and it can be easily recycled once it reaches the end of it's life. The manufacture of PVC also has a low carbon footprint when compared to other common building materials.
PVC Cable sheaths
PVC is by far the most common type of cable sheath and is ideal for general use cables. The PVC compound used for cable sheaths is generally very flexible, durable and long lasting making it a very popular choice. A disadvantage to using PVC however is that it is vulnerable to UV light and can become brittle and cracked when exposed to bright sunlight for long periods and as such is more suited for indoor use.
PVC is typically resistant to ignition and requires temperatures more than 150°C higher than wood to ignite. When PVC burns however it gives off harmful halogen based gasses including carbon dioxide, carbon monoxide and hydrogen chloride as well as creating a significant amount of smoke. As a result it's use in densely populated buildings is not recommended and current building regulations recommend using cable that isn't constructed from halogens.
PVC is ideal for everyday use indoors or in containment where low cost cable is required and smoke and gas emissions are not a consideration.
LSF (Low smoke and fume) Cable
LSF is an acronym used to describe cable sheaths that are made of PVC but have additional additives to reduce the amount of smoke produced when the polymer is burnt. As the polymer is still manufactured from halogens however the same harmful and corrosive gasses are produced that PVC produces. The building regulation that require use of halogen free cable are designed to protect against smoke as well as harmful gasses so LSF cable is not suitable for use when adhering to these regulations. Unlike halogen free cable, LSF doesn't follow a specification or stipulate use of a particular compound and as such the compound used to create LSF cable is determined solely by the manufacturer. This ambiguity means that we can't rely on LSF cable to meet any particular standards and such it should only be used where normal PVC cable would be suitable.
LSf cable is only really suitable where PVC would be used although it is slightly more expensive to produce and little benefit is gained from this.
LSHF (Low smoke halogen free) Cable
Cables with a low smoke halogen free (LSHF) sheath which can also be called LSZH, LSOH and LS0H are manufactured primarily from polyethylene which contains very little if any Chlorine. When burnt the compounds used to make LSHF cable produce very little smoke and almost no hydrogen chloride which is the most significant of the harmful gasses produced by PVC.
In order to be classed as LSHF a cable a cable must pass rigorous tests and adhere to the BS EN 50525-3-11 specification. This ensures that the EN specifications with regard to production of smoke, hydrogen chloride and other gasses are followed and that the cable meets building and fire regulations.
LSHF cable is ideal for use indoors in public buildings and densely populated areas and where building and fire regulations specify that it is used.
When installing LSHF data cable it is important to ensure it is contained within LSHF conduit as well as standard PVC conduit produces smoke and harmful gasses in the same way as PVC cable.
PE (Polyethylene) Cable
Polyethylene (PE) is a thermoplastic polymer which means it shares commonalities with what we perceive as everyday plastics. It is tougher and more rigid than PVC compounds but is less easily re-cycled and subsequently more expensive to produce. Unlike PVC polyethylene doesn't burn but rather melts and reforms once cooled. It's tough rigid properties make PE sheaths ideal for use outdoors or where a tougher sheath is required. Polyethylene on it's own however will degrade when exposed to UV light so additional ultra violet absorbers (UVA)s are added to stabilise the compound making it suitable for prolonged outdoor use.
Cable with a PE sheath is ideal for use outdoors either underground in containment such as ducting or above ground where it is exposed to bright sunlight or light amounts of wear.
HDPE (High Density Polyethylene) Cable
High density Polyethylene is a type of Polyethylene compound with a highly crystalline structure which makes it even more rigid and durable than standard Polyethylene. Polyethylene is commonly used to manufacture water and drainage pipes and parts. The HDPE compound used in cable sheaths retains some flexibility compared to the compound used in water pipes and also has UVA's added to stabilise it against UV light making it suitable for exposed use above ground. Cables with a HDPE sheath are often referred to as direct burial cables as they are durable enough to be buried directly into the ground with minimal risk of degradation or damage occurring.
Cables with a high density polyethylene sheath are ideal for use underground without containment (directly buried) or above ground exposed to bright sunlight or moderate wear.
Polypropylene is a thermoplastic polymer like Polyethylene but it is harder and less dense making it lighter. Polypropylene is a very versatile polymer which has strong resistance to water and other chemicals and is generally quite tough. It also has a high resistance to electricity. Polypropylene is most commonly found in household packaging. Polypropylene has good UV resistance but as it is less flexible the Polyethylene it is less suited to cable sheaths and is only used for thin layers of insulation.
PUR (Polyurethane) Cable
Polyurethane is a strong, versatile polymer that can be thermoplastic or thermosetting depending on the compound used. Polyurethane is halogen free, tough and flame resistant although not ideally suited for use in extreme temperatures due to it limited temperature range. It is however highly resistant to bacterial growth making it well suited for use in the food or medical industry. Generally though is not easily recycled and more expensive to produce than PVC and as such is not commonly found in use as an everyday cable sheath outside of specialist applications.
Cables with a Polyurethane sheath are typically designed for specialist applications and as such are best suited for the application for which they are designed.
TPE (Thermoplastic Elastomer)
Thermoplastic Elastomer is a low density flexible compound that is a combination of plastic and rubber. TPE withstands temperature extremes well and is resistant to bacterial growth and is also easier to recycle than Polyurethane. TPE is still relatively expensive to manufacture although it is becoming popular for use as a cable sheath in industrial or medical applications which will eventually reduce manufacturing costs. It is also easy to colour making it well suited to control cables that require colour coding.
Where available cables with a TPE sheath are well suited for use in extreme temperatures and industrial applications.
PTFE (Polytetrafluoroethylene) Cable
Polytetrafluoroethylene (PTFE) is a thermoplastic with excellent thermal, chemical and electrical resistance properties. PTFE does however have low resilience so cables with a PTFE sheath require careful installation and additional mechanical protection such as containment to prevent damage. Cables with a PTFE sheath are used in exterme environments where they are subjected to very high temperatures and chemicals which would otherwise corrode standard polymers.
Cables with a PTFE sheath are ideal for specialist applications in extreme temperatures and corrosive environments.
Silicone is a non-carbon based polymer that is highly resistant to heat, chemicals and bacterial growth. Silicone based polymers are commonly used in food and medical applications. Cables with a silicone sheath are can withstand a wide range of temperatures including high temperatures and temperatures well below freezing. Silicone also has excellent electrical and UV resistance and low thermal conductivity making it a very versatile material. When burnt silicone does not produce any smoke or toxic gasses as they do not consist of any organic halogen compounds. Silicone is however quite expensive to produce, has very limited options for recycling and only has average abrasive resistance which are limiting factors to it's use in common cable sheaths.
Silicone's high temperature tolerance and excellent resistance to fire, chemicals and UV make it ideal for use in specialist cabling applications. It's higher cost however makes it less useful for everyday cabling.
Rubber was in use as a cable sheath long before synthetic polymers such as PVC and PE became popular. Rubber sheathing was originally made from natural rubber but natural rubber is a valuable natural resource and is not very durable so modern rubber is now primarily made from more durable, synthetic thermoset compounds. Rubber is waterproof, chemical resistant, extremely flexible and retains it's flexibility over a wide range of temperatures. Cables that use a rubber sheath are typically found in specialist applications and extreme temperatures.
Cables with a rubber sheath are ideal for use where flexibility must be retained in extreme temperatures or where a chemical or water resistance is required.
CST (Corrugated Steel Tape) Cable
Corrugated steel tape is not strictly a cable sheath and is rather an additional layer of protection that is applied under a cable sheath to protect the inner cables. CST is most commonly found in fibre optic cables where the fragile glass cores require additional protection such as when used outdoors but where they high cost of steel wire would be excessive for high volume use. The corrugated steel tape is a semi-rigid layer applied directly beneath the sheath that is made of thin corrugated steel which provides good abrasion, impact and crush resistance to the cable. Cables using CST are also cost less and are more flexible and considerably lighter than cables that use steel wire for protection.
Cables with a CST protective layer are ideal for use outdoors where steel wire armour would be excessive but where some mechanical protection is required to protect the cable.
SWA (Steel wire armoured) Cable
Steel wire armour like corrugated steel tape is applied directly beneath the cable sheath to provide mechanical protection to the cable. A layer is formed by tightly wrapping individual steel wires around the inner sheath and finally applying an outer sheath over it. Steel wire provides the highest level of abrasion, crush and impact resistance making it suitable for use where the cable is exposed to heavy levels of direct wear or potential damage. The layer of steel wire however makes the cable significantly less flexible and heavier than other cables so it is only used where absolutely necessary.
Cable with an SWA protective layer is ideal for use outdoors where other forms of mechanical protection such as containment are not available and where the cable will be exposed to potential damage.
About Nathanael Owen
Nathanael Owen is Connectec's resident tech guru and usually first stop for technical support. When he's not on a support case he's usually neck deep in web development and marketing tasks.
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