A Guide to Cable Sheaths and Jacket Types

Get to know the various cable sheath types CST, LSF, PVC, SWA....

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19/02/   Nathan Owen     

There 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 sheaths that are used for cable manufacture.

PVC (Polyvinyl Chloride) Cable

About PVC

Polyvinyl Chloride (PVC) is one of the most widely used polymers in the world thanks to its 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 gases including carbon dioxide, carbon monoxide and hydrogen chloride as well as creating a significant amount of smoke. As a result, its use in densely populated buildings is not recommended and current building regulations recommend using cable that isn't constructed from halogens. 

Ideal for

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 gases are produced that PVC produces. The building regulations that requires 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.

Ideal for

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 gases produced by PVC. 

In order to be classed as LSHF a cable a cable must pass rigorous tests and adhere to the BS EN -3-11 specification. This ensures that the EN specifications with regard to production of smoke, hydrogen chloride and other gases are followed and that the cable meets building and fire regulations.

Ideal for

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 gases 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. Its tough rigid properties make PE sheaths ideal for use outdoors or where a tougher sheath is required. Polyethylene on its own however will degrade when exposed to UV light so additional ultra violet absorbers (UVA)s is added to stabilise the compound making it suitable for prolonged outdoor use.

Ideal for

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 UVAs 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.

Ideal for

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.

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PP (Polypropylene)

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 it 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.

Ideal for

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.

Ideal for

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 extreme environments where they are subjected to very high temperatures and chemicals which would otherwise corrode standard polymers.

Ideal for

Cables with a PTFE sheath are ideal for specialist applications in extreme temperatures and corrosive environments.

Silicone Cable

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 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 gases 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 its use in common cable sheaths.

Ideal for

Silicone's high temperature tolerance and excellent resistance to fire, chemicals and UV make it ideal for use in specialist cabling applications. Its higher cost, however, makes it less useful for everyday cabling.

Rubber Cable

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 its flexibility over a wide range of temperatures. Cables that use a rubber sheath are typically found in specialist applications and extreme temperatures.

Ideal for

Cables with a rubber sheath are ideal for use where flexibility must be retained in extreme temperatures or where 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 also cost less and are more flexible and considerably lighter than cables that use steel wire for protection.

Ideal for

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.

Ideal for

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.

XLP
Cross-linked polyethylene (XLP) compound was invented during the s and gained popularity during the s because it provided physical protection and flexibility, making it a staple in providing improved reliability in the electrical system. Cross-linked insulation was also affordable and could be mass produced by many suppliers. Therefore, it was the right compound at the right time.

In the early s there were growing concerns that cable failure was linked to the ingress of moisture and contaminants into the cable core. Over time, this caused stress points within the insulation that led to the &#;treeing&#; phenomena. Treeing, over time, erodes the insulation, leading to cable failure.

TR-XLP
Since the mid-s, TR-XLP has become the dominant insulation in utility distribution cable applications primarily due to lower first-cost and lower operating losses. TR-XLP compound continues to be improved and is provided by many cable manufacturers in the United States and the rest of the world.
TRXLPE compounds are rated 90ºC for normal operation, 130ºC for emergency operation and 250ºC short circuit.

EPR
The first versions of EPR (ethylene propylene rubber-insulated), introduced in the early s, demonstrated favorable characteristics that were quickly recognized by cable manufacturers. It presented weathering stability, heat resistance up to 160ºC, low temperature flexibility, and it was easily extruded.
Over the years, EPR insulation has become the standard insulation for most industrial applications, and in the last 20 years has become more popular with electric utility applications in network cables, substation underground feeder cables, and distribution cables because of its long-term reliability and strong performance over a 50-year period. Generally, EPR insulations retain their break-down (dielectric) strength in service over the life of the installation, given proper storage and handling of the cable prior to installation of the cable. The superior flexibility of the EPR-insulated medium voltage cable is an important factor in larger (kcmil-sizes) as those cables must be trained and coiled in vaults and other enclosures. The linemen expend less effort, reducing risk of muscle strains. This insulation has a solid history of reliability, superior flexibility, and strong performance in wet applications, as well as improved flame retardancy over TR-XLP insulations. EPR compounds are rated 105ºC for normal operation, 140ºC for emergency operation and 250ºC short circuit.

Losses
The modern electrical distribution system has many contributors to electrical losses such as transformers, service conductors, arresters, secondary conductors and primary conductors. Dielectric losses occur in both TR-XLP and EPR compounds and it is well documented that EPR has significantly higher losses than TRXLP insulation. Decisions on the insulation type to be used should not be based on a single attribute.

Additional Improvements to Cable Design
Other key developments that improved cable life have been the use of strand-fill, where a conductive elastomer (rubber) is pumped within the wires of the conductor, minimizing the effect of water at the most critical point between the conductor and the conductor shield. Another significant development was the almost universal adoption of Linear Low Density Polyethylene (LLDPE) encapsulating jackets, which surround the concentric neutrals, providing another barrier to water, but more importantly adding another layer of tough, physical protection to guard against handling and installation damage prior to installation by the utility.
Regardless of the insulation type specified on medium voltage cable; proper handling, storage, correctly-sized accessories, proper stripping tools, and safe work practices ensure a successful installation and help promote a long service life of the installation. We also encourage the use of solid conductors, strand-fill in stranded wire and the use of LLDPE encapsulating jackets to ensure maximum cable performance.

Medium Voltage Cable Specifications
The specifications that govern the manufacturing of medium voltage underground cables are AEIC CS8-13 and the latest versions of ANSI/ICEA S-94-649 &#;Standard for Concentric Neutral Cables Rate 5 through 46kV&#; and S-97-682 &#;Standard for Utility Shielded Power Cables Rated 5 Through 46kV.&#;
These standards emphasize performance-based requirements for the materials used in manufacture of cable and for the finished product. These specifications are applicable to TR-XLP (tree-retardant cross-linked polyethylene) and EPR (ethylene propylene rubber) and do notendorse one insulation compound over another.

Note: Contribution: Ed Tafoya, SW/SC Regional Sales Manager &#; Utility, for Border States Electric, shared some of his cable expertise in the creation of this article.

Cross-linked polyethylene (XLP) compound was invented during the s and gained popularity during the s because it provided physical protection and flexibility, making it a staple in providing improved reliability in the electrical system. Cross-linked insulation was also affordable and could be mass produced by many suppliers. Therefore, it was the right compound at the right time.Since the mid-s, TR-XLP has become the dominant insulation in utility distribution cable applications primarily due to lower first-cost and lower operating losses. TR-XLP compound continues to be improved and is provided by many cable manufacturers in the United States and the rest of the world.TRXLPE compounds are rated 90ºC for normal operation, 130ºC for emergency operation and 250ºC short circuit.The first versions of EPR (ethylene propylene rubber-insulated), introduced in the early s, demonstrated favorable characteristics that were quickly recognized by cable manufacturers. It presented weathering stability, heat resistance up to 160ºC, low temperature flexibility, and it was easily extruded.Over the years, EPR insulation has become the standard insulation for most industrial applications, and in the last 20 years has become more popular with electric utility applications in network cables, substation underground feeder cables, and distribution cables because of its long-term reliability and strong performance over a 50-year period. Generally, EPR insulations retain their break-down (dielectric) strength in service over the life of the installation, given proper storage and handling of the cable prior to installation of the cable. The superior flexibility of the EPR-insulated medium voltage cable is an important factor in larger (kcmil-sizes) as those cables must be trained and coiled in vaults and other enclosures. The linemen expend less effort, reducing risk of muscle strains. This insulation has a solid history of reliability, superior flexibility, and strong performance in wet applications, as well as improved flame retardancy over TR-XLP insulations. EPR compounds are rated 105ºC for normal operation, 140ºC for emergency operation and 250ºC short circuit.The modern electrical distribution system has many contributors to electrical losses such as transformers, service conductors, arresters, secondary conductors and primary conductors. Dielectric losses occur in both TR-XLP and EPR compounds and it is well documented that EPR has significantly higher losses than TRXLP insulation. Decisions on the insulation type to be used should not be based on a single attribute.Other key developments that improved cable life have been the use of strand-fill, where a conductive elastomer (rubber) is pumped within the wires of the conductor, minimizing the effect of water at the most critical point between the conductor and the conductor shield. Another significant development was the almost universal adoption of Linear Low Density Polyethylene (LLDPE) encapsulating jackets, which surround the concentric neutrals, providing another barrier to water, but more importantly adding another layer of tough, physical protection to guard against handling and installation damage prior to installation by the utility.Regardless of the insulation type specified on medium voltage cable; proper handling, storage, correctly-sized accessories, proper stripping tools, and safe work practices ensure a successful installation and help promote a long service life of the installation. We also encourage the use of solid conductors, strand-fill in stranded wire and the use of LLDPE encapsulating jackets to ensure maximum cable performance.The specifications that govern the manufacturing of medium voltage underground cables are AEIC CS8-13 and the latest versions of ANSI/ICEA S-94-649 &#;Standard for Concentric Neutral Cables Rate 5 through 46kV&#; and S-97-682 &#;Standard for Utility Shielded Power Cables Rated 5 Through 46kV.&#;These standards emphasize performance-based requirements for the materials used in manufacture of cable and for the finished product. These specifications are applicable to TR-XLP (tree-retardant cross-linked polyethylene) and EPR (ethylene propylene rubber) and do notendorse one insulation compound over another.

For more information, please visit FR Polyethylene Cable Compounds Exporter.