The Science of Cables
SELECT - Conductors - Insulation - Binders - Cabling - Shielding - Sheathing - Composite - Check List >>
The central component of any cable, the conductor is the term for the metallic wire or wires that carry the signal and/or power through the cable. The most common material used is copper, but other materials can be used to increase temperature rating or improve conductivity, solderability or signal performance. Stranding offers variation in flexibility against durability; conductor sizes are according to national and international standards.
There is a wide range of metals that can be used as a conductor, however Copper (Cu) is by far the most common due to its relative low cost and availability. Other common options such as aluminium, steel or tinsel wire (mixed strands of copper and cotton) may offer advantages in strength, weight or flex-life, however they almost always come at the cost of reduced conductivity. Plated copper such as Tin Plated Copper (TPC), Silver Plated Copper (SPC) and Nickel Plated Copper (NPC) offer additional features such as elevated temperatures and improved conductivity or solderability. Purer conductors such as Oxygen Free High Conductivity (OFHC) plated copper can improve the signal performance, and are often used for audio frequencies, whilst High Strengh Copper Alloy (HSA) conductors can provide a much improved dynamic performance over standard copper conductors.
A variety of other metals and alloys are often used for their unique conducting properties when exposed to heat. Commonly known as Resistance Wires, they are used in Thermocouple cables where combinations of resistance wires can be used to detect variations in temperature. Some of the most commonly used are Nickel-Chromium (NiCr), Copper-Nickel (CuNi) and Iron (Fe).
The simplest form of conductor is a single, solid strand, however although this offers the smallest diameter, the purest signal and the largest cross-sectional area, this is also the weakest option and solid conductors are prone to breaking after just a few bending cycles. To improve the durability and flexibility of a conductor it is common to strand multiple wires together, the more wires that are stranded together to make a given size, the more flexible the conductor will be. Metric sizes categorise the number of strands into Classes, the higher the class, the more strands in the conductor:
- Class 1 Solid, round.
- Class 2 Stranded conductor, 7 strands (larger sizes will be 19 strands).
- Class 5 Multi-stranded conductor for flexible general purpose installations.
- Class 6 Extra-multi-stranded conductor for improved flexibility / flex-life.
There are many different national and international standards for identifying the size of a conductor, and terminology such as BWG, SWG and Cmils can still be found. However most parties now standardise on either American Wire Gauge (AWG) or Metric (mm2) which is also referred to as Cross Sectional Area (CSA). It is also quite common to use both of these methods as they indicate subtly different sizes (see Habias AWG / Metric conversion table for details).
One important note regarding AWG sizes is that the higher the number, the smaller the wire. For example: AWG 2 is a large conductor with a diameter of 8,64mm. AWG 20 by contrast is a small conductor with a diameter of just 0,96mm.
The intended use of the cable is the key reason for selecting one type of conductor over another. Whilst the applications are as varied as the custom design cables made to meet them, some examples might include:
- Crimp terminations Use as few strands as possible and avoid rope-lay or bunched conductors.
- Soldered terminations Use tin or silver plated copper for best results and avoid nickel plated copper.
- Data/signal use Use solid, smooth-surfaced conductors and SPC or steel for best results.
- Dynamic use Use as many strands as possible and high strength copper alloy for best results.
- High temperature use Use silver plated (+200oC) or nickel plated (+260oC) for best results.
Insulation is necessary to provide electrical isolation between the conductor and the earth-plane. The choice of insulation can also have a significant impact on the overall performance of the cable. There is a wide range of materials that can be used for insulation; some can also be used for sheathing cables, all have their own advantages and limitations.
Measures the rugged properties of a material. High abrasion resistance is ideal in dynamic cables where cores are required to move in relation to one other, and to any shield as the cable is flexed. LSI 155 has excellent abrasion resistance.
Under fire conditions, many materials such as FEP, although very good in fire, will release toxic and corrosive gasses once they eventually ignite. Corrosive gasses can damage sensitive equipment such as circuit-boards and consideration to this should be given when installing cable in potentially sensitive areas. HFI 140 is a good example of low corrosivity.
The degree to which the insulation will burn and/or spread a fire once ignited. Most materials used by Habia will self-extinguish once the flame source has been removed. With many different national and international fire standards in use, Habia recommends IEC 60331 and IEC 60332 (in applicable parts) as these test the completed cables and are therefore more truly representative of the application. The other test that is commonly requested is flammability to UL 94. This method tests a small sample of the sheath material and does not assess the overall cable. PTFE is one of the best flame retardant insulations.
There are two key aspects to this. Flexibility is the degree to which a cable can bend whilst flex-life is the frequency over which a cable can be flexed without breaking. Some materials may have poor flexibility, but are actually so strong that they can exhibit a very good flex-life. Although not the most flexible by any means, ETFE is ideal as insulation for flexible cables as its combines abrasion resistance and flex-life.
Fuels and oils
Many fluids will actively break-down the chemical bonds of the insulation. This effect is often magnified when the fluids are at temperature (such as an engine-bay). PFA is amongst the best insulation materials for fuel and oil resistance as it is able to operate in fluids at very high temperatures.
As with fuels and oils, continued exposure to radiation will break-down plastic. Many different measurement scales exist, however the most common are Rads and Grays. Habia will quote the Total Integrated Dose (TID) in Grays (Gy) unless otherwise specified. HFI 260 is the best example of a highly radiation tolerant insulation.
This refers to the level of smoke that is generated by a material under fire conditions. This is of key importance where visibility must not be impeded (for example: the route to an exit). As with flexibility and flex-life there is no link between smoke corrosivity and smoke generation, although FEP is one of the most corrosive materials, it is also one of the best examples of a low smoke-generation insulation.
Where the insulation is concerned, this is the degree to which water can permeate through it, creating an electrical path from conductor to earth (known as arc tracking). ETFE is again one of the best materials for use in water.
The other side of smoke corrosivity, halogen content refers to the toxic gasses that can be emitted under fire conditions. Halogens can affect health and therefore halogen free cables should be installed in areas of high foot-traffic and/or enclosed spaces. HFI 140 is one of the best examples of a zero halogen insulation material.
Although often considered incidental in the design of a cable, there are many reasons for applying foils and tape binders, both production based to hold the cable together during manufacture and performance based, including physical protection, flexibility and roundness. Two main types are used, polyester and PTFE, with hard foil or soft tape options; other types are also available for specialist applications.
The two primary types of tapes and foils used by Habia Cable are polyester (also known as mylar) and PTFE.
Each is available in a hard foil and soft tape version. Polyester is typically used for general purpose and halogen free constructions whilst PTFE provides a high temperature option.
Other tapes and foil include:
- Al/Pr foil - A polyester-backed, aluminium foil used for electrical screening.
- Polyimide - Foil used for high temperatures and high radiation. Can be FEP coated for sintering.
- Low noise - Carbon-loaded tape that can reduce electrical noise and interference.
- Mica - Used as a flame barrier in Habiaflame2 constructions.
- Mµ-Metal - A metallic foil that provides magnetic shielding.
- Water-swellable - Used to absorb water and prevent it from tracking through the interstices of the cable and into the connector.
Cable manufacture is a process that has to be carried out in many stages. It is therefore often necessary to apply a foil or tape in order to physically hold a cable together as it moves between the different stages in production. As a general rule, any cable with 8 or more cores in the final layer of cabling, or a cable with a filling compound to remove all air-spaces in the cable will require a foil or tape to be applied for this purpose.
A typical overlap is between 25% and 50%.
Braided electrical screens and armours can be abrasive to the cores over which they are placed. The application of a foil or tape can often prevent wear and tear to the cores within the cable as it is flexed.
Some tapes and foils such as PTFE are very soft and low friction. This can enable the elements within a cable to move past one another, improving both the overall flexibility and flex-life of the cable.
The addition of a binder can also improve the roundness of a cable, particularly when used beneath a pressure extruded outer sheath. Pressure extrusion allows the sheath of the cable to be made perfectly smooth and round, however this pressure also forces the sheath material into the interstices of the braid and/or cores which makes it very difficult to remove the sheath by hand and virtually impossible to remove with automated cut-and-strip equipment. The addition of a binder gives a smooth surface over which to extrude, enabling the sheath to be removed with relative ease.
Cabling (also known as twisting) is the process where cores are wrapped around one another, enabling the cable to be flexed. Without this rotation the core/s on the inside of the bend would be placed under compression and the core/s on the outside of the bend would be placed under tension, causing deformation of the cable, damage to the other cores in the centre of the cable and also breakages within the connectors.
Most cables will be manufactured with alternating left-hand (S) and right-hand (Z) layers. This is done to make the cable evenly balanced which can prevent it from twisting up in a single direction under dynamic use. Each core will also have back-twist applied to further prevent this twisting process and to ensure that the cable is as dead as possible. It is most common to have the final layer as a left-hand layer, an example of which is a typical 19 core cable that would have:
- Centre - 1x core laid straight.
- First layer - 6x cores laid around the centre core with a right-hand lay.
- Second layer - 12x cores laid around the first layer with a left-hand lay.
The exception to the rule of alternating layers is where the application will require torsion to be applied to the cable (such as coiled/spiral cables). In this instance it is advantageous to have all the elements cabled in a single direction as this will help the cable to return to its original form each time, even when extended and retracted frequently.
The lay-length is the distance in mm or inches over which a core travels from its starting position in a layer, for example: 12 oclock on a clock face, around the cable and back to its original position at 12 oclock. A cable with a short lay length will have a more springy flexible feel to it, whilst a cable with a long lay length results in a stiffer cable. However cables with longer lay-lengths can be produced significantly quicker and use less material which provides benefits in both manufacturing time and cost, so there are good reasons for using a long lay-length where flexibility is not critical to the cable design. Habia Cable will nominally use a lay length of between 8x and 16x the cabled diameter, so for flexible cables, a lay-length close to 8x the cabled diameter will be used, whilst normal use cables will be closer to 16x the cabled diameter.
Twists per inch
Lay length is often specified as a given number of twists per inch. This relates to the number of times a core should travel from its starting position at 12 oclock, around the cable and back again over a given distance. A cable requirement of 3 twists per inch would therefore require the same core to rotate around the cable and return to the 12 oclock position 3 times over the distance of 1 inch (25.4mm) giving a lay length of approximately 8mm.
Habia Cable also has the capability to lay up to 8 components (depending on size) side by side for inclusion in a flat cable design. Flat cables provide a significant benefit with regard to bend radius if the cable is being flexed in a single direction, as the cable can be made with a noticeably smaller overall dimension and yet still contain several elements. However flat cables are not ideal for applications which require freedom of movement in more than one direction.
Shielding or screening as it is also known can be applied for mechanical, electrical and magnetic protection. From stand screening on current cable designs to optimum screening for high performance applications, Habia Cable can provide the optimum solution based on best value balanced against desired performance. Talk to our Technical Sales Staff for further information.
Braided screens of Tin Plated Copper (TPC) wires and Aluminium/Polyester (Al/Pr) foils provide protection from electrical interference either from other sources within the cable (such as power and data cores in the same cable) or from external sources. Braided screens offer a large amount of copper which is beneficial at lower frequencies whilst foil screens offer 100% coverage which is more effective at higher frequencies.
A combination of both foil and braid will therefore often provide the best screening over a wide range of frequencies. Manufacturers vary in the coverage they offer as standard on a braided screen, however most screens will be in excess of 70% optical coverage. Habia Cable standardises on a minimum coverage of 85%, although as electrical interference becomes more important, it is increasingly common to specify optimised braided screens which have an optical coverage of >90% and/or two layers of braided screens.
There are two common methods of measuring the performance of an electrical screen. Shielding effectiveness is measured in dB, where the higher the value, the better the screen.
The second method (used by Habia) is surface transfer impedance which is measured in mohms/m. In this instance, the lower the value, the better the screen.
This is the process whereby multiple braided screens and a mµ-metal foil screen are used in conjunction with one another to provide complete electrostatic and magnetic protection. This combination of screen does significantly limit the flexibility of the cable.
There are three commonly used methods of mechanical protection. Galvanised steel wire armouring is common for large industrial applications, but is not available from Habia Cable. Stainless steel wire braids are the most common form of outer protection and offers significant crush and cut-through resistance. Kevlar braids can also be used where the cable must remain flexible. Kevlar also offers significant cut-through resistance, along with excellent tensile strength.
Although the sheath (or jacket) offers isolation from the earth-plane for any shielding of the cable, it is primarily used to provide mechanical protection from a wide variety of hazards. There is a wide range of materials that can be used for sheathing cables, all with their own advantages and limitations. Abrasion measures the rugged properties of a material. Typically softer, more flexible sheaths will exhibit a high degree of abrasion loss. HFI 260 when used as a sheath exhibits possibly the best abrasion resistance.
Under fire conditions, many materials such as FEP, although very good in fire, will release toxic and corrosive gasses once they eventually ignite. Corrosive gasses can damage sensitive equipment such as circuit-boards and consideration to this should be given when installing cable in potentially sensitive areas. HFS 80 is one such example.
The degree to which the sheath will burn and/or spread a fire once ignited. Most materials used by Habia will self-extinguish once the flame source has been removed. With many different national and international fire standards in use, Habia recommends IEC 60331 and IEC 60332 (in applicable parts) as these test the completed cables and are therefore more truly representative of the application.
The other test that is commonly requested is flammability to UL 94. This method tests a small sample of the sheath material and does not assess the overall cable construction.
The flame retardancy of many sheaths such as TPS 100 can be improved by flame retardant (FR) additives, however these do affect other material properties.
There are two key aspects to this. Flexibility is the degree to which a cable can bend; flex-life is the frequency over which a cable can be flexed without breaking. Some materials may have poor flexibility, but are actually so strong that they can exhibit a very good flex-life. MPR 105 is an example of both good flexibility and flex-life.
Fuels and oils
Many fluids will actively break-down the chemical bonds of the sheath. This effect is often magnified when the fluids are at temperature (such as an engine-bay). Cross-linked materials such as TPS 125 XL are particularly good at resisting damage from chemicals.
As with fuels and oils, continued exposure to radiation will break-down plastic. Many different measurement scales exist, however the most common are Rads and Grays. Habia Cable will quote the Total Integrated Dose (TID) in Grays (Gy) unless otherwise specified. HFS 100 is an example of a highly radiation tolerant sheath.
This refers to the level of smoke that is generated by a material under fire conditions. This is of key importance where visibility must not be impeded (for example: the route to an exit). As with flexibility and flex-life there is no link between smoke corrosivity and smoke generation, so whilst materials such as FEP might be very corrosive, they actually generate very little smoke. HFS 107 XL is one of the best examples of a low smoke-generation sheath.
Where the sheath is concerned, this is the degree to which water can either be absorbed by the sheath (swelling its size) or permeate through it, flooding the cable interstices and allowing water to track back up to the connector. Habias TPU 90 is one of the best examples of a water resistant sheath.
The other side of smoke corrosivity, halogen content refers to the toxic gasses that can be emitted by the sheath under fire conditions. Halogens can affect health and therefore halogen free cables should be installed in areas of high foot-traffic and/or enclosed spaces. HFS 80 is one of the best zero halogen sheath materials.
The core of Habia Cables business model is the design and manufacture of high specification, custom designed cables for demanding applications. Habia Cables ability to take many and varied components and combine them into a working, composite cable that is fit for function both electrically and mechanically is one of the best in the industry. Composite or multicore cables offer a key benefit in umbilical and reeling cables as a single composite cable can do the work of a strain cable, electrical cable, power cable and even a number of hydraulic hoses.
Components can include (but are by no means limited to):
With a wide variety of sizes and impedances available, Habia Cable are able to combine any of the RG Style, Multibend, Speedflex or Speedfoam coaxes can be included within the design (Flexiform is not recommended for inclusion within composite cables due to its limited flex-life). Habia Cable is also frequently called upon to design customised coaxials for use within composite cables and these can be modified to feature varied impedances, additional screening or alternative sheaths and colours (including unsheathed coaxials).
As with the coaxial cables, data and ethernet pairs are available in a range of sizes and impedances. Perhaps the most common are 90 Ohms (USB) and 100 Ohms (Cat 5) however 77 Ohms, 120 Ohms and 125 Ohms are also often requested. Depending on the performance requirements of the cable, Habia Cable can make these components using either PE, PTFE or FEP dielectrics.
Power cores can be varied in size and colour coding. They can also be electrically isolated from the rest of the cable if required.
Probably the main component of any composite cable, signal wires can also be electrically isolated from the rest of the cable and are often specified as screened twisted pairs, triples and quads. Either colour coded or numbered (depending on size and cost) for ease of termination, Habia Cable can manufacture cables with hundreds of signal cores if required.
Can be applied as either a single, central strain cores or and overall braid (multiple strain wires throughout the cable are occasionally requested, but these are inadvisable as they often move within the cable when placed under strain, damaging the other components of cable as they do so). The level of load that can be supported varies from cable to cable, but Habia Cable have had experience with cables that can take loads of several tonnes.
Vent tubes are incorporated within cables for a variety of purposes as they are able to provide air in the cable for cooling, they can aid the buoyancy of a cable and they can carry high pressure air or oil for pneumatic and hydraulic use.
Once arranged in a suitable lay-up that can be produced by machine, the cable will be cabled together with back-twist and alternating layer directions to ensure the best possible construction. Over braids of copper and stainless steel wire or kevlar strands can be applied and (depending on the performance of the materials within the cable) one of a wide variety of inner and outer sheaths can be applied over the top of the whole construction. These sheaths can be marked with either Habia Cables standard printing, designed to simply identify the cable for future reference, or with the customers requested printing.
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From technical articles issued by Habia Cables
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