Routine tests may differ from one cable type to the next and will be clearly specified in the relevant cable standard. These tests are often non-destructive, some of which may be conducted in line during the manufacturing process.
Examples of routine tests include:
- Spark test on over sheath
- Dimensional tests
- Conductor resistance testing
- Cable markings and measurement
- Voltage test on complete cable
Type tests (short for Prototype) are predominately destructive tests, conducted to determine if the cable construction and materials are compliant with standard specifications. As inferred in the name, prototype tests are done to prove design and the required cable parameters – they are only required to be done once and are generally not repeated.
Type tests, routine tests or sample tests are categorised in the relevant standard to which the cable is manufactured to.
AC stands for an alternating current. Essentially the polarity of the supply is changing with time and as it does the current flows in one direction and then the other. Mains power generation is typically AC – most generators are based on an alternator which creates an alternating current as the wire stator turns within a magnetic field.
DC stands for direct current. Here the current flow is in the one direction only and does not alternate. This is typical of the sort of current produced by a battery. Power generated by photovoltaic panels is DC and would need to be converted with a power inverter to be used for standard mains applications. DC power, once generated, is very useful in speed control motors etc.
The most widely recognised International standards bodies are the IEC, the ISO and CENELEC
IEC is the International Electrotechnical Commission
ISO International Organisation for Standardisation
CENELEC is the European Committee for Electrotechnical Standardisation
Standards bodies with some international standing are as follows:
CSA is the Canadian Standards Association (Canada)
UL is the Underwriters Laboratories (USA)
CEBEC Comite Electrotechnique Belge Service de la Marque (Belgium)
DEMKO Danmark Electriske materaikontrol (Denmark)
SETI Electrical Inspectorate Sakiniementie (India)
IMQ Instituto Italiano del Marchio di Qualita (Italy)
KEMA KEUR NV tot Keuring van Elektrotechnische Materialen (Netherlands)
NEMKO Norges Electriske materllkiontrollanstalten (Norway)
SEV Schweizerischen Electrotechnischen Verein (Switzerland)
VDE Verband Deutscher Elektrotechnischer (Germany)
Within the UK, the official standards body is BSI, the British Standards Institute.
Each country will have its own national standards body with many of these standards being based on a harmonised version of the international standards.
Electrical conductivity and conductor resistivity are essentially the opposite of each other:
Electrical conductivity is the ability of a material to conduct an electrical current.
Conductor resistance is the inherent resistance to current flow in a conductor.
The more electrically conductive a material is the less resistance it offers to current flow. The more resistance the conductor is to current flow, the less conductive it is.
Due to its excellent electrical properties as well as ready availability, copper is the metal most frequently used for electrical conductors. In 1913 the IEC (International Electrotechnical Commission) established a standard for copper conductivity, the IACS (International Annealed Copper Standard), based on the resistivity of annealed copper being equal to 100 percent conductivity.
Although the unit of conductivity is the Mho, its reciprocal, the Ohm is more usually used to express both resistance and thus a measure of conductivity – the lower the resistance in Ohms, the more conductive the material.
A voltage drop in an electrical circuit normally occurs when a current passes through the cable. It is related to the resistance or impedance to current flow with passive elements in the circuits including cables, contacts and connectors affecting the level of voltage drop. The longer the circuit or length of cable the greater the voltage loss. The impact of a voltage drop can cause problems such as motors running slowly, heaters not heating to full potential, lights being dimmed. To compensate for voltage drop larger cross-sectional sized cables may be used which offer less resistance / impedance to current flow.
Voltage drop can be calculated from the formula:
Vd =mV/A/m x I x Ib ÷ 1000
Where:
mV/A/m = the voltage drop per metre per amp
I = the length of the circuit conductor
Ib = the design current
The allowable voltage drop for low voltage installations supplied directly from a public low voltage distribution system is 3% for lighting and 5% for other uses.
There are many different types of electrical cable used for applications across power distribution, control or signalling, and data transmission, and used in industrial, commercial and domestic installations. Electrical cables can be categorised in several different ways including by voltage rating, application, environment, industry, and material type, and determining any of these will help narrow down the search for the correct cable for any given purpose.
Typically, voltage rating categories for cable types include the following:
- Extra Low Voltage for supplies below 70V
- Low Voltage cables include voltages up to 1000V
- Medium Voltage Cables from 1000V to 35kV
- High Voltage cables from 35kV to 230kV
- Extra High Voltage above 230kV
The insulation layer is designed to withstand the electrical performance demands of the cable, so the choice of material type and thicknesses may vary. In some cases a higher voltage may require additional cable layers as determined by local specifications and national or international standards.
The materials used in cable construction are chosen for their electrical properties such as conductivity and insulation resistance. These materials and the precise construction may also influence reactance, impedance, capacitance and inductance values of the cables.
The intended application also determines the cable design and the materials used. For example:
- Overhead line wires need to be strong to support their weight between pylons or posts, and be corrosion resistant, but they don’t need a material insulation layer to protect against short circuit and electric shock if they are used in areas where the risk of contact or grounding does not exist.
- Underground cables must be insulated to protect against water ingress and possible mechanical damage. Cables suitable for direct burial will often have a metallic armour to provide extra protection.
- Cables for use in data-sensitive areas such as instrumentation cables often need to be screened using metallic tapes to protect against electrical noise (also commonly referred to as electromagnetic interference).
- Fire performance cables designed to support fire safety systems such as alarms and emergency lighting must be capable of withstanding fire conditions and maintaining functionality.
Different industries have their own particular requirements for electrical cable, for instance the mining industry requires cables that are resistant to the harsh a unique environments they operate in. Rubber insulation and sheathing is often used as this can offer additional flexibility but also need enhanced robustness and resistance to the chemicals they may be subjected to as a matter of course in operation. The mechanical properties such as resistance to abrasion and impact and the tensile strengths and the elasticity required must also be considered. Protection can include wire braiding, wire armouring, and metallic taping.
The chemical properties of the materials must be in compliance with national and international regulations such as RoHS and REACH, and be capable of withstanding exposure to the various chemical and environmental stresses they may come into contact with. Cables used for outdoor use must be weather resistant and capable of withstanding sunlight and ozone. Other considerations for material selection and construction include the range of temperatures the cables are required to operate in (both high and low temperatures).
Lastly, both cost and appearance (such as colour coding for easy identification) can influence cable construction but of course, these factors should play a secondary role to that of safe application.
The most common method of referring to conductor sizes uses the cross-sectional area, expressed in mm². The following AWG metric conversion table converts AWG to mm and inches, and also lists the cross sectional area.
AWG METRIC CONVERSION CHART (AWG TO MM)
| American Wire Gauge (AWG) | Diameter (in) | Diameter (mm) | Cross sectional area (mm2) |
| 0000 (4/0) | 0.460 | 11.7 | 107.0 |
| 000 (3/0) | 0.410 | 10.4 | 85.0 |
| 00 (2/0) | 0.365 | 9.27 | 67.4 |
| 0 (1/0) | 0.325 | 8.25 | 53.5 |
| 1 | 0.289 | 7.35 | 42.4 |
| 2 | 0.258 | 6.54 | 33.6 |
| 3 | 0.229 | 5.83 | 26.7 |
| 4 | 0.204 | 5.19 | 21.1 |
| 5 | 0.182 | 4.62 | 16.8 |
| 6 | 0.162 | 4.11 | 13.3 |
| 7 | 0.144 | 3.67 | 10.6 |
| 8 | 0.129 | 3.26 | 8.36 |
| 9 | 0.114 | 2.91 | 6.63 |
| 10 | 0.102 | 2.59 | 5.26 |
| 11 | 0.0.907 | 2.30 | 4.17 |
| 12 | 0.0808 | 2.05 | 3.31 |
| 13 | 0.0720 | 1.83 | 2.63 |
| 14 | 0.0641 | 1.63 | 2.08 |
| 15 | 0.0571 | 1.45 | 1.65 |
| 16 | 0.0508 | 1.29 | 1.31 |
| 17 | 0.0453 | 1.15 | 1.04 |
| 18 | 0.0403 | 1.02 | 0.82 |
| 19 | 0.0359 | 0.91 | 0.65 |
| 20 | 0.0320 | 0.81 | 0.52 |
| 21 | 0.0285 | 0.72 | 0.41 |
| 22 | 0.0254 | 0.65 | 0.33 |
| 23 | 0.0226 | 0.57 | 0.26 |
| 24 | 0.0201 | 0.51 | 0.20 |
| 25 | 0.0179 | 0.45 | 0.16 |
| 26 | 0.0159 | 0.40 | 0.13 |
While there is no right answer for the question. The choice of insulation depends on the application for which the cable is used. Given below is a comparative analysis for cable insulation and jacketing material properties.
Thermoplastic vs. Thermosetting
THERMOPLASTIC: A material which will soften, flow, or distort when subjected to sufficient heat and pressure. These compounds are heated and extruded over conductor. Likewise, the insulation on the finished product can be re-melted or soften if exposed to heat.
- Easy to manufacture
- Normally less expensive
- No cure required
- Will melt when subjected to heat
- Can be extruded in very thin walls
THERMOSETTING: A material which will not soften, flow, or distort when subjected to heat and pressure. Once extruded over conductor, these compounds will not re-melt, however, they can be burnt or deteriorate due to heat.
- Will harden and age when overheated
- Forgiving when exposed to overloads
- Better low temperature properties
- Higher temperature potential
- Usually more expensive
- Requires a cure process when extruded
- Not extruded smaller than 22 AWG in CV processes. Irradiated products can be extruded in smaller sizes.
Thermoplastic Compounds
POLYVINYL CHLORIDE PVC, sometimes referred to as vinyl or polyvinyl chloride, consists of three types of vinyl compounds – standard, semi-rigid and irradiated. Depending upon the formulation the rated temperature may vary from -55 C to 105 C. Typical dielectric constant values can vary from 2.7 to 6.5
STANDARD PVC, rated for 1000 volts or less, is used for hook-up, computer and control wires. Different compounds are used for 60C, 80C, 90C and 105C service along with commercial and military applications.
SEMI-RIGID PVC (SRPVC) is much tougher than standard vinyl. It has greater resistance to abrasion and cut-through and offers more stable electrical properties.
IRRADIATED PVC has improved resistance to abrasion, cut-through, soldering and solvents. Irradiation changes the vinyl from a thermoplastic to a thermosetting material.
POLYETHYLENE (PE) is a very good insulation as it offers a low dielectric constant, a stable dielectric constant over all frequencies and a very high insulation resistance. In terms of flexibility, polyethylene can range from stiff to very hard depending on molecular weight and density. Low density is the most flexible, while high density and high molecular weight formulations are very hard. Moisture resistance is excellent, however, both types are flammable. Brown and Black formulations have excellent weather resistance. The dielectric constant is 2.3 for solid insulation and 1.5 for cellular (foamed) designs.
RULAN is a flame retardant polyethylene which contains additives to inhibit the rate of burning. These additives have only a slight effect on physical or electrical properties of the insulation.
PROPYLENE (SOLID AND CELLULAR) is similar in electrical properties to polyethylene. This material is primarily used as an insulation. Typically, it is harder than polyethylene, making it suitable for thin wall insulations. UL maximum temperature ratings may be 60C or 105C. The dielectric constant is 2.59 for solid and 1.55 for cellular (foamed) designs.
KYNAR has great mechanical strength, superior resistance to abrasion and cut-through and substantially reduced cold-flow which makes it an excellent back plane wire insulation. Kynar is self-extinguishing, radiant resistant and rated at 135C.
TEFZEL (ETFE) is rated at 150C, has very good electrical properties, chemical inertness, high flex life and exceptional impact strength. Tefzel can withstand an unusual amount of physical abuse and is self-extinguishing. Tefzel is a registered trademark of DuPont Corporation.
HALAR (ECTFE) has a specific gravity of 1.68, the lowest of any fluorocarbon. Its dielectric constant and dissipation factor at 1 Mhz are 2.6 and 0.013 respectively. Halar chars, but does not melt or burn when exposed to direct flame and immediately extinguishes on flame removal. Its other electrical, mechanical, thermal and chemical properties are almost identical with Tefzel’s. Its temperature rating is -70C to 150C. Halar is a registered trademark of Ausimont Corporation.
TEFLON (FEP) is extrudable in a manner similar to PVC and polyethylene, allowing for long wire and cable lengths. FEP has excellent electrical characteristics, chemical inertness and a service temperature of 200C. Teflon is a registered trademark of DuPont Corporation.
TEFLON (TFE) is extrudable in a hydraulic ram type process. Lengths are limited due to the amount of material in the ram, thickness of the insulation, and preform size. TFE must be extruded over a silver or nickel coated wire, with ratings at 260C and 200C respectively. Teflon is a registered trademark of DuPont Corporation.
PFA is the latest addition to DuPont’s Teflon resins. Like the others it has outstanding electrical properties, high operating temperature (250 C), resistance to virtually all chemicals and flame resistance. PFA is a registered trademark of DuPont Corporation.
THERMOPLASTIC RUBBER (TPR) has properties similar to those of vulcanized (thermosetting) rubbers. The advantage is that processed like thermoplastics, it is extruded over the conductor. Like many conventional rubber materials, TPR is highly resistant to oils, chemicals, ozone and other environmental factors. It has low water absorption and excellent electrical properties, and is very flexible with good abrasion resistance.
Thermoset Compounds
CHLOROSULFONATED POLYETHYLENE (CSPE), better known as Hypalon, is sometimes used as a 105C rated motor lead wire insulation, but most often as a jacketing compound. Hypalon has excellent tear and impact strength, excellent abrasion, ozone, oil, and chemical resistance and good weathering properties. This material also has low moisture absorption, excellent resistance to flame and heat, and good dielectric properties. Hypalon is a registered trademark of DuPont Corporation.
SILICONE is a soft insulation which possesses a typical temperature range from -80C to 250C. It has excellent electrical properties plus ozone resistance, low moisture absorption, weather resistance and radiation resistance. Silicone typically has low mechanical strength and poor scuff resistance. While silicone rubber burns slowly, it forms a non-conductive ash which, in some cases, can maintain the integrity of the electrical circuit.
ETHYLENE PROPYLENE RUBBER (EPR) is a chemically cross-linked, thermosetting high temperature rubber insulation. It has excellent electrical properties combined with outstanding thermal stability and flexibility. EPR’s resistance to compression, cutting, impact, tearing and abrasion is good and is not attacked by acids, alkalis and many organic solvents. It is also highly moisture resistant. EPR has temperature ratings up to 150C.
CROSS-LINKED POLYETHYLENE (XLP) is a material which has greater resistance to environmental stress cracking, cut-through, ozone, solvents and soldering than either low or high density polyethylene. Sometimes designated as XLPE. Can be cross-linked either chemically or irradiated.
SEMI-BUTYL RUBBER(SBR) is flexible and offers good heat and moisture resistance at an economical cost. It must be jacketed for mechanical and chemical protection. SBR is suitable for 75C max temperature ratings.