FREQUENTLY ASKED QUESTIONS
As cable experts we get asked questions covering everything from general electrical information to more specific queries about cable construction, specific electrical wires for applications, and the cable industry as a whole. As part of the technical support services we offer our customers we’re always pleased to help and some of the more frequently asked questions, as well as some general technical information is explained below.
CABLE KNOWLEDGE FROM CABLE EXPERTS
A sound technical understanding of cables and their applications is important for a number of roles and responsibilities across the industries. Just as we make sure our staff benefit from cable training from our experts, we believe that our customers should have access to this insight too.
If you would like to know more, then contact us and learn about cables from experts.
Electricity is a versatile form of energy used to power anything from huge data centres and critical systems to many of our everyday electrical appliances. It can be generated and transferred very easily. Electricity, or electrical energy, can be created and converted from energy generated from other forms including thermal energy from burning fuels, wind energy harnessed by turbines, nuclear energy, and solar energy. This energy can either be used as it is generated or it can be stored in batteries or fuel cells.
Electricity is associated with electric charge which can be either a static charge or a dynamic charge. Although static charge electrical energy has its uses, it is the dynamic charge or flow of electric current which is most useful.
So what is electricity made of? At its simplest, all matter is made up of atoms, which are themselves made up of three particles: neutrons, protons and electrons. Neutrons have no charge; positively charged protons which are bound around the neutrons in the atomic nucleus; and negatively charged electrons which orbit the nucleus. The flow of electricity is associated with the loosely bound electrons flowing from one atom to the next. The more loosely bound the electrons in a material’s atomic structure are, the better the material is at conducting electricity. Metals are good electrical conductors whilst poor conductors or insulators such as plastics have tightly bound electrons in their atomic structure and don’t allow electric current to flow easily.
What is static electricity? Static electricity can be produced when two dissimilar materials are rubbed together, creating a potential difference between them, with one material becoming more negatively charged and one becoming more positively charged. This static electrical field around the material will remain until it is discharged when the current flows from the surface of the material through a conductor. Examples include the static electricity produced by combing your hair, or the shock you get when static energy built up on a car door is discharged. On a much larger scale, lightning is another example of static electricity discharge.
What is dynamic electricity? Dynamic electricity producing a constant current flow is generated primarily by turning a wire stator or copper disk in a magnetic field which in turn causes a current to flow in the wire. The turning force for the wire loop or copper disk can be generated by turbines powered by the energy in wind, steam or water. This is known as electromagnetic induction.
Other ways of generating an electric current include:
– Electrochemical such as in batteries and fuel cells.
– Photovoltaic used in solar energy cells in PV panels.
– Thermoelectric where temperature differences are converted into voltages across dissimilar metals.
– Piezoelectric which involves the mechanical strain of electrically anisotropic molecules
– Nuclear transformation alpha particle emission.
Electrical cables work by providing a low resistance path for the current to flow through. Electrical cables consist of a core of metal wire offering good conductivity such as copper or aluminium, along with other material layers including insulation, tapes, screens, armouring for mechanical protection and sheathing. These additional layers are designed principally to allow the metal core to continue to conduct electrical current safely in the environment it is installed in.
A good conductor is made of a material whose atomic structure has loosely bound electrons in its outer shell which can move across the atomic matrix of the material. This movement of electrons is known as the current flow. On the contrary, good insulators have tightly bound electrons which make it difficult for this current flow.
The simplest electrical cable may just be a metal conductor – these cables are used as overhead line wires manufactured without any insulating material around the conductor other than the air that surrounds the conductor. Overhead cables such as those used in the rail industry, require practical considerations to ensure that the cable is isolated from any means of accidentally grounding the electrical conductor. Cables used either at ground level, within reasonable reach, or underground need to be effectively insulated to both maintain current flow through the cable and to be safe for users.
The amount of current which can effectively flow through the electrical cable will be determined by a combination of factors:
– the cross-sectional area of the conductor
– the resistance of the conductor material
– the insulation material
– the installation method or environment.
The effect of resistance to current flow is the heating of the conductor and the surrounding insulation layers. Over-heating has the potential to cause failure of the insulation material resulting in current short circuiting, electric shock and even fire.
There are many different types of electrical cable used for applications across power distribution, control or signalling, data transmission and used in industrial, commercial and domestic installations. Electrical cables can be categorized 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. That’s why 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 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 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 and 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 cable bending radius is a measurement of the smallest radius a cable can be bent around without damaging the cable.
Factors which influence the minimum bending radius include the cable size, the cable construction, the conductor type and the sheathing and insulation types used.
The bending radius is normally expressed as a factor of the overall dimension of the cable for example, 6D or 6x the outer diameter of the cable.
EXCEEDING THE CABLE BENDING RADIUS
The relevant standards determine a minimum bending radius so as to protect the integrity and performance of the cable. Where the cable bending radius has been exceeded – whether through winding onto drums with a barrel diameter too small for the cable or during installation – the cable can show kinking or other sheath damage as an indication of other possible problems. Where it is suspected the cable bend radius has been exceeded it is recommended that testing, such as the sheath integrity test conducted by The Cable Lab, be undertaken prior to further install to determine if the cable performance has been impeded.
Electrical current flows from a point of positive charge to a point of negative charge whilst essentially the electrons flow in the opposite direction.
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. AC power transmission is also preferred for high voltage mains transmission because it is relatively easy to step down the voltages for various applications with transformers. The frequency of this alternating direction for mains supply in the UK is 50Hz, or 50 cycles per second.
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.
When the International Electrotechnical Commission (IEC) member countries and affiliate members are added together the IEC family covers more than 97% of the world’s population. The members are the national committees of the respective country, responsible for setting national standards and guidelines.
The IEC controls the publication of 212 standards associated with electric cables which come under the remit of the Technical Committee 20 of IEC. Of course, these countries do not exclusively use only IEC cable standards and have their own National types, however they do recognize many of the IEC standards and work towards the ongoing harmonization of standards and test methods etc.
Full affiliate members of the IEC include:
Algeria, Argentina, Australia, Austria, Belarus, Belgium, Brazil, Bulgaria, Canada, Chile, China, Columbia, Croatia, Czech Republic, Denmark, Egypt, Finland, France, Germany, Greece, Hungary, India, Indonesia, Iran, Iraq, Ireland, Israel, Italy, Japan, Korea Republic of (South Korea), Libya, Luxembourg, Malaysia, Mexico, Netherlands, New Zealand, Norway, Oman, Pakistan, Philippines, Poland, Portugal, Qatar, Romania, Russian Federation, Saudi Arabia, Serbia, Singapore, Slovakia, Slovenia, South Africa, Spain, Sweden, Switzerland, Thailand, Turkey, Ukraine, United Arab Emirates, United Kingdom, United States of America.
There are an additional 22 associate members:
Albania, Bahrain, Bosnia & Herzogovina, Cuba, Cyprus, Democratic People’s Republic of Korea (North Korea), Estonia, Georgia, Iceland, Jordan, Kazakhstan, Kenya, Latvia, Lithuania, Malta, Moldova, Montenegro, Morocco, Nigeria, Sri Lanka, The Former Yugoslav Republic of Macedonia, Tunisia and Vietnam.
Additionally there are 83 affiliate members:
IEC standards cover the whole spectrum from low voltage, medium voltage and high voltage power cables and cable accessories in various material types and for a wide range of applications including but not limited to fibre optic cables, mineral insulated cables, heating cables, ground lighting cables for aeronautics, data cables, power control and instrumentation cables for shipboard and offshore applications.
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)
BSI the British Standards Institute.
Each country will have its own national standards body with many of these standards being based on a harmonized version of the international standards.
This is a term for the maximum current carrying capacity, in amps, of a particular device. The current carrying capacity is normally associated with electrical cable and is determined as the maximum amount of current a cable can withstand before it heats beyond the maximum operating temperature. The effect of resistance to current flow is heating and this is dependent upon the size of the conductor, the insulation material around the conductor, and the installation environment.The larger the conductor size the lower the resistance to current flow, meaning less heat associated with this resistance. Increasing the conductor size increases the current carrying capacity. Similarly, the higher the temperature resistance of the insulating material, the higher the ampacity or current carrying capacity. A 90°C rated insulation will have a higher current carrying capacity than a 70°C rated insulation.
The installation environment and the temperature of this environment affects the ability to dissipate heat away from the cable and so also affects the current carrying capacity. Cable used in air or ground at lower temperature will have a higher current carrying capacity than cable in air or soil at higher ambient temperatures.
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
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.
An Ohm is the SI unit for electrical resistance and is symbolized by the Greek letter Ω.
The Ohm is related to the current and voltage in a system: a current of 1 amp through 1 ohm of electrical resistance produces a voltage of 1 volt across it.
The formula for this is I=V/R where:
I = the current through the conductor
V = the voltage measured across the conductor
R = the resistance of the conductor
Materials with a low resistance make good conductors – examples include copper and aluminium – whereas materials with very high resistance which make good insulators, such as PVC (Polyvinyl Chloride) and PE (Polyethylene).
Conductors are typically measured in Ohms (Ω) whereas insulators are measured typically measured in Mega Ohms MΩ.
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.
An ampere, or amp as it is more commonly referred to as, is the standard unit of current. It is determined as the amount of current which flows when a potential difference of one volt is applied across a resistance of 1 ohm.
Current is the measure of the amount of electrical charge moving through a specified point in a unit of time. An ampere of current is flowing when a charge of 1 coulomb passes a point in a second.
The symbol for an ampere is A.
The voltage rating of a cable is the highest voltage that may be continuously applied to a cable construction in compliance with the relevant cable standard or specification.
Voltage rating figures for cables are normally expressed in A.C. RMS. (Alternating Current Root Mean Square) and are written as a figure Uo/U (Um)
Uo = Rated voltage phase to Earth
U = Rated voltage phase to phase
Um = Maximum system
The volt (referred to with the symbol V) is the Standard International (SI) unit of electromotive force or the potential difference required to carry 1 ampere of current through a resistance of 1 Ohm.
Voltages are sometimes expressed in units representing power-of-10 multiples or fractions of one volt:
A kilovolt (kV) is equal to one thousand volts.
A millivolt (mV) is equal to one-thousandth of a volt.
Two conductors which are separated by a distance can store an electric charge between them. A cable or harness with two or more wires can also store a charge and this can affect the way the cable performs. Capacitance describes the ability of two conductors, separated by an insulating material, to store charge.
Capacitance in cables is usually measured in pf/m (pico farads per meter) or pf/ ft (pico farads per foot). The lower the capacitance the better the cable performance.
Capacitance is a particular problem with data or signal cables. When a voltage signal is transmitted through a twisted pair or coaxial type cable, a charge builds up across the insulation between the conductors. The charge that builds up in the cable over a period of time is due to the inherent capacitance this results in a delay causing interference in the signal transmission. Digital data pulses which are square in shape are transformed to form a shape similar to “saw teeth” due to the ramp up and discharge, this may result in the circuitry failing to recognize the digital pulses.
There are a number of ways to reduce the capacitance in cable design including:
– Increase the insulation thickness
– Decrease the conductor diameter
– Use an insulation with a lower dielectric constant
Usually a combination of all three is used as either method has its limitations.
Having a metallic shield over the cable introduces a further capacitance, that of core to shield, which can significantly increase the overall capacitance of the cable.
The fault current is the electrical current which flows through a circuit during an electrical fault condition. A fault condition occurs when one or more electrical conductors short to each other or to ground. The fault types are phase to ground, double phase to ground, three phase to ground, phase to phase and three phase.
A fault current is usually several times larger in magnitude than the current which normally flows through the circuit in a non-fault condition.
Fault interruption devices include fuses, circuit breakers and relays.
The dielectric strength of a material is a measure of the electrical strength of an insulator. It is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed in terms of Volts per unit thickness. The higher the dielectric strength of a material the better an electrical insulator it makes.
IEC 60243 is a standard referred to for a method of testing dielectric strength of a material. The test is conducted in either air or oil and involves placing the test material between two electrodes and increasing the voltage between the electrodes until an electrical burn-through punctures the sample or decomposition occurs. Usually the specimen is between 0.8 and 3.2mm thick. Samples which are over 2mm in thickness are usually tested in oil to prevent flash over before breakdown.
The dielectric strength is then calculated by dividing the breakdown voltage by the thickness of the sample.
Most plastics have good dielectric strengths in the order of 10 to 30kV/mm.
Low Density Polyethylene LDPE = 27kV/mm
Polypropylene PP = 22kV/mm
Polyvinylchloride PVC = 14kV/mm
When a current flows through a conductor it produces a magnetic field around the conductor. If this induced magnetic field changes with a changing current it produces an electromotive force (EMF) of 1 Volt which opposes the change. Any sudden change is opposed by the induced current. This is known as self -induction.
Mutual induction is the effect of inducing this EMF upon another conductor in the same magnetic field. An electrical transformer works on this principle of induction, the primary coil windings inducing voltage in the secondary coil windings
The symbol representing Inductance is L.
The SI unit of inductance is the henry and is defined as ‘1 henry induces an electromotive force of 1 volt in a closed circuit with a uniform change of current of one amp per second’.
Impedance is measured in Ohms and represents the total resistance that the cable presents to the electrical current passing through it. Impedance is associated with AC circuits.
At low frequencies the impedance is largely a function of the conductor size (resistance), but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable’s impedance. Matching impedance is very important, for example, if the system is designed to be 100 Ohms, then the cable should match that impedance, otherwise error-producing reflections are created at the impedance mismatch, seen as lower return loss in bidirectional signal cables.
The symbol for Impedance is Z.
Attenuation is generally associated with data cables and refers to any reduction in signal loss, calculated as a ratio of the power input signal to output signal, which is measured in decibels per unit length (db/ft). Attenuation is very dependent on signal frequency, a cable that performs very well with low frequency data may demonstrate poor performance at higher data rates, cables with lower attenuation provide improved performance.
Attenuation occurs on computer networks for several reasons including:
– Range for wireless or length of run for wired networks
– Interference from other networks or physical obstructions for wireless systems
– Wire size, thicker wires are better.
Reducing attenuation in an electrical system and improving performance can be achieved by increasing the power of a signal through a signal amplifier or repeaters.
The mains supply to most homes is a single phase alternating current (AC) supply. Unlike the current supply from a battery which is a direct current (DC) supply, the current is constantly alternating between zero and peak values.
The speed at which this cycle changes is known as the frequency of the supply. In the UK this supply frequency is 50Hz or 50 times per second.
For most domestic purposes this alternating supply is sufficient but for many commercial and industrial purposes it is necessary to improve power and efficiency by using a three phase supply. With a three phase supply each phase set to be separated by 120°.
A three-phase system is usually more economical than single-phase as reduced conductor material is required to transmit electrical power.
Electromagnetic interference is abbreviated as EMI. EMI is the disturbance which is unintentionally generated by an external source that effects the electrical circuit by electromagnetic induction, electrostatic coupling or conduction. This is a particular problem with sensitive equipment where transmission signals may be corrupted or distorted. Data transmission may also result in an increase in error rate or total loss of data.
Electromagnetic interference can be reduced by ensuring that all electronic equipment is operated with a good electrical grounding system. Cables used to connect the electronic or computer systems should if possible be shielded such as the CY control cable or our range of instrumentation cables. The use of specialized components and circuits to reduce EMI through the use of filters, capacitors and inductors can also be installed in the circuit path.
Sunlight is converted into electrical energy by photo-voltaic (PV) cells. The PV cells consist of layers of semi-conductive material designed to be either N-type (negative) or P-type (positive). The light (photons) which is absorbed is used to excite electrons from their atomic structure which then creates a potential difference between the two semi-conductive layers, typically in the region of 0.5V per cell.
The photo-voltaic cells are connected together and mounted into a support structure or frame called a photo-voltaic module these modules are combined together to form a photo-voltaic array.
The photo-voltaic arrays produce direct current (DC) and can be connected in series or parallel arrangements using PV1-F Photo-voltaic cable to produce the required voltage and current combinations.
Electrical energy is generated from wind in the way most electrical energy is generated – using an electrical generator which converts mechanical energy into electrical energy. An electric generator is based on a magnetic core turning inside a wire coil which produces an electric current. Wind turbines generate the mechanical energy required to turn the magnetic core within the wire coil windings similar to how diesel engines or steam powered electrical generators work.
The modern day wind turbine is designed to catch the prevailing wind in the most effective way, controlling the direction and pitch of the blades as the wind changes direction and speed. Typical wind turbines can operate in a wide range of wind speeds. There is a minimum speed required to turn the turbine and start producing electricity and also typically a cut out speed to protect the equipment from excessive wind speeds.
Transformers are used to regulate and convert the power produced so that it is compatible with the requirements of the end user.
As current passes through a wire it encounters resistance to the current flow, this causes some of the electrical energy to be converted into heat energy, which is in turn dissipated to the surrounding area.
The effect of this resistance and the resulting electricity or power loss can be reduced by increasing the size of the conductor.Yet increasing the conductor size has several disadvantages; firstly the conductor is more expensive and heavier, requiring additional support; and secondly there is a limit to how effective this is for AC transmission.
However, increasing the voltage will increase the ‘pressure’ to overcome this resistance to current flow, resulting in a much more efficient means of transmitting power. AC voltages can be easily increased using transformers.
AC power has been traditionally used for the transmission of power because the voltages can be easily converted with step-up or step-down transformers. AC systems do however have problems and suffer losses through induction (electromagnetic fields).
Increasingly, high voltage DC power is being used to carry electricity over long distances and this can prove to be a more efficient means of transmission. Generated AC power is converted to HVDC by rectifiers for transmission purpose and then converted back to AC with the use of inverters for consumption purposes.
Definitions vary somewhat but a general guide to the voltage categories are as follows:
Low Voltage: up to 1000V
Medium Voltage: between 1000 V and 35 kV
High Voltage: between 35 kV and 230 kV
Extra High Voltage: from 230 kV and above
The earliest use of recognizable electrical cable was probably the early commercialized Telegraph lines such as that strung between Washington, D.C, to Baltimore, Maryland in 1844
These early cables were made of iron and were difficult to produce. In order to improve the production by lubricating the iron surface of the wires, copper sulphate was used to apply a thin copper coating. The superior conductive property of the copper was soon realized and copper eventually replaced these early iron conductors. By 1913 the International Electrotechnical Commission established IACS (International Copper Standard) as a benchmark for the resistivity of copper as being equal to 100 percent conductivity.
In the 1880’s the first insulated cables were insulated with gutta percha, a natural latex material produced from the sap of trees of the same name. This insulation needed to be kept constantly wet or it would dry out and fail to insulate the conductors. This material was largely replaced by rubber and vulcanized bitumen.
By the 1890’s mass impregnated paper insulation was being used on cables and voltages as high as 10kV.
In 1906 armored cables were introduced which had flexible sheathing and two cloth covered, rubber insulated conductors.
In the 1930’s the first trials with PVC insulation were being made in Germany and by the end of the second world war there were significant varieties of synthetic rubbers and polyethylene.
By the 1950’s PVC was commercially viable and replaced rubber cables in many areas particularly in domestic wiring, aluminium was also starting to be used widely as an alternative conductor
The 1970’s heralded XLPE insulation replacing paper insulated cables in medium voltage applications.
In the 1980’s optical fibres were being introduced in overhead lines for data transmission and condition monitoring, and further use of XLPE in high voltage transmission lines between 66 and 240kV. Also high temperature super conductivity materials were discovered. PVC alternatives (LSZH cables) which were safer in fires were being developed in response to a number of tragic public fires which demonstrated the dangers of smoke and toxic gases from various materials including PVC’s.
By the 1990’s polymers were being extended to use in EHV, extra high voltage, applications and the use of optical fibres in overhead lines was becoming widespread.
Cable ratings determine the parameters within which a cable can be safely used. The most typical cable ratings are temperature, voltage and current.
Temperature rating is usually defined as one of the following:
– Max conductor temperature rating
– Minimum installation temperature rating
– Minimum flexible temperature rating.
Voltage is usually defined in terms of the following:
Uo = The R.M.S value between any insulated conductor and the earth or metal covering.
U = The R.M.S value between any phase conductor and another phase conductor or system of single insulated conductors.
(Um) = Maximum system voltage
The current rating is usually defined in terms of:
– Normal maximum continuous current rating
– Short circuit current rating
Cables may also be de-rated depending on the method of installation, for example, a cable installed in a thermally insulated wall will have a lower current rating than a cable of the same size and type which is installed in free air or in an open cable tray.
Cables installed in the ground at a higher than normal ambient temperature will have a lower current rating.
Copper and aluminium are most frequently used as the electrical conductors in electrical cables due to their low resistance and excellent conductivity. These metals are both ductile and relatively resistant to corrosion, but they also have different properties which make them useful for various applications. Copper is the most conductive of the two metals, in fact of the commonly found pure metals, only silver is more conductive but it is considerably more expensive and not as strong.
Copper is determined as the standard for electrical conductivity – the International Annealed Copper Standard (IACS) with a copper resistivity of 1.724µΩcm at 20°C is assigned the 100% value. The addition of impurities or the work hardening of the copper through drawing down will adversely affect the conductivity of the copper. Whilst copper alloys are sometimes produced to improve the hardness of the copper where ductility is not desired, or to enhance the tensile strength, flex endurance and temperature resistance, the consequence of these additional alloying materials is to decrease the conductivity.
Aluminium is abundantly available and offers a cheaper alternative to copper for conductors. The demand for copper is variable and the price fluctuates considerably whereas the price of aluminium is much more stable. Whilst an aluminium conductor is only about 61% as conductive as the same sized copper conductor it is also three times lighter in weight which makes it much easier to handle. For this reason aluminium finds favor in large size cables and cables for overhead power distribution.
The difference in the conductivity means that a much larger size aluminium conductor needs to be used to match the conductivity of the equivalent copper conductor. Using a larger size conductor has the add-on effect of requiring a greater amount of insulation material to adequately cover the conductor and the extra cross-sectional size of the cable may be restrictive in certain applications.
Other differences between the two include the tensile strength – copper has approximately twice the tensile strength of aluminium, but it is worth noting that given the equivalent aluminium conductor is bigger and lighter it often doesn’t require the same degree of tensile strength. Copper is more thermally conductive than aluminium but again, when the larger conductor sizes are factored in the differences are reduced. The better the thermal conductivity the better the short circuit performance of the conductor.
In some cases copper-clad aluminium conductors consisting of an aluminium core with a heavy skin of copper bonded to the aluminium can be used. Whilst not in widespread use, this conductor type does combine the advantages of the lighter weight aluminium with the more conductive copper. The ductility is however that of aluminium and not the improved performance of copper. This conductor type has found some favor with coaxial cables as a lightweight center conductor. The lighter weight wire allows the use of lower density dielectric material for better attenuation.
PVC (Polyvinyl chloride) is widely used in electrical cable construction for insulation, bedding and sheathing. It was the 1950s when PVC started to replace rubber insulated and sheathed cables in general household wiring due to its ease of processing. PVC is cost-effective and also has excellent ageing properties and typically exceeds a 25 to 30 year service life.
It’s considered to be one of the most versatile of the common thermoplastics due to the fact that its properties can be easily modified – although PVC is inherently hard and rigid it is easily modified with plasticizers, stabilizers, lubricants and various other ingredients and fillers that aid processing and enhance various properties. It is also easy to process and recycle when used as a thermoplastic type.
Cable with a PVC insulation or sheathing is flame retardant, which is an important consideration for electric cables in most applications. PVC can be made resistant to a wide range of chemicals including oils, acids and alkalis, and is tough, durable and resistant to abrasion. The addition of various additives can improve its temperature range, typically from -40 to 105°C, as well as the resistance to sunlight, reduced smoke emission and improved water resistance.
As an insulation material cables often come down to a choice between XLPE vs PVC – between a thermoplastic and a thermoset material. There are thermoset versions of PVC which are cross-linked, typically with electron beam technology but they are more expensive to use and so when specified they are typically in high-spec applications in industries such as defense and automotive. The thermoset or cross-linked PVC has improved temperature resistance, is tougher, and has a better dielectric strength, which means that a thinner coating or insulation layer can be applied making the overall cable dimension smaller.
XLPE or Cross-linked polyethylene is a thermoset insulation material. Crosslinking polymers is a process which changes the molecular structure of the polymer chains so that they are more tightly bound together and this crosslinking is done either by chemical means or physical means. Chemical crosslinking involves the addition of chemicals or initiators such as silane or peroxide to generate free radicals which form the crosslinking. Physical crosslinking involves subjecting the polymer to a high energy source such as high-energy electron or microwave radiation.
Polyethylene (PE) material itself has excellent dielectric strength, high insulation resistance, and a low dissipation factor at all frequencies making it an ideal insulator, however it is limited in its temperature range. Cross-linking the PE to become XLPE increases the temperature range of the insulation whilst maintaining the electrical properties.
XLPE VS PVC CABLE INSULATION
XLPE is suitable for voltage ranges from low to extra high voltage, surpassing other insulation materials such as PVC, Ethylene Propylene Rubber (EPR) and silicone rubbers. Cross-linking the polyethylene also enhances the chemical and oil resistance at elevated temperatures and makes it suitable for use as a Low Smoke Zero Halogen material.
The mechanical properties of the XLPE are superior to many other insulations, offering greater tensile strength, elongation and impact resistances. The addition of carbon black can be used to further enhance hot deformation and cut through resistance. The XLPE insulation will not melt or drip, even at the temperatures of soldering irons, and it has increased flow resistance and improved ageing characteristics.
Improved water-tree resistance is another benefit of XLPE insulation for LV cables and MV cables over PE insulation. Water treeing is a defect which is the result of imperfections in the insulation where fracture lines occur and grow in the direction of the electric field, increasing with electrical stress. It should be noted that this effect is not limited to PE materials.
Rubber has been used as cable insulation and sheathing material long before other insulation such as PVC and PE can to be commonly applied. It remains widely used across domestic and industrial applications.
Initially, natural rubbers were used but these have been largely replaced by various synthetic rubbers. All rubbers are thermoset or cross-linked by a process referred to as Vulcanization. As thermoset materials they do not soften or melt when exposed to heat.
The properties of these base rubbers can be significantly changed through the addition of various additives including fillers, vulcanizing agents, accelerators, antioxidants, and antiozonants.
Typical rubber cable compounds include Natural rubber, SBR or Styrene-Butadiene Rubber, Butyl, EPR or Ethylene Propylene Rubber, Silicone, Polychloroprene, Chlorosulphonated Polyethylene (PCP) and Fluorocarbon.
The principle advantage of all rubber cables over other insulated cables is the excellent flexibility in temperature range. They also have very good water absorption properties. Many rubber cables also have superior abrasion resistance and weathering resistance making them particularly suitable in harsh environments as trailing leads for portable electrical appliances, power tools, pumps and generators. Rubber cables are also compounded to give excellent resistant to oils and other chemicals.
MDPE stands for medium density polyethylene. This material is used principally as a sheathing material on larger size cables with higher voltage ratings, such as our BS6622 11kV MDPE sheathed cable.
Polyethylene materials all have excellent insulation resistance, dielectric strength, low dissipation factors, and abrasion resistance. They are classified by their density which is associated with the crystallinity levels of the polyethylene. The higher the crystallinity or density, the greater the toughness of the cable sheath.
WHY USE MDPE CABLES
MDPE has very high resistance to abrasion: it is extremely hard and has a low dielectric constant with superior oxidation resistance. The hardness of the MDPE cable sheathing protects the cable from sharp objects dropped or loaded onto the cable. This means MDPE finds particular usage in transmission cables in the toughest of environments, notably those with the high ambient temperatures found in tropical and subtropical countries. At these high ambient temperatures alternative sheathing materials such as PVC would be soft and prone to damage easily during laying of the cables. When compared with LDPE (low density polyethylene) it has significantly more strength enabling it to be pulled under heavy load for cabling applications, whilst compared to HDPE (high density polyethylene) it has improved resistance to cracking.
Various additives can be applied to MDPE compounds to improve fire retardant properties, UV and weathering resistance and the chemical degradation.
Ethylene Propylene Rubber is a generic term for a wide range of polymers based on copolymers of ethylene and propylene. It is one of a number of rubber insulation materials. The EPR polymers can be tailored for different applications.
EPR is widely used as an insulation material for electric cables due to its high dielectric strength but it is also used as a sheathing material exhibiting excellent ozone and weathering resistance. EPR has a wide thermal range typically in the region of -55°C to 150°C. Unlike other organic rubbers, the copper conductor does not need to be tinned to prevent deterioration of the rubber.
EPR rubber is noticeably softer than Natural rubbers and Styrene-Butadiene rubbers so can be used as a replacement material in many applications. Where greater hardness is required, the EPR compound can be blended with polyethylene (PE) or polypropylene (PP) to achieve improved physical properties. Mechanical properties include resistance to compression, cutting, impact, tearing and abrasion.
Although EPR does not offer a good resistance to oils, it is resistant to a wide range of other chemicals including many acids, alkalis and organic solvents. It is also highly resistant to moisture.
Like XLPE insulation, EPR insulation is suitable for many higher voltage applications and whilst its dielectric properties are not as good as those of XLPE it does have some important advantages over XLPE including extra flexibility, reduced thermal expansion, and low sensitivity to water treeing.
When metal is cold worked or formed it becomes work hardened or strain hardened. Copper conductors go through a considerable amount of work hardening as the copper rod is drawn down through ever decreasing die sizes until the required conductor dimension is achieved. Copper in this state is known as hard drawn copper.
Hard drawn copper is difficult to work with and stranding and bunching of the finer wires in this state would be very difficult. By heat treating the copper at the correct temperatures the ductility can be restored to make the cable soft and flexible again. The heat treating process is known as annealing and the resulting metal is known as soft annealed copper. The degree of annealing is controlled by temperature and time, copper wire is used with different degrees of annealing depending on the application.
Hard drawn copper has significantly higher tensile strength than soft annealed copper and is used as overhead wire whereas the soft annealed copper is flexible and has somewhat improved conductivity over hard drawn copper conductor.
Annealed copper – Tensile strength 300-400 ksi (kilopound per square inch). Conductivity 100.00 % IACS
Hard drawn copper – 500-700 ksi. Conductivity 97 % IACS
The plastic or polymers used in cable insulation are either thermoplastic or thermoset. Thermoplastic material is softened by heating and can be shaped, with the shape then maintained by cooling. The important characteristic of thermoplastic material is that this process can be repeated with the material re-softened and reshaped over and over again as required. These thermoplastic materials lend themselves to recycling and reuse.
Thermoset materials are also softened by heating and can be shaped and then cooled to retain a new shape however unlike thermoplastic material, it is only possible to do this once. This is due to a chemical reaction that has taken place during the polymerisation.
Examples of thermoplastic types are PVC (Polyvinyl Chloride) and PE (Polyethylene).
Examples of thermoset types include rubber insulations such as silicone rubbers and EVA (Ethylene-Vinyl Acetate).
PE and PVC may also be cross-linked making them thermosetting types. PVC and XLPE materials which have been cross-linked to make them thermoset materials also have enhanced resistance to temperature, improved dielectric strengths and resistances to certain chemicals.
The operating temperature of an electrical cable normally refers to the minimum and maximum temperature that the cable can safely operate at for a sustained period of time. This operating temperature is determined by the insulation and/or sheathing material around the cable.
Each material type will have an upper and lower range of temperatures within which it continues to be suitable for use. This varies widely depending on the material type as well as whether or not the cable is required to be flexible at these temperatures. Generally, materials soften at higher temperatures and become rigid at lower temperatures making the material less suitable for applications involving flexing at either low or high temperatures.
A typical PVC insulation material has a temperature range of -15°C to 70°C for applications. Silicone rubber typically has a temperature range of -60°C to 180°C for fixed applications
Cables are braided for one of two reasons, either to electrostatically screen the cable or to provide mechanical strength to the cable.
Applying a braid of metallic wires in the cable’s construction to achieve electrostatic screening and/or mechanical strength as opposed to applying metal tapes is that the braiding maintains the cables flexibility. The design of the crossing, interwoven wires allows for bending and stretching of the braiding without buckling, folding or kinking in the way the tapes might do as a result of a flexible application.
Where the braiding is designed to provide an electrostatic screen to ensure signal integrity it is composed of an excellent electrical conductor such as copper, tinned copper or aluminium. If the braiding is designed to provide mechanical strength or toughness it can be composed of a number of different materials, such as steel wires, nylon strands or glass fibers.
When applied as a covering to the cable a braid can also serve to provide increased protection against hot surfaces, offering resistance to abrasion and cutting, or helping prevent attack by rodents (see how we helped design a cable for Network Rail with a glass fiber glass braid for precisely this reason). Braiding can be used in creating cable harnesses to group cables together.
The term FTP stands for foil twisted pairs. FTP networking cables often support Ethernet LAN. The cables are designed and constructed with a twisted pair or multiple twisted pairs of cores with an overall foil tape shield wound around the assembly. Twisting the cores together and covering with the foil shield helps to reduce cross–talk and electromagnetic interference.
UTP CABLE AND STP CABLE
FTP cable also known as F/UTP cabling is one of the three main types of twisted pair wiring. The other two types are UTP cable, where the twisted pairs are unshielded, and STP cable with twisted pairs screened with a braid.
FTP CABLE VARIATIONS
S/FTP cable which incorporates a combination of both a foil shield and an overlapped braided shield.
F/FTP cable which is individually foil pairs and over all foil shield
U/FTP cable which is individually foil shielded pairs without over all foil shield.
ADDITIONAL NETWORK CABLE DESIGNATIONS
F/UTP: Overall foil screen / Unscreened individual twisted pairs
S/UTP : Overall braided screen / Unscreened individual twisted pairs
SF/UTP : overall braided and foil screened / Unscreened individual twisted pairs
U/UTP : No overall screen / no individual screen, twisted pairs
Drain wires are used in cables in conjunction with a metallic shield to ensure effective grounding. The drain wire serves to complete an electrical circuit from the shield and carry unwanted electrical noise to ground away from the circuit. The drain wire is connected to ground and is in contact throughout its length with the metallic side of the shielding tape. It would be difficult to connect the tape with an earth terminal without the use of a drain wire.
The drain wires are usually tinned copper conductors. The tin coating helps prevent dissimilar metal reaction between the copper conductor and an aluminium screen. Instrumentation cables requiring electromagnetic protection or for use in intrinsically safe circuits are examples of cables with drain wire in their construction.
Cables for external use have been designed to survive the adverse conditions in the outdoor environment. The outer layers of the cable must serve to protect the cable from external influences such as mechanical damage, water, extremes of temperatures, rodent or insect attack, UV exposure from sunlight, and ozone in the atmosphere.
There are a wide range of cables suitable for outdoor use provided they are protected from direct sunlight or other external influences. Protection can come from placing the cables in metal conduit, plastic ducting, or where directly buried, through steel wire armouring.
Unprotected outdoor cables must, as a minimum, be weather resistant, which includes protection against the typical ambient temperature range, UV light, ozone and water. There are several different methods and material types used for outdoor cables. Materials such as Ethylene propylene rubber (EPR), Polychloroprene (PCP) and Fluorocarbon naturally have a very good weathering resistance, whilst others such as PVC and Polyethylene can be made resistant with the addition of specific additives or stabilizers such as carbon black.
Cable joints are used to connect low voltage, medium voltage and high voltage cables. There are several different types of joints and the right joint will depend on the size, shape and configuration of the cable, the voltage rating, the structure, the insulation type, the particular application, and the number of cores to be jointed.
The joint should provide electrical insulation and mechanical protection, it may also need to provide a barrier to water ingress.
The conductors may be joined by either welding, crimping, soldering or using mechanical connectors. The jointing insulation and shroud applied over the conductors must be compatible with the cable materials and may include heat or cold shrinkable insulation, molded types or special tapes.
The structure of the jointing will depend upon whether the intention is for a simple straight-through connection between two cables or if there is a requirement to have branch of connections to other cables. Cable Joints are available as part of our range of cable accessories – speak to our sales team for more information.
There are several different ways to terminate cables, with Rados Cable offering a range of cable accessories for this purpose. The termination method will depend on the system installed, the type of cable, and the connector. Using the proper termination method is essential for maintaining the electrical and mechanical integrity of the cable.
A solder type connection allows for a strong, solid mechanical and electrical connection. The solder is applied with a soldering iron and care must be taken that this is hot enough to ensure a proper liquid flow of solder around the jointing parts.
Crimps are applied by mechanical force around the conductor ends, ideal for terminating solid and stranded conductors. Crimps may also grip both the insulation and the conductor. The choice of crimp size and crimp tool is important to make sure that the cable is neither under crimped which would result in a poor or loose connection or over crimped which would result in damage to the cable and crimp.
Insulation displacement is a means of making a connection without having to cut the cable. Connection pins are pushed through the sheathing and insulation and onto the conductor. This type of termination / connection is only suitable for certain cable types, typically flat ribbon type cables.
Direct connection uses connector blocks or junction blocks, ideal for solid and stranded connectors in particular. The insulation must be adequately stripped back to get the right connection but no more than this to ensure bare conductor is not exposed outside the terminal point. A connector pin is then screwed down onto the exposed section of the conductor end. This is a robust type of termination which allows for quick easy replacement of the cable.
Cables designed to be submerged in water or in constant contact with water are usually designed to be both laterally and longitudinally watertight. Laterally watertight ensures that water can’t penetrate into the cores of the cable. Longitudinally water tight cable is designed with a barrier to the spread of moisture along the cable length.
Longitudinal and lateral water-tightness can be achieved in a number of ways including the use of water-blocking or water swell-able tapes and water swell-able powders. Lateral water tightness can also be achieved with certain water resistant rubber sheathing materials.
BS 5308 PART 1 CABLES
Multi-core pairs in cables manufactured BS 5308 Part 1 cables are unscreened and identified by insulation color in the following sequence.
|Pair Number||‘A’ Wire||‘B’ Wire|
Screened pairs in BS5308/PAS5308 cables are identified either by colour insulation or by a numbered polyester film which serves as part of the screen insulation in which case each pair has one black and one blue cable in the pair.
BS 5308 PART 2 CABLES
Multi-core pairs in cables manufactured BS 5308 Part 1 cables are unscreened and identified by insulation color in the following sequence.
|Pair Number||‘A’ Wire||‘B’ Wire|
AWG or American Wire Gauge is the US standard measure for the diameter of electrical conductors. The American Wire Gauge chart is based on the number of dies originally required to draw the copper down to the required dimensional size. It means the higher the AWG number is, the smaller the wire diameter is. Our Belden cables and the pairs in instrumentation cable are some of the electrical cables where the conductor size is expressed as an AWG figure. Our Tri-Rated cable, compliant with American standard UL 758, can be converted to AWG cable conductor sizes if required.
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)|
If this AWG metric calculator doesn’t provide you with the information you need, please get in touch with our technical experts who will be pleased to answer your questions or calculate the appropriate AWG/metric size for your installation.
Cable Lay refers to the lay length or length of twist or to the method and type of lay of electric cores or cables, sometimes known as cabling. Lay length is defined as the distance required to complete one revolution of the strand around the diameter of the conductor. When a conductor has more than one layer, it usually refers to the lay length of the outer layer.
There are several reasons for twisting cables together, including increasing flexibility, strength, concentricity and reducing cross-talk. The cable lay will depend on the reason for the twisting, the diameter of the cables, if there is a required orientation for cores and the number of cores of layers being twisted or laid up.
The cable lay may be left hand lay, known as S stranding or right hand lay, known as Z stranding. Multiple layers may be wound in alternating directions or the same direction. In some configurations the lay is first in the left hand direction and then in the right hand direction known as SZ stranding.
Earthing is a safety measure to prevent electric shock or damage to equipment by providing a low resistance path for electric current to flow to earth in the event of a fault. For example if there is an electrical fault in an appliance such as a cooker then the fault current flows to earth through a protective conductor. A protective device such as a fuse or relay switch in the consumer unit switches off the electric supply to the cooker rendering it safe.
Bonding is simply a term used for connecting together all the metallic parts that are not supposed to be carrying electric current to the same electrical potential. This means that no electrical current can flow between these parts. The primary reason for bonding is to prevent a person getting a shock when they touch two metal pieces of equipment at different potentials. By earthing these bonded elements it protects people and equipment from harmful electrical faults.
There are many different environmental and operational conditions which are likely to influence the longevity of electrical cables in service. The insulation and sheathing materials of cables may degrade over time when exposed to heat, UV light, ozone, various chemicals, excessive flexing, or mechanical action, not to mention in certain situations cables may be exposed to attack by termites and rodents.
When a current passes through the cable conductor it generates heat – the higher the current the more heat will be generated. This will have a significant impact if the conductor is undersized or continuously at or near the cable’s maximum permissible (rated) load, degrading the insulation and sheathing materials over time until they become dangerous and require replacement.
Although it is primarily the condition of the insulation and sheathing materials rather than the actual conductors that determine the longevity of the cables, water ingress and poor fixings can also cause corrosion and damage.
The standards that cables are manufactured to do not specify a particular life expectancy. Some cable manufacturers will determine a likely life expectancy based on typical conditions. For example a household fixed wiring cable with typical electrical loading, wired using the appropriate wiring guidelines, could be expected to last 20 years. However, in some cases cables which have not been used excessively have been found in relatively good condition up to 50 years after installation.
There are many different environmental and operational conditions which are likely to influence the longevity of electrical cables in service.
Cable size selection is based on three main factors:
– Current carrying capacity
– Voltage regulation
– Short circuit rating
The current carrying rating is determined by the conductor size and the thermal heating of the cable. The cable spacing, application and insulation materials are relevant to the dissipation of this heat.
Voltage regulation is not usually a problem with well-designed electrical power systems but the voltage drop incurred with excessively long cable runs needs to be accounted for.
Guidance on cable size selection for various temperature ratings and wiring methods, along with guidance on voltage drop calculations, are contained in BS 7671.
Short circuit ratings are based on the maximum current withstand capability of the cable in a short circuit condition. The cable should be capable of withstanding this current without thermal damage until the fault condition can be switched to safety through a device like a circuit breaker of fuse.
Cable drums come in many different sizes and weight depending somewhat on the materials they are made of. Usually materials include plywood, timber, plastic or metal, depending on the weight and type of cable they are expected to support and whether they are designed to be reusable and/or returnable. Additionally, the choice of material for the drums may depend on whether the drums and cables are being stored indoors or outdoors.
Drum sizes are typically determined by the flange height or drum diameter, the drum barrel or drum core, the width of the drum and the inside width.
Plastic drums range from 400 mm to 1000 mm and carry loads of up to 850 kgs.
Plywood reels range from 125 mm to 1500 mm and carry loads of up to 2 tons.
Wooden drums range from 250 mm to 4500 mm and carry loads of up to 60 tons.
Steel drums can vary in size from 630 mm to 10000 mm and carry loads of up to 250 tons.
How should electrical cable drums be stored?
The appropriate storage of cable drums and the electrical cable it contains will depend on both the type of drum and whether or not the cable itself is designed for indoor or outdoor use. The following points should be noted:
– Plywood drums are not suitable for outside storage unless protected from moisture.
– Wooden drums are not suitable for long-term storage outdoors unless protected from moisture.
– Cable drums should be stored on firm and well-drained ground.
– Cable should be covered from direct sunlight.
– Protective lagging or covering should be used to protect against UV and weathering effects.
– Any cable stored outdoors should be sealed to protect from water ingress.
– Cables should be protected from mechanical damage.
– Drums should be stored on their edges and never on the flanges.
– Drums should be secured and prevented from rolling into one another, causing damage due to misalignment.
BS 8512 is the British Standard for code of practice for handling, installation and disposal of cables on wooden drums.
Electrical cable drums can be heavy and difficult to handle manually. It is important that a proper risk assessment is done before handling drums. Plywood drums should not be rolled on the ground. Wooden cable drums are not meant to be rolled over significant distances. When rolling drums, the following considerations should be taken into account:
– The mass of the drum
– The visibility in the direction in which the drum is being rolled
– The condition of the ground (which should be smooth, hard and flat)
– The method of rolling and the direction of lay of the cable on the drum
– The condition of the drum
– Any additional considerations flagged by the risk assessment
Particular care should be taken with full drums of cable. When lifting drums, use appropriate lifting equipment, rated to the load they are intended to carry. When lifting with a crane, it is important to make sure the drums are properly supported with a through shaft and spreader bar to prevent damage to the drum or to the cable.
When using a forklift truck to lift the cable, the drum must be handled correctly, lifting from the right direction. The cable should never be lifted by the flange on the flat and must be lifted on its edge.
When transporting cable drums, it’s essential to make sure that drums are adequately strapped to prevent rolling and are secured to the vehicle.
Electrical cables are tested for compliance against parameters which have been designed to determine their particular suitability. These tests are incorporated into a wider schedule of tests in the applicable cable standard including those on the components of the cable as well as the complete cable. These tests include electrical tests, mechanical tests and chemical tests.
A number of tests on electrical cables are conducted during the manufacturing process, described as non-destructive tests such as tests for absence of faults, voltage testing and overall dimension tests. Other tests require more invasive destructive testing on samples, such as tensile strength and elongation testing, vertical flame testing, acid gas emission tests, and impact tests at cold temperature.
Electrical cables are also subject to various ageing tests to simulate how the cables are likely to perform over time, such as air ageing, compatibility testing, continuous flexing tests and UV stability tests.
Routine tests are tests conducted on each batch of cable and required to confirm ongoing conformity. They form part of Rados Cable’s QA procedures. 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
– Copper wire screen resistance
– Partial discharge test
– 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.
Examples of type tests include:
– Corrosive and acid gas
– Mass of zinc coating for galvanized wire armor
– Smoke emission
– Flame propagation test for multiple cables
– Shrinkage test on insulation
– Abrasion test
Type tests, routine tests or sample tests are categorized in the relevant standard to which the cable is manufactured to.
A spark test is an inline voltage test used either during cable manufacturing or during a rewinding process. Spark testing is primarily for low voltage insulation and medium voltage non-conducting jacket or sheath. The test unit generates an electrical cloud around the cable which in high frequency AC units appears as a blue corona around the cable. Any pin holes or faults in the insulation will cause a grounding of the electrical field and this flow of current is used to register an insulation fault.
Spark testers are usually fitted with counters indicating the number of faults. Different spark test voltages are applied which are determined by the cross-sectional area of the conductor and insulation material, with the appropriate voltage specified in the relevant cable standard.
There are many reasons why a cable may fail in service, with the failure at its most serious resulting in fire or other serious fault.
Some of the main causes of cable failure include:
The service life of a cable can be significantly reduced if it has been expected to operate outside of the optimal operating conditions it was designed for. The ageing process usually results in embrittlement, cracking and eventual failure of the insulating and sheathing materials, exposing the conductor and risking a potential short circuit, a likely cause of electrical fire.
If cable selected is not appropriate for the application, it is more likely to fail in service. For example, a cable which is not robust enough for the environment, either mechanically tough enough to wear and abrasion or chemically resistant to the ambient conditions, is more likely to fail than one whose construction is suitable for the installation environment.
If the cable is damaged either during installation or in subsequent use, the integrity of the cable will be affected and reduce its service life and suitability.
Degradation of the cable sheath:
There are several reasons why the sheathing material may degrade, including excessive heat or cold, chemicals, weather conditions, and abrasion of the sheath. All of these factors can ultimately cause electrical failure as the insulated cores are no longer protected by the sheathing as originally designed.
Moisture in the insulation:
Moisture ingress can cause significant problems including short circuit and corrosion of the copper conductors.
Heating of cable:
Excessive heating of the cable will cause degradation of the insulation and sheathing material and premature failure. The heat may come from an external source or may be generated by the resistance to current flow in the conductor – a particular problem if the cable is overloaded and/or underrated for the application.
Electrical overloading normally occurs when the cable is underrated for the application or when too much load is being placed on the cable. In domestic applications this is often a result of plugging too many appliances into the one socket and overloading the wiring to that individual socket, extension adapter or gang socket.
Rodents frequently attack the outer layers of cables. This damage can be extensive, significantly reducing the sheathing or insulation properties of the cable, another likely source of electrical fires.
UV exposure can have a significant influence on electrical cable insulation and sheathing. Cables likely to be exposed to UV light should either be designed with UV resistant materials with a suitable carbon black content, or protected from exposure with a protective covering such as installing inside cable conduit so not in direct sunlight. UV exposure frequently causes cracking of the insulation and therefore potential short circuit failures.
Electrical cables are installed in a wide variety of environments and it is often necessary to provide protection for these cables to prevent mechanical and environmental damage. Some of the methods for protection include:
Reinforced Plastic Spiral Binding: used to group cables together so they don’t snag. This offers light mechanical protection.
Braided Sleeving: Flexible braiding such as polyamide fibres which offers protection from heat and abrasion.
Plastic conduit: Lightweight tubing suitable for light mechanical protection and chemical resistance. This type of conduit is typically used in domestic applications direct into plaster. It is also available in more flexible versions and versions which include a metal sleeve (primarily for electromagnetic (EMC) screening.)
PTFE Conduits: these are used for protection against extreme conditions and offer excellent chemical resistance, high and low temperature resistance, very good tensile and fatigue strength and resistance to fire, moisture, vibration and abrasion.
Metal conduit: This is a heavier duty conduit tubing usually galvanised to prevent corrosion. This offers significant mechanical protection and fire resistance. May also be available in flexible versions.
Cable ducts: Cable ducting is also a means of offering mechanical and environmental protection to cables, ducts can be plastic, metal or concrete and can be of sufficient size to offer protection to many different cables and electrical circuits.
Other cable accessories are available to offer protection to cables at particular points such as in wiring panels and lighting fixtures and include edge protectors and grommets.
There are several different fire performance tests for cables. The purpose of the test is to verify that the cable will continue to maintain electrical continuity or functionality for a defined period of time in a simulated fire condition. These cables are used to provide power to fire survival equipment, fire alarms and emergency lighting etc.
IEC 60331 – INTERNATIONAL FIRE PERFORMANCE STANDARD
International standard for Fire Performance tests is IEC 60331 consisting of the following parts under the general title: Tests for electric cables under fire conditions – Circuit integrity:
Part 1: Test method for fire with shock at a temperature of at least 830°C for cables of rated voltage up to and including 0.6/1kV and with an overall diameter exceeding 20 mm.
Part 2: Test method for fire with shock at a temperature of at least 830°C for cables of rated voltage up to and including 0.6/1kV and with an overall diameter not exceeding 20 mm.
Part 3: Test method for fire with shock at a temperature of at least 830°C for cables of rated voltage up to and including 0.6/1kV tested in a metal enclosure.
Part 11: Apparatus – Fire alone at a flame temperature of at least 750°C
Part 21: Procedures and requirements – Cables of rated voltage up to and including 0.6/1kV
Part 23: Procedures and requirements – Electric data cables
Part 25: Procedures and requirements – Optical fibre cables
NOTE Parts 21, 23 and 25 relate to fire-only conditions at a flame temperature of at least 750 °C.
BS 6387 – BRITISH FIRE PERFORMANCE TEST
One of the longest established British Standards for fire performance testing is BS 6387. This covers cables with a diameter of up to 20 mm. The standard describes three which the cable must pass, C, W and Z. The tests are conducted on a special fire test rig and the cable is energized to 600V. The cables electrical continuity is indicated by a series of light bulbs which are connected through fuses. A failure will be indicating by the fuse blowing and the light failing on one or more of the bulbs.
BS 8491 – FIRE PERFORMANCE STANDARD FOR LARGER CABLES
The British Standard which caters for cables with a diameter greater than 20 mm is BS 8491. The test method in BS 8491:2008 involves subjecting the cable under test to radiation via direct impingement, corresponding to a constant temperature attack of 842 °C; to direct mechanical impacts corresponding to a force of approximately 10N; and to direct application of a water jet simulating a firefighting water jet.
The test method given in this standard includes three different test duration to allow testing of cables intended for different applications:
F30 – for 30 minutes duration
F60 – for 60 minutes duration
F120 – for 120 minutes duration.
BS EN 50200 – HARMONIZED FIRE PERFORMANCE STANDARD
The harmonized standard covering test requirements for fire performance for these smaller cables is BS EN 50200. The test to BS EN 50200 includes direct flame at a notional temperature of 842°C with mechanical shock to the back board on which the test cables are mounted. There is also a separate test for the cable included in Annex E which includes a water, fire and mechanical shock element.
BS EN 50200 FIRE PERFORMANCE CABLE CLASSIFICATIONS:
PH15 – 15 minutes duration
PH30 – 30 minutes duration
PH60 – 60 minutes duration
PH120 – 120 minutes duration
There are several different types of cable gland and choosing the right one for your application is very important. When choosing a suitable compatible gland you need to consider the following:
– The type of cable the gland will be connected to (for example SWA armored cable requires a gland such as the BW brass cable gland)
– The material the cable is made of, the construction and cross-sectional size (for instance, is the cable screened or braided?)
– The color requirement (if any) for the cable gland
– The location of the cable and gland (are there any restrictions on the installation space, electromagnetic interference, or environmental considerations to bear in mind?)
– Does the gland need to be water resistant (cable glands are rated IP68 for watertight, dust-proof seals, or IP69K where total immersion in water and resistance to direct water jet pressure is needed).
– If the gland is required to provide mechanical protection
– If the cable and gland is to be used in hazardous areas (explosive areas should use glands for intrinsically safe circuits such as the HSK-K-EXE-Active plastic gland)
– Should the gland provide electrical earthing or grounding
– Will there be issues with dissimilar metal reaction (nickel-plated cable glands offer chemical stability)
– If the cable is an armored cable then consideration must be given to the size of the inner bedding of the cable, the armor material, and the short circuit rating of the armor.
– The material of the mating or housing that the gland is being attached to needs to be considered for compatibility.
– The depth and size of the gland thread must also be considered – metric or PG?
– Are stopper plugs needed in the gland to close off unused cable entries?
GENERAL ORDER RELATED QUESTIONS