Electricity is a versatile form of energy used to power anything from huge data centers 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.
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.
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
Capacitance describes the ability of two conductors, separated by an insulating material, to store charge. 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 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 that 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 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.
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.
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 (see our FAQ on ‘what is electricity’ for more information on atoms) 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.
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.
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.
Examples:
Low Density Polyethylene LDPE = 27kV/mm
Polypropylene PP = 22kV/mm
Polyvinylchloride PVC = 14kV/mm
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.
The earliest use of recognizable electrical cable was probably the early commercialized Telegraph lines such as those 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 International Copper Standard (IACS) as a benchmark for the resistivity of copper as being equal to 100 percent conductivity.
Sunlight is converted into electrical energy by photovoltaic (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 photovoltaic cells are connected together and mounted into a support structure or frame called a photovoltaic module and these modules are combined together to form a photovoltaic array.
The photovoltaic arrays produce direct current (DC) and can be connected in series or parallel arrangements using PV1-F Photovoltaic 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.
The most widely recognized International standards bodies are the IEC, the ISO, and CENELEC.
IEC is the International Electrotechnical Commission
ISO is the International Organization for Standardization
CENELEC is the European Committee for Electrotechnical Standardization.
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
There are many different types of electrical cables used for applications across power distribution, control or signaling, and 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, 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.
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.
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.
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 cables 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.
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 Polyvinyl Chloride (PVC) and Polyethylene (PE).
Conductors are typically measured in Ohms (Ω) whereas insulators are measured typically measured in Mega Ohms MΩ.
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, while 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 in a cyclical wave form shown below.
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°. As shown below
A three-phase system is usually more economical than single-phase as reduced conductor material is required to transmit electrical power.
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.
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
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 aluminum. 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. Braiding can be used in creating cable harnesses to group cables together.
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.
Polyethylene is a semi-crystalline polymer available in a wide variety of versions with differing chemical structures, molecular weights and densities determined by the various methods of polymerization.
All polyethylenes have excellent dielectrics – high dielectric strength, low dielectric constant, and low dissipation factor at all frequencies. This makes it an ideal insulation across a range of different cable types including telephone and high speed transmission, high frequency signal and control cable, low, medium and high voltage power cables, overhead line wire and service drop cables.
Whilst PE naturally has poor fire resistance it can be significantly improved by the addition of fillers – both halogenated types and halogen free types. PE can also be compounded to include additives which enhance other properties such as resistance to sunlight, weathering and chemical degradation. PE is a hard and abrasion-resistant material which makes it useful as a sheathing material in various applications but where a more flexible material is required the addition of small amount of butyl or ethylene propylene rubber (EPR) can improve flexibility. The toughness of the PE also makes it suitable for direct burial in the ground.
The temperature range is typically -65°C to +75°C but cross-linking the polyethylene (to make XLPE) can extend this temperature range to +90°C.
Polyvinyl chloride (PVC) 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 years of 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 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.
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.
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 resistant 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 International Electrotechnical Commission (IEC) established a standard for copper conductivity, the International Annealed Copper Standard (IACS), 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.
Copper and aluminum 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.
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 aluminum 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. However, it is worth noting that given the equivalent, aluminum conductor is bigger and lighter and it often doesn’t require the same degree of tensile strength. Copper is more thermally conductive than aluminum 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 aluminum conductors consisting of an aluminum core with a heavy skin of copper bonded to the aluminum can be used. Whilst not in widespread use, this conductor type does combine the advantages of the lighter weight aluminum with the more conductive copper. The ductility is however that of aluminum 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.
