Frequently Asked Questions
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.
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.
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 & Herzegovina, 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 fiber 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 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.
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 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.
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.
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 insulations and medium voltage non-conducting jacket or sheaths. 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:
Ageing:
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.
Application:
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.
Mechanical failure:
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:
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 adaptor or gang socket.
Rodent attack:
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:
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: This is used to group cables together so they don’t snag. This offers light mechanical protection.
Braided Sleeving: Flexible braiding such as polyamide fibers 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 galvanized 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.
