There are 1001 questions that could be asked about cables and supplying electrical power to projects. Below are those most frequently asked of us. If you can’t find what you’re looking for, contact your local branch here. They will be happy to help!
How do electrical cables work?
An electrical cable is a set of one or more wires running side by side or together, used as an electrical conductor to carry electric current. Cables normally have layers of insulating materials to protect the wires from loss of performance and damage and the operators from electric shocks.
Cables are used to connect two or more devices, enabling the transfer of electrical signals, power, or both from one device to the other. An electrical cable is an assembly consisting of one or more conductors with their own insulations and optional screens, individual coverings, assembly protection and protective covering.
One or more electrical cables and their corresponding connectors may be formed into a cable assembly, which is not necessarily suitable for connecting two devices but can be a partial product (e.g. to be soldered onto a printed circuit board with a connector mounted to the housing). Cable assemblies can also be a cable tree or cable harness, used to connect many terminals together.
How do I choose the right cables?
Essentially a suitably qualified competent person should be consulted to ensure that the correct type of cable is specified for the project construction, application, environment and location. Suitably competent persons include mechanical and electrical consultants, electrical design engineers and electricians.
How are cable sizes selected?
Guidance on appropriate cable size selection can be found in the National Wiring regulations – BS7671, 18th Edition for the UK. Details of the standards can be found here. Factors requiring close attention for cables are voltage regulation, current carrying capacity and short circuit rating.
How is current carrying capacity calculated?
The insulation material and temperature rating of a cable are the main determining factors in the current carrying capacity, because the capacity – or ampacity – is the maximum current (in amperes) that it can continuously carry without exceeding its temperature rating. And the temperature rating is dependent on the insulation material.
How are cables jointed?
Cables are jointed by stripping the insulation from the ends of the conductors, then connecting them together using a method like crimping, soldering, or welding, and finally applying a protective insulation sleeve over the joint, which can be heat-shrinkable or cold-shrinkable depending on the application, to ensure a secure electrical connection and prevent damage from environmental factors like water ingress; the specific method of jointing used depends on the cable type and voltage level required.
Why do some cables have braiding?
Cables are braided to either provide mechanical strength or electrostatically screen cables. Applying a braid of metal rather than solid pieces helps maintain cable flexibility. Where the braiding is designed to provide an electrostatic screen to ensure signal integrity it is made of copper, tinned copper or aluminium. If the braiding is designed to provide mechanical strength it can be made of a number of different materials, such as steel wires, nylon strands or glass fibres. When applied as a covering to the cable a braid can also provide increased protection against hot surfaces, and help prevent abrasion and cutting.
What is a cable spark test?
A spark test for cables is a voltage test that uses an electrical current to detect defects in the cable’s insulation or coating. It’s often performed during the manufacturing or rewinding of cables. A high-voltage electrical current is passed through the cable. If there’s a defect in the cable, the current will arc through it, creating a spark. The spark can be seen or heard Spark tests are mainly used for low voltage insulations and medium voltage non-conducting jackets or sheaths. They can help identify pinholes, cracks, or voids in the cable
What is impedance?
In simple words, impedance is the measure of resistance to the flow of the electric current that is presented by the circuit or the network whenever the voltage signal is applied. It is denoted by Z = VI. Impedance is relevant to AC circuits and is measured in Ohms. The amount of current which can flow through the cable is determined by a number of factors – the cross-sectional area of the conductor, the resistance of the conductor material, insulation material and the installation method or environment.
What is capacitance?
Capacitance is the amount of charge that can be stored at a given voltage by an electrical component called a capacitor. The unit of capacitance is the Farad (F) and a 1F capacitor charged to 1V will hold one Coulomb of charge. The lower the capacitance figure is, the better the cable performs.
What is voltage rating?
A voltage rating is the maximum voltage that an electrical component, such as a cable or appliance, can safely handle without damage. It essentially indicates the designed operating voltage limit for that device. Voltage rating is a key specification used to ensure proper selection and usage of electrical equipment within a given system. When used in a DC system, it is generally accepted that a cable’s voltage rating is 1.5 times the AC voltage rating.
What is the difference between AC and DC?
In direct current (DC), the voltage is always constant, and the electricity flows in a certain direction. In contrast, in alternating current (AC), the voltage periodically changes from positive to negative and from negative to positive, and the direction of the current also periodically changes accordingly.
What are voltage ratings - from low to extra high?
Generally, voltage categories are:
Low Voltage (LV): up to 1000V
Medium Voltage (MV): between 1000 V and 45 kV
High Voltage (HV): between 45 kV and 230 kV
Extra High Voltage (EHV): from 230 kV and above
Low Voltage cables are often used in fixed wiring; Medium Voltage cables are for power distribution – local grid power and heavy industrial equipment. High Voltage and Extra High Voltage cables are overhead lines for power distribution, often installed on pylons.
What is Voltage Drop?
Voltage drop occurs when voltage in an electrical circuit passes through a cable. It is related to the resistance (impedance) to current flow with passive devices in circuits affecting the level of voltage drop, which includes cables and connectors. The longer the circuit or length of cable the greater the voltage loss. Voltage drop can cause motors to run slower, heaters not performing fully, lights dimming. Larger cross sectional cable sizes may be used to allow for voltage drop, as these will have less resistance (impedance) to current flow.
