From general knowledge to specialist cable questions - we've tried to answer them all!
What is voltage rating?
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
What are cable ratings?
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.
What are intrinsically safe cables?
Intrinsic safety is a protection technique for the safe operation of electrical and electronic equipment in hazardous areas where any ignition source needs to be eliminated.
Electrical cables are essentially sealed and so may be regarded as intrinsically safe in certain applications if protected from damage, external electrical or magnetic fields (EMI), and isolated from non-intrinsically safe circuits. The junctions and switches pose a potential hazardous ignition point in these environments and circuits. Intrinsically safe cables are usually coloured blue so as to identify the potential risk of the electric circuit and the need for special consideration. The cables are often protected by tough sheathing materials such as polyurethane to prevent any mechanical damage which could compromise electrical safety. Examples of intrinsically safe cables include the French Standard NF M87-202 instrumentation cables.
How should I select cable glands?
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 armoured 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 colour 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 armoured cable then consideration must be given to the size of the inner bedding of the cable, the armour material, and the short circuit rating of the armour
- 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?
Our technical experts are available to provide advice on the most appropriate cable glands and other cable accessories for your cable installation.
How are cable sizes selected?
Cable size selection is based on three main factors:
Current carrying capacity
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 the National Wiring regulations (BS7671). Assistance in determining the correct cable size for your application can be sought from the experts in The Cable Lab.
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.
What are electrical cable type tests?
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 galvanised wire armour
Flame propagation test for multiple cables
Shrinkage test on insulation
Type tests, routine tests or sample tests are categorised in the relevant standard to which the cable is manufactured to.
What is ampacity?
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.
If you need any help, please contact us to get support. We’ll do our best!