CLASSIFICATION OF ELECTRICAL CABLES
The constructional feature of electric cables has been discussed in the previous article POWER CABLE CONSTRUCTION EASY EXPLANATION101, here, we shall proceed to discuss the electrical cable classifications.
Table of Contents
Classification based on application
Wiring cables
These electrical cables are used for internal wiring of the buildings and other protected installations and have two components viz. conductor and insulation. PVC as insulation material and annealed copper (solid or stranded) as a conductor are commonly used for wiring cables. The voltage grade of these cables is up to 1100 Volts.
Control cables
These are designed for control purposes or measuring circuits for carrying signals of direct current up to 220 Volts and alternating currents up to 440 volts. These electrical cables are available with armor and without armor. In these electrical cables, PVC, XLPE, EPR, Neoprene, etc. are used as insulation. Control cables are available in 0.5/0.75/1.00/ 1.5/2.5 mm2 size copper conductors (solid/stranded) from 2 cores to 61 cores.
Power Cables
Electrical cables are used for the distribution and transmission of electrical energy. These electrical cables either single-core or multicore are particularly useful in power stations, substations, house service connections, street lighting, etc. They can be installed indoors or outdoors, in the air, in cable ducts, or underground.
Special purpose cables:
These are based on special applications such as pilot cables, instrumentation cables, submarine cables, etc.
Classification based on conductor:
Solid conductor in the electrical cable:
Solid copper conductors are permitted up to 16 mm2 in British Standards, but with the exception of mineral-insulated electrical cables are rarely used above 6 mm2 because of reduced flexibility. However, IEC 228 provides instructions for the use of solid conductors up to 150 mm2 for special applications. These are mostly used in DC applications.
Stranded conductor in the electrical cable:
With alternating current, there is a tendency for more of the current to be carried on the outside of the conductor than in the center (skin effect), and to overcome this problem the larger sizes of conductors are frequently stranded.
Classification as Per Designed Voltage
Electrical power cables are generally classified according to the rated voltages as given below:
Low voltage cables
These electrical cables are available up to and including 1100 volts. These cables are normally used for domestic wiring, lighting, and powering small appliances. These cables are more flexible and easy for termination and connections.
Medium voltage cables
These electrical cables are rated from 3.3 kV up to and including 33 kV. This type of cable is normally used for industrial applications, extensively used in utility distribution and commercial buildings.
High voltage cables
These electrical cables are rated above 33 kV and up to and including 132 kV. These cables are used for the transmission and distribution of electricity, also used in large industrial applications. They are designed for handling high potential stress and generally have a long service life.
Extra high-voltage cables
These electrical cables are rated above 132 kV and up to and including 800 kV, mostly used for long-distance transmission or interlinking power based region-wise and very specific industrial applications. These cables require very good insulation to handle the high electrical stress. Regular inspection and maintenance are required as these installations involve high risks.
Classification as per the type of Insulation Used
Electrical power cables are generally classified according to the type of insulation used as given below:
PILC Cables
For many years, the superior insulation material for power cables from low voltage to high voltages was oil-impregnated paper. Oil-impregnated paper has excellent electrical properties and a high degree of thermal overloading capacity without excessive deterioration of the insulation.
However, PILC cables have the following disadvantages.
- Prone to moisture and damage.
- Low current carrying capacities.
- Low operating temperatures.
- Heavier weight making it difficult to handle during installation.
- Migration of impregnating compounds that do not permit laying cables vertically or on steep slopes.
- Due to the above disadvantages, the use of PILC cables is limited.
PVC Cables
PVC is a general-purpose thermoplastic used for wires and cable insulation and is a suitable alternative to paper insulation. Polyvinyl chloride is applied as a continuous seam-free extrusion in the insulation and sheath of cables.
PVC cables have the following properties and advantages:
- The insulation resistance and breakdown strength of these cables remain practically unaffected by moisture.
- There is no impregnating compound present in these cables, hence these cables can be laid vertically and on any steep slopes without any issue.
- These cables can withstand a high transient conductor temperature without any physical deformation of insulation.
- These cables are practically resistant to all chemicals normally encountered in the installations.
- These cables are flame retardant since PVC ignites with great difficulty and that too when directly exposed to a flame.
- These cables are lighter in weight and hence easy to install and handle.
- Small bending radii permit the termination of these cables in and around small spaces. This eases the termination of PVC cables in switchboards, control panels, etc.
- PVC cables have a smooth outer surface which results in an aesthetic appearance when installed. However, the PVC outer sheath is tough and also abrasion-proof.
The main disadvantage of PVC is that it becomes brittle when subjected to a very high-temperature variation.
The most popularly used PVC cables are general-purpose PVC cables and fire retardant (FR-PVC) cables. PVC insulation is essentially suitable for any voltages up to 11 kV.
XLPE Cables
Polyethylene has a linear molecular structure. Polyethylene molecules, not chemically bonded, are deformed when subjected to high temperatures. This linear structure is therefore changed into cross-linked structure by the use of special processes.
This thermosetting XLPE insulation material provides extraordinary electrical, thermal, and mechanical properties to the cables, like low dielectric loss, excellent dielectric strength, higher continuous current rating, high resistance to thermal aging, etc.
The following are the main advantages of XLPE cables over PVC cables:
Excellent electrical & physical properties
High resistance to thermal deformation and the aging property of XLPE cables provide greater continuous and short-circuit current capacity ensuring a higher degree of reliability over a wide range of temperature variations as compared to PVC cables.
| Permissible maximum conductor temperature | XLPE | PVC |
| Continuous Duty | 90°C | 70°C |
| Short Circuit | 250°C | 160°C |
Higher current carrying capacity
The current carrying capacity of XLPE cables of the same size is approximately 20 to 30% higher than that of PVC due to higher operating temperatures.
Resistant to heat
With a cross-linked molecule structure, XLPE cables are excellently ozone-resistant, provide outstanding stability, and are resistant to heat.
Less dielectric loss
XLPE cables have a lower dielectric loss, and lower permittivity as compared to PVC cables.
Less Weight
The XLPE cables are comparatively lighter in weight than PVC cables, because of less specific gravity. Therefore, it becomes easy to handle, lay, and install. The cable requires less support due to its low weight.
More Rigid
XLPE cable has higher mechanical properties and is more robust as compared to PVC cables due to the thermosetting process.
