If you can't explain it simply, then you don't know it well enough. — Albert Einstein

Types of underground cables

We have already seen what Underground cables are, and where they’re used. Moving forward, we must understand various types of Underground cables available to us.

Classification of underground cables

The classification of Underground cables can be done on the basis of several criteria. Various aspects are taken into account while classification and these include:
  1. Number of conductors in the cable
  2. Voltage rating of the cable
  3. Construction of cable
  4. Type and thickness of insulation used
  5. Installation and Laying of the cables

Classification based upon number of conductors in the cable

  1. Single core cable
  2. Three core cable
Typically, an Underground cable has either one, three or four cores. These cables are of course, constructed accordingly.
Underground cables are usually employed to deliver 3 phase power. A 3 cored cable is preferred up to 66 kV. Beyond that, insulation required for the cable is too much. For higher voltages, 3 cored constructions become too bulky, and hence, even with some limitations we employ single cored cables

[Also read: Electric power transmission system]

Classification based upon voltage rating of the cable

  1. Low tension cables: These have a maximum voltage handling capacity of 1000 V (1 kV)
  2. High tension cables: These have a maximum voltage handling capacity of 11 kV.
  3. Super tension cables: These have a maximum voltage handling capacity of 33 kV.
  4. Extra high tension cables: These have a maximum voltage handling capacity of 66 kV.
  5. Extra super voltage cables: These are used for applications with voltage requirement above 132 kV.

Classification based upon construction of the cable

1. Belted cable

In such cables, the conductors (usually three) are bunched together and then bounded with an insulating paper ‘belt’. In such cables, each conductor is insulated using paper impregnated with a suitable dielectric. The gaps between the conductors and the insulating paper belt are filled with a fibrous dielectric material such as Jute or Hessian. This provides flexibility as well as a circular shape. As we discussed earlier (in Construction of Cables), the jute layer is then covered by a metallic sheath and armouring for protection. One particular speciality of this cable is that its shape may not be perfectly circular. It is kept non-circular to use the available space more effectively.
underground cables belted type
There are some limitations of such construction. Since the electric field is tangential, the insulation provided is stressed. As a result, the dielectric strength falls over time. Hence, such construction isn’t preferred for voltage levels above 11 kV.

2. Screened cable

Further divided as H-type and S.L. - type cables.
  • H-Type Cables: It was first designed by M. Hochstadter. The three cores are individually insulated with paper and then covered by a metallic screen / cover. These metallic covers are perforated. As a result, such construction allows the three metallic screens to touch each other. These three metallic covers are then grouped together in a metallic tape usually made of copper. A lead sheath surrounds this construction. The metallic covers and the sheath are grounded.
    The obvious advantage is the electric stresses are radial, not tangential and hence of lesser magnitudes. Also, the metallic covers improve the heat dissipation.
  • S.L Type Cables: It is similar to the H type cables, with the difference that each of the three cores has its own lead sheath. With this provision, the need for the overall sheath used previously is eliminated. The advantage of such a construction is that the chances of a core-to-core breakdown are greatly minimized. Also, the flexibility of the cable is improved.
    The limitations are severe. Such construction is limited for voltages up to 66kV only. The individual sheaths are thinner, and if there are constructional defects, moisture may enter the cable and reduce its dielectric strength.
H type and SL type screened underground cables
  • H.S.L. Type Cables: This type of cable is combination of H type and S.L. type cable. In these cables each core is insulated with impregnated paper and provided with seperate lead sheaths.

3. Pressure cables

For voltages beyond 66 kV, the electrostatic stresses in the cables exceed the acceptable values and solid cables become unreliable. This occurs mainly because voids are created when voltages exceed 66 kV. Hence, instead of solid cables, we use Pressure cables. Typically, such cables are either oil filled or gas filled.
  • Oil Filled Cables: Oil is circulated under suitable pressure through ducts provided for such purpose. This oil supply and pressure are maintained through reservoirs kept at proper distances. The oil used is the same that is employed for impregnation of paper insulators.
  • Gas Filled Cables: Pressurized gas (usually dry nitrogen) is circulated around cables in an air-tight steel pipe. Such cables are cable of carrying higher values of load current and can operate at higher values of voltage. But the overall cost is more.
oil filled and gas filled underground pressure cables

[Also read: Types of conductors used in overhead power lines]

Classification based upon insulation of the cable

Various insulating materials used in cable construction are Rubber, Paper, PVC, XLPE (Cross linked Polyethene) etc. Such classification is based upon operating temperature limitations. Following are some insulating materials used and their maximum operating temperatures.
Insulation materialMax. operating temperature

Classification based upon installation and laying of the cable

  • Direct Buried: As the name suggests, the conductors are buried underground in a trench without additional accessories. Sometimes cooling pipes are added if required. Once the cables are installed, there’s no visible sign above the ground.
  • Trough: Concrete troughs are dug and cables are installed in them. They’re visible on the surface. Maintenance is easier.
  • Tunnels: Sometimes, tunnels are dug up for this purpose. Such construction is mainly employed if a river needs to be crossed or if the intended power distribution is to a major city. Maintenance and future expansion is easier, but initial cost is higher.
  • Gas Insulated Lines: This is a relatively new technology. For cables operating at higher voltages and currents, and handling high power, such gas insulated line construction is safer. It is being employed nowadays for advanced projects.

Underground Power Cables

In addition to the overhead lines we come across every day, electrical power can also be both transmitted and distributed using Underground cables. These underground cables, of course, come with their own set of advantage and limitations. Aside from better general appearance and lesser interference with other amenities, the advantages include smaller voltage drops and lesser probability of fault occurrence. On the other hand, they have higher production and installation costs, and hence are used wherever overhead lines aren’t viable due to practical limitations or risks involved. Hence we employ them in specific places such as urban areas with high population densities and across water bodies (as submarine cables).
A typical underground cable will consist of a conductor/s covered by a number of insulating and protective layers necessary for its satisfactory operation. Underground cables construction is explained below:

Construction of underground power cables

construction of underground power cable
Construction of underground power cable
  • Conductor: Usually, 1 or 3 conductors (depending upon the application) are used. These conductors are stranded to reduce skin effect, proximity effect and to keep it flexible. Conductors are made from electrolytic grade pure copper or aluminium.
  • Conductor screen: It is a semi-conducting tape or an extruded layer of a semi-conductive compound. Conductor screening is generally used in MV and HV cables to maintain uniform electric field and minimise electrostatic stresses.
  • Insulation: It is provided to withstand the electrostatic stress. Various types (and thickness) of insulators like VIR (Vulcanized India Rubber), Impregnated paper, PVC (polyvinyl chloride) and XLPE (Cross linked polyethylene) are utilised depending upon the applied voltage.
  • Insulation screen: A layer of semi-conductive material generally used in MV and HV lines. It serves the similar purpose as that of conductor screen.
  • Metallic Sheath: It provides protection to the cable from moisture and other chemicals (acids or alkalies) present in the environment or soil. It’s usually made up of Aluminium or Lead. It also provides a path for fault and leakage currents as the sheath is earthed at one cable end.
  • Bedding: It is a low grade insulator like Jute or Hessian which protects the metallic sheath from corrosion and from mechanical injury due to armouring.
  • Armouring: It provides mechanical protection from various stresses the cable may get exposed to during its installation and operational life. It’s usually a steel tape wound around the Bedding layer.
  • Serving: Another layer of low grade insulator like Jute or Hessian or a thermoplastic compound like PVC is again provided to protect the steel from atmospheric contaminants and agents.
[Also read: Corona Discharge in overhead power lines]

Types of underground cables

Underground cables are usually classified according to their Voltage ratings. They’re grouped as follows:
  1. Low tension cables which have a maximum voltage handling capacity of 1000V
  2. High tension cables which have a maximum voltage handling capacity of 11kV
  3. Super tension cables which have a maximum voltage handling capacity of 33kV
  4. Extra high tension cables which have a maximum voltage handling capacity of 66kV
  5. Extra super voltage cables which are used for applications with voltage requirement above 132kV.

Classification of 3 phase underground cables

1. Belted cables: As the name suggests, it has an additional layer of oil-impregnated paper which is wound around the insulated conductors. Such an arrangement is useful for low and medium voltage levels up to 11 kV.
2. Screened cables: Used only in particular applications with specialised construction, these Underground cables can be further divided as H-type and S.L-type cables.
3. Pressure cables: These are used when the voltage requirement exceeds 66kV and solid cables can't be used. Either pressurized gas or pressurized oil is used in these cables.

Sag in transmission lines

Overhead power lines (transmission and distribution lines) are suspended on pole/tower supports. Suspended conductors are subjected to mechanical tension which must be under safe value. Excess mechanical tension may break the conductor. Therefore, a conductor between two supports must not be fully stretched and allowed to have a dip or sag.
The difference in level between the points of support and the lowest point on the conductor is called as sag.
Keeping the desired sag in overhead power lines is an important consideration. If the amount of sag is very low, the conductor is exposed to a higher mechanical tension which may break the conductor. Whereas, if the amount of sag is very high, the conductor may swing at higher amplitudes due to the wind and may contact with alongside conductors. Lower sag means tight conductor and higher tension. Higher sag means loose conductor and lower tension. Therefore, a suitable value of sag is calculated so that the conductor remains in safe tension limit keeping the sag minimum.
  • The tension at any point on the conductor acts tangentially. Therefore, the tension at the lowermost point on the conductor is horizontal.
  • The horizontal component of tension at any point on the conductor is constant.
  • The tension at the support points is nearly equal to the horizontal component of tension at any point on the conductor.

Calculation of sag

The tension on a suspended conductor is governed by the conductor weight, effects of wind, ice loading and temperature variations. Generally, conductor tension is kept less than 50% of its ultimate tensile strength. The value of sag is calculated for two different scenarios - (i) supports are at equal levels and (ii) supports are at unequal levels.

When supports are at equal levels

Consider a conductor suspended at supports of equal heights as shown in the figure below. A and B are the support points and O is the lowest point on the conductor.

sag in power lines when supports are at equal levels

  • l = length of the conductor span
  • w = weight per unit length of the conductor
  • T = tension in the conductor
Consider a point P on the conductor. Considering the lowest point O as the origin, let the coordinates of point P be x and y. Assume the curvature is so small that the curved length is equal to its horizontal projection (i.e. OP = x). The forces acting on the conductor portion OP are -
  1. the weight w.x acting at a distance x/2 from the point O
  2. the tension T acting at the point O
Equating the moments of the two forces about point O, we get,
T.y = w.x * x/2
or, y = w.x2 / 2T
The maximum sag (dip) is represented by the value of y at either of the support points. At support point A,
x = l/2 and y = S (sag)
therefore, sag S = w(l/2)2 / 2T
therefore, sag S = w.l2/8T

When supports are at unequal levels

We generally encounter supports of unequal heights in hilly areas. The following figure shows a conductor suspended on supports at unequal heights.

sag in power lines when supports are at unequal levels

  • l = length of the conductor span
  • w = weight per unit length of the conductor
  • T = tension in the conductor
  • h = difference in levels between the two supports
  • x1 = distance of support at lower level (A) from the origin O
  • x2 = distance of support at higher level (B) from the origin O
From the above calculation of sag in the previous point, y = w.x2 / 2T
Now, at support A, x = x1 and y = S1
Therefore, S1 = w.x12 / 2T
sag in power lines equation

Values of S1 and S2 can be easily calculated from x1 and x2.

[Also read: Insulators used in overhead power lines]

Effect of wind and ice loading

The above calculations of sag are only true for still air and normal temperature conditions. However, in actual practice conductors are suspended to wind pressures and often loaded with ice coating in cold areas. The force due to wind is assumed to act horizontally to the conductor. And the force applied by ice loading is vertically downwards. Therefore, the total force on the conductor is the vector sum of vertical and horizontal forces.

In this case, weight of the conductor per unit length will be,
wt = sqrt.[(w + wi)2 + (ww)2]
  • w = weight of the conductor per unit length
  • wi = weight of the loaded ice per unit length
  • ww = wind force per unit length
When a conductor has wind as well as ice loading,
  • The conductor sets itself in a plane at an angle θ to the vertical plane.
    Where, tan θ = ww / (w + wi)
  • In this case, sag is given by S = wt.l2 / 2T. But, here, S represents the slant sag, i.e. sag is in the plane where the conductor has set itself. The vertical sag is equal to S.cosθ

Signs of electrical hazards and precautions

Electricity is essential for life in the modern world, but it can also pose deadly hazards. Below is some information about the potential hazards of contact with electricity, as well as precautions you can take to avoid electrocution or shock injury.
signs of electrical hazards and precautions

Dangers of electricity

Anybody who comes into contact with electricity can sustain serious burns or shock. In some cases, electrocution can even be fatal. Most electrical injuries occur when people encounter downed power lines, frayed power cords or malfunctioning electrical appliances. People may also be injured if they come into contact with water that is touching a source of electricity.

Identifying potential hazards

To keep yourself safe from electrical injuries, look for the following signs of electrical hazards:
  • Appliances located near any source of water, including bathtubs, sinks and spills. Water is a conductor of electricity and its presence raises the risk of an electrical injury.
  • Power cords and electrical wires close to any source of water.
  • Power cords and wires close to sources of heat. Heat may damage cords and wires, making injury more likely.
  • Downed electrical lines, especially after inclement weather. If you see something that looks like a downed power line, call the power company to report the incident immediately.
  • Worn electrical wires or power cords. A worn wire or cord doesn't provide proper insulation and can raise the risk of injuries or fires.
If you notice any of these potential electrical hazards, avoid contact with the area until the risk has been mitigated.

[Also read: Safety tips while using portable generators.]

Keeping children safe

Because of their age and/or lack of experience with electricity, children face a greater risk of sustaining electrical injuries than adults. To keep your children safe, teach them about the dangers of electricity. Make sure they know the basics of staying safe around electrical appliances, power cords and power lines. Children should also be taught to avoid contact with cords and electrical appliances any time they are wet or touching water.
Instruct your children to report any damaged electrical lines or cords to you immediately. If your children are very young, install protective covers over your electrical outlets and keep cords and other electrical dangers out of reach. If an incident involving electricity does occur, seek medical attention immediately.

Corona rings and Grading rings

Corona rings are toroidal shaped metallic rings which are fixed at the end of bushings and insulator strings. They are also called as anti-corona rings. Corona rings are used to prevent corona discharge that occurs in high-voltage power lines. Corona discharge or corona loss is a significant issue with very high voltage power lines which causes power loss. One way to reduce corona discharge is using corona rings.

How corona rings work?

Corona discharge occurs when the electric field (potential gradient) at the surface of conductors exceed a critical value, called as critical disruptive voltage. The value of critical disruptive voltage varies with atmospheric condition. Its value is roughly 30 kV/cm. The electric field greatest where the curvature is sharpest. Therefore, corona discharge occurs first at the points where the curvature is greatest - i.e. at suspension points, corners and edges. Corona rings are installed at these points to prevent corona formation.

corona discharge on a corona ring
Corona discharge on a corona ring
Image source: Wikimedia commons

A corona ring is electrically connected to the conductor, encircling the points where corona discharge may occur. Therefore, the corona ring distributes the electric field (or charge) across a wider area, because of its smooth round shape. Hence, it reduces the potential gradient below the critical disruptive value.
Manufacturers usually recommend aluminum corona rings to be installed at the conductor end of the string insulators for lines above 230 kV and on both ends of the insulator for 500 kV.

What are grading rings?

Grading rings are very similar to corona rings. In fact, one can say, grading ring and corona ring are two different names for the same device. The difference is due to their main purpose of use and their placement. Grading rings encircle insulators rather than conductors. Their main purpose is to reduce the potential gradient along the insulator. Grading rings help in equalizing the potential distribution over a string of suspension insulator. Hence, grading rings improve the string efficiency and prevent insulation breakdown. Grading rings also serve the purpose of corona rings to some extent.
corona rings and grading rings