Underground cables play an important part in electrical installation. In this article I will explain what you should know about underground cables. Let’s get started.
When are overhead cables better than underground cables?
The overhead power lines are better than underground cables in transmitting a large amount of power at large distances. This is due to:
- Cost effectiveness.
- Can be used for high and extra high voltage power transmission.
- Flexibility and can be modified as loads change.
- Easy to detect faults.
Note that overhead power lines total cost is less than underground cables, however it is only suitable outside cities. While inside cities the only option of high and medium voltage transmission is underground cables.
Let’s go in some details:
As the wires carry a very lesser current than the underground cables the power loss is minimum in the overhead transmission system as compared to the underground cables on the same distance.
Also, the systems where we need to access the mainline at many points, the underground cables pose a high difficulty to get the work done. While in the case of overhead transmission lines, if we need to access the mainline it is way easier.
To get a connection from underground cables we must unwrap the already insulated wire, make a connection, insulate the new connection up to a certain distance out of the ground and fill the pothole.
Above all of this, the digging process should be really professional and subtle. Because if during the process of digging and locating the wire, one mistakenly penetrates through the live wires, it may cause the loss of life. Also, the laying of the underground system is not economical.
So, we can conclusively say that Overhead Transmission cables are better than Underground cables in the aspects of:
- Power Transmission
- New connection arrangement
- Economic factors
When are underground cables better than underground cables?
Underground cables are better than overhead cables inside cities, due to their high safety and low maintenance. The underground cables are not exposed thus zeroing out the probability of risk of fire and electrocution.
Moreover, the underground cables have much more life expectancy. So, the expenses of replacing the cable system are also postponed too. So, the underground cables are better than overhead cables in the aspects of:
- Safety, this is why cables are suitable for cities.
- Maintenance cost is lower than overhead lines.
- Longer life than OHTL.
Overhead power lines are preferred due to its low cost, flexibility and faults detection ease. But OHTL is not the choice near or inside cities and residential areas.
- Overhead power lines are better than the underground cables as they fulfill the main purpose of their existence i.e., the efficient power transmission over large distances.
- Underground cables are safer than overhead cables, but still due to the aspects of Power transmission, Economic factors, and the ease of connection access Overhead Power transmission cables are better than the former.
Underground Cable Laying Methods
Underground cables laying at a certain depth is mainly used to transmit the power safely. There are 3 different methods to lay the underground cables.
- Direct Laying
- Solid System
The direct laying of the underground cables is the most used laying process. Some prerequisites of this process are:
- The cable must have bituminous paper covered on it.
- Hessian tape used should be for the bituminous paper covering to keep it in its place.
The process of laying the cable by the direct laying method is as:
- A trench is dug at the depth of 1.5 meters with a width of 0.45 m.
- A sand bed is prepared for the cable.
- Then the cable is laid on the sand bed.
- Then another layer of sand bed that is no more than 0.1 m in thickness.
- These sand beds are actually the protection from the moisture that may cause corrosion and eventually lead to electrolysis. They also provide a reasonable heat sink for the heat generated in the wire due to the high current.
- Then this trench is filled with some soil and the shield for the mechanical damage is some concrete blocks.
- In the case of multiple wiring systems through the same trench, the trench is widened, and also depth is increased. The reason for spacing is to deal with the heat in the wires and get the other cables saved from the damage in one cable. The distance is varied from 0.3 meters to 0.4 meters that is for both the vertical and horizontal spacing.
Draw-in laying system:
As obvious from the name of this process, the wires are drawn in through some holes under the ground for laying of the cables. But these manholes are not dug barely through the soil horizontally along with the earth but need a little preparation for the manholes before drawing in the wire.
These holes are made in concrete blocks or cast-iron blocks. The concrete blocks are more used than the cast iron blocks and have 3 holes for the cables and 4th for the pilot cable and relay cable, both in the same hole.
The procedure is as follows:
- A suitable trench is dug and concrete blocks (sometimes pipes) are laid in this trench. The blocks must have holes for passing the cable through them.
- The cables are passed through the holes.
- The diameter of the holes should be enough to carry and pull the wire through it.
This method is performed by laying the cable in solid materials. These materials could be asphalt, stoneware, or cast iron.
This method is not used nowadays due to its non-economical way of laying and the laborer must have a proper skill of this method.
Why underground cables are laid at a certain depth?
Underground cables are laid at a certain depth to make sure that the outer conditions do not affect the cables. Normally, the main effect is due to vibrations produced by the vehicle and working machines.
Deep laying provide mechanical protection against these loads, and also provide thermal conductivity by the soil to reduce cable temperature.
Bedding, Bricks, and Sand on the Underground Cables
The purpose of sand bedding is to save the cables from moisture. The bricks are used to perform the duty of shield for the cable to protect it in case of digging activity near the laying location.
If some one is digging in the same location of the cable laying, bricks prevent his digging tool from hitting the power cable, and from causing electric shock to him or short circuit.
What happens if an underground cable gets wet?
Many of underground cables in my work are fully immersed in wet locations due to heavy rain season. However, there is no worries about this if and only if the cable insulation is not damaged. Once the insulation gets any damages, the cable gets short circuited.
The underground cable is highly insulated. The conductor of wire is not exposed to the outer world. Even if the cable’s premises are soaked in the water, the inner conductor doesn’t get short-circuited with the outer conducting elements along with the water.
The special case could be as mentioned above, if the cable was damaged and its conductor was exposed and then the cable’s premises are soaked in water, it may cause a short circuit.
So, generally, the cables will not be harming people and infrastructure if exposed to water and were not already damaged.
Why are Underground Cables Stranded Conductors?
The main reason for using a stranded conductor is to ensure the flexibility of the cable. If solid conductors are used, this will cause problems at laying, as these cables are never laid in a straight trench. Trenches are a little curved, flexibility is needed to deal with these curves.
How do I check the underground cable faults?
If you think that your underground cable is not working properly, you need to apply some tests to make sure that the underground cables are working fine or not.
Murray loop test:
Murray loop test is applied to employ the Wheatstone’s bridge. Murray loop test is used to detect
Let’s take a look at this figure. Say, Rx is the faulty cable. The wire connecting Rc and Rx is a low-resistance wire. To find the location of the fault, we can use the following equation: Lx =(Rb/Ra+Rb).2L
Where Lx is the distance where the fault has occurred, and L is the total length of the faulty cable. Ra and Rb are variable resistors. The values of Ra and Rb are changed until the G shows no deflections. At those values of Ra and Rb , we can calculate the value of Lx.
Varley loop test:
This test is used to locate the short circuit and the ground faults in underground cables.
firstly, the connections are made like this. We can find the resistance using the following equation: Rx =(R2/R1+R2).2r
Where Rx is the resistance of the faulty cable and 2r is the sum of the resistances of the faulty cable and the galvanometer. To find the exact distance of fault from the test site use this formula:
Rx =Rb(Rc+Rx). Ra S1/( Ra+Rb ), where Ra, Rb, and Rc are constant and S1 is varied until the galvanometer is showing no deflection.
Now the resistance of faulty cable can be found by:
Rx =Ra(S2– S1)/(Ra+Rb) ,Where S2 is the value of the resistor S in the second figure when the galvanometer is not deflecting anymore. The distance of the fault can be found by: LX=Rx/2. There are other tests for underground fault detection but the best detection systems have been mentioned
Why lead sheath is used in underground cables?
The lead sheath provides protection against moisture ingress and to prohibit the entry of Hydrocarbons like chemicals, gases, oils and solvents. If the outer insulation is breached.
Lead sheathed cables are suitable to workplace environment of oil and gas industries as well as petrochemical industries.
The outer surface of lead makes a protective layer to suppress the entry of reactive material that does lead to loss of conductor by the process of electrolysis. Lead also provides protection from corrosion.
Conclusively, we can say that the lead provides:
- Protective layer in case the outer insulation fails its own purpose.
- Protection against moisture.
- Can be used as earthing.
- Increase cables corrosion resistance.
Overhead vs. Underground Cables
|Overhead Cables||Underground Cables|
|Construction||L complicated in manufacturing||More complicated.|
|Installation||The cables mounted on high steel and aluminum towers.||Cables are laid underground.|
|Location||Mainly used to transmit power outside cities and residential areas.||Transmits power inside cities and residential areas.|
|The insulators are used to connect the conductor with the towers.||No concept of towers is there in this method.|
|Insulation||Conductors without insulation is used.||A very sophisticated kind of insulation is used.|
|Conductor size||The size (cross-sectional area) of wire is way lesser than the underground cables.||Cross-sectional are wire is higher than that of overhead cables.|
|Environmental effect||The conductor may get damaged due to the environmental factors.||The conductor is not damaged by environmental factors.|
|This circuitry has Insulator dominant characteristics.||The circuitry has capacitive characteristics. (Circuitry means the transmission circuit from source to the end)|
|High voltage is used for the Power transmission.||High current is used for the Power transmission.|
|Proximity effect||Proximity Effect is negligible i.e., 0.||Proximity effect is much higher than OHTC.|
|Branching||Extension or getting a branch from the cable is easy.||Extension or getting a branch from the cable is headache.|
|Corona losses||Corona discharge is present. A reasonable corona discharge is present in windy areas.||No concept of corona discharge. As the underground cables are filled with materials like sand and cement. So, air has no reach to the conductor.|
|Falling risk||Risk of falling||No falling risk.|
|Maintenance||Easy to repair.||Difficult to repair.|
|Fault location||Easy to locate the fault.||Locating of fault is difficult.|
|Current capacity||Higher current capacity due to good cooling.||Lower current capacity.|
|Manufacturing Cost||Lines of overhead cables are cheaper.||Lines of underground cables are expensive as many layers of insulators are present over the core conductor.|
|Voltage levels||High and ultra high voltages power transmission up to 400 KV||Not suitable for ultra high voltages. Up to 66kv|
|Life Time||Life of cables is lesser than underground cables as the environmental factors lead to metallic loss of the conductor.||Life of the cable is even up to 40-50 years as the metallic loss is 0 unless manually done.|
|Installation||Easier and straight forward||Need digging activity|
|Cooling||Easier by natural air||Depend on soil heat conductivity.|