Construction of 11 KV, 3.5 core underground cable | What is 3.5 core? (Article)

Underground cable has many advantages over overhead lines but what strength do an underground cable needs, how is the practical construction of underground cable and what is 3.5 core cable to get the answer read this article.

We will first see what are the strengths that are needed by underground cables. We are going to put that cable underground so we need some strengths and they are as follows,

  1. Weather proof insulation

This is needed because the cable should sustain everything like water, soil, high temperature, low temperature and everything that is in the weather.

  1. Mechanical and tensile strength

When cable is going underground it should have some amount of pressure carrying capacity and it should also have some tensile strength.

  1. A bit of extra insulation

This insulation is provided under the Steel armour so that it will avoid the short circuit possibility between the armour and the conductors.

  1. core insulation

Core insulation is must. Because if your cores or simply the conductors they are not insulated from each other then they will result into a short circuit, so the core insulation should be there.

If you have these four things, then you are ready to lay a cable underground.

Look at the following image it shows the detail construction of a 3.5 core armoured cable.

Number 1 represent the weather proof coating.

Number 2 represent the Steel armour which is provided to give the required mechanical and tensile strength.

Number 3 represent the extra insulation provided between Steel armour and the cores.

number 4 represent the insulation provided to conductors or cores

And number 5 represent the bundled conductors.

  • Why 3.5 Core cable?

Now this is about the construction but one question still remains and that is what is 3.5 core cable and what is the purpose of that extra 0.5 Core?

No worries, we’ll make it clear

Now let’s say you are having a three-phase supply and all of your three phases are carrying equal amount of current as shown in the image 1.  when all your phases are carrying equal amount of current that means your system is perfectly balanced and there is no out of balance current flowing through your system, at this time you can use 3 core cable.

Image 1

But what is your system is not balanced at this time your phases will carry different current as shown in image 2.

Image 2

Now there is some extra current flowing through your system you can call it as an out of balance current and you need something to carry that, and to carry that extra current we generally provide a conductor with the three phase conductors and that conductor is called as Neutral. The greater the degree of imbalance, the larger the neutral current.

When there is some degree of unbalance and the amount of fault current is very small, then 3.5 core cables are used. In these types of cables, a neutral of reduced cross section as compared to the 3-main conductors is used, which is used to carry the small amount of unbalanced currents

So, I hope this article helped you, we have also made a video on this in which you will be able to see the practical construction of a 3.5 core armoured cable, you can click the following link to go to that video.

Practical construction of a 11KV underground cable | 3.5 core cable



Construction of 11 KV, 3.5 core underground cable | What is 3.5 core? (Article)

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Gaurav J
About The Author
- Electrical Engineer. Interest & Study in Electrical Engineering. Currently working with a High-Voltage Switchgear Industry.