If you are an Electric Engineering student, then you might have encountered suspicion regarding the earthing cable size calculation. It is a very easy process indeed.

The word earthing is a very familiar word in our electrical field. For instance, we all know that when a device suffers from a short circuit fault, its load current increases many times over the originally rated current. At that very moment, the earthing overcomes the excess current and protects the main device from accidents due to a short circuit. Learning about the earthing cable size calculation method is the very first criteria of today’s article. In the end, I would like to highlight a little more about earthing.

Earthing cable

earthing cable size calculation- earthing cable
Earthing Cable

In fact, in order to understand earthing, you must first hear a story. Let’s get started. One day a wedding was taking place at a community center. Many people came there to eat. Again, there was a naughty boy who spread rumors that there was a bomb in the community center. For instance, there is only one way out of the community center. So, everyone panicked and tried to get out through the door. There was a lot of competition as to who would come out first. As a result, there was a blockage at the door. At last, a person found a secret door to exit. He was able to get out of there effortlessly.

The same goes for earthing cable. Here, when a fault occurs in the power system, leakage current will flow. Now, this excess current looks for an opportunity to escape. Again, that is why the earthing cable is arranged so that the power can easily get out of this path. Here, you can compare the hidden door of the community center with the earthing cable and you can compare the person with the leakage current. Now, let’s come to the main question. Finally, I hope you understand what an earthing cable is, but let’s talk about how to select this cable.

Earthing Cable Selection Formula and Method:

Earthing cable size selection depends on your fault current (KA) and fault clearing time. Of course, we are already aware of the fault current. Now, I want to say something about fault clearing time.

What is Fault Clearing Time?

Fault clearing time is the time it takes for a circuit breaker or any protective system to be sensed after a fault has occurred, for the relay to be effective, and for the arc to be suppressed. Accordingly there is the need of much time to clear the fault.

Now, we will know your much-awaited formula which will work in earthing cable selection.

The formula is:

Earth conductor size PE(sqmm) = 1.5 × √ (I(Fault-A)2× t(s)) / k

  • Here, 1.5 is the safety factor. You can take the safety factor at your own convenience. However, there is no obligation.
  • I (Fault-A) is the amount of fault or leakage current.
  • t (s) is the fault clearing time.
  • K is the cable material content.

Below are the values ​​of K of some of the materials:

Copper 205, Aluminium 226, GI (Galvanized Iron) 80

aluminium bar
Aluminium Bar

In the end, let’s make the matter clearer by doing math.

Let us select GI earth conductor size for 20kA fault current and fault clearing time 0.5 sec.

Earth Conductor size, √(200002 × 0.5) / 80 

= 20000 × 0.707 / 80 

= 176.8 Sqmm

Approximately 265 Sqmm when multiplied by the safety factor.

GI housing is usually available in the market. on the whole, its cable is also available. However, in this case, 30×10 sqmm, 20×15 sqmm cable selection will be better.

Check More Articles:

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  3. Direction of current flow and electron flow | Why are they opposite to each other?

Courtesy: Iqbal Mahmood

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