WHAT IS GROUNDING?
Grounding is nothing but connecting the
1. Non-current carrying electrical point (neutral) or
2. Metallic enclosure of any equipment to the earth.
The first one is called “Neutral Grounding” and the second one is called “Body Earthing or grounding“
We have known what is grounding, now comes the most important question
“Why do we need grounding?”
1. Purpose of Neutral Earthing or grounding: Neutral earthing is done for mainly two reasons.
a) To keep the neutral point of any star connected 3 phase system at earth potential.
b) To provide a complete path for current during earth fault so that it can be detected.
The image shows a neutral point with no earth connection i.e. “Unearthed Neutral”
We can also see that if any earth fault occurs at any phase the fault current shall have no path to go, causing a huge potential rise and an undetected hanged earth fault.
Now let us see the previous system with neutrality connected to the earth. Here it can be seen clearly that Earth’s fault current has a complete path to return through the neutral point via earth.
This current can easily be detected by placing a current transformer at the neutral. There are many other methods of detecting earth faults also.
Please refer to our article CT connection to sense earth fault/Earth Faults Detection Philosophy for these details.
So from the above observation, it seems to be obvious that ungrounded neutral should not be used anywhere.
It is true that almost in every case neutral point of any star-connected system may be a generator or transformer is earthed. But where delivering power is more important than detecting earth fault, for those cases the neutral remains open. Let me explain.
In a power plant, there are emergency diesel generators that deliver power when the power plant fails for safe shutdown of the power plant. Basically in a power plant the turbine and the main generator are the most expensive equipment, so during any disturbance, it is necessary to safely shut down those two pieces of equipment. During a complete blackout of a power plant, the emergency dg feeds power, then it is more important that the DG must deliver power without any interruption. That’s why the neutral of the DGs is kept open so that it doesn’t trip for earth leakage or earth fault.
So, we can say neutral earthing is done for equipment safety i.e. to detect an earth fault and isolate it.
Now we shall see the second type of grounding i.e. body grounding or body earthing.
2. Purpose of Body Earthing or grounding: We have already mentioned that connecting the metallic enclosure of equipment to the ground is called Body Earthing. It is done for human safety, which is the most essential criterion of industrial practice.
Every Electrical equipment has winding inside it which is charged with rated voltage and carries current as per its rating. The windings are insulated for their rated voltage, hence current is confined only in the cross-section of the conductor. But there are two situations when this consideration may not remain valid.
CASE 1- What will happen if the insulation of the outermost winding gets damaged?
In that case, the voltage of the winding will try to break down the airgap with the enclosure or the insulating material.
If the insulation is damaged enough and the voltage is also high enough then it will become capable to break down the airgap insulation between the winding and enclosure.
If the enclosure is not earthed, and at this condition, an operator touches the enclosure then the winding will cause a leakage current through the airgap, the operator’s body to the ground. Thus the person will experience electrical shock, due to leakage current.
But if the enclosure was earthed then?
Then this leakage current would flow through the earthed conductor to the ground. If the leakage current is high enough or the earth fault detection system is sensitive enough (See article- CT connection to sense earth fault/Earth Faults Detection Philosophy, Restricted Earth Fault Protection Philosophy) then it shall be treated as earth fault and by earth fault relay the faulty machine shall be isolated.
CASE 2: The second possibility is that if the conductor inside the equipment got broken and touched the enclosure then the voltage will appear in the enclosure. If the enclosure is earthed then it will be treated as an earth fault causing the earth fault relay to operate and isolate the fault, but if the enclosure is not earthed then accidental contact with that equipment and the operator will experience a fatal shock.
Conditions are similar to the picture shown above, only in the shown picture current is flowing due to insulation damage, whereas here current will flow as a conductor.
So we can hereby conclude that all metallic enclosures of Electrical equipment shall be connected to the earth. Basically, in industry, all metallic structures, and enclosures, are suitably earthed.
GENERAL METHODS OF GROUNDING:
Ground mat/grids are buried underground. Grounding systems are comprised of horizontal and vertical conductors.
Horizontal conductors cover the area that is to be grounded. And vertical conductors basically kept releasing the earth fault current to deeper ground. Many configurations of horizontal conductors are used like simple peripheral configuration, mesh configuration, etc. depending on the voltage level. Detailed discussion about the grounding conductor configuration selection is beyond scope of this article, we shall give details in the upcoming articles.
For neutral point earthing at least two vertical electrodes are buried. These two electrodes are connected to the main grounding grid. And neutral point is connected with these electrodes by G.S flat or cable as applicable.
From the main ground grid, risers are kept. Equipment is connected to the risers as required.
This is the very basic method of grounding. But there are many other factors for a safe grounding design which shall be discussed in the upcoming articles.