Why is double earthing necessary for 3-phase machines and equipment?

Why Earthing Important?

The main purpose of the earthing of electrical equipment is to avoid the danger of getting electric shock. The proper earthing also provides fire protection. The large current flowing through the undefined high resistance path may increase the chance of fire due to heat, and also it can cause electric shock or electrocution due to rise in voltage between  the undefined path with respect to ground.

The metallic enclosure of the electrical appliance must be earthed to provide low resistance path to leakage current in order to protect the human being from getting electrocution and also to protect the electrical system.

When any live conductor in the enclosure touches to the body of metallic enclosure, it attains the live potential. If anyone touches the enclosure it may electrocute. To avoid this untoward incident, the body of the enclosure is earthed.

The earth has very low resistance, and thus the current starts flowing from the enclosure body to earth. The earth fault current trips the protection relay and thus the power supply is cut off. One earthing point is sufficient to provide earth fault protection, then why the metallic enclosure of the electrical equipment should be earthed from two distinct point of the enclosure.

Why Double Earthing?

The electrical equipment enclosure earthed at two separate points with two separate earth pits provides the following advantages.
  • When the electrical equipment enclosure is earthed at two points on the enclosure body, it establishes the two  current path at the time of attaining of live potential. In the case of discontinuity of one circuit, the earth current keep on flowing through the other circuit and thus, the equipment remains fully protected against the earth fault.double earthing of panel
  • The two points earthing provides two parallel path for earth fault current. The two parallel path is connected to two separate earth points. Thus the equivalent resistance of the two parallel path gets reduced and it offers low resistance path to the fault current. If the resistance of one path is 0.4 ohms and the other path is 0.3 ohms, the equivalent resistance of the two parallel path is [(0.4 x 0.3)/(0.4+0.3)]=0.17 ohms. Thus the effective path resistance of the two parallel circuit gets reduced and it provides better reliable protection.2 point earthing reduces the effective earth resistance

To summarize,  the electrical equipment is earthed at two distinct points for the following reasons.

  • The earthing remains intact in case of broken of one of the earthing points.
  • To have very low earthing resistance.
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