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

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.

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.

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.