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How to test three-phase electrical supply

A three-phase electrical supply consists of three active conductors and an earth ground.

A three-phase induction motor cannot function correctly if its electrical supply is not within certain parameters.

Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or ground, voltage within 12% of nominal values, and each phase within 5% of each other.

In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the direct-on-line motor starter.

Figure 1:

      A  B  C        earth/
      O  O  O        ground
       /  /  /
      /  /  /
      O  O  O

Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.

Note that listed voltages are for countries which use 120 or 240 volts only!