How to test three-phase electrical supply

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

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

Typical parameters are 415 volts between phases, 240 volts from any phase to earth, voltage within 12% of 415/240 volts, 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
        /  /  /
       /  /  /
       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.