Answer MET Question 50

 Question: Explain the effect of making incorrect phase and starter connections.

Answer: When the phase sequence of a three-phase system is incorrect, the connected three-phase motors and other rotating equipment runs in the opposite direction. In many cases, this can cause a hazardous condition that may destroy product, damage machinery, and injure personnel.
Incorrect phase sequence within a three phase system is called phase reversal. Phase reversal usually occurs as a result of mistakes made during equipment installation, maintenance, or modifications to the facility power system.
A three-phase monitor relay with a phase reversal protection should be used in applications where three-phase motors and other rotating equipment must not run in the reverse direction. The relay will not energize if the phase sequence is reversed and it can stop machinery if the phase reversal occurs during operation.