Loss of Phase is defined as the loss of one or more but not all phases of the polyphase power source.

• A. Loss of Phase
• B. Loss of Ground
• C. Phase-to-phase short
• D. Reversal of rotation

Answer: (A)

In a polyphase system, loss of phase refers to a condition where one or more phases drop out while the others remain energized. This is commonly called single phasing. It’s the scenario described by the term because only part of the three-phase power is lost, leaving an incomplete supply to the motor. This is distinct from a loss of ground (a fault to earth), a phase-to-phase short (two phases shorted together), or reversal of rotation (which happens if two supply lines are swapped and the motor runs in the opposite direction).

When a phase is lost, the motor can continue to run on the remaining phases, but with significantly lower torque and higher current in the remaining windings. The result is overheating and potential damage if the condition persists. Protective systems in NFPA 20 environments are designed to detect loss of phase and trip the pump to prevent damage.