What is single phase preventer?

Answer 1

Anti-single phasing relays or single phasing preventer are sometimes required for critical loads and circuits.These are sometimes required because the normal overload protection doesn't trip on time. For large air conditioning compressors, these are sometimes, included.

Answer 2

Single phase preventer is kind of safety relay to protect 3 phase equipment from burning out, should there be a phase missing, in a three phase system. This device constantly takes the feedback from all three phases and if any one phase is out, it then triggers a relay contact which can be used as interlock in the control circuit of any 3 phase equipment, particularly 3 phase motors.

3 Phase AC induction motors continue to run even if a phase is out, but drawing abnormal current, thus heating up and ultimately resulting into winding burn out.