USCG Exam PracticeMotor controllers and remote control
In an ungrounded three-phase AC system aboard a vessel, how many motor-running protective devices are required, and in which conductors must they be placed?
- A. Three devices, one in each ungrounded conductor
- ✓ Two devices, in any two ungrounded conductorsCorrect
- C. Two devices, one in an ungrounded conductor and one in the grounded neutral
- D. Three devices, one in each phase plus one in the neutral
Why B is correct
46 CFR §111.70-1(b) states that in ungrounded three-phase AC systems, only two motor-running protective devices need be used, placed in any two ungrounded conductors. The exception applies when a wye-delta or delta-wye transformer is used.
Cited:46 CFR §111.70-1