USCG Exam PracticeElectrical measuring devices and instrumentation
On a shipboard ungrounded delta power system, the ground-detection lamp on Phase A dims noticeably while the lamps on Phases B and C brighten. This indication means:
- A. Phase A voltage has increased above normal, overdriving its lamp
- ✓ A ground fault exists on Phase A, and the fault must be located before a second ground causes a phase-to-phase shortCorrect
- C. The insulation on Phases B and C has failed simultaneously, increasing current through those lamps
- D. The system neutral has been inadvertently connected to ground, shifting the reference point
Why B is correct
In an ungrounded delta system, ground-detection lamps are connected one per phase to ground. A ground fault on one phase effectively grounds that phase, reducing the voltage across its lamp (dimming it) and increasing the voltage across the other two lamps (brightening them). The fault must be found and cleared before a second ground on another phase creates a phase-to-phase fault.