USCG Exam PracticeElectrical troubleshooting and maintenance
On a shipboard ungrounded delta power system, a single ground fault occurs on one phase. Which of the following best describes the immediate operational consequence and the required follow-up action?
- A. The faulted phase breaker trips immediately, isolating the fault; no further action is required until the next scheduled maintenance period
- ✓ Vital loads continue to operate because the single fault does not complete a circuit; however, the fault must be located and cleared before a second ground fault on another phase creates a phase-to-phase short circuitCorrect
- C. The system neutral is driven to full phase voltage, causing immediate overvoltage damage to all connected loads; the system must be de-energized immediately
- D. The ground-detection lamps all extinguish simultaneously, and the system automatically transfers to the emergency generator
Why B is correct
DOE-HDBK-1011 Vol.4 §15-3 explains that in an ungrounded delta system a single ground fault does not immediately trip the system, allowing vital loads to continue operating. However, the fault must be found and cleared before a second ground on another phase creates a phase-to-phase short circuit. The ground-detection lamps signal the fault condition to the watchstander. 46 CFR §111.05-23 requires ground indicators to be readily accessible at the distribution switchboard to support this monitoring function.