USCG Exam PracticeElectrical measuring devices and instrumentation
A current transformer (CT) is installed on a high-current bus to feed the switchboard ammeter. While the bus is energized, the technician must remove the ammeter for calibration. Before disconnecting the meter leads, the technician must first:
- ✓ Short-circuit the CT secondary terminals, then disconnect the meterCorrect
- B. Open the CT primary circuit by tripping the bus breaker
- C. Switch the ammeter to its highest voltage range to protect the movement
- D. Disconnect the potential transformer (PT) secondary to de-energize the panel
Why A is correct
A CT secondary must never be left open while primary current flows. With the secondary open-circuited, all the primary magnetomotive force drives the core into saturation and induces a lethal high voltage across the open secondary terminals. The correct procedure is to short the CT secondary before removing any burden (the ammeter), keeping the secondary circuit closed.