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, a technician removes the ammeter for calibration without first shorting the CT secondary terminals. What is the immediate hazard?
- A. The CT primary winding will overheat and fail because current has no return path
- ✓ A lethal high voltage will develop across the open CT secondary terminalsCorrect
- C. The bus voltage will collapse because the CT provides the reference for the protection relays
- D. The ammeter will be permanently magnetized when reconnected because of residual flux
Why B is correct
With primary current flowing, an open CT secondary forces all the magnetic flux to drive a voltage across the open secondary terminals; this voltage can reach lethal levels. The CT secondary must always be short-circuited before any secondary device is disconnected.