USCG Exam PracticeElectrical measuring devices and instrumentation
A current transformer (CT) secondary circuit becomes open while the primary conductor is carrying full load current. The immediate hazard is:
- A. The CT core saturates and the primary circuit trips on overcurrent
- B. The secondary current drops to zero, causing the associated meter to read low and mask an overload condition
- ✓ A lethal high voltage develops across the open CT secondary terminalsCorrect
- D. The primary current increases because the CT no longer presents a burden to the circuit
Why C is correct
With the secondary open, there is no secondary current to oppose the primary magnetomotive force. The core flux rises to saturation on each half-cycle, inducing a very high voltage spike across the open secondary terminals — sufficient to be lethal and to damage insulation. A CT secondary must never be left open while primary current flows.