USCG Exam PracticeElectrical measuring devices and instrumentation
A clamp-on ammeter is used in preference to a series-connected ammeter for measuring load current in a large feeder cable primarily because:
- A. The clamp-on meter reads RMS current more accurately than a series ammeter on AC circuits
- ✓ The clamp-on meter can measure current without opening the circuit, avoiding interruption of the loadCorrect
- C. The clamp-on meter has a lower insertion resistance than a shunted series ammeter
- D. The clamp-on meter is the only instrument capable of measuring three-phase current simultaneously
Why B is correct
A series ammeter requires the circuit to be opened to insert it. A clamp-on ammeter senses the magnetic field around the conductor and reads current without breaking the circuit, which is essential when the load must remain in service.