Electrician Red Seal · Question
An industrial facility uses a 480 V, 3-phase ungrounded Delta system for motor loads. A maintenance electrician measures 277 V between phase A and ground, 277 V between phase B and ground, and 0 V between phase C and ground. What is the likely cause of these readings?
In an ungrounded Delta system, a single ground fault does not immediately cause an overcurrent device to trip. Instead, it places the faulted phase at ground po
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Question: An industrial facility uses a 480 V, 3-phase ungrounded Delta system for motor loads. A maintenance electrician measures 277 V between phase A and ground, 277 V between phase B and ground, and 0 V between phase C and ground. What is the likely cause of these readings?
Answer options:
- A phase-to-phase fault. ✅ A ground fault on phase C.
- An open phase conductor.
- Improper bonding of the system.
Correct answer: A ground fault on phase C.
Explanation: In an ungrounded Delta system, a single ground fault does not immediately cause an overcurrent device to trip. Instead, it places the faulted phase at ground potential. The other two phases then present their phase-to-ground voltage, which for a 480V system is approximately 277V (480/√3) with respect to the faulted phase which is now at ground potential.
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