An electrician is commissioning a new three-phase 480 V delta primary to 208Y/120 V secondary transformer. After energizing the primary, the electrician measures the line-to-line secondary voltages and finds phase A-B at 208V, B-C at 208V, but C-A at 180V. What is the most likely cause of this discrepancy?

Question: An electrician is commissioning a new three-phase 480 V delta primary to 208Y/120 V secondary transformer. After energizing the primary, the electrician measures the line-to-line secondary voltages and finds phase A-B at 208V, B-C at 208V, but C-A at 180V. What is the most likely cause of this discrepancy?

Answer options:

• The transformer is overloaded. ✅ One of the primary fuses for the transformer is blown.
• The transformer has an internal short circuit.
• Incorrect tap settings on the primary side.

Correct answer: One of the primary fuses for the transformer is blown.

Explanation: Uneven secondary voltages, specifically one phase significantly lower, are a classic symptom of an open circuit in one of the primary phases. A blown primary fuse would result in this condition, as it starves one leg of the delta primary, leading to an unbalanced magnetic field and uneven secondary voltages.