If a transformer coil short-circuits, the resistance between the coils becomes __________.

• A. Very low
• B. High
• C. Finite
• D. Zero

Answer: (D)

Transformers keep the primary and secondary windings electrically isolated under normal conditions. When a coil shorts, that insulation fault creates a direct conductive path that effectively ties the winding ends together. Because of that closed, low-resistance path, the resistance you measure between the two windings collapses to essentially a direct short. In other words, the loop between the coils becomes a near-zero resistance path, aside from tiny contact or conductor resistances. So the expected reading is essentially zero ohms. In real equipment you might see a small nonzero value due to connections, but functionally it behaves as a short.