What does transformer impedance percentage indicate for fault levels?

• A. Lower percent impedance means higher fault current; higher impedance reduces fault current and voltage drop
• B. Higher percent impedance means higher fault current
• C. Impedance has no impact on fault levels
• D. Impedance only affects efficiency

Answer: (A)

Transformer impedance percentage is a measure of the short-circuit impedance of the transformer, expressed as a percentage of its rated voltage. It tells you how much the transformer itself will oppose a fault current.

When a fault happens, the current that can flow from the source is limited by the total impedance the current must pass through. The transformer's impedance is a key part of that total. If the transformer has a low impedance percentage, there’s less opposition to current flow, so the fault current is high. If the impedance percentage is high, the transformer provides more opposition, which reduces the fault current.

So a lower percent impedance means higher fault current; a higher impedance means the fault current is reduced. This is why transformer impedance is a central factor in fault calculations and protection coordination.