If the source impedance increases, what happens to the short-circuit current at the bus?

• A. The short-circuit current decreases.
• B. The short-circuit current increases.
• C. The short-circuit current remains unchanged.
• D. The short-circuit current becomes negative.

Answer: (A)

When a fault occurs at a bus, the current that can flow is limited by the source’s Thevenin impedance seen from the fault location. The fault current magnitude is basically the available voltage from the source divided by that impedance. So, if the source impedance increases, the current that can flow during the short decreases.

In simple terms, the stronger (lower impedance) the source, the larger the fault current; the weaker (higher impedance) the source, the smaller the fault current. This also means that as impedance grows, the breaker and equipment have a smaller current to interrupt, which changes protective coordination and ratings. A fault current becoming negative isn’t a typical outcome for a standard bus short, and the current wouldn’t stay exactly the same or increase with higher impedance.