During a fault with a short circuit, the current drawn from the source is best described as which of the following?

• A. Normal operating current
• B. Very high current limited only by source impedance
• C. Zero current
• D. Steady current equal to load current

Answer: (B)

When a short circuit occurs, the path for current becomes very low in impedance. The source then pushes as much current as its own internal impedance allows, rather than the current determined by the normal load. In other words, the fault current is set mainly by the source impedance (and any impedance in the fault path), so the current becomes very large: approximately I_fault ≈ V_source / Z_source. This is why protective devices must quickly interrupt this high fault current.

That’s why the best description is a very high current limited only by the source impedance. It isn’t normal operating current, it isn’t zero, and it isn’t simply the load current, because the short bypasses the load and drives a large current limited only by the source’s own impedance.