Discussion
In which type of fault maximum short-circuit current occurs?
Answer: A
The keyed answer is the three phase fault to ground.
A symmetrical three phase fault short-circuits all three phases together, so the full system voltage drives current through only the positive sequence impedance. In most systems this gives the largest fault current, which is why circuit breakers are rated on the three phase fault level.
In unsymmetrical faults the negative (and for ground faults the zero) sequence impedances come into the circuit and usually limit the current. A single line to ground fault can exceed the three phase value only in special cases, such as at the terminals of a solidly grounded generator.
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