A(n) ? fault current will be approximately 87% of the 3-phase bolted fault current

• A. Line to line
• B. Line to ground
• C. Three-phase bolted
• D. Open circuit

Answer: (A)

The main idea is how fault type changes the available current path in a short-circuit. A solidly grounded, three-phase (bolted) fault uses all three phases together and taps the full positive-sequence network, giving the largest fault current. A line-to-line fault, however, involves only two phases and does not recruit the zero-sequence path through the ground. When you work through the symmetrical-components model, the combination of the positive and negative sequence networks for a line-to-line fault yields a current magnitude that is about 0.87 of the three-phase fault current—roughly 87%. That’s why line-to-line faults have significantly high but slightly lower current than a full three-phase fault.

Open circuit would mean almost no fault current, and line-to-ground faults typically engage the zero-sequence path and produce a different magnitude than the line-to-line case, not the 0.87 factor.