Which fault is typically a symmetrical three-phase fault that affects all three phases?

Which fault is typically a symmetrical three-phase fault that affects all three phases?

• A. LLL: Three-phase fault
• B. LG: Single-phase-to-ground
• C. LLG: Two-phase-to-ground
• D. L-L-ground

Answer: (A)

Symmetrical faults are those where all three phases are involved in the same way. The classic example is a solid short that ties all three phase conductors together with no path to ground. In this situation, each phase carries a very large current and the phase voltages remain balanced (roughly 120 degrees apart), so the disturbance looks the same in every phase. This uniform behavior lets the positive- and negative-sequence parts of the system interact in a straightforward, symmetrical way, which is why this type of fault is used as the standard example of a symmetrical fault. Other fault types involve only one or two phases and often include a ground path, which makes the currents and voltages—and thus the faulted conditions—unbalanced. That’s why they’re not symmetrical across all three phases.

Symmetrical faults are those where all three phases are involved in the same way. The classic example is a solid short that ties all three phase conductors together with no path to ground. In this situation, each phase carries a very large current and the phase voltages remain balanced (roughly 120 degrees apart), so the disturbance looks the same in every phase. This uniform behavior lets the positive- and negative-sequence parts of the system interact in a straightforward, symmetrical way, which is why this type of fault is used as the standard example of a symmetrical fault.

Other fault types involve only one or two phases and often include a ground path, which makes the currents and voltages—and thus the faulted conditions—unbalanced. That’s why they’re not symmetrical across all three phases.