In a three-phase system, which voltage references may be used for short-circuit calculations?

In a three-phase system, which voltage references may be used for short-circuit calculations?

• A. Line-to-line voltages are used for all faults.
• B. Line-to-neutral voltages are used for all faults.
• C. Line-to-line voltages are used for some faults and line-to-neutral for others.
• D. Either line-to-line or line-to-neutral values may be used, depending on fault type.

Answer: (D)

The key idea is that the voltage you use in a short-circuit calculation is the voltage across the fault itself, which depends on how the fault is connected. If the fault is between two lines (line-to-line), the relevant voltage is the line-to-line voltage. If the fault is between a line and ground (line-to-neutral), the relevant voltage is the line-to-neutral voltage. Because line-to-line and line-to-neutral voltages are simply different ways of expressing the same three-phase system, you can use either reference as long as you match it to the fault type. This is why you may apply line-to-line values for line-to-line faults and line-to-neutral values for line-to-ground faults, and, when appropriate for the fault being analyzed, you can use either reference consistently. The important point is: choose the voltage reference that directly corresponds to the nodes involved in the fault.

The key idea is that the voltage you use in a short-circuit calculation is the voltage across the fault itself, which depends on how the fault is connected. If the fault is between two lines (line-to-line), the relevant voltage is the line-to-line voltage. If the fault is between a line and ground (line-to-neutral), the relevant voltage is the line-to-neutral voltage. Because line-to-line and line-to-neutral voltages are simply different ways of expressing the same three-phase system, you can use either reference as long as you match it to the fault type. This is why you may apply line-to-line values for line-to-line faults and line-to-neutral values for line-to-ground faults, and, when appropriate for the fault being analyzed, you can use either reference consistently. The important point is: choose the voltage reference that directly corresponds to the nodes involved in the fault.