In per-unit impedance calculations, which expression correctly gives Isc (per unit) in terms of the base values?

• A. Isc (pu) = Vphase / Zpu.
• B. Isc (pu) = Zpu / Vbase.
• C. Isc (pu) = Vbase × Zpu.
• D. Isc (pu) = Vbase / Zpu.

Answer: (D)

In a per-unit system, current and impedance are tied to the base values so that circuits scale consistently. For a short-circuit, the actual current drawn is I_actual = V_base / Z_actual. Writing Z_actual in terms of the per-unit impedance gives Z_actual = Z_pu × Z_base, and Z_base = V_base^2 / S_base, while the base current is I_base = S_base / V_base. When you convert to per-unit current, Isc_pu = I_actual / I_base, which simplifies to 1 / Z_pu. Expressing this with the base voltage explicitly, Isc_pu can be written as Isc_pu = V_base / Z_pu, and since V_base in per-unit terms is 1 p.u., this is equivalent to 1 / Z_pu. So the current in per-unit terms is determined by dividing the base voltage by the per-unit impedance.