Question

How did your professor explain it?

Read this: A Practical Guide to Short-Circuit Calculations by Conrad St. Pierre, ISBN:0-9717506-0-2

In IEC 600909-1 there are these explanations:

2 Factors used in IEC 60909-0

Art. 2.1.2:

"IEC 60909-0 therefore recommends a calculation method with the equivalent voltage source cUn / 3 at the short-circuit location. This method, described in IEC 60909-0, is an approximation method without special conditions of operation. The aim of this standard is to find the maximum short-circuit currents with sufficient accuracy, mainly taking into account safety aspects and as far as possible economical aspects."

Art. 2.1 Voltage factor c for the equivalent voltage source at the short-circuit:

2.1.1 General The magnitude of a short-circuit current:

"The introduction of a voltage factor c is necessary for various reasons (IEC 60909-0, 1.3.15). These are:

-voltage variation depending on time and place;

- changing of transformer taps;

- neglecting loads and capacitances by calculating according to IEC 60909-0 (see 2.3.1);

- the subtransient behaviour of generators, power-station units and motors."

In LV systems some of these "reasons" are less determinant conditions.