Calculating short-circuit currents by hand

You won’t do many short-circuit hand calculations, but it’s useful to go through the exercise to develop an understanding of the fault behavior of typical electrical equipment. There are five you should get to know . . . .

Short-circuit currents are calculated based on a system’s configuration in order to determine which circuit-protection elements and ratings to use. Different types of components factor differently when calculating short-circuit currents. Some components determine the “amount” of short-circuit current “available” during a fault. Some components add to that value. Some components reduce that value.

There are five important components that constitute a majority of the equipment encountered in nuclear plants. Understanding how they behave during a fault and how they are factored into short-circuit evaluations helps you to gain insight into how a system functions. Doing fault analysis by hand essentially consists of constructing a Thevenin equivalent circuit from the point of the fault and calculating a current using Ohm’s Law, but actual circuits get so complex so quickly that short-circuit analysis is mostly done with software.

Hand calculations and rules-of-thumb are conservative, and they are refined when calculated using software and vendor-specific information from datasheets and nameplates. But when designing new branch circuits, they are a good place to start.

The five main components that you should become familiar with are:

  1. utilities,
  2. transformers,
  3. motors,
  4. cables, and
  5. generators.

To read how they are used in a short-circuit hand calculation, click here.