What is Short circuit study

The purpose of a short circuit study is to determine whether or not electrical equipment is rated properly for the maximum available fault current that the equipment may see.  The study is performed using computer software first by modeling the system (conductors, transformers, generators, utility sources, etc.) and then by simulating faults.

The Short Circuit Study:

• Develops a computer model of the electrical system.
• Calculates maximum fault levels at various locations.
• Evaluates the application of breakers and fuses.
• Identifies problem areas in the system.
• Provides recommended solutions.

Why it is required

There are essentially four types of faults: three-phase, single line-to-ground, double line-to-ground, and line-to-line.  Each of these types of faults can result in different magnitudes of fault current.  In all types, however, there is a common element: an abnormally low-impedance path for current to flow.  Such a condition can lead to extremely high currents.

Whenever a fault occurs in an electrical power system, relatively high currents flow, producing large amounts of destructive energy in the forms of heat and magnetic forces. A short circuit study ensures that protective device ratings within a power system are adequate for maximum currents that flow during a fault.

The short circuit study can lesson the risk your company faces and help avoid catastrophic losses. It's about safety & reliability.  Far too often, the electrical system goes through changes without serious consideration of short circuit levels, and equipment ratings.  Even new buildings and installations are not immune.  The results can be expensive at best, disastrous at worst.

A severe fault (short circuit) in your system can have catastrophic consequences.

It's a matter of "when" a fault will occur .

What are short circuit ratings?

It's not uncommon for plant engineers to be unfamiliar, or downright confused, when it comes to understanding short circuit ratings.  If you find yourself in this category, here are some things to consider:  

Circuit breakers and fuses come with an over current rating (or size), and a short circuit interrupting rating.  The over current rating specifies the amount of electrical current the device should tolerate without the fuse blowing, or circuit breaker tripping.  The short circuit rating is the maximum electrical current the device can tolerate before it fails.  

Consider for example, a 1000 amp circuit breaker.  At some value of current exceeding 1000 amps, you'd expect the breaker to trip .  If 1500 amps passes through the breaker for a certain period of time, it should trip.  But what happens if 17,000 amps flows through the device?  Will the breaker trip, or explode?   The answer depends largely on the manufacturer's short circuit interrupting rating for that device.  If the short circuit rating was 20kA it should trip, while a 15kA rated breaker would very likely explode. 

The short circuit study examines this for you.  Fault levels are calculated at points throughout the system, and compared to the short circuit interrupting ratings of the devices used at each location.