Short Circuit Current Calculations – Infinite Bus Method

The short circuit is essentially an abnormal condition within a power system in which a heavy amount of current flows through the circuit.

A short circuit usually occurs as a result of a fault in a power system. The fault may be a conductor breaking and falling to the ground, or two or more electrical conductors coming in contact with each other.

Such faults result in the formation of a low resistance path for the current. This is a short circuit condition.

That is why short circuit calculations are mandatory as well as recommended practice of electrical preventive maintenance according to NFPA. (NFPA 70B chapter 9)

Impact of Short Circuit Current

A short circuit is followed by a flow of extremely high current known as short circuit current. The high magnitude of the short circuit current makes the working environment critically dangerous.

The excessive heat generated from the high current causes the conductors to burn or catch fire. Not only these currents damage equipment like generators, motors, and other electrical appliances but can also burn the motor windings.

Another hazardous effect of short circuits is arc flashes that destroy the equipment and can prove lethal to surrounding people and equipment.

It is, therefore, necessary to perform short circuit calculations in order to be prepared for an unfortunate short circuit event.

Having adequate knowledge of short circuit current helps in determining the protection of a system before an incident occurs.

Infinite Bus Method

Data required:

The infinite bus calculations are performed across a 3-phase transformer in a power system. Therefore, we should have the data on the transformer KVA rating, primary and secondary voltage, and percentage impedance. This data can be easily obtained from the transformer nameplate.

Calculation

This calculation is performed in two simple steps which are as follows:

Step 1:  

Calculate the full load ampere (Current) rating at the transformer secondary.

Where :

Step 2: 

 Calculate the short circuit current on the secondary of the transformer.

Where :

Limitations of Infinite Bus Method:

Infinite bus method, being a simple method of calculation, overlooks some important factors and has its limitations, which are:

• The effect of current contributed by the motor.
• The decrease in current due to source and line impedance. 

Short Circuit Current and Arc Flash Studies:

Short circuit calculations using infinite bus methods are not suitable for arc flash studies.

Infinite bus method gives the worst possible current in the event of a short circuit, therefore a relay or protection system configured using infinite bus method current will trip the circuit in minimum time. 

However, for a smaller value of fault current, the inverse time characteristics of that relay will delay the operation of protection systems (delayed tripping time).

The prolonged time will release greater incident energy in the event of an arc flash at such a current value. 

We, at AllumiaX, have a standard procedure to make use of 2 cases:

1. First, we study using the normal short circuit current given by the infinite bus method.
2. Then a second study is carried out by using depressed currents at 40% to 50% of the worst possible current to cater to the prolonged trip time.  

AllumiaX, LLC. specializes in conducting arc flash studies and complies with the global standards set by OSHA and NFPA. Following the acts suggested above, we provide "Arc flash Reports" along with "Arc Flash Labels" indicating an equipment risk category along with associated PPE clothing required for employees to interact. Moreover, we also provide training on arc flash, educating the employees on how to work with equipment.

We boast a group of talented and skilled engineers who are always ready to serve our client's requirements. We perform various power system studies including Short Circuit Studies, Selectivity and Protective Device Coordination, Load Flow Studies, Snubber Circuit Studies, Transient Stability Studies, and others.