What is Short Circuit Analysis? What does it do, how is it done and why is it necessary?

Short circuit analysis is the process of calculating the short circuit current that may occur at a certain point when a fault occurs in an electrical installation. Briefly, the answer to the question of what is short circuit analysis is; It is an engineering study that evaluates whether the equipment can withstand this stress by calculating the possible fault currents in the system in advance. This analysis is not just a numerical calculation; It is a critical system review that forms the basis of facility safety, equipment selection and protection approach.

Security is at the center of the question of what short circuit analysis is for. When a short circuit occurs in a system, the current may exceed normal operating values. This situation creates serious thermal and dynamic stresses on breakers, busbars, cables, transformers, current transformers and switchgear equipment. If these equipment are not selected according to the fault current that may occur, the fault will not only be limited to a power cut; Consequences such as equipment disintegration, fire, arcing risk and personnel hazard may occur.

For this reason, short circuit analysis is not only for theoretical calculations; It is done to choose the right equipment. The short-circuit breaking capacity of a breaker, the short-time withstand current of a busbar, the thermal resistance of a cable or the mechanical strength of a cell can only be evaluated correctly if the expected fault current is known. Equipment selection without knowing the short circuit current can turn into one of the riskiest weaknesses of the design.

To explain the question of how to perform short circuit analysis simply, the source side of the system is determined first. Grid supply, transformer impedance, generator contribution, cable and busbar impedance, motor contributions and the entire electrical path to the fault point are taken into account. The short circuit current is then calculated based on the equivalent impedance for certain fault types. That is, the analysis reveals how much current all sources in the system can carry to the fault point.

The most well-known situation in short circuit analysis is the three-phase short circuit calculation. Three-phase bolted fault is often considered the heaviest and highest current producing fault scenario. For this reason, it is one of the first values ​​to be considered in terms of equipment durability and breaker selection. However, short circuit analysis is not limited to three-phase faults only. Phase-phase, phase-earth and, in some applications, double phase-earth faults are also evaluated. Because each type of fault can have a different effect on the system.

When talking about short circuit current, it is often not enough to talk about a single number. When the fault first occurs, an asymmetric current may occur in the system due to DC offset, and the peak value of this current is different from the symmetric RMS value seen later. Therefore, for some equipment, not only the symmetric short circuit value but also the asymmetric or peak current effect is important. This distinction creates a serious engineering difference, especially in medium voltage and high strain applications.

X/R ratio is one of the important parameters of short circuit analysis. Because the asymmetric behavior and peak value of the fault current is directly related to the resistance-reactance relationship of the system. In systems with a high X/R ratio, the DC component current seen at first may last longer and the mechanical impact effect on the equipment may be higher. Therefore, not only the question of how many kA is output, but also the waveform in which this current is formed is important.

Transformer impedance is one of the main determinants in short circuit calculation. As the percentage impedance of a transformer increases, the maximum short circuit current that can occur on the secondary side is limited to a certain extent. Therefore, the short circuit behavior of two transformers of the same power may differ depending on the impedance value. In short circuit analysis, impedance percentage as well as transformer power is used as critical data.

Engine contribution is also at a level that cannot be neglected in some systems. Especially in industrial facilities with large motors, in case of a fault, the motors can contribute to the system for a short time and increase the fault current level. For this reason, it may not always be enough to take only the grid and transformer side into account. In large industrial facilities, analysis without considering the engine contribution may be incomplete.

Short circuit analysis and protection coordination are directly related. When making relay and breaker settings, the maximum and minimum fault currents that may occur in the system should be known. Otherwise, the protection device may not detect the fault quickly enough or unnecessary trips may occur. Therefore, short circuit analysis is like the preliminary step of relay coordination. It is difficult to make the right protection choice without knowing the fault current.

Short circuit analysis also plays a critical role in facility expansion or revision works. Adding a new transformer to the existing system, connecting a generator, increasing engine power or parallel feeding may increase the current fault level. This may cause the cutters or cells that previously seemed suitable to no longer be sufficient. Therefore, when the system changes, the short circuit analysis should also be updated.

A short circuit analysis is not done only on a single panel; It is usually done for different points of the system. Main distribution point, transformer secondary, sub-panels, motor control centers, medium voltage cells and critical load points can be examined separately. Because the fault current varies throughout the system due to the distance to the fault point and the impedance in between. It may be higher near the source and lower downstream.

Short circuit analysis and short circuit resistance label are not the same thing. The analysis calculates the actual level of failure that may occur in the system. The equipment label tells you how much current the device can withstand or interrupt. The correct approach in engineering is to ensure that the calculated fault current does not exceed the endurance or breaking capacity of the equipment. In other words, analysis and equipment data gain meaning together.

In summary, short circuit analysis; It is a basic engineering study that verifies system safety, equipment selection and protection structure by calculating possible fault currents in an electrical facility. Three phase, phase-phase and phase-ground faults; symmetric and asymmetric currents; Variables such as transformer impedance, cable-bus impedance and motor contribution are the main parts of this study. A correctly performed short circuit analysis is necessary not only to complete the project, but to make the facility truly safe. If short circuit analysis, relay coordination, equipment suitability and MV/HV field safety will be evaluated together in your facility LV/MV/HV project design and consultancy with HV/MV testing, maintenance and repair studies can support this process technically.