What Is Short-Circuit Analysis? What Does It Do, How Is It Performed

Short-circuit analysis is the calculation of the short-circuit current that may occur at a specific point when a fault occurs in an electrical installation. In short, the answer to the question of what short-circuit analysis is: it is an engineering study that calculates possible fault currents in advance and evaluates whether equipment can withstand this stress. This analysis is not only a numerical calculation; it is a critical system review that forms the basis of facility safety, equipment selection and the protection approach. For related context, see What Is an OLTC? What Does It Do, How Does It Work and For What Purpose Is It Used?.

Safety is at the center of the question of what short-circuit analysis does. When a short circuit occurs in a system, current may rise far above normal operating values. This creates serious thermal and dynamic stresses on circuit breakers, busbars, cables, transformers, current transformers and switchgear. If this equipment has not been selected according to the fault current that may occur, the fault is not limited to a power outage; it may lead to equipment rupture, fire, arc risk and personnel danger. For related context, see What Is Power Factor Correction? What Does It Do, How Does It Work and Why Is It Necessary?.

For this reason, short-circuit analysis is performed not only for theoretical calculation but also for correct equipment selection. The short-circuit breaking capacity of a circuit breaker, the short-time withstand current of a busbar, the thermal withstand of a cable or the mechanical strength of an MV cubicle can be evaluated correctly only if the expected fault current is known. Equipment selection made without knowing the short-circuit current may become one of the riskiest weaknesses of the design. For related context, see What Is a Power Quality Analyzer? What Does It Do, How Does It Work and What Does It Measure?.

To explain simply how short-circuit analysis is performed, the source side of the system is first identified. Utility supply, transformer impedance, generator contribution, cable and busbar impedance, motor contributions and the entire electrical path up to the fault point are considered. Then, short-circuit current is calculated for specific fault types through equivalent impedance. In other words, the analysis reveals how much current all sources in the system can feed into the fault point. For related context, see What Tests and Maintenance Are Required for Power Quality Analyzers?.

The most widely known case in short-circuit analysis is the three-phase short-circuit calculation. A three-phase bolted fault is often accepted as the most severe fault scenario that produces the highest current. Therefore, it is one of the first values checked in terms of equipment withstand and circuit breaker selection. However, short-circuit analysis is not limited only to three-phase faults. Phase-to-phase, phase-to-earth and, in some applications, double phase-to-earth faults are also evaluated separately. Because each fault type may create a different effect on the system.

When short-circuit current is mentioned, speaking of a single number is often not enough. When the fault first occurs, an asymmetrical current may appear in the system due to DC offset, and the peak value of this current is different from the symmetrical RMS value seen later. Therefore, for some equipment, not only the symmetrical short-circuit value but also the asymmetrical or peak current effect is important. Especially in medium-voltage and high-stress applications, this distinction creates a serious engineering difference.

The X/R ratio is one of the important parameters of short-circuit analysis. Because the asymmetrical behavior and peak value of the fault current are directly related to the system's resistance-reactance relationship. In systems with a high X/R ratio, the initial DC-offset current may last longer and the mechanical impact on equipment may be higher. Therefore, not only the question of how many kA occurred but also the waveform behavior of that current is important.

Transformer impedance is one of the main determining factors in short-circuit calculation. As the percentage impedance of a transformer increases, the maximum short-circuit current that may occur on the secondary side is limited to a certain extent. Therefore, the short-circuit behavior of two transformers with the same power rating may differ according to their impedance value. In short-circuit analysis, the transformer power rating and the impedance percentage are both used as critical data.

Motor contribution may also be at a non-negligible level in some systems. Especially in industrial facilities with large motors, motors may contribute to the system for a short time during a fault and increase the fault current level. Therefore, considering only the utility and transformer side may not always be sufficient. In large industrial facilities, analysis performed without considering motor contribution may remain incomplete.

Short-circuit analysis is directly related to protection coordination. Maximum and minimum fault currents that may occur in the system must be known when setting relays and circuit breakers. Otherwise, the protection device may not detect the fault fast enough or nuisance trips may occur. Therefore, short-circuit analysis is like the preliminary step of relay coordination. It is difficult to make the correct protection selection 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 motor power or using parallel supply may increase the existing fault level. This may cause circuit breakers or MV cubicles that were previously considered suitable to become inadequate. Therefore, when the system changes, the short-circuit analysis should also be updated.

A short-circuit analysis is not performed only in a single panel; it is generally performed for different points of the system. The main distribution point, transformer secondary, sub-panels, motor control centers, medium-voltage switchgear and critical load points can be examined separately. Because fault current changes throughout the system depending on the distance to the fault point and the impedance in between. It may be higher at points close to the source and lower downstream.

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

In summary, short-circuit analysis is a fundamental engineering study that verifies system safety, equipment selection and the protection structure by calculating possible fault currents in an electrical installation. Three-phase, phase-to-phase and phase-to-earth faults; symmetrical and asymmetrical currents; variables such as transformer impedance, cable-busbar 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 genuinely 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 and HV/MV testing, maintenance and repair works can technically support this process.