Short-Circuit Analysis and Relay Selectivity Studies

Short-circuit analysis and relay selectivity studies are among the fundamental engineering studies for safe operation, equipment protection and energy continuity in electrical installations. Especially in industrial facilities, production lines, hospitals, data centers, shopping centers, hotels and large commercial buildings, it is difficult to ensure healthy operational safety without technically verifying which protection device will trip in which order during a fault. For related context, see Electrical Engineering Service and Technical Consultancy.

What is short-circuit analysis Short-circuit analysis is the process of calculating expected fault currents in fault scenarios such as three-phase short circuit, phase-to-ground, phase-to-phase and two-phase-to-ground faults that may occur in an electrical system. These calculations are made by taking transformer rating and impedance, grid short-circuit power, cable lengths and cross-sections, motor contributions, generator/SPP contributions and system topology into account. For related context, see Periodic Inspection and Periodic Examination Service.

What is relay selectivity study Relay selectivity (protection coordination) study is the setting and coordination work that ensures the protection device closest to the fault trips first in the event of a fault, while upstream protection devices operate only when necessary. The aim is to isolate only the faulty zone without de-energizing the entire facility. For related context, see High Voltage Installations Engineering and Consultancy.

Why are these studies important If the short-circuit level is calculated incorrectly, the selected circuit breaker or protection device may be insufficient during a fault. If selectivity is not performed correctly, the main circuit breaker may trip because of a small fault and the entire facility may be de-energized. This can lead to production loss, equipment damage, occupational safety risks and unplanned downtime costs. For related context, see Electrical Contracting Services.

Within the scope of the service, first the single-line diagram, transformer data, MV/LV panel data, cable lists, protection device types, existing relay brand-model information and existing setting values, if available, are collected. Field data are compared with project documents; if there is missing or outdated information, it is revised for model accuracy.

During the modelling stage, grid supply, transformers, generators, motor loads, cables, busbars and protection devices are represented in the software environment. Operating scenarios such as normal operation, generator supply, ring opening/closing condition and parallel transformer operation are evaluated separately, and fault currents are calculated for each scenario.

The outputs of short-circuit analysis are not limited to giving a fault-current value. The breaking capacity, thermal-dynamic withstand adequacy, busbar/cable stresses and suitability of the measuring ranges of protection devices are also checked. In this way, it becomes clear whether the equipment selection is sufficient from an engineering perspective.

In relay selectivity studies, protection relays, fuses, MCCB/ACB electronic trip units and, when necessary, motor protection devices are evaluated together. Tripping thresholds and delays are coordinated on time-current curves (TCC); overcurrent, short-circuit, earth fault and directional protection functions are optimized according to the application.

When carrying out a selectivity study, not only theoretical coordination but also operational priorities are taken into account. For example, continuity is prioritized on feeders supplying critical loads, while special protection approaches may be required for fire pumps, process safety loads, UPS supply paths or critical production lines. Therefore, the study is not a standard setting-copying task; it is an engineering optimization according to the facility's operating scenarios.

In new facility installations, short-circuit and selectivity studies are critical for correct equipment selection. In revision projects, after transformer capacity increase, panel replacement, new machine addition, generator integration or SPP connection, it must be checked whether the existing protection levels have been disrupted. Existing settings may become insufficient or overly sensitive after a system change.

At the reporting stage, the assumptions and acceptances used, system model, short-circuit calculation results, equipment suitability assessment, coordination curves, relay setting recommendations and revision items to be implemented are presented clearly. If requested, the 'existing condition' and 'recommended condition' can be provided comparatively so that investment priorities can be determined.

Field implementation and commissioning support are also important parts of this service. To ensure that relay settings do not remain only in the report, coordination is provided with field teams, relay parameter entries are checked, a primary/secondary test plan is created if necessary, and the expected tripping behavior of protection functions is verified. In this way, the analysis study turns into a protection system that works in practice.

Short-circuit analysis and relay selectivity studies make a major difference not only in MV systems but also in LV main distribution panels, MCC panels and critical sub-distribution systems. Especially in transformer-fed facilities with high short-circuit power, LV-side protection coordination can be vital to prevent fault propagation and preserve business continuity.

As Pow-Sys Güç Sistemleri, we provide this service through a systematic process consisting of field data collection, single-line verification, software modelling, short-circuit calculations, protection coordination, relay setting optimization, reporting and field implementation support. According to the need, we can carry out the service in an integrated manner with relay tests, power quality measurements, harmonic analyses, thermal inspections and periodic maintenance works.

If you would like to receive detailed information about short-circuit analysis and relay selectivity studies, request a technical assessment for your existing facility or ask for a proposal for your new project, you can contact us. When your single-line diagram and basic equipment list are shared, we can quickly clarify the scope and create an applicable work plan.