Table of Contents
Introduction
In a previous article, we discussed the various types of faults encountered in an electrical distribution network. In this article, we focus on one of the most severe types — the short-circuit fault. For the smooth and reliable operation of any electrical distribution system, it is essential to provide adequate protection against abnormal conditions. These abnormal conditions are collectively referred to as faults, and among them, the most critical and potentially damaging is the three-phase short circuit. The intensity of fault is measured in terms of fault level.
To design a safe, reliable and efficient system, engineers must know the fault levels that will occur under different fault scenarios. This knowledge is critical for selecting protective devices, ensuring equipment can withstand fault stresses, and maintaining overall system stability.
A short circuit happens when a low-resistance path is created between conductors carrying current, or between a conductor and ground. The resulting surge of current flows almost instantaneously, with the potential to damage equipment, trip protective devices, or cause fires and explosions. The magnitude of the current is called the fault level.
Accurate short-circuit fault current calculation is therefore a cornerstone of electrical engineering practice, supporting protection system design, operational safety, and compliance with standards such as IEC 60909 and IEEE 551.
Need to Calculate Fault Level
Short-circuit current calculation is fundamental to designing a safe and reliable electrical network. Knowing the prospective fault level at each point in the system enables engineers to make informed decisions and meet regulatory requirements. Key reasons to perform these calculations include:
- Equipment rating selection – ensuring circuit breakers, busbars, and cables can withstand the prospective fault current for the designated time, usually 1 or 3 sec. Making and breaking capacity is also determined based on the short-circuit current.
Tip: Compare the calculated prospective short-circuit current (PSC) with device interrupting ratings and thermal withstand limits to select a suitable device.
- Protection coordination – set relays, fuses and breaker time-current characteristics so they operate selectively (discrimination) and clear faults in the correct order and time.
- System safety – limiting the risk of damage to people and equipment by isolating the fault before it reaches the peak. Accurate fault currents let you size protective device settings and determine required arc-flash mitigation.
- Compliance – meeting requirements of electrical codes and standards governing the system design & safety.
Types of Short Circuit Faults
Below are the different scenarios tabulated for reference:
| Fault Type | Description | Severity |
| Three-phase fault (3Φ) | All three phases shorted together, symmetrical | High |
| Line-to-ground (LG) | One phase shorted to ground | Low |
| Line-to-line (LL) | Any two phases shorted together | Medium |
| Double-line-to-ground (LLG) | Two phases shorted to ground | Medium |
For pictorial representation, see diagrams below:
Key Parameters in Fault Level Calculation
Accurate short-circuit current estimation requires detailed knowledge of the network’s electrical characteristics. The following parameters are essential before starting any calculation:
- System voltage (V) – The nominal operating line-to-line voltage at the fault location, adjusted for voltage correction factors per standards (e.g., ±10% in IEC 60909 for max/min cases).
- Source impedance (Zs) – from generators, transformers, and upstream network, seen from the fault point.
- Transformer parameters – MVA rating, nominal voltage ratio, and percentage impedance (%Z) from the nameplate.
- Generator parameters – MVA rating and subtransient reactance (%Xd″), which governs the initial short-circuit current magnitude.
- Cable impedance – Positive-sequence resistance (R) and reactance (X) per unit length. Cable length must be factored in to obtain the total impedance.
- Motor contribution – Large induction motors and synchronous motors can feed additional fault current back into the system for a short duration after fault inception.
Rule of thumb: Estimate 4–6 × motor full-load current for induction motors, decaying within a few cycles.
Tip: An induction motor feeds fault current immediately after a fault because it spinning rotor still has stored kinetic energy. The stator windings are still linked to the rotating rotor, so the motor’s magnetic field continues to cut the stator conductors. This makes the motor act briefly like a generator, delivering 4–6 times its full-load current into the fault.
The contribution decays quickly—typically within 3–6 cycles—yet can significantly raise the initial short-circuit current, so it must be included in fault level calculations.
Methods of Fault Level Calculation
We will calculate the fault level in the simplest way possible. This will be a good estimate for all practical purposes.
- Key Formula from Ohmic Method
ISC: short‑circuit current (A)
VLL: line-to-line RMS voltage at fault point (V)
ZTotal: total Thevenin impedance up to fault (Ω)— includes upstream source,
ransformer(s), cables
- Simplified Formula
IFL – Full load current
Zpu – Per unit imedence
Short circuit voltage: Voltage required to circulate nominal current.
During the short circuit, there are three stages in the whole phenomenon. First is sub-transient, then it is transient & final stage is steady state. The sub-transient stage is the stage when the current is at peak and can cause maximum damage, and then it decays down in the transient stage and finally settles in the steady state. So we can replace % impedance with % sub transient reactance (Xd”) for calculating short circuit fault current for sources like Transformers and Generators.
Name plate of all the source equipments, be it generators or transformers, carries the value of % sub transient reactance that can be used for the above calculation. The same can also be referred to from the equipment technical data sheet.
To understand the behavior of fault current in the different stages, refer to the diagram below:
Let us decode these stages.
- Sub-transient Period (first few cycles)
- Duration: ~1–3 cycles (0.02–0.06 seconds at 50 Hz)
- Dominated by: Machine’s sub-transient reactance (Xdʺ)
- Current magnitude: Highest possible — can be 5–8× the full-load current for synchronous machines.
- Reason: Both stator and rotor contribute heavily, plus induction motors feed into the fault momentarily.
- Impact: Maximum mechanical and thermal stress on equipment.
- Transient Period
- Duration: ~0.1–0.5 seconds
- Dominated by: Machine’s transient reactance (Xdʹ)
- Current magnitude: Falls from sub-transient value but still above steady-state level.
- Reason: Damper winding effect fades, field winding still contributing.
- Impact: Still high enough to trip protective devices.
- Steady-State Period
- Duration: After ~0.5 seconds until fault cleared
- Dominated by: Machine’s synchronous reactance (Xd)
- Current magnitude: Lowest fault current stage — determined by the network’s % impedance.
- Impact: Sustained heating if the fault is not cleared quickly.
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Steps for Practical Calculation of Fault Level
- The following steps are to be followed:
- Identify system voltage (VLL).
- Get the source or transformer rating (MVA or kVA).
- Note the % impedance of the transformer or upstream source.
- Apply the formula above for ISC.
- If there are multiple impedance elements in series, sum them first before calculation
Example for Fault Level Calculation
To understand it simply, refer to the diagram below:
Consider 1000kVA two-number DG sets with having % impedance of 10%. They are connected as shown in the above single-line diagram to a common bus. Let us calculate the fault level at different points in the above circuit.
Full load current of each DG set IFL = kVA*1000/(1.732*VLL) Amp
= 1000*1000/(1.732*415)
= 1391 Amp
Short circuit fault current of each DG set ISC = IFL/%Z
= 1391/0.10
= 13910 Amp or 13.9kA
For ease of calculation, consider the impedance of bus bars negligible. Fault current at different points will be as follows:
- Fault Currents at different points
- Fault current at point A If both DG sets are running in parallel =ISC1+ISC2 = 13.9+13.9 kA=27.8kA
- Fault current seen by the Circuit Breaker Q1 will be 13.9kA, as only DG2 fault current will pass through it
- Consider the fault at point C. In this case, the bus bar & Circuit Breaker Q will see a fault current of 27.8kA
We can determine the fault current at any point and calculate the fault level of busbars and circuit breakers, making the necessary selections accordingly.
The example presented here has been deliberately simplified for conceptual clarity. In practical, complex network configurations, it is essential to account for the combined effect of various impedance components within the system. For the purpose of illustration, the calculation is based on the most severe fault condition. Software is available to the network analysis and find the level of fault current at various points in the electrical network. In scenarios where the fault location is situated farther from the source, the fault current magnitude will be appreciably lower due to the additional impedance introduced by the intervening cables.
Selection of Electrical Equipment Based on Fault Current
When selecting electrical equipment for a power system, it is essential to ensure that all components can safely withstand and interrupt the maximum possible fault current at their point of installation.
- LV Switchgear (IEC 61947-2)
- Breakers (ACBs/MCCBs/MCBs)
- Rated ultimate breaking capacity Icu ≥ Ik″ at system voltage.
- Rated service breaking capacity Ics: choose ≥ 50–100% of Icu per project policy.
- Making capacity Icm ≥ calculated Ipeak at local X/R.
- Short-time withstand if used as an incomer with a delay): Icw ≥ expected through-fault for the delay time.
- Verify selectivity/energy-let-through with downstream devices from manufacturer tables/curves.
- Fuses (IEC 61269)
- Breaking capacity ≥ Ik″; check I²t let-through vs cable and busbar thermal limits.
- Ensure fuse–breaker coordination where mixed.
- Breakers (ACBs/MCCBs/MCBs)
- MV Switchgear (IEC 62271-100/200)
- Circuit breakers: Rated short-circuit breaking current ≥ Ik″.
- Making current: ≥ 2.5 × rated short-circuit breaking current (50 Hz); also confirm vs Ipeak from X/R.
- Short-time withstand (thermal/mechanical) Ik for 1–3 s ≥ through-fault for relay delay.
- Contactors/fuse-contactors: verify making/breaking classes and back-up fuse ratings.
- Busbars & Busways
- Thermal short-time withstand Ith for clearing time (usually 1–3 s) ≥ through-fault.
- Peak (dynamic) withstand Ip ≥ calculated Ipeak at that location.
- Check temperature rise limits at rated current and enclosure derating.
- Cables & Conductors
- Thermal withstand during faults:
- Verify S ≥ I × √t / k for PE/NE and, where required, phase conductors (S in mm²).
- Confirm installation derating (ambient, grouping, soil resistivity) and check let-through I²t from protective devices.
- For armoured cables acting as CPC/PE, verify return path impedance and bonding.
- Thermal withstand during faults:
- Safety Margin
- Apply a reasonable safety factor, typically 10–25% to account for calculation uncertainties, system expansion, or network changes.
Conclusion
Accurate fault level calculation is essential for designing safe and reliable power system design. By applying these simplified formulas, engineers can quickly estimate the worst-case three-phase short‑circuit current, select suitable protective devices, and ensure equipment ratings meet safety requirements. While detailed software analysis may refine results, these methods provide a solid first-pass estimate that is sufficient for most practical applications. Always verify critical installations with comprehensive studies, but keep this guide handy for quick, on-site decision‑making.
References
Electrical Power Systems by C.L. Wadhwa – Clear explanation of symmetrical components and fault calculations.
Power System Analysis by J.D. Glover, M.S. Sarma, and T.J. Overbye – Covers both per-unit method and symmetrical components for short circuit analysis.
Modern Power System Analysis by D.P. Kothari and I.J. Nagrath – Includes worked-out examples of various fault types.
https://www.se.com/ww/en/tools/npag-online/pdf/A3-Fault_Calculations.pdf
Nice information