Short-circuit currents are important considerations in power system design, protection, and equipment specifications.
When a fault occurs, such as a
- Line-to-Line Fault or
- Line-to-Ground Fault
there are no simple & uniform waveforms associated with the current.
Instead, it is made up of complicated components that vary with time. These are generically classified as
- Symmetrical short-circuit currents &
- Asymmetrical short-circuit currents,
with different characteristics in electrical engineering applications.
International standards such as
- IEC 60909 and
- IEEE 141/399
define these currents exactly for their accurate short-circuit diagnosis, resulting in safety and effectiveness in the system operation.
Understanding the fundamental difference between these 2 types is essential for:
- Selecting the suitable breaker ratings.
- Performing accurate and reliable relay coordination.
- Defining the resist capability of the cables, busbars & other switchgear.
What is Symmetrical Short-Circuit Current?
Symmetrical short-circuit current (Ik) is the RMS value of the entire alternating current (AC) component after the transient DC component has got dissipated.
Characteristics:
- A pure sinusoidal waveform with the zero axis at its center.
- Begins with the decline of the exponential DC offset, which typically takes three to five cycles.
- Constant in size while the fault condition is steady-state.
Standard:
- Ik (IEC 60909)
- IAC (IEEE)
Applications:
- Used to coordinate relays, particularly following DC filtering.
- Equipment, busbars, and conductors are rated mechanically and thermally.
- Determining the available fault MVA in system studies.
What is Asymmetrical Short-Circuit Current (ASC)?
The asymmetrical short-circuit current consists of a symmetrical AC component and a DC offset caused by the fault initiation angle and system impedance characteristics.
Components:
AC Component: the symmetrical part (Ik).
DC Component: The DC offset is a transient that is non-repetitive and exponentially decreasing. It depends on the following:
- Fault inception angle
- System X/R ratio (reaction/resistance)
Magnitudes:
- RMS asymmetrical current at 1/2 cycle = 1.6 x Ik
- Peak asymmetrical current (Ip) = 2.5-2.7 x Ik.
Application:
- Determines breaker making & breaking capacity.
- During a fault, magnetic forces act on the equipment.
- Specifies the transitory mechanical stress conditions.
Time-Domain Evolution of Fault Current
| Time Period | Current Characteristics |
| At fault inception (t=0) | High DC offset + rising AC → Highly asymmetrical |
| 1–3 cycles | DC offset decays exponentially → AC dominates |
| After 5–6 cycles | Only symmetrical AC remains (steady-state fault) |
The time constant for the DC decay is determined by the system’s L/R ratio, where:
- L – System Inductance
- R – System Resistance
Difference Between Symmetrical & Asymmetrical Fault Currents
Symmetrical vs Asymmetrical Fault Currents
| Aspect | Symmetrical Current (Ik) | Asymmetrical Current (Ip) |
| Nature | Pure AC (RMS value) | AC + transient DC component |
| Occurrence | After DC decay (steady-state) | Immediately after fault inception |
| Waveform | Symmetrical sine wave | Offset sine wave, initially non-symmetric |
| Influencing Factors | System impedance | X/R ratio, fault angle, breaker timing |
| Utilized for | Thermal/mechanical ratings, relay settings | Breaker design (making/breaking), transient stress |
| Typical Magnitude | Base RMS current (ex: 25 kA) | RMS ≈ 1.6 x Ik; Peak ≈ 2.7 x Ik |
| Duration | Continuous during fault | Present only during initial few cycles |
| Standard Reference | IEC 60909: Ik | IEC 60909: Ip (peak); Ik (initial symmetrical RMS) |
It is essential to the safety and dependability of electrical systems to understand the distinction between symmetrical & asymmetrical short-circuit currents; it is not merely academic.
In the case of a malfunction, the asymmetrical current is responsible for determining the peak stress that occurs within milliseconds. On the other end, the symmetrical current is responsible for determining the long-term current that the equipment must consistently experience.
Applications:
- Asymmetrical peaks must be able to be ignored and eliminated by switchgear in order for it to be considered appropriate.
- The implementation of symmetrical RMS heating is the objective of conductors and busbars.
- Protection relays are configured with the help of filtered symmetrical RMS currents during the configuration procedure.
Given the guidelines made by the IEEE and the International Electrotechnical Commission (IEC) 60909, it is therefore necessary to conduct a comprehensive evaluation of both elements in order to design the substation, conduct protection studies, and choose the equipment.