ANSI/IEEE vs IEC Short Circuit Calculations Methodology

The differences between IEC methodology and ANSI/IEEE methodology in calculating short-circuit currents are:

1. DC Decay Component. ANSI/IEEE requires the calculation of a single Thevenin equivalent fault point X/R ratio, based on separately derived R and X values at the fault point. The X/R ratio is used to calculate a single equivalent dc decay for multiple sources at the fault location.

IEC methodology uses a unique R/X ratio, calculated from the complex form of the R and X values at the fault location for each contribution, and uses this unique ratio for calculating the asymmetrical fault currents from each machine to the fault point.

IEC methodology is CURRENT based, while the ANSI/IEEE methodology is IMPEDANCE based.

2. DC Decay Component for Parallel or Meshed paths. Both IEC and ANSI/IEEE recognize that the transient solution to the short-circuit current calculation must be uniquely accomplished when parallel or meshed paths are involved.

IEC uses completely different procedures for calculating the dc decay current component when meshed or parallel paths are involved compared to ANSI/IEEE.

3. AC Decrement Component. ANSI method globally adjusts the machine sub-transient impedances when considering different moments of time during the fault.

IEC method modifies the prospective short-circuit currents available from each machine based on the transfer impedance between the active source and the specific fault location in question and the defined contact breaking time.

IEC methodology is more computationally intensive than the ANSI/IEEE methodology.

The results from IEC and ANSI calculations cannot be directly compared. While both calculate a withstand duty, the IEC and ANSI methodologies are fundamentally different.