The differences between IEC methodology and ANSI methodology in calculating short-circuit currents are:
1. DC decay component. ANSI requires 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 calcuate 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 methodology is impedance based.
2. DC decay component for parallel or meshed paths. Both IEC and ANSI recognize that the transient solution to the short-circuit current calculation must be uniquely accomplished when parallel or meshed paths are involved.
IEC use completely different procedures for calculating the dc decay current component when meshed or parallel paths are involved compared to ANSI.
3. AC decrement component. ANSI method globally adjusts the machine subtransient 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 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.
If an equipment is rated in accordance with IEC Standards, then the IEC methodology must be used to calculate the fault duties; and when equipment is rated in accordance with the ANSI Standards, then the ANSI methodology must be used to calculate the fault duties.
Source: SKM Power Tools for Windows