IEC 60909 vs ANSI/IEEE C37: Two Methods for Short-Circuit Calculation Compared

If you calculate the short-circuit current at an 11 kV bus using IEC 60909 and then recalculate the same bus using ANSI/IEEE C37, you will get different numbers. Not wildly different — typically 5-15% — but enough to change equipment ratings, protection settings, and project costs.

These are not competing methods trying to solve different problems. They are two parallel methodologies developed by different standards bodies to answer the same question: what is the maximum fault current that can flow at a given point in a power system? The differences arise from how each method handles pre-fault voltage, impedance corrections, motor contributions, and DC offset decay.

Understanding these differences is essential for any engineer who works on international projects, or who needs to reconcile calculations from different software packages that may use different methodologies without making it obvious.

The Fundamental Methodological Difference

Both methods use Thevenin's theorem to reduce the power system to an equivalent voltage source behind an equivalent impedance at the fault point. The fault current is then V/Z. But they differ in how they define V and how they modify Z.

IEC 60909-0: Voltage Factor Approach

IEC 60909 does not use the actual pre-fault voltage at the fault point. Instead, it applies a voltage factor c to the nominal voltage:

Equivalent voltage source = c x Un / sqrt(3)

IEC 60909-0, Clause 5, Table 1 — Voltage factor c

The voltage factor accounts for voltage variations, transformer tap positions, and the subtransient behaviour of generators and motors — all in a single multiplier. This simplifies the calculation because you do not need to know the actual operating voltage; you apply the factor to the nominal voltage and get a conservative result.

ANSI/IEEE C37: Pre-Fault Voltage Approach

The ANSI/IEEE C37 method uses the actual pre-fault voltage at the fault point (or an assumed pre-fault voltage, typically 1.0 per unit of the bus nominal voltage). There is no voltage factor equivalent.

Instead, ANSI/IEEE C37 applies impedance multiplying factors to different types of machines (generators, synchronous motors, induction motors) to account for how their contribution changes over time. The calculation is performed at three distinct time intervals:

• First-cycle (momentary): 0-0.5 cycles — maximum asymmetrical current for bus bracing and instantaneous relay settings
• Interrupting (contact parting): 1.5-4 cycles — symmetrical current at the time circuit breaker contacts part
• Time-delayed (30 cycles): steady-state — for coordination of time-delayed relays

IEEE C37.010, Table 9 — Multiplying factors for rotating machine reactances

Key Differences: Point by Point

1. Motor Contribution Handling

This is the largest source of divergence between the two methods.

IEC 60909 treats motors as voltage sources behind their subtransient reactance. Induction motors contribute to the initial symmetrical short-circuit current Ik" with a ratio based on their locked-rotor current:

• Large motors (per unit power > 100 kW): included individually
• Small motors: may be combined as an equivalent motor
• Motor contribution decays rapidly — IEC 60909 accounts for this with the factors mu and q

IEC 60909-0, Clause 9 — Short-circuit currents of asynchronous motors

ANSI/IEEE C37 separates motor contributions by type and applies different multiplying factors at each time interval:

The ANSI method increases the effective impedance (reduces contribution) of motors at the interrupting time because motor contribution decays significantly by the time breaker contacts part. IEC 60909 handles this decay through the breaking current factor mu.

2. DC Component Treatment

Both methods recognise that the DC offset in the fault current depends on the X/R ratio at the fault point, but they handle it differently.

IEC 60909 calculates the peak short-circuit current ip as:

ip = kappa x sqrt(2) x Ik"

where kappa is a function of R/X ratio (IEC 60909-0, Clause 8.1). For a typical industrial system with X/R = 15, kappa is approximately 1.8, giving a peak factor of approximately 2.55.

ANSI/IEEE C37 calculates the asymmetrical current using a multiplying factor based on X/R ratio applied to the symmetrical (RMS) current. The first-cycle asymmetrical current is used for bus bracing and momentary ratings. For X/R = 15, the asymmetry multiplying factor is approximately 1.55 (applied to RMS symmetrical).

IEC 60909-0, Clause 8.1, Figure 9 — Factor kappa for peak short-circuit current

3. Impedance Correction Factors

IEC 60909 applies correction factors to transformer and generator impedances to account for operating conditions:

• Transformer correction factor KT = 0.95 x c_max / (1 + 0.6 x xT) where xT is transformer reactance in p.u.
• Generator correction factor KG accounts for subtransient reactance and voltage regulation

IEC 60909-0, Clause 6.3 — Impedance correction factors for transformers

ANSI/IEEE C37 does not apply these correction factors. Transformer impedances are used at their nameplate values. The pre-fault voltage assumption and the machine multiplying factors are considered sufficient to cover the same variability.

Worked Example: Same System, Both Methods

System: 11 kV bus, supplied by a 40 MVA, 33/11 kV transformer (Z = 10%), grid fault level 500 MVA at 33 kV. Motor load: 5 MW of induction motors (average X" = 0.17 p.u.).

Base Values

• Base MVA: 40 MVA
• Base voltage: 11 kV
• Base impedance: 11^2 / 40 = 3.025 ohms
• Base current: 40 / (sqrt(3) x 11) = 2.099 kA

Grid Contribution (Both Methods)

Grid impedance at 11 kV: Z_grid = (11^2 / 500) = 0.242 ohms = 0.080 p.u.

Transformer impedance: Z_T = 0.10 p.u.

Total source impedance: Z_source = 0.080 + 0.10 = 0.180 p.u.

Motor Contribution

Motor MVA (at X" = 0.17): S_motor = 5 / 0.85 = 5.88 MVA (assuming 0.85 pf)

Motor impedance: Z_motor = 0.17 x (40/5.88) = 1.156 p.u.

IEC 60909 Result

Voltage factor c_max = 1.10

Ik" (source) = (1.10 x 1.0) / 0.180 = 6.11 p.u. = 12.83 kA

Apply transformer correction factor: KT = 0.95 x 1.10 / (1 + 0.6 x 0.10) = 0.985

Corrected source impedance: 0.180 x 0.985 = 0.177 p.u.

Ik" (source, corrected) = 1.10 / 0.177 = 6.21 p.u. = 13.04 kA

Ik" (motors) = 1.10 / 1.156 = 0.951 p.u. = 2.00 kA

Total Ik" = 13.04 + 2.00 = 15.04 kA

ANSI/IEEE C37 Result (First-Cycle)

Pre-fault voltage: 1.0 p.u.

I_sym (source) = 1.0 / 0.180 = 5.56 p.u. = 11.67 kA

I_sym (motors) = 1.0 / 1.156 = 0.865 p.u. = 1.82 kA

Total first-cycle symmetrical = 11.67 + 1.82 = 13.49 kA

ANSI/IEEE C37 Result (Interrupting, 5-Cycle Breaker)

Motor impedance multiplied by 1.5 for large induction motors:

I_sym (motors, interrupting) = 1.0 / (1.156 x 1.5) = 0.577 p.u. = 1.21 kA

Total interrupting symmetrical = 11.67 + 1.21 = 12.88 kA

Results Comparison

Why the Results Diverge

The differences are not errors — they are deliberate design choices by each standards body:

1. Voltage factor vs actual voltage: IEC 60909's c = 1.10 adds a 10% margin to account for voltage variations and tap positions. ANSI assumes 1.0 p.u. pre-fault voltage. This single factor accounts for most of the difference.

2. Transformer correction: IEC 60909 slightly reduces transformer impedance (KT < 1.0), which increases fault current. ANSI uses nameplate impedance without correction.

3. Motor treatment: IEC 60909 applies the voltage factor to motor contributions, increasing them. ANSI increases motor impedance at the interrupting time, decreasing them.

4. Philosophy: IEC 60909 aims for a single conservative value (Ik") that covers worst-case conditions. ANSI provides multiple values at different time points, allowing more precise equipment selection.

When to Use Each Method

Use IEC 60909 When:

• The project is in any country outside North America
• Equipment is rated to IEC standards (IEC 62271, IEC 60947)
• The client specification calls for IEC calculations
• You need a single conservative value for equipment specification
• International projects with multiple stakeholders

Use ANSI/IEEE C37 When:

• The project is in the United States, Canada, or countries that adopt ANSI standards
• Equipment is rated to ANSI/IEEE standards (IEEE C37 series)
• The client or utility requires ANSI-basis calculations
• You need time-separated values (first-cycle, interrupting, 30-cycle)
• The project involves US-manufactured switchgear

Practical Recommendations

1. State your basis: every short-circuit study report should clearly state which standard was used for the calculation and which standard the equipment ratings are based on.

2. Do not convert between methods: multiplying an ANSI first-cycle result by 1.1 does not give you an IEC 60909 result. The methodologies are too different for simple conversion factors.

3. Software transparency: when using calculation software, verify which method it implements. Some packages default to IEC 60909, others to ANSI. Some allow selection but do not clearly indicate which is active.

4. Motor contribution sensitivity: for industrial plants with large motor loads (>30% of total fault contribution), the difference between methods can exceed 15%. Pay particular attention to motor modelling in these cases.

The calculation method is not a preference — it is determined by the project location, the equipment standards, and the client specification. Know which one you are using, and make sure everyone on the project team knows too.