Short Circuit Calculation Methods: IEC vs ANSI

Understanding short circuit calculation methods is essential for engineers working in power system design, protection, and fault analysis. Whether you’re designing a substation, verifying protection settings, or conducting a fault study, accurate short circuit analysis ensures system reliability and safety.

Two dominant standards guide these calculations: IEC (International Electrotechnical Commission) and ANSI (American National Standards Institute). Each has unique principles, assumptions, and applications.

Why Short Circuit Calculations Matter

Short circuit faults are unexpected connections between phases or to the ground. These faults result in extremely high currents that can damage equipment and endanger lives. Electrical engineers use short circuit calculation methods to predict fault currents and select appropriate equipment ratings and protection devices.

These calculations are required by:

• Power system studies
• Equipment manufacturers
• Utility standards
• Grid code compliance

By using standardized methods like IEC 60909 or ANSI C37, engineers can ensure consistent and safe system design across the globe.

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Overview of Short Circuit Calculation Methods

Short circuit calculations aim to find:

• Initial symmetrical short circuit current (I”k)
• Peak asymmetrical fault current (Ip)
• Steady-state fault current (Ik)
• Interrupting current for breakers
• Fault MVA at buses

Both IEC and ANSI short circuit calculation methods follow different assumptions and techniques to estimate these values.

IEC Short Circuit Calculation Method (IEC 60909)

The IEC 60909 standard is widely used in Europe, Asia, and other parts of the world. It focuses on calculating fault currents under the assumption that the network is in steady-state operation.

Key Assumptions in IEC Method

• Voltage at the fault location is nominal
• Fault is a symmetrical three-phase fault (for worst-case)
• Internal generator impedances are represented as sub-transient reactance
• Motors can feed into the fault (contributing current)
• No load current is considered during the fault

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Types of Fault Currents in IEC

Equipment Modeling in IEC

• Transformers: Modeled with impedance from test data
• Generators: Modeled using sub-transient, transient, and synchronous reactances
• Lines and cables: Modeled with positive sequence impedance
• Motors: Considered based on rated power and contribution factor

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ANSI Short Circuit Calculation Method (ANSI C37 Series)

The ANSI method, primarily used in North America, is based on a momentary, asymmetrical fault current approach. It considers both the symmetrical and DC offset components of the current.

Key Assumptions in ANSI Method

• Prefault voltage is 1.05 per unit
• Fault is a bolted three-phase short circuit
• Equipment is modeled using ANSI-standard impedance tables
• Loads are generally ignored during faults
• Asymmetry is important in breaker sizing

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Types of Currents in ANSI

Equipment Modeling in ANSI

• Generators: Modeled using ANSI impedance tables
• Transformers: Impedances from nameplate or ANSI data
• Transmission lines: Modeled using X/R ratios
• Motors: Included based on size and system location

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IEC vs ANSI: Key Differences in Short Circuit Calculation Methods

Practical Example: IEC vs ANSI Calculation

Let’s take a simple example to compare both short circuit calculation methods.

Assume:

• 11kV Bus
• Fault at the bus
• Transformer: 10MVA, 11/0.4kV, 6% impedance
• Generator: 20MVA, X”d = 0.2 p.u

IEC Calculation

• Use fault voltage = 11kV
• Calculate Zfault = Transformer Z + Generator Z
• I”k = √3 × V / Zfault

ANSI Calculation

• Use fault voltage = 1.05 × 11kV
• Convert all impedances to a common base
• Apply X/R ratio for asymmetry
• Calculate momentary and interrupting currents

While both methods may yield similar results in terms of RMS symmetrical currents, ANSI will show a higher momentary current due to DC offset. IEC will provide more details on peak and steady-state current values.

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Which Short Circuit Calculation Method Should You Use?

The choice between IEC and ANSI short circuit calculation methods depends on:

1. Geographic Location
IEC is standard in Europe, Asia, and parts of Africa. ANSI dominates in North America.

2. Industry Standards
Utilities or industrial facilities often follow a regional standard. Manufacturers of protective relays and breakers design products based on the applicable method.

3. Design Objective
For peak current and mechanical stress design, IEC is often better. For breaker interrupting rating and coordination, ANSI may be more practical.

4. Software Tools Used
ETAP, DIgSILENT, SKM, and others support both standards. Be sure to select the correct method based on project scope.

Short Circuit Calculation Challenges in Modern Grids

With increasing renewable energy integration, the short circuit behavior of grids is changing. Inverter-based resources (IBRs) contribute very low or controlled fault currents, making traditional short circuit calculation methods less applicable.

IEC has introduced updates (e.g., IEC 60909-0) to account for converter-based generation. ANSI methods are also evolving but often require user adjustments.

Understanding how these modern sources impact fault currents is now an essential part of short circuit studies.

Conclusion: Choosing the Right Short Circuit Calculation Method

Both IEC and ANSI short circuit calculation methods are robust and widely accepted. However, they are based on different philosophies. IEC uses a steady-state approach with more types of fault current defined. ANSI focuses on practical equipment ratings, emphasizing peak and interrupting current values.

Knowing the difference is crucial for:

• Equipment selection
• Protection coordination
• Grid code compliance
• Safety assurance

As electrical networks become more complex, engineers must choose the right method or combine insights from both to ensure accurate and safe system design.

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