Short Circuit Current Results

Transformer Data

Full Load Current: 0 A

Transformer Impedance: 0%

Short Circuit Current

Available SCC: 0 A

Asymmetrical SCC (with X/R): 0 A

Compliance Check

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Calculation Details

Calculation details will appear here after performing calculations.

Short Circuit Current Decay

This graph shows the theoretical decay of short circuit current over time, considering the system X/R ratio.

Short Circuit Analysis Guide

Short circuit current (SCC) is the current that flows through an electrical circuit during a fault condition when the impedance is drastically reduced. It's crucial for:

  • Selecting properly rated protective devices (circuit breakers, fuses)
  • Ensuring equipment can withstand fault conditions
  • Maintaining system safety and reliability

The magnitude of short circuit current depends on:

  • System voltage
  • Transformer characteristics (kVA rating, % impedance)
  • Conductor impedance between source and fault
  • System X/R ratio (affects asymmetry)

Transformer impedance (%Z) is a key parameter in short circuit calculations:

  • It represents the percentage of rated voltage required to produce full-load current when the secondary is short-circuited
  • Typical values range from 2% to 10% for power transformers
  • Lower %Z means higher available fault current
  • The %Z value is usually marked on the transformer nameplate

The basic formula for short circuit current at transformer secondary is:

Isc = Ifl / (%Z / 100)

Where:

  • Isc = Short circuit current
  • Ifl = Full load current
  • %Z = Transformer impedance percentage

The X/R ratio (reactance to resistance ratio) affects the asymmetry of short circuit current:

  • Higher X/R ratios result in greater DC offset
  • This creates higher peak currents during the first few cycles
  • The asymmetrical current can be significantly higher than the symmetrical RMS value
  • Equipment must be rated to withstand these peak currents

The asymmetrical current multiplier can be calculated as:

Multiplier = √2 × [1 + e-(2π)/(X/R)]

Where:

  • X/R = Reactance to resistance ratio
  • e = Base of natural logarithm (~2.71828)
  • π = Pi (~3.14159)

Safety Standards Reference

NEC (National Electrical Code)
  • Article 110.9 - Equipment must have an interrupting rating sufficient for the available fault current
  • Article 110.10 - Circuit protective devices must be coordinated with equipment short-circuit current ratings
  • Article 240 - Overcurrent protection requirements
IEC (International Electrotechnical Commission)
  • IEC 60909 - Short-circuit currents in three-phase AC systems
  • IEC 62271 - High-voltage switchgear and controlgear standards
  • IEC 61439 - Low-voltage switchgear and controlgear assemblies
Typical Equipment Ratings
Equipment Type Typical SCC Rating Range
Residential Circuit Breakers 10kA - 22kA
Commercial Circuit Breakers 18kA - 65kA
Industrial Circuit Breakers 42kA - 200kA
Fuses 10kA - 300kA