Short-Circuit Fault Current Estimator (simplified) mechanism

Model scope

The honest-label limitation is appropriate, but the instrument needs explicit phase-system, voltage-reference, impedance-reference, fault-type, and impedance-unit definitions. The formula should be corrected before release to use percent impedance divided by 100 and to include the optional cable impedance.

• The transformer formula Isc=Irated/(%Z) is incorrect when %Z is entered as a percentage: 5% must be converted to 0.05, so the formula is Isc=Irated/(%Z/100).
• Isc=V/Zsource is only valid for a consistently defined single-phase equivalent circuit. For a three-phase bolted fault using line-to-line RMS voltage and per-phase impedance, the standard formula is Isc=VLL/(sqrt(3)×Zsource).
• The optional added cable impedance is not included in the stated formula. It must be combined with the transformer equivalent impedance, with both impedances expressed on the same per-phase base and in compatible units.
• The specification does not define whether the system is single-phase or three-phase, whether voltage is line-to-line or line-to-neutral, or whether impedance is per phase. These choices change the result.
• A complete symmetrical-fault calculation normally requires the relevant sequence/source impedance and fault type; the simplified formula should explicitly be limited to a bolted three-phase fault using an equivalent positive-sequence impedance.

Units and conventions

• Test vectors assume a balanced three-phase system, line-to-line RMS secondary voltage, and a bolted three-phase fault.
• Transformer rated current is Irated=kVA×1000/(sqrt(3)×VLL).
• Transformer base impedance is Zbase=VLL²/(kVA×1000) ohms per phase, and transformer impedance is Ztr=(%Z/100)×Zbase.
• Added cable impedance is assumed to be ohms per phase at the fault location and is added arithmetically to the transformer impedance; this assumes purely resistive or already-magnitude-combined impedance.
• Outputs are RMS symmetrical current in amperes, not peak current, asymmetrical peak current, or arc-flash incident-energy current.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "transformer_kVA": 100,
    "percent_impedance": 5,
    "secondary_voltage_VLL_V": 480,
    "added_cable_impedance_ohm_per_phase": 0
  },
  "expected_outputs": {
    "rated_current_A": 120.28,
    "transformer_impedance_ohm_per_phase": 0.1152,
    "total_source_impedance_ohm_per_phase": 0.1152,
    "available_symmetrical_fault_current_A": 2405.7
  },
  "working": "Zbase=480²/(100×1000)=230400/100000=2.304 ohm. Ztr=(5/100)×2.304=0.05×2.304=0.1152 ohm. Zsource=0.1152+0=0.1152 ohm. Irated=100000/(1.73205×480)=100000/831.384=120.28 A. Isc=480/(1.73205×0.1152)=480/0.199526=2405.7 A."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

A zero-percent-impedance input causes division by zero and must be rejected. The convention probe also exposes a large result discrepancy: 7216.87836487032 A for a three-phase 480 V line-to-line interpretation versus 12500 A for a single-phase interpretation.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json