Transformer Secondary Fault Duty vs AIC Checker: Which Screening Protects Your System?

Transformer secondary fault duty determines relay coordination and protects secondary circuits from excessive currents rapidly.

An AIC checker screens protective devices ensuring interrupting capacity matches prospective fault currents at equipment.

Transformer Secondary Fault Duty vs AIC Checker (secondary fault current and interrupting rating adequacy)

Transformer rated apparent power (kVA)

Transformer secondary line-to-line voltage (V)

Transformer impedance (%Z)

4 % (typical small LV transformer) 5.75 % (common standard) 6 % 7 % 8 % Custom %Z

Protective device interrupting rating (kA)

5 kA 10 kA 14 kA 18 kA 25 kA 35 kA 42 kA 65 kA 85 kA 100 kA Custom kA

Advanced options

System type Three-phase Single-phase

Motor contribution factor (%)

Design margin factor (per unit)

Asymmetry factor (multiplier)

Optionally upload a nameplate or one-line diagram photo so AI can suggest reasonable input values.

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Enter transformer and device data to evaluate secondary fault duty and interrupting rating adequacy.

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Formulas used

• Secondary full-load current (three-phase): I_FL = (S_kVA × 1000) / (√3 × V_LL) [A]
• Secondary full-load current (single-phase): I_FL = (S_kVA × 1000) / V_LN [A]
• Ideal transformer-limited symmetrical short-circuit current: I_sc,sym = I_FL × (100 / Z%) [A]
• Including motor contribution: I_sc,sym,total = I_sc,sym × (1 + Motor_factor% / 100) [A]
• Estimated asymmetrical initial fault current: I_sc,asym,total = I_sc,sym,total × Asymmetry_factor [A]
• Required symmetrical interrupting rating at secondary: I_req,kA = (I_sc,sym,total × Design_margin_factor) / 1000 [kA]

Comparison criterion: the protective device interrupting rating (AIC) in kA symmetrical must be greater than or equal to the interrupting rating I_req,kA for the device to be considered adequate at the transformer secondary.

Typical reference values for transformer %Z and low-voltage breaker AIC ratings

Transformer rating (kVA)	Typical %Z (LV dry-type)	Typical secondary voltage (V)	Common LV breaker AIC ratings (kA sym)
75–225	3.5–5.75 %	208 / 480	10, 14, 18
300–750	5.75–6.0 %	480	18, 25, 35
1000–2500	5.75–8.0 %	480 / 600	35, 42, 65
Above 2500	6.0–10.0 %	480 / 600	65, 85, 100

What does this calculator check?

It estimates the maximum three-phase short-circuit duty at the transformer secondary terminals based on transformer kVA, secondary voltage and %Z, optionally including motor contribution and design margins, and compares the resulting interrupting rating with the installed device AIC.

Why is transformer %Z important for secondary fault duty?

The transformer percent impedance (%Z) determines how much fault current the transformer will allow for a given system voltage. Lower %Z means higher available fault current and a more demanding interrupting rating requirement for downstream devices.

How should I use the design margin and motor contribution factors?

The design margin is typically set between 1.2 and 1.3 to account for calculation tolerances and future system changes. The motor contribution factor approximates the additional short-circuit current supplied by motors connected on the transformer secondary, often taken between 0 and 30 % in preliminary studies.

Does the asymmetrical current affect the AIC comparison?

Breaker AIC ratings are generally expressed in symmetrical kA, but standards require devices to withstand associated asymmetrical duties based on X/R. This tool reports an estimated asymmetrical current for reference but checks adequacy using symmetrical current versus symmetrical AIC rating.

Fundamental Concepts: Fault Duty, AIC, and Why Screening Matters

Fault duty is the calculated prospective current available at a point on the secondary side of a transformer when a bolted three-phase fault occurs. It is directly used to size protective devices, verify interrupting ratings, and ensure switchgear withstand capability. AIC (Ampere Interrupting Capacity, sometimes called AICC or interrupting rating) is the declared maximum fault current an overcurrent protective device can safely interrupt without catastrophic failure. Screening via transformer secondary fault duty uses transformer kVA, impedance and system voltages to compute available fault currents. AIC checking compares those computed currents to device interrupting ratings and switchgear withstand limits. Both processes are complementary; one determines the hazard, the other evaluates device adequacy.

Key Parameters and Their Technical Roles

Transformer nameplate and electrical quantities

• Rated kVA (S): transformer power rating influencing rated current.
• Rated secondary voltage (V): line-to-line voltage on secondary side.
• Percent impedance (%Z): the transformer's impedance expressed as a percentage of rated voltage drop at rated current.
• X/R ratio: determines DC offset and peak asymmetrical current following fault inception.
• Short-circuit MVA (MVAsc): indicates the transformer's ability to deliver fault MVA at its terminals.

Protective device attributes

• AIC / interrupting rating: must exceed prospective fault current.
• Time-current characteristic (TCC): determines coordination and clearing time.
• Let-through I2t and energy withstand: important for downstream equipment damage assessment.

Formulas Used for Transformer Secondary Fault Duty Calculation

All formulas below are presented as plain HTML text. Values typical for analysis are provided after each formula.

1) Rated secondary current (I_rated)

I_rated = (S * 1000) / (sqrt(3) * V)

Where:

• S = transformer rating in kVA (typical: 500 kVA, 1500 kVA, 2500 kVA)
• V = secondary line-to-line voltage in volts (typical: 400 V, 480 V)
• sqrt(3) ≈ 1.732

2) Prospective 3-phase short-circuit current at secondary (I_sc)

I_sc = I_rated * (100 / %Z)

Alternative combined formula:

I_sc = (S * 1000) / (sqrt(3) * V) * (100 / %Z)

Where %Z is the transformer's percent impedance (typical: 3%–8% for distribution transformers).

3) Short-circuit apparent power (MVA_sc)

MVA_sc = sqrt(3) * (V / 1000) * I_sc

Where:

• V in volts (divide by 1000 to get kV)
• I_sc in kA (divide amperes by 1000)

4) Asymmetrical peak current due to DC offset (I_peak)

I_peak ≈ sqrt(2) * I_sc * (1 + e^(-π / (X/R)))

Where:

• X/R = reactance-to-resistance ratio of the fault path (typical: 5–15 for distribution transformers)
• e is the base of natural logarithm (~2.718)

5) I2t energy (simplified for steady symmetrical current)

I2t = I_rms^2 * t

Where:

• I_rms is the RMS current through the device during fault (A)
• t is clearing time in seconds (derived from TCC)

Typical Values and Reference Tables

The following tables provide common transformer and device values used in screening and verification tasks.

Screening Methods: Secondary Fault Duty Calculation vs AIC Checker

Both approaches are required in a thorough protection study, but they address different questions:

• Secondary fault duty calculation answers: "What current can the transformer deliver into a bolted fault at this secondary bus?"
• AIC checking answers: "Can each protective device safely interrupt the calculated prospective fault current?"

Common screening workflows:

1. Compute prospective fault currents at all secondary busses using the formula in the previous section.
2. Calculate asymmetrical peak currents and I2t for devices that have time-dependent characteristics.
3. Collect AIC and let-through data from manufacturers for each device.
4. Compare prospective currents to device AIC and check coordination using TCC plots.
5. Alter device selection or add upstream limiting features (circuit breaker with higher AIC, current-limiting fuse, transformer impedance change) as required.

Advantages and limitations

• Transformer secondary fault duty calculations are straightforward and conservative when using percent impedance and rated values, but can overestimate when multiple sources or impedances are present.
• AIC checkers provide rapid pass/fail screening for device interrupting capability, but may not capture transient details (DC offset, inrush, switching sequences) without additional modeling.
• Both should be used together: compute fault duty, then apply AIC screening, then iterate if equipment ratings are inadequate.

Detailed Example 1: 1500 kVA Transformer Feeding a Main Switchboard

Scenario and assumptions:

• Transformer: 1500 kVA, 13.8 kV / 480 V, %Z = 5.75%
• Secondary bus: 480 V line-to-line
• Main breaker: LV power breaker at bus with AIC rating 65 kA RMS
• X/R ratio for fault path: 9 (typical for medium distribution)
• Assume bolted three-phase fault on busbar

Step 1 — Compute rated secondary current

I_rated = (S * 1000) / (sqrt(3) * V)

Numeric substitution:

I_rated = (1500 * 1000) / (1.732 * 480) ≈ 1,804.2 A

Step 2 — Compute prospective short-circuit current at secondary

I_sc = I_rated * (100 / %Z) = 1,804.2 * (100 / 5.75) ≈ 1,804.2 * 17.391 ≈ 31,372 A

So I_sc ≈ 31.4 kA RMS.

Step 3 — Compute peak asymmetrical current considering DC offset

I_peak ≈ sqrt(2) * I_sc * (1 + e^(-π / (X/R)))

Compute exponent: e^(-π/(9)) ≈ e^(-0.349) ≈ 0.705

I_peak ≈ 1.414 * 31.372 * (1 + 0.705) ≈ 1.414 * 31.372 * 1.705 ≈ 75.6 kA

Peak asymmetrical current ~75.6 kA (instantaneous peak). This is useful when evaluating switching device dynamic stresses; however AIC ratings are typically given in kA RMS symmetrical.

Step 4 — Compare to device AIC

Main breaker AIC = 65 kA RMS. Prospective I_sc RMS = 31.4 kA RMS. Since 31.4 kA < 65 kA, the breaker passes the AIC check for interrupting capability.

Step 5 — Evaluate let-through energy and coordination

Even if the breaker interrupts the RMS fault current, verify that upstream and downstream devices coordinate. The breaker clearing time from its TCC and the resulting I2t determines thermal energy let-through. If using a downstream MCCB with lower AIC, check that the upstream breaker will clear first or that the downstream device has adequate AIC.

Final assessment

• Transformer prospective current is 31.4 kA RMS; main breaker AIC 65 kA is adequate.
• Peak asymmetrical current is high (75.6 kA), so mechanical and dielectric forces must be considered for bus & cables; ensure mechanical ratings comply with standards.
• Confirm coordination with TCC plots; if downstream devices have lower interrupting ratings, consider current-limiting fuses or upgrade devices upstream.

Detailed Example 2: Parallel Transformer Scenario and AIC Screening

Scenario and assumptions:

• Two parallel transformers supply a 480 V bus: Transformer A = 1000 kVA, %Z_A = 4.5%; Transformer B = 1000 kVA, %Z_B = 6.0%.
• Both secondaries tied in parallel through tie breakers; a bolted three-phase fault occurs on the common bus.
• Main switchgear breaker AIC rating = 65 kA.
• X/R assumed similar; evaluate combined contribution using MVA method.

Step 1 — Compute individual rated secondary currents

I_rated_A = (1000 * 1000) / (1.732 * 480) ≈ 1202.8 A

I_rated_B = 1202.8 A (identical kVA)

Step 2 — Compute individual prospective currents

I_sc_A = 1202.8 * (100 / 4.5) ≈ 1202.8 * 22.222 ≈ 26,729 A (26.7 kA)

I_sc_B = 1202.8 * (100 / 6.0) ≈ 1202.8 * 16.6667 ≈ 20,046 A (20.0 kA)

Step 3 — Combine contributions

For parallel sources at the same voltage, currents superimpose vectorially; with both contributing mostly symmetrical currents, the RMS resulting current is approximately arithmetic sum if phases align:

I_sc_total ≈ I_sc_A + I_sc_B ≈ 26.7 kA + 20.0 kA ≈ 46.7 kA RMS

Note: This simple summation is conservative; more accurate combination uses Thevenin equivalents and system impedance. For screening, summation is acceptable to ensure margins.

Step 4 — Compare to device AIC

Main switchgear AIC = 65 kA, prospective I_sc_total = 46.7 kA < 65 kA, pass for interrupting capability.

Step 5 — Consider fault current contribution after breaker clearing and tie opening

If one transformer breaker opens faster than the other, the remaining transformer may feed a reduced fault. Protective coordination must ensure selective tripping. Also confirm that parallel transformers have compatible impedances to prevent adverse circulating currents during normal operation.

Final assessment

• Parallel operation increases prospective fault current significantly compared to a single transformer.
• AIC of the switchgear must be selected considering the worst-case combined contribution.
• Designers should check transient behavior, differential protection, and bus mechanical ratings when currents exceed certain thresholds.

Practical Considerations, Limitations and Best Practices

1) Coupling with upstream network and motor contributions

Utility source impedance, motors infeed, and parallel sources modify prospective currents. Always obtain utility short-circuit data and include large motor locked-rotor contributions where relevant.

2) Use of current-limiting devices

Current-limiting fuses can dramatically reduce let-through energy and peak currents, allowing downstream equipment with lower AIC to be protected. When using current-limiting devices, consult manufacturer I2t and let-through curves.

3) DC offset and asymmetry

Standard AIC ratings are specified in RMS symmetrical terms. However, mechanical and dielectric stresses are influenced by asymmetrical peaks. Use X/R-based peak calculations and compare with device capability notes in standards.

4) Transformer impedance tolerances and temperature

%Z varies with manufacturing tolerance and temperature; conservative design uses nameplate worst-case (lowest %Z yields highest fault current). Verify with vendor data when operating near limits.

5) Equipment withstand vs device interrupting capacity

• Even if a breaker interrupts the fault, bus bars, cable terminations, and enclosures must be rated for the mechanical and thermal stresses. Check bus short-circuit rating per IEC/IEEE and manufacturer documentation.

Standards, Guidance Documents, and Authoritative References

Industry standards and normative documents should be referenced when performing fault duty and AIC screening. Key documents include:

• IEC 60076: Power Transformers — provides transformer design, testing, and impedance guidance. See https://www.iec.ch/
• IEC 60909: Short-Circuit Currents in Three-Phase AC Systems — methodology for calculating fault currents. See https://webstore.iec.ch/
• IEEE Std 141 (Red Book): Electric Power Distribution for Industrial Plants — practical guidance on short-circuit studies and coordination. https://standards.ieee.org/
• IEEE Std C37.x series: Circuit Breaker and Switchgear standards and ratings. https://standards.ieee.org/
• NFPA 70 (National Electrical Code) and NFPA 70E: Safety-related work practices and equipment ratings. https://www.nfpa.org/
• Manufacturer technical guides: Siemens, ABB, Schneider Electric provide application notes on AIC, let-through energy, and transformer fault calculations. Examples: https://www.siemens.com/ ; https://new.abb.com/ ; https://www.se.com/

Checklist for Compliance and Verification

1. Collect transformer nameplate data: kVA, V, %Z, vector group.
2. Obtain utility short-circuit data and confirm system topology.
3. Calculate prospective fault currents at key points using the formula set above.
4. Compute asymmetrical peaks and I2t where necessary.
5. Collect AIC and let-through data from manufacturers for all overcurrent protective devices.
6. Perform TCC coordination and confirm selective clearing times.
7. Document that equipment interrupting ratings exceed prospective currents with appropriate safety margins.
8. Where margins are insufficient, specify upgrades: higher AIC breakers, current-limiting fuses, or increased transformer %Z (rare; typically change transformer only if justified).
9. Retain records and reference standards used for audit and safety compliance.

Final Technical Guidance for Design Engineers

• Always treat transformer secondary fault duty as the primary computation for prospective current sources; treat AIC checking as a verification layer.
• When using AIC checkers (software or spreadsheet tools), ensure they use accurate source impedance, include motor contributions, and model parallel sources correctly.
• Prefer conservative assumptions for %Z and worst-case parallel contributions unless manufacturer or utility data proves otherwise.
• Consider mechanical constraints and bus short-circuit withstand numbers in addition to interrupting capability; coordination is not just interruption but also ensuring physical integrity.
• When in doubt, request short-circuit test data from transformer vendors and perform solution-level modeling in power system analysis tools (e.g., ETAP, SKM PowerTools, CYME) for final verification.

Useful external links for deeper reading

• IEC 60909 short-circuit calculations overview: https://www.iec.ch/
• IEEE Power Engineering resources: https://standards.ieee.org/
• NFPA Codes and Standards: https://www.nfpa.org/
• Transformer technical pages (examples): https://www.abb.com/transformers, https://www.siemens.com/transformers
• ETAP short-circuit study guide: https://etap.com/ (vendor tool documentation)

Summary of the Screening Decision Logic

1. Compute the prospective fault current(s) using transformer kVA, V, and %Z.
2. Evaluate asymmetrical peaks and energy for device stress assessment.
3. Compare RMS prospective currents against device AICs; require device AIC >= prospective RMS current with defined margin.
4. If device fails AIC check, apply mitigation: upgrade device, add current-limiting device, or revise system impedance.
5. Document and validate with standards and manufacturer certifications.

Adhering to these steps ensures that transformer secondary fault duty calculations and AIC screening together protect the electrical system from catastrophic failures, maintain equipment integrity, and comply with international standards.