Padmount Fuse Lab

How to read the results

Every result carries one status, so verified conclusions are never mixed with assumptions:

• ✓Pass / ✕Fail: verified method and verified data (for example the published Bay-O-Net curve against the C57.12.00 withstand duration).
• ✓Provisional pass / ✕Provisional fail: the method or data are not yet verified. IEEE C57.109-2018 is not in the project files, so its curves are provisional.
• TEaton table / TDiffers from table: what Eaton's published correlation tables say, on their stated basis.
• –Unavailable: data missing or outside a valid domain. Nothing is assumed; the reason is shown. An unavailable check is never counted as a pass.

"No device combination satisfies all requirements" is a valid answer. The tool then lists which requirements conflict and what could change.

Loading words, used consistently

Loading is always stated as a percent of transformer rating. 200 % loading means twice the nameplate load, which is the same as a 100 % overload. 160 % means 1.6 times nameplate.

• Continuous loading: the load the unit carries indefinitely.
• Preload and initial thermal state: the load (and oil temperature) before an event. Eaton's dual-sensing selections assume 75 % preload at 35 °C ambient.
• Temporary and emergency loading: planned overloads for hours, such as 200 % for 2 h or 160 % for 7 h.
• Transformer magnetizing inrush: the short current surge when the transformer is energized. Eaton's criterion is 12 times full-load current for 0.1 s.
• Motor and other load starting: currents drawn by connected loads when they start; they add to load, not to inrush.
• Cold-load pickup: after an outage, loads that normally cycle all come on together (loss of diversity). Eaton's optional criterion is 3 times full-load current for 10 s. If your restoration envelope already contains the inrush, do not count inrush twice.

A dual-sensing Bay-O-Net link senses both current and oil temperature. Its published curves (R240-91-51) are for 25 °C oil with no initial load and stop at 300 s. Longer overloads depend on the oil temperature, which Eaton publishes only through its selection tables and TransFusion. The tool therefore checks short events (inrush, cold load, user points up to 300 s) on the curve and compares longer ones with Eaton's tables. It never builds a thermal memory by adding fractions of curve times.

Why two fuses, and how they must coordinate

The Bay-O-Net expulsion link clears overloads and secondary faults. It can only interrupt up to its assembly rating (2,500 A at 15.5 kV in mineral oil). The ELSP partial-range current-limiting fuse in series clears high-current internal faults, but it cannot interrupt below its minimum interrupting current. They must hand over cleanly. Eaton's rules (TD132004EN) for the crossover of the link's total-clearing curve with the ELSP's minimum-melt curve are:

1. R1: crossover above the maximum through-fault current, I_TF = I_FL × 100 / %Z, so the ELSP never melts for a secondary fault;
2. R2: crossover at or above the ELSP minimum interrupting current;
3. R3: crossover below the Bay-O-Net interrupting rating;
4. R4: at 300 s, the link's maximum clearing current is no more than 90 % of the ELSP's minimum-melt current.

The tool holds both sets of curves: the link's total-clearing curve (R240-91-51) and the ELSP's minimum-melt curve (R240-91-168 Rev 02), both published for 25 °C with no initial load. TD132004EN does not state which curves its rules assume; the tool uses these published curves. It finds the crossover, applies R1–R4 directly, and in Select mode pairs each link with the smallest ELSP that meets all four. TD132004EN takes the smallest rating that meets the rule; the tool passes over a rating that meets a rule by less than 5 % (its usual "marginal" band) for the next one that meets them by more, and says so.

On that basis the tool reproduces 18 of the 21 rows of Eaton's dual-sensing combination tables at 12–12.47 kV (16 of 21 with the literal smallest-rating rule: at 225 kVA three-phase and 333 kVA single-phase the smaller ELSP meets the rules by under 5 %, and Eaton also lists the next size). The three that differ all pair the C14 link with two 125 A ELSPs in parallel: on the published curves their crossover (about 970 A) is below the pair's 1,100 A minimum interrupting current, so item 5 of the rules is not met, and the tool picks the 2 × 150 A pair instead. Eaton should be asked about this.

Electrical coordination is separate from physical fit, fluid compatibility, manufacturer qualification and utility acceptance. Similar ampere ratings or similar-looking curves do not make two devices interchangeable.

Line current and winding current are different

The primary fuse sees the line current. The transformer is damaged by winding current. For a three-phase delta-primary, grounded-wye unit:

• Three-phase secondary fault: line and winding per-unit currents are equal.
• Line-to-line secondary fault: the winding carries 0.866 of the three-phase value; one primary line carries the full three-phase per-unit value (factor 1.155), which favours protection.
• Line-to-ground secondary fault: with the transformer's zero-sequence impedance equal to its positive-sequence impedance (Z0 = Z1), the winding carries the same per-unit value as for a three-phase fault, but each of the two primary lines carries only 1/√3 ≈ 0.577 of the winding per-unit current. The fuse sees less current and is slower: this is usually the governing case.

For a single-phase unit with a 120/240 V secondary, a 120 V (half-winding) fault puts the primary at one-half of the faulted half-winding's per-unit current (ampere-turn balance). The half-winding current itself depends on the winding design and must come from the manufacturer: it is not a fixed fraction of the full-winding fault.

The single-phase templates are connected line-to-line on the 12.47 kV system, so the source loop for a secondary fault is Z1 + Z2 of the system. TransFusion's single-phase input is fixed to a wye (phase-to-ground) connection, so it does not represent this.

Transformer short-circuit withstand

IEEE C57.12.00-2021 (7.1.3.1, 7.1.4, Table 12) sets the duration a distribution transformer must withstand at its maximum short-circuit current, calculated with transformer impedance only:

• Category I (single-phase 5–500 kVA, three-phase 15–500 kVA): t = 1250 / I² with I in per unit, not above the Table 12 multiple (40, 35 or 25 ×).
• Category II (single-phase 501–1667 kVA, three-phase 501–5000 kVA): 2 s.

If the impedance is so low that the terminal fault exceeds the Table 12 multiple, the transformer itself is outside its standard capability and no fuse can correct that.

Between rated current and the maximum, C57.12.00 refers to the manufacturer; IEEE C57.109 gives through-fault protection curves. C57.109-2018 is not in the project files, so those curves are drawn dashed and every result that uses them is provisional.

Impedance has a tolerance (C57.12.00 9.2: ±7.5 % above 2.5 %, ±10 % at or below). The tool checks the low end (highest current) and the high end (lowest current, slowest fuse); either can govern. If you enter a guaranteed minimum, the nominal and high-end values are derived from it. No tolerance is applied to a manufacturer half-winding impedance.

Secondary arc flash

1. Bolted three-phase fault at the chosen location (source, transformer and cable impedances added as complex numbers).
2. Arcing current and the reduced arcing current (IEEE 1584-2018 variation factor).
3. The current each protective device sees: the primary link sees the arcing current times the voltage ratio.
4. Clearing time from the link's total-clearing curve at that current, never from the bolted-fault clearing time.
5. Incident energy and arc-flash boundary for both arcing currents; the larger governs.

Lower fault current does not mean lower hazard. A longer secondary cable reduces the current, but the primary link then takes much longer to clear, and the energy can rise sharply. When the arcing current falls below the link's 300 s clearing current, the arc duration is not bounded by the published curve and the tool says so rather than inventing a number.

The tool imposes no arc-duration limit. IEEE 1584-2018 discusses using about 2 s where a person can move away; if you apply a limit, you must enter it and its justification, and both values are shown. Shorter clearing because of preload is not credited.

Single-phase 120/240 V. The IEEE 1584-2018 model was built from three-phase tests and does not cover single-phase systems directly, but the guide gives a method for them: find the arcing current from the single-phase bolted fault current at the system voltage, then use the three-phase equations for the clearing time and incident energy. The guide states the result is expected to be conservative (wording as quoted by EasyPower; the standard's text is not in the project files). The tool does this for the 240 V line-to-line fault. A 120 V line-to-neutral arc is below the model's 208 V lower limit, so it is not calculated.

TransFusion and this tool

Eaton's TransFusion program selects a link and ELSP from kVA, voltage, minimum impedance, protection type, overload level, temperature class, ambient and preload. It shows raster charts and does not publish its selection rules or the meaning of its overload levels ("Least", "Moderate" and so on). This tool records TransFusion results for the 12.47 kV templates as comparison references and explains differences; agreement with TransFusion is not treated as proof.

Typical reasons the results differ: the tool checks C57.12.00 withstand across the impedance tolerance band and for line-to-ground faults (the TransFusion reference runs on record show one unshifted damage curve; TransFusion does not document its basis); single-phase line-to-line units are entered in TransFusion as phase-to-ground; and the tool refuses to judge long-time loading from curves that end at 300 s.

Scope of Rev 1 and what comes next

• Devices: Eaton dual-sensing 358C Bay-O-Net links (C03–C14 at 15.5 kV; C16CB/C18CB are 8.3 kV only) and 15.5 kV ELSP backup fuses. Other families and manufacturers are added only when their original data are verified.
• Not modelled: C57.109 curves (provisional), transformer thermal loading and aging (C57.91), link oil-temperature behaviour, chronological simulations, ground faults beyond the secondary cable, motor/generator/battery/PV contributions, and library breakers as the secondary main.
• Planned: C57.109 verified against the standard; a shared C57.91 thermal engine and timeline; more device families and manufacturers; a utility-practice library built from original dated documents.