Transformer Fusing 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 fully verified for this case: for example a 120 V half-winding fault, a kVA not listed in C57.12.00 Table 12, a C57.109 point above 25× (where the figures stop), or a C57.109 shortfall only at or below 5× (the overload region, see below).
• 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.

External fusing: S&C SM-4 and SM-5 power fuses

A pad-mounted transformer can instead (or also) be fused upstream: in pad-mounted or metal-enclosed gear, on a riser pole or in a station. The tool holds S&C's SM refill units, which fit the outdoor SM-4 and SM-5 power fuses (Specification Bulletin 242-31) and the indoor SM-4Z and SM-5S used in enclosed gear (252-31). Choose the scheme at the top of the inputs:

• Internal: the Bay-O-Net link with its ELSP, as in earlier revisions.
• External: the S&C fuse is the only primary fuse. Its total-clearing curve is checked against the C57.12.00 withstand points and the C57.109 range, its minimum-melt curve against inrush and cold load, and its interrupting rating against the available fault.
• Both: the internal pair protects the transformer. The S&C fuse upstream is the "protected" device: for every secondary fault, up to the larger of I_TF and the largest calculated fault, the link must clear before the S&C fuse can melt.

Curves. S&C publishes Standard Speed (TCC 153-4 minimum melt, 153-4-2 and 153-4-4 total clearing) and Slow Speed (119-4, 119-4-2, 119-4-4) curves, with total-clearing curves that differ for 25–34.5 kV. The tool uses S&C's own point data for each sheet and was checked against the drawn sheets (median difference under 1 % in current). The curves start at no initial load and 25 °C and run to about 600 s. Minimum-melt curves are plotted to minimum test points (tolerance plus 10 % in current for 10E–400E); total-clearing curves to maximum test points.

Preload. S&C asks for the protected fuse's minimum-melt curve to be adjusted for preloading (Information Bulletin 242-190) but needs no other "safety zone" for its silver elements. The tool applies a melting-time allowance instead of reading S&C's adjustment chart: by default the link must clear within 75 % of the S&C minimum-melt time. The value is marked as an estimate and can be changed.

Long durations. Beyond the curves, loading is judged from S&C's peak-load capability tables (continuous; daily and emergency for ½ to 8 h, at 70 % preload and 30 °C, derated 0.5 % per °C above 30 °C). Eaton's 200 % for 2 h and 160 % for 7 h criteria start from 75 % preload, so those comparisons are provisional.

Interrupting ratings are the symmetrical values S&C assigns for X/R = 15. S&C lists higher ratings for lower X/R and lower system voltages; the tool does not use them, and flags a source X/R above 15.

Coordinating-speed (TCC 173-4 / 179-4), capacitor-speed and parallel (two-holder 720E) refill units are not included.

Fuse families in the library

Inside the transformer the tool models two kinds of protection. A Bay-O-Net expulsion link (replaceable through the tank wall or cover) clears overloads and low-current faults and must be in series with a partial-range (backup) current-limiting fuse that clears high-current internal faults. A full-range current-limiting fuse clears every current that melts it and stands alone.

• Dual sensing 358C: responds to current and to oil temperature (Eaton R240-91-51).
• Current sensing 353C: responds to current only (R240-91-50). As drawn by Eaton, the 65 A link's curve joins the 40 A link's below about 0.64 s.
• Dual element 108C and the Kearney 124080 line: Eaton states the Kearney links are electrically equivalent to the 108C links (TD132015EN), so they use the same curves; the two largest Kearney links carry lower label ratings (30 and 35 A) than their Cooper equivalents (40 and 50 A).
• High ampere overload 361C (65, 100, 125 A): an integral cartridge for large units, in a silver-plated flapper assembly; not rated at 23 kV (R240-91-53).
• ELSP backup fuses in three voltage classes, 8.3, 15.5 and 23 kV (R240-91-167, -168, -169). The class must cover the winding voltage; Select pairs each link with the smallest ELSP of that class that meets the coordination rules.
• NX (4.3, 15.5 and 23 kV) and ELX (8.3, 15.5 and 23 kV, dry-well canister) full-range fuses. Their curves run to 1000 s. The 5.5 and 8.3 kV NX curves are not in the library yet.

Damage-curve crossing. For each fault basis the checks state the multiple of rated current above which the protecting fuse's total-clearing curve stays under the C57.109 curve, and the TCC marks it. The range runs from 2× (where the curve starts) to the lower of the curve's end and the largest through fault of that type, since a larger fault cannot occur. Every band where the fuse is outside the curve is listed: with the frequent-fault (mechanical) curve of Category II-IV units a fuse can be inside over a middle band and outside again near the top. When the fuse is never inside the check says so, and when it is inside from the start of its published curve (for example a link's 300 s point) that is reported as the curve start, not a crossing. Single-phase 120 V (half-winding) faults are evaluated with the exact 0.5 primary factor; without the half-winding impedance the end of the range is approximate. Link curves are 25 °C with no initial load; a preloaded link clears sooner, so preloaded crossings (as TransFusion plots them) are somewhat lower. Below the crossing the transformer relies on overload and secondary protection. The fuse rating as a percent of rated current is listed under Loading, with both ratings where the same curve is sold under two (Kearney and Eaton dual-element links). The fuse ratio (long-time melting current divided by rated current) is listed under Loading for comparison with a sizing table.

Line-to-ground single-phase units. Choose the line-to-ground connection and enter the winding (line-to-neutral) voltage; the source fault current is then the line-to-ground value.

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 C57.109-2018 gives the through-fault protection curves that the tool checks over the whole attainable fault range (next section).

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.

Through-fault protection curves (IEEE C57.109-2018)

IEEE Std C57.109-2018, Guide for Liquid-Immersed Transformers Through-Fault-Current Duration, gives a duration limit for every symmetrical through-fault current from 2 to the maximum, as a per-unit multiple of the winding's rated current. The tool builds the curves from the guide's construction (Clause 4, Tables 1-5) and checked them against the guide's Figures 1-6, digitized from the licensed copy. The standard's text is not reproduced here; refer to the standard for its wording.

• Thermal curve (all categories): from 2× to 5× the durations of the guide's Table 2 (2× 1800 s, 3× 300 s, 4.75× 60 s, joined on log-log scales, reaching 50 s at 5×); above 5× t = 1250 / I².
• Category I (Figure 1): one curve for frequent and infrequent faults, continued as 1250 / I² up to the C57.12.00 Table 12 multiple (25, 35 or 40×; never above 40×, 0.78 s).
• Category II, frequent faults (Figure 2, typically more than 10 in the transformer's life, counting faults above 70 % of the maximum): above 70 % of the maximum current the duration drops to t = K / I² with K = 2 I_max², the maximum current for 2 s; I_max = 100 / Z% with transformer impedance only. Infrequent faults (Figure 3): the thermal curve alone, which may also serve as the backup curve where frequent faults are cleared by high-speed relaying.
• Category III (Figures 4 and 5): as Category II with the threshold at 50 % and "frequent" meaning typically more than 5 faults in the life; I_max uses the transformer plus system impedance.
• Category IV (Figure 6): always the frequent-fault curve with the 50 % threshold and transformer plus system impedance.
• System impedance (Categories III and IV): the value in the transformer specification; if it is not known, the guide's Table 4 (63 kA for systems below 48.3 kV maximum voltage). Choose it under Transformer. The tool adds it to the transformer impedance as a plain sum (same X/R), the basis the guide states.

Impedance tolerance. The mechanical (frequent-fault) portion depends on the impedance. The tool uses the lower envelope over the tolerance band: K and the threshold from the high end of the band (the smallest I_max), and never less than 2 s up to the largest current the low end of the band allows.

Table 2 and the figures. Between 2× and 5× the guide's figures draw a straight log-log line from 2× / 1800 s to 5× / 50 s, which lies above the Table 2 values (369 s against 300 s at 3×, about 23 % longer). The tool uses Table 2, the shorter and more conservative duration, and states the margin against the curve as drawn when the two differ in outcome. The guide notes that currents at or below 5× are more likely overloads than faults and refers to the loading guide (C57.91), so a shortfall only in that region is reported as a provisional fail.

Which current. The curves are in winding current. The fuse sees line current: on a delta-wye unit a secondary line-to-ground fault gives 0.577 of the winding per unit in the primary lines, so the chart draws both the three-phase basis and the line-to-ground basis. Above 25× the figures show no curve for Categories II-IV; the thermal line is continued there and a governing point on it is provisional.

Applicability. The guide's curves apply to transformers built from the early 1970s; for older units, and as a precaution for early-1970s units, the manufacturer must confirm the capability.

The "C57.109 through-fault range required" box in Select makes the range checks mandatory; it is off by default, so studies made with Rev 2 select the same fuses and the C57.12.00 withstand point stays the required protection criterion unless you choose otherwise.

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 fuses (link and/or external fuse) see the arcing current times the voltage ratio.
4. Clearing time from each primary fuse's total-clearing curve at that current (the faster governs), 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.