Overhead Utility Distribution Protection, IEEE C37.60 Auto-Recloser & Expulsion Fuse TCC
Distribution Recloser Fuse-Saving Coordination Formula
Calculate Fast-Trip Fault Joule Let-Through I²t (kA²·s), Fast-Clear-to-Fuse-MMT Ratio (<= 0.75), Slow-Curve-to-Fuse-TCT Backup Ratio (>= 1.35), and Fuse-Saving Margin (ms).
Close-in 4,600 A feeder faults outrun a 2.5-cycle vacuum recloser—melting 65T lateral fuses in 31 ms BEFORE the fast curve clears and turning temporary squirrel faults into truck rolls!
Because `80%` of overhead `13.8` kV / `34.5` kV distribution faults are **temporary lightning flashovers or tree-branch slaps**, **IEEE `C37.60` Pole-Top Vacuum Auto-Reclosers** run a `2`-Fast + `2`-Delayed sequence (`101-A-fast` -> `0.5` s dead-time reclose -> `117-C-slow`) to **save downstream lateral cutout fuses (`Fuse-Saving`)** on transient faults while letting the fuse blow selectively on permanent faults! Selective coordination at Lateral Fault Current `I_f` (`A`) demands two **Time-Current Characteristic (`TCC`)** margins: `1)` **Fast-Trip Fuse-Saving `t_{fast} / t_{MMT} <= 0.75`** (`giving positive Fuse Thermal Damage Margin Delta_t_{save} = (0.75 * t_{MMT} - t_{fast}) * 1,000` ms below Fuse **Minimum Melting Time `t_{MMT}`** so `I^2 * t` heating never fatigues the silver/tin link!), and `2)` **Delayed Slow-Curve Selectivity `t_{slow} / t_{TCT} >= 1.35`** above Fuse **Total Clearing Time `t_{TCT}`** so permanent lateral faults blow ONLY the lateral tap without locking out the entire backbone feeder!
Verify on a Mid-Feeder `100T` Lateral Tap (`I_f = 1,850` A)—Lateral_Fault `I_f` = 1,850 A (B2), Recloser_Fast_Total_Clear `t_{fast}` = 0.048 s (`2.9` cycles, B3), Fuse_Min_Melt `t_{MMT}` = 0.086 s (B4), Fuse_Total_Clear `t_{TCT}` = 0.135 s (B5), and Recloser_Slow_Curve `t_{slow}` = 0.310 s (B6): Fast-Trip Joule Let-Through `I^2 * t` equals `0.164` kA²·s (E2), Fast-to-MMT Ratio equals **`0.558`** (`<= 0.75` fuse saved! E3), Slow-to-TCT Ratio equals **`2.30`** (`>= 1.35` selective! E4), with **`+16.50` ms** of clean fuse-saving cushion (E5)!
| IEEE C37.60 Recloser-Fuse TCC KPI (75% MMT Damage Limit, 1.35x TCT Slow Margin) | Formula | Verified Output |
|---|---|---|
| Fast-Trip Joule Impulse (kA²·s) | ((Fault_Current_A / 1000) ^ 2) * Fast_Clear_s | 0.164 kA²·s (0.889 kA²·s close-in) |
| Fast-to-MMT Fuse-Save Ratio | Recloser_Fast_s / Fuse_Min_Melt_MMT_s | 0.558 SAVED (1.355 > 0.75 MELTED!) |
| Slow-to-TCT Selectivity Ratio | Recloser_Slow_s / Fuse_Total_Clear_TCT_s | 2.30 SELECTIVE (3.08 selective) |
| Net 0.75·MMT Thermal Margin (ms) | (0.75 * Fuse_MMT_s - Recloser_Fast_s) * 1000 | +16.50 ms SAFE (-18.75 ms NUISANCE BLOW!) |
IEEE C37.60 Fast/Slow Recloser-to-Expulsion-Fuse TCC Coordination Formula
Validate `Fault_Current I_f` in `[50, 40,000]` A, `Recloser_Fast` in `[0.015, 2.0]` s, `Fuse_MMT >= 0.008` s, `Fuse_TCT > Fuse_MMT`, and `Recloser_Slow > Recloser_Fast`!
=LET(if_a, B2, tf_s, B3, tm_s, B4, tt_s, B5, ts_s, B6, IF(OR(if_a < 50, if_a > 40000, tf_s < 0.015, tf_s > 2, tm_s < 0.008, tt_s <= tm_s, ts_s <= tf_s), "Invalid Recloser Input", ROUND((0.75 * tm_s - tf_s) * 1000, 2)))Close-In Substation Lateral (`I_f = 4,600` A, `MMT = 0.031` s vs `Fast = 0.042` s) Coordination-Limit Check
Check a tap `400` m outside the substation fence (`B2 = 4,600` A) where vacuum-interrupter mechanical floor clamps `t_{fast}` at `B3 = 0.042` s (`2.5` cycles) while the inverse fuse `MMT` plunges to `B4 = 0.031` s (`B5 = 0.052` s, `B6 = 0.160` s): Fast-to-MMT Ratio E3 crosses unity to **`1.355`** and Thermal Margin E5 flips negative to **`-18.75` ms**—proving `Fuse-Saving` fails above the maximum coordination current unless utilities add a series current-limiting reactor or upsize to a `140T` link!
=IF(ISTEXT(E5), E5, ROUND(B3 / B4, 3))Try the example
Use the original values or change them to test your understanding.