Recloser-to-Fuse Coordination Screen

A recloser's fast curve is supposed to clear a temporary fault before the branch fuse melts, so a tree limb costs nobody a fuse change. Whether it does is a comparison of two published curves at one fault current, and the multiplier that makes the comparison honest is the part crews get wrong. Fuse saving works when the fast-curve time, MULTIPLIED BY A HEATING FACTOR that accounts for the element retaining heat between operations, still sits below the fuse's minimum-melt time at the maximum fault current on the branch. Two fast operations heat the element more than one, which is why the factor rises with the number of fast shots -- roughly 1.2 for one and 1.35 for two are the figures in common use, and both are entered here rather than assumed, because they belong to the fuse family. A comparison made without the factor looks comfortable and is not. The band is bounded at BOTH ends and that is the part missed. Coordination holds only between the minimum fault current where the fuse still clears inside the recloser's slow curve and the maximum fault current where the heated fast curve still beats minimum melt. Outside that window the scheme does not work, and on a modern feeder with high available fault current at the head, the upper bound often falls inside the zone the scheme is supposed to protect. Both ends are therefore screened here: the fuse-saving check against minimum melt and the fuse-blowing check of total clearing against the slow curve. The margin is reported in cycles as well as seconds because that is the unit protection people argue in. This is a comparison of times a reader takes off the published time-current characteristics at one current. NO CURVE DATA IS SHIPPED -- recloser and fuse curves are manufacturer publications that change, and a copy would go stale silently -- and it does not sweep the current range to find where coordination begins and ends, which is what a protection study does. It ignores asymmetry, pre-loading of the fuse by load current, ambient temperature, and fuse damage curves distinct from minimum melt, and it says nothing about sectionalizers downstream, substation relays upstream, or distributed generation changing the current the fuse actually sees. A screen, not a study: the manufacturer's time-current curves, IEEE C37.230, and the utility's protection engineer govern.

heated fast-curve time = recloser fast-curve time x fuse heating factor, which must stay below the fuse minimum-melt time; coordination ratio = minimum melt / heated fast; the fuse-blowing check is fuse total clearing time below the recloser slow-curve time; margin is reported in seconds and in cycles at the entered system frequency.

The fuse-saving coordination criterion with fuse heating factors, by name, and IEEE C37.230 named. Heating factors near 1.2 for one fast operation and 1.35 for two are in common use and are ENTERED. NO RECLOSER OR FUSE CURVE DATA IS SHIPPED: both times are read off the manufacturer's published time-current characteristics. A screen, not a protection study. The manufacturer's curves, IEEE C37.230, and the utility's protection engineer govern.

One multiplication and one ratio on times the reader takes off published curves; no time-current characteristic is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: fault_current_a, fast_curve_s, slow_curve_s, fuse_min_melt_s, fuse_total_clear_s, heating_factor, system_frequency_hz, heated_fast_s, coordination_ratio, margin_s, margin_cycles, longest_fast_curve_s, blowing_margin_s

• Curve times are read off the manufacturer's characteristics no curve data is shipped, because it changesthe manufacturer's time-current curves
• One fault current, not a sweep finding where coordination begins and ends is a protection studyIEEE C37.230
• No asymmetry, pre-loading, or ambient correction and no fuse damage curve distinct from minimum meltthe fuse manufacturer's data