C. Protective devices (fuses, breakers, reclosers)
Switchgear + breaker ratings + interrupting capacity, breaker-failure backup + auto-reclosing schemes.
Lessons
Switchgear & circuit breakers — LV / MV / HV across air, vacuum, SF6
Carry + interrupt + isolate. Three ratings: continuous A, interrupting kA, voltage class. Arc-interrupt physics: contact part → arc → current zero → race between dielectric recovery and TRV. LV: MCB/MCCB (LSIG)/ACB with air + arc chute. MV: vacuum dominates (>95%, 10⁻⁵ Pa metal-vapor). HV: SF6 (GWP 23500 — phasing to CO2-vacuum & g3). Metal-clad (IEEE C37.20.2), arc-resistant (C37.20.7), GIS (IEC 62271-203). NETA ATS/MTS.
Breaker failure / reclosing / sync check (50BF / 79 / 25)
Trip-loop completers: 50BF detects breaker failure → trips adjacent breakers / DTT. 79 reclosing handles 80-90% transient faults (single-shot transmission, 3-shot distribution). 25 sync-check verifies Δθ / Δf / ΔV before close.