Overcurrent & coordination
ANSI 50/51, TCC curves, pickup + time-dial, CTI.
Lessons
Overcurrent protection & TCC coordination
ANSI 50/51, IEEE TCC curves, pickup + time-dial settings, and coordination of cascaded protective devices with CTI.
Differential (87) and distance (21) protection
KCL-based unit protection plus impedance-plane mho zones for transmission lines — the two fastest, most selective protections on the grid.
Motor protection — ANSI 49 / 50 / 51 / 46 / 27
Layered relay protection for industrial motors: thermal overload (49), instantaneous + time overcurrent (50/51), ground (50G/51G), negative-sequence (46), undervoltage (27), phase reversal (47), start-counter (66), motor differential (87M).
Generator protection — 30+ ANSI functions
Zoned protection for large synchronous generators: stator (87G, 64G, 27TN, 50/51V), rotor / field (40, 64R), system (32, 81, 24, 78), mechanical (38, 49, 46, 87T). All in one microprocessor relay (SEL-300G / GE G60 / ABB REG670).
Transformer protection — 87T / 87REF / 63
ANSI 87T differential primary, 87REF restricted earth fault, percentage-restraint bias + 2nd/5th harmonic blocking, sudden-pressure (63), Buchholz, thermal (26/49). One modern numerical relay (SEL-787 / GE T60 / RET670 / 7UT85).
Line protection — 87L + 21 + 67 + pilot schemes
Transmission-line protection: 87L line differential (100% coverage via comms), 21 distance multi-zone (Z1≤80% / Z2 120% / Z3 220%), 67 directional OC, pilot schemes POTT / DCB / DUTT / DTT. SEL-411L / 421, GE D60, REL670, 7SA8.
Bus protection — ANSI 87B differential
Substation bus differential applying KCL across all feeder CTs. High-impedance vs low-impedance numerical 87B (SEL-487B / REB670 / B30 / 7SS8), zone-interlocked OC for distribution, multi-zone topology, 87B + 50BF coordination.
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.
CT / VT (PT) characteristics and saturation
ANSI C-class CTs (C400, C800), accuracy classes, burden, saturation effects on differential / distance / overcurrent protection. Inductive VT vs CCVT. Ferroresonance on ungrounded systems. Modern relay tolerance.
Arc-flash analysis (IEEE 1584 + NFPA 70E)
Incident energy I_E (cal/cm²) at working distance, arc-flash boundary AB, NFPA 70E PPE categories 1-4 (>40 cal/cm² prohibited). IEEE 1584-2018 calculation flow. Mitigation hierarchy: de-energize → maintenance mode → arc relay → AR switchgear → PPE.
System grounding methods
Solid / low-Z NGR / high-Z HIRG / ungrounded — sets I_gf, V_rise on healthy phases, detection method (51G vs 59N), and ride-through capability. NEC 250.20 LV default solid; 250.21 HIRG for industrial 480 V+. Ungrounded mostly phased out.
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.
Substation ground grid — IEEE 80 step / touch voltage design
Protect personnel by limiting STEP (foot-to-foot 1 m) and TOUCH (hand-to-feet) voltages below shock thresholds during ground faults. Dalziel I_body = 0.116/sqrt(t_s). V_step/V_touch derived. GPR = I_g·R_g. Wenner 4-probe ρ (IEEE 81). Sverak: V_mesh = ρ·K_m·K_i·I_g/L_M. 3-10 m mesh of 4/0-500 MCM Cu, rods at perimeter, fence bonded with outer ring. Validate by fall-of-potential test (NETA ATS/MTS).
NEC Article 430 — motor branch circuit & feeder sizing
Use TABLE FLC (NEC 430.247-430.250) for conductors + branch OCPD; NAMEPLATE FLA only for overload (430.32). Conductors: 125% × FLC (430.22). Branch OCPD: 250% inv-CB / 300% NTD fuse / 175% DE fuse — × FLC (430.52). Overload: 115-125% × FLA. Disconnect ≥115% FLC within sight (430.102). Feeder: 125% LARGEST + 100% OTHERS (430.24). VFDs: Article 430 Part X.