E. Grounding (NEC 250, IEEE 80)
NEC 250 grounding + bonding, system grounding methods, substation ground grid design (IEEE 80).
Lessons
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.
NEC Article 250 — grounding and bonding
The most-tested PE blueprint topic. 250.4(A) five general requirements (earth is NOT an effective ground-fault current path). 250.20 which AC systems must be grounded. 250.50/52 grounding electrode system. Table 250.66 GEC sizing + rod/pipe/plate reduction. 250.118/Table 250.122 EGC types + sizing. 250.28/96/102 bonding. All values verified against NEC 2020.
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).