NETA ETT Level 4 cheat sheet

Arc flash boundary: distance at which incident energy equals 1.2 cal/cm² (5 J/cm²); use IEEE 1584 or NFPA 70E tables.

Approach boundaries (NFPA 70E): limited (shock protection) for <750 V: 1 ft (0.3 m); restricted: avoid contact; prohibited: 0 ft.

Incident energy (cal/cm²) ≈ 4.184 × (k × I_bf × t / D²) where k depends on electrode configuration; use IEEE 1584 for 0.208–15 kV.

PPE categories: 1 (4 cal/cm²), 2 (8), 3 (25), 4 (40); above 40 cal/cm² requires de-energization or remote operation.

Lockout/tagout: verify absence of voltage with a tester rated for the system; test before and after on a known live source.

Step and touch potential: touch potential = voltage across body from hand to feet; step potential = voltage between feet ~0.8 m apart; both reduced by equipotential grounding.

Bonding jumpers must be rated for available fault current and connected before working on de-energized lines near energized circuits.

Arc flash risk increases with gap size, available fault current, and clearing time; current-limiting fuses reduce incident energy by faster clearing.

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R (DC and resistive AC).

AC impedance: Z = R + jX; |Z| = √(R² + X²); phase angle θ = arctan(X/R).

Series RLC resonance: f_r = 1/(2π√(LC)); at resonance X_L = X_C and Z = R (minimum).

Three-phase power: P = √3 × V_L × I_L × cosθ; reactive power Q = √3 × V_L × I_L × sinθ; apparent S = √3 × V_L × I_L.

Delta-wye transformation: R_A = (R_AB × R_AC)/(R_AB + R_BC + R_CA) for wye equivalent.

Thevenin equivalent: V_th = open-circuit voltage; R_th = resistance looking into terminals with sources deactivated.

Superposition applies to linear circuits: sum responses from each independent source acting alone.

Power factor correction: capacitor kVAR = P × (tan θ1 − tan θ2) to improve from cosθ1 to cosθ2.

Per-unit system: base power S_base (MVA) and base voltage V_base (kV); Z_base = V_base² / S_base; I_base = S_base / (√3 × V_base).

Transformer per-unit impedance on its own base converts to system base by Z_new = Z_old × (S_new/S_old) × (V_old/V_new)².

Power flow: P = (V1V2/X) sin δ; Q ≈ (V1²/X) − (V1V2/X) cos δ for a lossless line; maximum power at δ = 90°.

Voltage regulation = (|V_no-load| − |V_full-load|)/|V_full-load| × 100%.

Symmetrical components: positive (1), negative (2), zero (0) sequences; Ia = I1 + I2 + I0; zero-sequence requires ground return path.

Fault current from a source: I_f = V / (√3 × Z_total) for three-phase fault; X/R ratio determines DC offset and peak.

Load flow bus types: slack (V, δ specified), PV (P, V), PQ (P, Q).

Transmission line surge impedance Z_c = √(L/C); surge impedance loading SIL = V² / Z_c.

Insulation resistance (IR) test voltage: 500 V for <1 kV equipment; 1 kV for 1–2.5 kV; 2.5–5 kV for >2.5 kV; follow manufacturer.

Polarization index (PI) = IR at 10 min / IR at 1 min; PI ≥ 2.0 is generally acceptable for Class B/F insulation; <1.5 indicates moisture or contamination.

Dielectric absorption ratio (DAR) = IR at 60 s / IR at 30 s; DAR ≥ 1.4 acceptable.

Temperature correction: IR halves for every 10 °C rise above 20 °C (rule of thumb); correct to 20 °C for comparison.

Guard terminal shunts surface leakage current; use for cables with contaminated terminations.

DC hipot test: apply voltage gradually to specified test level (e.g., 2 × rated + 1 kV for cables) for 5–15 min; leakage current should be stable.

Tan delta (dissipation factor) test: measures dielectric losses; increases with moisture, aging, and contamination; typical limits <0.5% for new HV insulation.

Step voltage test: apply voltage in steps (e.g., 1 kV increments) and plot leakage current; a sudden increase indicates impending failure.

Turns ratio test: applied voltage ratio should match nameplate within 0.5%; deviation indicates shorted turns or tap changer issues.

Winding resistance test: measure DC resistance of each phase; imbalance >2% between phases indicates loose connection or shorted turns.

Insulation power factor (tan delta) test: measures dielectric losses; values >1% may indicate moisture or aging; compare to baseline.

Sweep frequency response analysis (SFRA): compares fingerprint of winding impedance vs frequency to detect deformation or displacement.

Dielectric tests: applied voltage (low-frequency) and induced voltage tests verify insulation strength; partial discharge test detects voids.

Oil tests: dissolved gas analysis (DGA) for arcing (C2H2), overheating (C2H4, CH4), and partial discharge (H2); moisture <20 ppm for 69 kV class.

Bushing test: power factor and capacitance; capacitance change >5% or PF >0.5% indicates deterioration.

Tap changer inspection: check contact wear, diverter switch timing, and transition resistance; infrared scan for hot spots.

Contact resistance test (micro-ohm): measure across closed contacts; compare to manufacturer's limit (e.g., <50 µΩ for MV breakers); increase indicates pitting.

Motion analysis: measure contact travel, velocity, bounce, and overtravel; velocity out of tolerance causes excessive arcing or mechanical stress.

Timing test: measure open/close times and pole discrepancy; pole discrepancy >2 ms (or per manufacturer) requires adjustment.

Vacuum bottle integrity: apply AC hipot (e.g., 10 kV for 15 kV class) across open contacts; leakage or breakdown indicates loss of vacuum.

SF6 gas tests: moisture (<500 ppmv for new gas), purity (>97%), and decomposition products (SO2, HF) indicate internal arcing.

Minimum pick-up voltage: close coil should operate at 85% of rated control voltage; trip coil at 70% (or per standard).

Mechanical operation test: perform several open/close cycles to verify linkage, latch, and auxiliary contacts.

Infrared thermography: scan breaker terminals and bus connections under load; ΔT >10 °C between phases indicates loose connection.

CT ratio and burden: secondary current 5 A or 1 A; burden (VA) must not exceed CT rating to avoid saturation.

CT polarity: subtractive polarity means primary current into H1 produces secondary current out of X1; verify with DC kick test.

VT burden: total VA of relays and meters must not exceed VT rating; excessive burden causes voltage error.

Overcurrent relay coordination: time-current curves (TCC); pickup and time dial settings ensure downstream clears first (selectivity).

Differential protection: operates on difference between currents entering and leaving protected zone; set above maximum unbalanced current (e.g., 0.2–0.4 pu).

Distance relay zones: Zone 1 (80% of line), Zone 2 (120–150%), Zone 3 (remote backup); reach settings depend on line impedance.

Ground fault protection: zero-sequence CT or residual connection; pickup typically 10–20% of CT rating for sensitive ground fault.

Relay testing: secondary injection test verifies pickup, drop-out, and timing; primary injection verifies CT ratio and wiring.

Insulation resistance test: 500 V or 1000 V DC for LV cables; 2.5–5 kV for MV; measure after 1 min and 10 min for PI.

DC hipot test: for MV cables, test voltage = 2 × (rated line-to-ground voltage) + 1 kV for 5–15 min; leakage current should be stable and low.

Very low frequency (VLF) test: 0.1 Hz AC hipot for aged cables; test voltage typically 2–3 × U0 for 15–60 min; reduces space charge effects.

Tan delta test: measures dielectric losses at VLF or power frequency; increase with moisture and aging; typical acceptance <0.5% for new XLPE.

Partial discharge (PD) test: detects voids and defects; PD inception voltage and magnitude (pC) indicate severity; >100 pC may require action.

Time domain reflectometry (TDR): locates faults by measuring reflection time; distance = (velocity factor × c × time)/2.

Cable fault locating: Murray loop or bridge method for low-resistance faults; impulse generator for high-resistance faults.

Sheath test: verify jacket integrity with insulation resistance test; low resistance indicates damage and moisture ingress.

Insulation resistance (IR) test: measure winding-to-ground and winding-to-winding; PI ≥2.0 acceptable; correct to 40 °C per IEEE 43.

Winding resistance test: measure phase resistances; imbalance >5% indicates shorted turns or connection fault.

Polarization index (PI) = IR at 10 min / IR at 1 min; for motors, PI <1.5 indicates moisture or contamination.

Surge comparison test: applies surge to each phase and compares waveforms; difference indicates turn-to-turn or phase-to-phase insulation fault.

Motor current signature analysis (MCSA): detects broken rotor bars, eccentricity, and load issues from current spectrum.

Vibration analysis: overall vibration velocity (mm/s) and spectral peaks identify bearing defects, misalignment, and looseness.

Core loss test (loop test): measures core losses; high losses indicate lamination shorting.

DC step voltage test: applies increasing DC voltage to detect insulation weaknesses; leakage current should be linear with voltage.

Ground resistance: measure with fall-of-potential (3-point) method; spacing ≥5× electrode depth; typical target <5 Ω for industrial, <25 Ω for residential.

Ground grid design: touch and step voltage limits per IEEE 80; E_step = (1000 + 6C_s ρ_s) × 0.116/√t; E_touch = (1000 + 1.5C_s ρ_s) × 0.116/√t.

Ground fault protection: zero-sequence CT or ground return CT; pickup typically 10–20% of CT rating; time delay for coordination.

Power quality: THD (total harmonic distortion) = √(Σ V_h²)/V_1 × 100%; IEEE 519 limits THDv to 5% at PCC for <69 kV.

Voltage unbalance: NEMA MG-1 derates motor if unbalance >1%; unbalance = maximum deviation from average / average × 100%.

Harmonic filters: passive (tuned LC) or active; mitigate resonance and reduce THD.

Grounding electrode system: ground rod, plate, or pipe; resistance to ground depends on soil resistivity (ρ), electrode length, and diameter.

Transient overvoltage: surge protective devices (SPD) clamp voltage; coordinate with insulation withstand levels.