Safety, Reliability, and Power Quality Standards
This domain tests the standards that define safe construction, measurable reliability, and acceptable voltage quality, and the reporting conventions that make those numbers comparable between utilities.
Safety codes
- NESC (IEEE C2) governs clearances, grades of construction, strength requirements, and grounding for supply lines; state adoption varies but the technical content is standard.
- NFPA 70E governs arc-flash risk assessment: incident energy calculation, the 1.2 cal/cm2 threshold for arc-rated clothing, and the limited, restricted, and arc-flash approach boundaries. The former PPE category approach is being displaced by incident-energy methods.
- IEEE 80 governs substation grounding, with step and touch potential limits derived from body current tolerance.
Reliability indices (IEEE 1366)
- SAIFI = total customer interruptions / total customers served.
- SAIDI = total customer-minutes of interruption / total customers served.
- CAIDI = SAIDI / SAIFI, the average restoration time.
- MAIFI covers momentary events; ASAI is availability. Major event days are excluded using defined statistical methods so that storm years do not swamp comparisons.
Power quality
- ANSI C84.1 Range A is plus or minus 5% at the service entrance on a 120 V base (114-126 V); Range B allows wider excursion but not as normal practice.
- IEEE 519 limits voltage THD at the point of common coupling and current distortion scaled by the short-circuit ratio; inverter-based resources also fall under IEEE 1547 ride-through requirements.
- IEEE 1159 classifies disturbances as sags, swells, interruptions, transients, and waveform distortion; flicker is measured as Pst and Plt.
Worked example
A utility serves 1,000 customers. Event one interrupts 200 customers for 2 hours; event two interrupts 50 customers for 4 hours. SAIDI = (200 x 120 + 50 x 240) / 1,000 = (24,000 + 12,000) / 1,000 = 36 minutes. SAIFI = 250 / 1,000 = 0.25 interruptions per customer. CAIDI = 36 / 0.25 = 144 minutes, or 2.4 hours.
Traps that cost marks
- Reporting SAIDI including major event days and comparing it to a utility that excludes them.
- Confusing SAIDI (duration) with CAIDI (restoration time).
- Applying a single 5% harmonic limit at all voltage levels and ignoring the short-circuit ratio.
- Quoting ANSI C84.1 Range B values as normal service voltage.
- Treating arc-flash PPE categories as equivalent to calculated incident energy.