Solar PV Fundamentals and Safety
Domain 1 tests how a PV cell converts sunlight into DC electricity, how the solar resource is quantified, and the safety controls NCCER expects on every PV jobsite - electrical, fall, and environmental.
Measuring the resource
- Irradiance is instantaneous power density in W/m². Irradiation (insolation) is energy accumulated over time, kWh/m²/day.
- Peak sun hours (PSH) = daily insolation ÷ 1000 W/m², so a site at 5.5 kWh/m²/day gets 5.5 PSH.
- Standard Test Conditions (STC): 1000 W/m², 25 °C cell temperature, AM 1.5 spectrum. NOCT/NMOT describes module temperature at 800 W/m², 20 °C ambient, 1 m/s wind.
Temperature changes everything
Module voltage rises as cells cool. A crystalline module with a −0.29 %/°C Voc coefficient is about 10 % above its STC Voc at −10 °C. Maximum system voltage is a cold-temperature calculation, not an STC calculation.
Safety fundamentals
- An illuminated array is energized. Opening the DC disconnect does not de-energize the conductors between array and disconnect.
- DC arcs do not cross zero and can sustain; NEC 690.12 rapid shutdown limits conductor voltage near the array.
- NFPA 70E governs arc-rated clothing, shock protection, and lockout/tagout. Rubber insulating gloves must be voltage-rated for the system and air-tested before use.
- Fall protection is required at 6 ft under OSHA 1926.501; ladders are set at 4:1 and extend 3 ft past the landing.
Worked example
A 12-module string uses 41.2 V STC modules with a −0.29 %/°C coefficient. At a record low of −10 °C: ΔT = 35 °C, correction = 35 × 0.29 % = 10.15 %. Voc(cold) = 41.2 × 1.1015 = 45.4 V per module, so the string reaches 545 V. Against a 600 V inverter limit that leaves 55 V of margin - acceptable. At STC the string reads only 494 V, so an STC-only check understates the real risk by 51 V.
Traps
- Confusing irradiance (W/m²) with irradiation (kWh/m²/day).
- Assuming the array is dead because the inverter is off.
- Checking maximum voltage at 25 °C instead of the lowest expected ambient.