NABCEP PVCMS cheat sheet

PPE for energized DC work: arc-rated clothing per incident-energy analysis, voltage-rated gloves with leather protectors, and eye protection; test gloves before each use by inflation.

Open-circuit voltage at the coldest expected cell temperature: Voc_cold = Voc_STC × [1 + (β_Voc/100) × (T_cold − 25)], where β_Voc is the Voc temperature coefficient in %/°C (typically negative).

String sizing rule: maximum string Voc_cold must not exceed inverter maximum DC input voltage; minimum string Vmp at the hottest cell temperature must remain above the inverter MPPT lower limit.

DC arc flash is sustained above roughly 80 V; treat all PV source and output circuits as energized until the array is isolated, the inverter is off, and conductors are tested and grounded.

Lockout/tagout sequence for a string inverter: open the AC disconnect, open the DC disconnect, verify zero energy at the inverter terminals, then isolate and label the array circuits.

Rapid shutdown: conductors more than 30 V within the array must be reduced to 30 V or less within 30 seconds of shutdown initiation; verify per the module or MLPE listing.

Conductor ampacity: continuous current is multiplied by 1.25 for the 125% factor, then the result is adjusted for ambient temperature and conduit fill using the applicable correction tables.

Voltage drop on a DC run: Vdrop = 2 × L × I × R_conductor / 1000 for single-phase two-wire circuits, with L in feet and R in ohms per 1000 ft; keep total drop typically under 2–3%.

Grounding: equipment grounding conductors are sized per the applicable table for the overcurrent device rating; array frames, module clamps, and racking are bonded to the EGC, not used as the only grounding path.

Commissioning order: visual inspection, then insulation resistance (megohmmeter) test on isolated conductors, then open-circuit voltage and short-circuit current measurements, then inverter startup and monitoring verification.

Insulation resistance test: apply typically 500 V or 1000 V DC for one minute between conductors and ground with the array isolated; record resistance in megohms and compare to the system baseline.

Irradiance correction for field Isc: Isc_corrected = Isc_measured × (1000 / G_measured), where G_measured is in W/m²; expect measured Isc to track irradiance within about ±5%.

String Voc check: measured string Voc should be within roughly ±5% of the calculated string Voc at the measured cell temperature; a low string indicates a failed module or open connection.

Polarity check: verify positive and negative conductors at each string and at the combiner before energizing; reversed strings can destroy inverters and bypass diodes.

I-V curve tracing: compare the field curve to the expected STC curve; step or stair-step shape indicates mismatch or bypass-diode activation, while a shifted curve indicates irradiance or temperature differences.

Inverter commissioning: confirm grid voltage and frequency windows, anti-islanding settings, and trip times match the local utility interconnection agreement before closing the AC disconnect.

Monitoring baseline: record production, irradiance, and module temperature at commissioning so later performance-ratio comparisons have a valid reference.

Performance ratio: PR = (E_AC / (P_STC × H_poa)) × 100%, where E_AC is AC energy in kWh, P_STC is array rating in kW, and H_poa is plane-of-array irradiance in kWh/m²; typical good values are 75–85%.

Thermal imaging: scan modules under load at irradiance above about 500 W/m²; a hot cell or substring indicates a failing cell, open bypass diode, or bad connection; a uniformly warm module may indicate reverse current.

Visual inspection checklist: cracked glass, delamination, snail trails, burnt connectors, corrosion at terminations, animal nesting, and vegetation shading.

Connector maintenance: use only matched connector types from the same manufacturer; mismatched MC4-style connectors are a leading cause of series arc faults and fires.

Torque verification: re-torque module clamps and grounding lugs to manufacturer specifications during periodic maintenance; loose bonding connections raise resistance and create heat.

Bypass diode failure: a module substring with a shorted diode shows reduced Voc by roughly one-third; an open diode produces a hot spot under shading and is found by thermal scan or I-V curve.

Soiling loss: clean when production loss exceeds the economic cleaning threshold; in high-dust areas, cleaning frequency may be monthly, while in temperate areas it may be annual.

Inverter maintenance: check fan operation, filter condition, and DC/AC terminal torque; review fault logs for ground-fault, arc-fault, and grid-trip events and trend them over time.