NABCEP PVCMS flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Voc cold temperature formula

Voc_cold = Voc_STC × [1 + (β_Voc/100) × (T_cold − 25)]. Use the module datasheet temperature coefficient β_Voc in %/°C and the coldest expected cell temperature to size string maximum voltage.

Maximum system voltage

The highest DC voltage the module and conductors are rated to withstand, commonly 1000 V or 1500 V. String cold Voc must not exceed the inverter maximum DC input or this rating.

Rapid shutdown threshold

Conductors more than 30 V within the array must be reduced to 30 V or less within 30 seconds of shutdown initiation. Verify the module or MLPE listing and test at commissioning.

Continuous current 125% factor

PV source and output circuit conductors are sized at 125% of the continuous current, then adjusted for ambient temperature and conduit fill using the applicable correction tables.

Arc-rated PPE requirement

Energized DC work requires arc-rated clothing selected from an incident-energy analysis, voltage-rated gloves with leather protectors, and eye protection. Inspect gloves by inflation before each use.

DC voltage drop formula

Vdrop = 2 × L × I × R / 1000 for a two-wire DC circuit, with L in feet and R in ohms per 1000 ft. Keep total DC voltage drop typically under 2–3%.

String sizing MPPT window

Minimum string Vmp at the hottest expected cell temperature must stay above the inverter MPPT lower limit. Maximum string Voc at the coldest temperature must stay below the inverter maximum DC input.

Lockout tagout sequence

For a string inverter: open AC disconnect, open DC disconnect, verify zero energy at inverter terminals, then isolate and label array circuits. Test conductors before touching them.

Equipment grounding conductor sizing

The EGC is sized from the applicable table based on the overcurrent device rating. Module frames, clamps, and racking are bonded to the EGC, not used as the only grounding path.

DC arc flash threshold

DC arcs can be sustained above roughly 80 V. Treat all PV source and output circuits as energized until isolated, tested, and grounded, regardless of inverter status.

Insulation resistance test

Apply typically 500 V or 1000 V DC for one minute between isolated conductors and ground, then record resistance in megohms. Compare to the system baseline to detect moisture or insulation damage.

Irradiance-corrected Isc

Isc_corrected = Isc_measured × (1000 / G_measured), where G_measured is in W/m². Measured Isc should track irradiance within about ±5% when the array is healthy.

String Voc field check

Measured string Voc should be within roughly ±5% of the calculated string Voc at the measured cell temperature. A low reading indicates a failed module or open connection.

Commissioning test order

Visual inspection, then insulation resistance on isolated conductors, then open-circuit voltage and short-circuit current, then inverter startup and monitoring verification. Each step depends on the prior one.

I-V curve interpretation

Compare the field I-V curve to the expected STC curve. A step or stair-step shape indicates mismatch or bypass-diode activation; a shifted curve indicates irradiance or temperature differences.

Polarity verification

Confirm positive and negative conductors at each string and at the combiner before energizing. Reversed strings can destroy inverters and bypass diodes and create reverse current through modules.

Anti-islanding verification

Confirm the inverter trips within the utility-specified voltage and frequency windows and times before closing the AC disconnect. Settings must match the interconnection agreement.

Commissioning monitoring baseline

Record production, irradiance, and module temperature at commissioning. This baseline enables valid performance-ratio comparisons and fault detection during later maintenance.

Megohmmeter test voltage

Use typically 500 V or 1000 V DC for PV array insulation resistance testing, applied for one minute with the array isolated and conductors disconnected from the inverter.

Open-circuit voltage measurement

With the string isolated and inverter disconnected, measure Voc across the string conductors. Compare to the temperature-corrected expected value to confirm string integrity.

Short-circuit current measurement

With the string isolated, measure Isc using a clamp meter rated for DC. Correct to 1000 W/m² and compare to the datasheet value; low current indicates soiling, shading, or a failed module.

Performance ratio formula

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. Good values are 75–85%.

Thermal imaging conditions

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.

Connector mismatch hazard

Use only matched connector types from the same manufacturer. Mismatched MC4-style connectors are a leading cause of series arc faults and fires in PV arrays.

Bypass diode failure signs

A shorted bypass diode reduces substring Voc by roughly one-third. An open diode produces a hot spot under shading and is found by thermal scan or I-V curve tracing.

Soiling loss decision

Clean when production loss exceeds the economic cleaning threshold. High-dust areas may need monthly cleaning; temperate areas may need only annual cleaning.

Torque verification

Re-torque module clamps and grounding lugs to manufacturer specifications during periodic maintenance. Loose bonding connections raise resistance and create heat.

Inverter fault log review

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 to catch developing failures.

Visual inspection checklist

Look for cracked glass, delamination, snail trails, burnt connectors, corrosion at terminations, animal nesting, and vegetation shading. Document findings with photos and location.

Ground-fault detection

Inverter ground-fault protection trips when current on the grounded conductor exceeds the threshold. Investigate moisture intrusion, damaged insulation, or a failed module before resetting.

Arc-fault circuit protection

AFCI-equipped inverters detect series arc signatures and trip. Frequent trips often indicate a loose or mismatched connector; locate and repair the connection rather than repeatedly resetting.

Module cleaning method

Clean with water and a soft brush during low irradiance, avoiding abrasive tools and high-pressure spray. Never clean hot glass with cold water, which can crack modules.