Before a new string is connected to the inverter, which two checks should be made at its ends with a DC voltmeter?
- Operating current and frequency
- Power factor and frequency
- Open-circuit voltage and polarity
- Insulation thickness and polarity
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C. Open-circuit voltage and polarity
Measuring Voc confirms that the right number of modules are in series and connected, and checking polarity confirms positive and negative are not swapped. Connecting a reversed or over-voltage string can destroy equipment.
Reference: Commissioning: string verification
A monitoring portal shows one of three identical inverters producing far less than the other two on a clear day. What does this suggest?
- Normal variation that needs no further attention
- A utility voltage problem affecting the whole site
- A seasonal change in the available solar resource
- A fault or shading affecting that inverter's array
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D. A fault or shading affecting that inverter's array
Identical equipment under the same sun should produce nearly the same, so an outlier points to a problem on that unit such as a failed string, shading or soiling. A site-wide cause would pull all three down together.
Reference: Performance monitoring: comparing like equipment
Which instrument is used to measure the operating current of a string without opening the circuit?
- A clamp-on ammeter rated for AC only
- A clamp-on ammeter rated for DC
- An insulation resistance tester
- A non-contact voltage tester
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B. A clamp-on ammeter rated for DC
A DC clamp meter uses a Hall-effect sensor to read current through the closed jaw, so the circuit stays intact. An AC-only clamp relies on a changing magnetic field and reads nothing useful on steady direct current.
Reference: Test instruments: DC clamp meters