To pass International Electrotechnical Commission (IEC) standards for insulation resistance testing, PV systems with an open circuit voltage rating greater than 120 Vdc must have an insulation resistance greater than 1 MΩ.
[pdf] The NFPA and IEEE recommend a ground resistance value of 5 ohms or less while the NEC has stated to “Make sure that system impedance to ground is less than 5 ohms specified in NEC 50.56. In facilities with sensitive equipment it should be 5ohms or less”.
[pdf] Ground your off-grid solar system properly to protect against lightning strikes and electrical faults. Install copper-clad ground rods at least 8 feet deep and connect them to your solar array frames, inverter, and battery bank using 6 AWG copper wire.
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