Some systems require the DC positive conductor to be connected to the grounding system. This is referred to as a “positive ground system.” It’s a common grounding approach in telecommunication installations, and is required by one manufacturer of solar-electric modules (SunPower). In a positive ground system, the breakers or fuses are installed in the negative wires (not the positives), since the negative is now the “ungrounded” conductor.
Regardless of whether the system is positive or negative ground, all the metallic enclosures and components still must be connected together to the same grounding system. This also applies to PV module frames, mounting racks, and wind turbine towers and anchors. All of the electrical systems (AC and DC) must share the same grounding system to be code compliant.
After finally getting a handle on what grounding means, you find out the NEC allows ungrounded systems as well! In the past, ungrounded systems were generally limited to small solar-electric systems operating at less than 50 VDC, but ungrounded systems are becoming more common with specific high-voltage, batteryless grid-tie systems. What’s the deal?
An NEC-compliant “ungrounded” system only eliminates the DC negative or DC positive (in positive ground systems) conductor’s connection to the grounding system. Instead, both conductors are considered “hot” and must include overcurrent protection and a means for disconnection, so breakers are commonly used, and installed in each conductor. This doubles the number of overcurrent protection devices, which means higher costs and somewhat lower performance for low-voltage battery systems. Reduced system performance is not an issue for a high-voltage, batteryless grid-tie systems since the currents are generally low. In all ungrounded systems, the PV module frames, the mounting structure, and all electrical enclosures and components must still be connected together and to a grounding electrode—just like with any normally grounded system.
RE equipment used in an ungrounded system should be specifically designed and listed for being installed in this application. The installation of products that require grounding in an ungrounded system can result in hazards to the installer or to the system user, and can damage devices connected to the system.