CODE CORNER: An Earth-Shifting Change in Grounding

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The sections of Article 690 that deal with grounding PV systems have changed significantly over the last several code cycles, but never so much as in 2017. This is primarily due to the introduction of a newly defined term—“functional grounded.” The implications are extensive—a wholesale simplification and homogenization of grounding the DC side of PV systems, along with the shortening (and even elimination) of many parts of Article 690, resulting in a single comprehensive theory and methodology that works for all types of PV systems.

Grounding Electrical Systems

“Grounded, solidly” is defined in the National Electrical Code (NEC) as “connected to ground without inserting any resistor or impedance device.” Most of the AC systems that PV systems are connected to—especially in residential applications—are “solidly grounded.” This is accomplished in a 240/120 VAC split-phase service by the main bonding jumper, which connects neutral to ground in the service equipment. This is a solid bond, and the connection is usually made with equipment supplied by the enclosure manufacturer. It may be a wire between the neutral and grounding bus bars; a strap that bonds the neutral bus to the chassis of the box; or a green screw that runs through the neutral bus bar and into the chassis.

Historically, there have been some solidly grounded PV systems in which one of the DC conductors—often, but not always, the negative—is mechanically connected to ground, using means similar to those described above for AC systems. However, with growth in PV system installations and realizations of some of the associated risks and hazards they present, requirements for DC ground-fault protection appeared in the NEC in the 1990s. One common way to implement DC ground-fault protection is with inverter-integrated systems, which typically consist of a fuse that acts as the main bonding jumper, forming the connection between the grounded and grounding conductors.

However, it’s hard to consider a fuse, which can melt open in a ground-fault scenario, a solid connection to ground. It makes more sense to call them “functional grounded PV systems” in that they have “an electrical reference to ground that is not solidly grounded.” Further clarification in the definition’s Informational Note states that in a functional grounded PV system, the connection to ground is often through a “fuse, circuit breaker, resistance device, non-isolated grounded AC circuit, or electronic means that is part of a listed ground-fault protection system.” This list essentially covers 99.9% of all PV systems installed in the last 10 years or more. Grounded or ungrounded—whatever we used to call them—they are “functional grounded” now.

Now, the only type of PV system permitted to be solidly grounded has only one or two PV source circuits, and no DC circuits on or in a building. This limits these applications to small, load-focused, PV-direct systems and perhaps some smaller battery-based systems.

Comments (1)

Douglas Grubbs's picture

The last paragraph of the Grounding Electrical Systems seems like something new but it is not new except terminology. Solidly Grounded (2017) means the same as Grounded without GF protection (2014).
However, GF protection in 2017 now requires more Isolation of the PV current carrying conductors.
While older inverters are galvanically isolated, almost all Solar Controllers (MPPT & PWM) are not. This makes the 1A fuse solution incompatible with DC Coupled battery systems that must remain grounded.
Article 712 defines Grounded Two-Wire Battery Systems as "a system with a solid connection or reference-ground from one of the current carrying conductors and the equipment grounding system" which means the battery bank remains bonded to ground excluding the use of 1A fusing of the GEC with most DC Coupled PV systems.
Morningstar's GPFD-150V and GFPD-600V meet 2017 GF protection by disconnecting all of the PV current-carrying conductors of the array without affecting the DC ground at the battery.
Therefore, it is possible to have a DC Coupled functionally grounded PV system that meets Part V of the 2017 NEC with a Solidly Grounded Battery Bank.

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