Ungrounded arrays can be just as safe—or safer—than grounded arrays. If installed correctly, with proper equipment grounding, ungrounded arrays may provide the safest installation possible: The ungrounded system inverters include an enhanced ability to detect ground faults, and there is reduced possibility of shock to personnel installing or servicing the system.
Without a connection to ground on the DC side, the largest shock potential is directly from positive to negative, with very little shock potential from either ungrounded conductor to ground (or anything bonded to ground, such as a metallic enclosure). To understand why this is beneficial, imagine an installer working on a roof with a negative-grounded system, positioning an energized, positive PV source circuit home run in a metal junction box. If the installer’s hand grazes against the metal box, the box completes the circuit between the positive and negative, which creates a serious shock hazard. (Safe working procedures should prevent this, but these procedures—such as not connecting any power sources until all conductors are wired at each termination—are not always followed.) If the system was ungrounded, the potential between the positive conductor and the metal junction box would have been close to zero volts—a high-resistance circuit with little shock potential. While no installer should work on energized conductors, ungrounded systems provide an additional level of safety. Homeowner safety is increased by enhanced ground-fault protection (see the ”Ground-Fault Protection” sidebar).
Reasons why U.S. PV systems have typically been grounded include a long history of grounded electrical systems in the United States; the topology of the majority of grid-tied inverters available for the U.S. market; the way integrated ground-fault protection detection devices work (see sidebar); and the past difficulties of meeting NEC 690.35 requirements for PV wire, since few modules were manufactured with it.
The majority of inverters on the U.S. market are transformer-based. A transformer is a heavy component, made of wire wound around iron or steel cores, used to match the inverter voltage with the AC voltage of the utility grid. A transformer has primary and secondary side windings, and the transformation occurs as current is induced from one side to the other. The conductors on either side aren’t touching, which means that a fault on the DC side of the system doesn’t transmit to the AC side (a situation that would be termed “DC injection”). There is no physical connection between conductors on the two sides.
A transformerless inverter, on the other hand, uses a higher-efficiency process of electronic switching in which the conductors are not isolated between the DC and the AC sides. If DC positive or negative in a transformerless inverter system was to become grounded, the ground-fault protection device wouldn’t allow the inverter to operate (see sidebar). Transformerless inverters are therefore installed with ungrounded PV arrays—those with no DC system ground, but always with equipment grounding.
Transformerless inverters generally weigh less, are more compact, and are slightly more efficient than transformer-based inverters. Many are now available in the United States (see the “2012 Grid-Tied Inverter Buyer’s Guide” in HP147). Manufacturers offering residential-sized transformerless inverters include Exeltech, Ingeteam, Kaco, OPTI-Solar, Power-One, REFUsol, SMA America, and SolarEdge. These inverters offer a range of DC voltage windows similar to transformer-based inverters, and most come with integrated fusing and DC disconnect options (for both the positive and negative DC conductors). If external fusing and disconnecting means are necessary, then special equipment, such as combiner boxes and disconnects with separate fusing, and disconnecting means for both the positive and negative conductors will be required. Battery-based systems could also be ungrounded, but Code-compliant GFP equipment for ungrounded charge controllers isn’t currently available.
There is no doubt that the number of ungrounded PV systems will continue to grow, claiming space on U.S. roofs and solar fields. While Europe may have paved the way for ungrounded systems, they are here to stay in the United States.
Rebekah Hren is a licensed electrical contractor, NABCEP-certified PV installer, and ISPQ-certified PV instructor for Solar Energy International. She lives off-grid and has experience installing and designing PV systems ranging from 10 watts to 4 megawatts. Rebekah works for North Carolina solar project developer O2energies.
Ungrounded System Inverter Manufacturers:
Exeltech • exeltech.com
Ingeteam • ingeteam.com
Kaco • kaco-newenergy.com
OPTI-Solar • opti-solar.com
Power-One • power-one.com
REFUsol • refusol.net
SMA America • sma-america.com
SolarEdge • solaredge.com