New Grounding Options

New Grounding Options

by Don Loweburg

Grounding metal enclosures, raceways, module frames, and mounting structures in electrical systems provides essential protection from electrical shock and fire. The National Electrical Code (NEC) dictates the basic methods for accomplishing this safety requirement. For PV arrays, an often-used method of meeting this requirement is to run a ground wire from each PV module frame, and connect it to the racking system and to the electrical system’s equipment-grounding conductor (see this issue’s Code Corner for a discussion of this method). For system installers, this method adds time and expense to the installation.


But in 2006, two manufacturers introduced new Underwriters Laboratories-listed grounding products that eliminate the need to run a wire to each module frame. Both Wiley Electronics’ WEEB (washer, electrical equipment bond) product and UniRac’s grounding clips are listed to UL Standard 467, which covers bonding washers and grounding devices. In addition, Sharp Solar recently introduced their SRS racking system with integral module grounding, though UL approval is still pending.

UniRac Grounding ClipWEEB ground


WEEB’s grounding method uses a special stainless-steel bonding washer. The washer has piercing teeth on both sides, situated so that when the washer is placed between the module frame and the racking system, a water- and airtight, sealed electrical connection between the module frame and racking is created. Tightening the module hold-down nuts to the required torque is critical to making a good ground connection when using these devices. The racking structure is then connected, using appropriate lugs and wire, to the equipment ground of the system. 
UniRac’s ground clips work similarly to the WEEBs, with the clip being sandwiched between the rack frame and the module frame. When the module frame’s hold-down is tightened, the piercing teeth complete the ground connection between the module frame and the rack frame.


Neater, Cheaper PV Installations


These new systems offer the potential for better-looking installations—and labor and material savings, since installation is usually quicker and the need for copper wire is reduced. Reports from the field estimate that these products can reduce time spent grounding arrays by approximately 50 percent. August Goers, an installer with Luminalt in San Francisco, reports that “the clips drastically reduce installation time and cost because we can complete the entire racking ground system before placing the modules. It also reduces the amount of tin-coated copper lugs used.”


But others say that time and cost reductions really depend on the installation specifics. “Our first ‘try-it-out’ WEEB installation was a big off-grid job, involving twenty-seven BP160 modules on three trackers,” says Allan Sindelar of Positive Energy in Santa Fe, New Mexico. “[Using the WEEB method] turned out to be not much of a cost or labor savings because of the multiple tracker layout. But a later installation of 40 roof-mount modules as four rows of ten modules made for a significant wire and labor savings.”


Code Contentions


Not all installers and PV professionals are comfortable with the use of grounding washers. William Miller of Miller Power and Communications in Atascadero, California, expresses his concern around the fact that modern grid-connected PV systems operate at up to 600 volts DC, posing an extreme hazard if the system isn’t adequately grounded.


Thomas W. Bowes, assistant director of the Detroit JATC (an IBEW union training center) and PV installation instructor, shares Miller’s concern. In his recently published paper, Bowes cites several sections of the NEC that could be interpreted to cast doubt on the use of grounding washers (see Access).


“Even though this method (grounding washers) is available, it is rarely used in the field because of the difficulties in establishing and maintaining a solid, low-impedance grounding connection between electrical devices and their associated mounting racks,” says Bowes in his report. “In fact, general practice in the industry is to require a properly sized copper equipment-grounding conductor instead of any other means recognized by the NEC.” Bowes says he favors the use of a ground wire because this is the general practice, industry-wide, and that this method has been reliable. He questions the ability of other methods to establish and maintain a solid, low-impedance grounding connection. He does not, however, cite any NEC sections that specifically prohibit the use of grounding washers.


Brian Wiley, developer of WEEB, responded to Bowes’s assertions (see Access). In his response, Wiley engages in a bit of the “code dance” with Bowes by stating his interpretations of the NEC articles that allow ground washers. The most convincing part of Wiley’s response is his report of the actual tests performed as part of the UL 467 listing process. According to Wiley, “WEEB products are certified to carry a current of 1,530 amps for 6 seconds…results [that] have been tested by Intertek ETL, a nationally recognized testing laboratory.” He also points to WEEB’s “long-term reliability,” citing accelerated lifetime tests conducted in-house in which the WEEB product was subjected to thermal cycle tests and salt water environment tests that “indicate exceptional reliability,” especially when compared to the lay-in lug method.


Phil Crosby, product development manager at UniRac, says that UniRac’s grounding clips have undergone similar rigorous testing by the company. According to Crosby, the washers tested as good as or better than other approved grounding methods.


But Bowes says that “it is one thing to do a bench evaluation of a product under ideal conditions in a controlled environment, but something quite different to consider the field application of the product and try to examine it in light of how it will actually be used.”


Proposed code changes to NEC Section 690.43, Equipment Grounding (Revised), due in 2008, seek to clarify this contention. The salient change that would specifically speak to using ground washers would read, “Devices listed and identified for grounding the metallic frames of PV modules are permitted to ground the exposed metallic frames of PV modules to grounded mounting structures.”


Grounding Details


Most parties do agree on one particular safety issue that may arise with either the traditional lay-in lug or the new clip-grounding approaches. Ground faults can occur if a module frame becomes energized due to faulty equipment or installation work. When removing a ground-faulted module from an energized PV array, an extreme shock hazard will exist if the module equipment ground is removed before the power wiring is opened. 
This is a major safety concern, but it has nothing to do with grounding methods. Rather, this safety issue is directly a result of the fact that PV modules cannot be easily turned off. This reality must be understood and respected by all installers. The solution requires safe work practices and knowledgeable, experienced installers. All module manufacturers, in their instructions, require modules to be covered during service. As an extra precaution, a separate, temporary ground jumper can be attached to the module frame and rack before the module is lifted from the rack and disconnected from the power circuit wiring. Because safety is paramount, servicing PV systems and arrays should only be done by qualified persons.


Moving Forward


The future of the PV industry depends on the safety and reliability of installed systems. Manufacturers, installers, and inspectors must continually strive for high standards. And although intelligent, well-meaning people may not always agree, engaging an issue from a conversational context often produces great results and better, safer, and more durable products. After all, today’s innovation may likely be tomorrow’s tradition.

Introducing Inspectors to Innovation

Inspectors are trained to look for an equipment ground wire connected to each module frame during field inspections, so it is prudent to clearly document your intention to use any new grounding approach. Installers planning to use grounding washer products should include explicit reference to the grounding method in their plans. This can be done with a note on the electrical one-line diagram that is required for most projects. Also include installation instructions for the grounding washer, UL listing information, and a copy of the module installation directions.


Few module manufacturers explicitly allow bonding washers in their instructions. By providing advance notice to the inspector and full documentation before the inspection, my experience is that there will be no grounding corrections from the inspection. I’ve been successful gaining approval from the inspectors in all four of the local jurisdictions (Central California) in which I work.
Installer August Goers says he’s had similar experiences. “We work mainly in San Francisco, which has very strict grounding policies. When obtaining the permit for our first job with the UniRac clips we brought in a sample UniRac rail, ground clip, and module clamp for the head inspector to see. He approved our use of the product and we haven’t had any problems with inspectors.”

 


Home Power 121 / October & November 2007