Solar, Storage

Bonded PV Arrays Could Provide Safer Solar Power

Most electricians are referred to as “indoor wiremen” and are experts at installing electrical equipment indoors, but some lack experience working with exposed cables and outdoor equipment bonding. Traditional methods of bonding and grounding include the extensive use of bare copper conductors for equipment bonding. While copper is preferred for bonding steel, it is not the best choice around exposed aluminum members. Aluminum and copper should not come into direct contact due to galvanic corrosion issues. Over time, copper has a tendency to erode aluminum and can even cause structural failure of thin aluminum extrusions when in direct contact.

Even though the National Electric Code (NEC) supports the bonding of PV module frames to their metal support structures, this method has only recently become widely accepted by the PV industry and authorities having jurisdiction (AHJs). While this practice was common for PV system installations during the 1980s and 1990s, extra focus on bonding aluminum PV module frames in the mid-2000s caused many jurisdictions to question whether this practice was appropriate. During the mid-2000s, methods that connected each PV module to an equipment grounding conductor became the widely accepted method. This was primarily due to installation information found in many PV modules’ manufacturing literature for products listed to UL 1703, the safety standard for PV modules.

Over the past few years, a new safety standard has been under development, UL 2703, entitled, “Mounting Systems, Mounting Devices, Clamping/Retention Devices, and Ground Lugs for Use with Flat-Plate Photovoltaic Modules and Panels.” One of the main reasons for the development of this standard was to have a method to evaluate bonding methods that a mounting system may employ to address the requirements of NEC 690.43.

Review of TUV Field Evaluation Report

In November 2014, TUV performed testing on bonding equipment in place at the Mandalay Bay Convention Center 6.4-MW PV array in Las Vegas, Nevada.  The equipment at the test site was the Unirac RM Roof Mount low-sloped roof mounting system. The field evaluation report provided by TUV, dated December 1, 2014, was based on the procedure set forth in the Bonding Path Resistance test in Section 13 of UL 2703.

Test site on the Mandalay Bay Convention Center rooftop in Las Vegas. Credit: Unirac.

This test requires a 4-wire design that uses two conductors to inject current into the test sample and another two conductors to measure the resultant voltage drop from the current injection point to the current receiving point. The worst case scenario was chosen for the bonding path test by injecting current at one corner of a 150 x 150-foot PV array and receiving the current at the opposite corner of the same array approximately 210 feet away. A simple diagram of the test setup, provided in the TUV report, is shown in Figure 1.

Figure 1: Test Setup for 150-foot by 150-foot Array. Credit: Unirac.

This bonding test shown in Figure 1 was duplicated with another test using a 6 AWG solid copper equipment grounding conductor. Since the copper conductor would typically be run around the perimeter of the array, rather than through the array, the length of copper used was 315 feet to be consistent with conventional installation methods. The test setup the solid copper wire is shown in Figure 2.

Figure 2: Test Setup for 315-foot and 150-foot Solid 6 AWG Copper Wire. Credit: Unirac. 

During the test, an 11-amp current and a 33-amp current were injected into the grounding circuit for each set of conditions. These two currents were chosen to coincide with the requirements of UL 2703 and to go well beyond those requirements, by a factor of 3. These currents simulate not just a single ground fault in one string, but simultaneous ground faults in 3 strings, which is much more than the standard requires. It is interesting to note that the change in current had no impact on the ground path resistance of either the bonded array or the 6 AWG copper conductor.

The key result from these tests was that the bonding path resistance for the bonded array was 0.0034 ? (3.4 milliohms) versus 0.1245 ? (125 milliohms) for the 6 AWG copper conductor. The typical published resistance values for 6 AWG copper is approximately 400 m? per 1000 feet so a 315-foot conductor should have resistance of approximately 126 m?, exactly what was measured. Clearly, the bonded array performed superior to the solid copper conductor, by a factor of 36. This means that a bonded RM PV array has 2.7 percent of the resistance of a solid copper 6 AWG conductor and will therefore carry 36 times as much current in a fault condition. Since fault currents are typically quite low in a PV array, and sometimes go undetected depending on the accuracy of a ground-fault detector, the RM bonding method is more likely to detect faults, resulting in a safer installation.

In addition to the worst-case test using a 150×150-foot array, testing was performed on a smaller 75 x 150-foot array as seen in Figure 3. This testing was done to determine if there was any appreciable change in the results from the bonding path resistance test. In this case, the solid 6 AWG copper conductor was only 150 feet long for this test given the small array section. Given the shorter bonding path for both the array section and the solid copper, the bonding path resistance was 2.2 m? for the array section and 57 m? for the solid copper. These results are expected given the shorter paths of both systems. The array section path distance was reduced from roughly 210 feet with the 150 x 150-foot array down to about 167 feet. This shorter distance was roughly 80 percent of the 210-foot distance and the ground path resistance reduced to 65 percent of the original path resistance.

Figure 3: Test Setup for 75-foot by 150-foot Array. Credit: Unirac.

The reason for large improvement in ground path resistance with the bonded array is because the resistance of a single path is simply the series resistance of the conductor — normally a copper conductor like solid 6 AWG. A bonded metal structure has literally thousands of current paths from one point to another point. Each of these different paths is in parallel, rather than series, with one another. Parallel resistors add according to the reciprocal of their resistance so the resistance always goes down with parallel resistance paths. With thousands of parallel paths, the resistance drops significantly.

Closeup of the bonded array at the test site in Las Vegas. Credit: Unirac.

Finally, with thousands of paths, comes the benefit of redundancy. With a bonded metal array, dozens of connections can be broken and there will be almost no change in ground path resistance. The many other paths simply take slightly more current. With a single equipment grounding conductor, a single connection can be broken and everything that the wire was connected to can become ungrounded. When a conductor is run 315 feet, rather than 3 feet from the bonded array to a ground bus in a combiner box, the probability that a failure occurs in that 315 feet is much more likely than a failure in the shorter 3-foot section.

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In Conculusion

In summary, bonding systems, tested to UL 2703, represent a large improvement in the performance and ease of meeting the equipment grounding requirements of the NEC. AHJs need to understand the benefits of these types of bonding and grounding configurations since these tests show that they are superior to conventional methods often used in the field today. The benefits of a bonded PV array are not only in ground-fault detection, but also in lower touch voltages, and better redundancy. In addition to better compliance with the intent and language of the NEC, bonding methods are much easier for the installer to install properly.

One of the major field problems with PV systems today is that archaic bonding and grounding methods are often difficult to perform properly in the field. This means that mistakes and bad connections are extremely common. PV array mounting methods that use products specifically designed to create electrical bonds at each connection point will ensure long-lasting safety for the exposed metal parts of a PV array.

Given the likelihood that exposed cables may become damaged over the 20-30 year life of a PV system, having well-bonded and grounded PV array structures is key to the long-term safety of PV systems.


PV System Equipment Bonding and Grounding Requirements

Article 690 in 2014 version (and the 2011 version) of the National Electric Code (NEC) and states the following:

(A) Equipment Grounding Required. Exposed non–current-carrying metal parts of PV module frames, electrical equipment, and conductor enclosures shall be grounded in accordance with 250.134 or 250.136(A), regardless of voltage.”

This section references two important sections in Article 250, Grounding and Bonding, sections 250.134 and 250.136(A), restated below:

“250.134 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed). Unless grounded by connection to the grounded circuit conductor as permitted by 250.32, 250.140, and 250.142, non–current carrying metal parts of equipment, raceways, and other enclosures, if grounded, shall be connected to an equipment grounding conductor by one of the methods specified in 250.134(A) or (B).

(A) Equipment Grounding Conductor Types. By connecting to any of the equipment grounding conductors permitted by 250.118.

(B) With Circuit Conductors. By connecting to an equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.

Exception No. 1: As provided in 250.130(C), the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.

Exception No. 2: For dc circuits, the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.

Informational Note No. 1: See 250.102 and 250.168 for equipment bonding jumper requirements.

Informational Note No. 2: See 400.7 for use of cords for fixed equipment.”

“250.136 Equipment Considered Grounded. Under the conditions specified in 250.136(A) and (B), the normally non–current-carrying metal parts of the equipment shall be considered grounded.

(A) Equipment Secured to Grounded Metal Supports. Electrical equipment secured to and in electrical contact with a metal rack or structure provided for its support and connected to an equipment grounding conductor by one of the means indicated in 250.134. The structural metal frame of a building shall not be used as the required equipment grounding conductor for ac equipment.”