Inspectors, plan reviewers, and PV installers frequently call and email questions about current Code requirements and questions about the 2014 NEC requirements and how they differ from 2011 and previous editions of the Code. This article will address the existing requirements to ground those ungrounded PV arrays and transformerless inverters. It will look at the new requirement in the 2014 NEC for the Rapid PV Shutdown System in Section 690.12. Also, the continuing issues associated with PV module grounding will be discussed.
Ungrounded PV arrays and transformerless inverters
Grounding an ungrounded array sounds counterintuitive and sometimes is confusing. Ungrounded PV arrays are directly linked to transformerless inverters that are also known as non-isolated inverters. Section 690.35 appeared in the 2005 Code and addresses requirements for installing an ungrounded PV array. This Code allowance for an ungrounded PV array came about because of the need to introduce the transformerless inverter into the North American market. These inverters have been used in Europe for decades. See photo 1.
Photo 1. Is this array grounded or ungrounded? See the three-line diagram (supplied with the permitting package) for the answer.
A very short history lesson is in order. For about 110 years the United States, North America, and a few other parts of the world have used grounded premises electrical systems where one of the circuit conductors is grounded (connected to earth). This would include the ac neutral conductor as well as one of the dc circuit conductors in a PV array. In order to convert direct current from the PV array to alternating current that can be used to run ac loads or to connect to the ac utility grid, some sort of switching device must be used. Because one of the dc circuit conductors is grounded and one of the ac circuit conductors is grounded, it is not possible to use a simple solid-state switching device to convert dc to ac because the grounded circuit conductor on the dc side would short to ground the ungrounded conductor on the ac side when the switch was activated. Therefore, PV inverters in North America were generally required to include a transformer to isolate the ac grounded conductor from the dc grounded conductor. The transformer was required in addition to the solid-state switching device. Obviously, inverters are far more complicated than this simple description.
Much of the rest of the world uses ungrounded electrical systems where none of the circuit conductors in the premises wiring system are connected to ground. For example, many countries in Europe have used ungrounded electrical systems for more than a century. In Germany, three-phase, four-wire ac power is fed to each residence and the fourth wire, while it is a neutral, is not grounded. When PV systems were initially designed and installed in these countries, there was no requirement to ground one of the dc conductors. Without a grounded dc circuit conductor, the inverter could be considerably simplified because the transformer was no longer required. These transformerless inverters (aka non-isolated inverters), lacking the large, heavy, expensive transformer could be more efficient, smaller in size, lighter in weight, cheaper to ship, and easier to install as well as potentially lower in cost than a similar inverter with a transformer.
When an ungrounded PV array is being discussed, the term “ungrounded” refers to only the circuit conductors. In an ungrounded PV array, neither the positive nor the negative circuit conductors are directly connected to ground either in the array wiring or in the inverter. This is different from a grounded PV array where one of the circuit conductors, either the positive conductor or the negative conductor, is grounded directly inside the utility-interactive inverter as part of the ground-fault circuit protection device required by Section 690.5 in the Code. On some stand-alone PV systems, the ground-fault protection device is a separate device or part of the charge controller, and one of the circuit conductors is grounded by that device.
Equipment grounding required on all systems.
In both ungrounded PV arrays and grounded PV arrays, there is always a requirement for equipment grounding conductors that will connect all exposed non-current-carrying metal surfaces that could possibly be energized (under fault conditions) to the grounding system. These equipment grounding conductors would be connected to the exposed metal PV module frames and to any metal rack members that could be accidentally energized. See Section 690.43.
Additionally, under the 2008 and 2014 Codes, but not under the 2011 NEC, the grounded and ungrounded PV array exposed metal surfaces must be connected to earth directly by a grounding system that is in addition to the requirement for an equipment grounding system. See Section 690.47(D).
Ungrounded PV Arrays similar to Grounded PV Arrays – Except.
From an installation or inspection point of view, both grounded and ungrounded PV arrays are similar. However, there are differences in the details. A grounded PV array will have white or gray insulated conductors in the dc source circuit conduits indicating the grounded circuit conductor where the ungrounded PV array will have no white or gray insulated conductors.
In both systems, each ungrounded conductor will have a disconnect and overcurrent protection (where required). In the grounded system, this will mean one of the circuit conductors will have a disconnect and overcurrent protection; where in the ungrounded system, both of the circuit conductors will have disconnects and overcurrent protection.
Transformerless versus transformer inverters.
The more common inverter which includes a transformer will typically have equipment grounding conductor (EGC) terminals for each dc input circuit and each ac output circuit. There will also be a grounding electrode conductor (GEC) terminal on the inverter because the Section 690.5 internal ground-fault protection device circuit makes a grounded conductor-to-ground bond inside the inverter. Underwriters Laboratories Standard UL 1741 requires this GEC terminal on these types of inverters. However, a transformerless inverter or a non-isolated inverter has no internal grounded conductor-to-ground bond and therefore will require no GEC terminal. The ground-fault protection requirements in 690.35 for the transformerless inverter result in the inverter detecting ground faults using different methods than the 690.5 ground-fault protection devices. See photo 2.
Photo 2. Do these inverters have a transformer or not? See the label and the instruction manual (supplied with the permitting package) for the answer.
Where the grounding electrode conductor terminal is present, it must be connected to a grounding system according to the requirements established in Section 690.47 (C). Also note 110.3(B).
Rapid PV Shutdown 690.12 2014 NEC
This requirement was established to enhance the safety for first responders entering a building or structure that has a PV system. Research indicated that voltages in excess of 30 volts (V), ac or dc, in a wet environment present a shock hazard. Also, circuits with a power level in excess of 240 volt-amps (VA) represent a potential hazard. With these two values in mind, Section 690.12 was added to the 2014 NEC in the following form (quoted from NFPA 70 – 2014 NEC):
“690.12 Rapid Shutdown of PV Systems on Buildings. PV system circuits installed on or in buildings shall include a rapid shutdown function that controls specific conductors in accordance with 690.12(1) through (5) as follows.
(1) Requirements for controlled conductors shall apply only to PV system conductors of more than 1.5 m (5 ft) in length inside a building, or more than 3 m (10 ft) from a PV array.
(2) Controlled conductors shall be limited to not more than 30 volts and 240 volt-amperes within 10 seconds of rapid shutdown initiation.
(3) Voltage and power shall be measured between any two conductors and between any conductor and ground.
(4) The rapid shutdown initiation methods shall be labeled in accordance with 690.56(B).
(5) Equipment that performs the rapid shutdown shall be listed and identified.”
As usual, the Code does not specify how this requirement is to be achieved. Eventually, there will be a standard that will address these requirements, particularly if new equipment is to be developed. However, we can determine which circuits will probably be affected and estimate how the shutdown might be initiated.
It seems evident, that some sort of external, readily accessible, highly visible switch or pushbutton or some other device will be used to initiate the rapid shutdown. Although, it could be tied to the ac premises wiring and be initiated when the premises wiring was disconnected from utility grid, that would not work in stand-alone, off-grid systems and the shutdown would be slower because of the need to wait for the utility company to disconnect the ac grid power. See photo 3/3A for a solar shut-off device by MidNite Solar Inc. that can be used with remote contactors to shut down parts of the PV system.
Photo 3/3A. Solar Shut-off Control Box for first responders by MidNite Solar Inc.
Energized circuits that will obviously be affected include each PV source circuit away from the array more than the distance specified and the output of any dc combiner where such an output is on the roof and away from the PV array. Less obvious, is the dc input to the utility interactive inverter that can remain energized for up to five minutes after the dc input is removed due to the energy storage capacitors internal to the inverter. Also, the outputs of any battery bank exceeding the distance and voltage and VA limits must also be disconnected.
In the stand-alone system, or in the multimode utility-interactive system, the stand-alone ac outputs from the inverter must also be de-energized. This might be achieved by simply turning off the inverter or disconnecting all inputs. In a utility interactive PV system, the ac output circuit of the inverter is energized by both the inverter and the utility grid. The exact language of the code in 690.12 does not specify ac or dc PV conductors and both can be dangerous. It is not clear at this point how these circuits should be treated. Without dc inputs, the inverters will source no power to the ac outputs. However, these output circuits will still be energized by the utility grid until the utility power is disconnected. It is noted that these ac inverter output circuits would be no more dangerous than the numerous ac branch circuits throughout the building that would also be energized until the main disconnect were opened or the utility power were removed.
And, first responders should always note placards (required by Code) indicating the presence of supply-side connected PV systems where the ac output circuits will not be de-energized by opening main service disconnect for the building or structure. Removal of utility power from the building or structure will deenergize the circuits but that disconnecting process is not always as timely as one would like.
Grounding PV Modules
UL Standard 1703. We currently have some degree of confusion and conflict in the UL standards for grounding PV modules. UL Standard 1703 for flat plate PV modules was revised in 2012 to specifically require the following statements to be in the instruction manual for each certified/listed PV module:
48.1(B) 2) “The module is considered to be in compliance with UL 1703 only when the module is mounted in the manner specified by the mounting instructions below.”
48.1(B) 3) “A module with exposed conductive parts is considered to be in compliance with UL 1703 only when it is electrically grounded in accordance with the instructions presented below and the requirements of the National Electrical Code.”
The Nationally Recognized Testing Laboratory (NRTL) (UL, CSA, ETL, TUV, etc.) certifying or listing the PV module is required to ensure that these statements are in the instruction manual. The standard has the following phrase to indicate this is a requirement for the NRTLs:
48.7 The contents of the instruction manual and subsequent revisions to the instruction manual shall be verified for compliance with this standard by inspection.
Should these statements not be in the module instruction manual, AHJs should query the NRTL.
Unfortunately, not all module instruction manuals have the statements because some modules have not been through the certification/listing process since May 2012 when the standard was changed. And then there is the UL Outline of Investigation UL 2703 for PV racks which further complicates the situation.
UL Outline of Investigation 2703. An outline of investigation is not a standard. It is a work in progress and 2703 is no different than other outlines of investigation. A Standards Technical Panel is actively developing it and at this point there is not complete agreement on the specific requirements for grounding and mounting PV modules and how the final requirements should be established. However, since the outline of investigation is a published document, several NRTLs are using this outline to certify/list PV racking systems. These certified racking systems have instructions for grounding the modules using some sort of a structural grounding clip or device and instructions for mounting the modules also typically using a top clip mounting system. See photo 4. It is not clear in the existing wording in the outline that the outline specifically allows a racking system to be certified as a method of mounting and grounding all PV modules. The outline certainly will allow racking systems that meet the requirements to be used as equipment grounding conductors and the outline will also verify the mechanical integrity of the racking system.
On the other hand, as noted above, UL 1703 requires a specific mounting and grounding system to be specified in the instructions for that PV module. Only a very few PV module instruction manuals show any mounting system other than using the four bolt holes that are associated with the back of the module. And only a few show any other grounding method than using the marked grounding holes that have been drilled into the frame. These mounting holes and grounding points are the only ones tested and evaluated through the certification/listing process.
Photo 4. Does the PV module instruction manual describe this top clip mounting system? Should it have been field modified by drilling two new holes?
Uncertainties exist when the requirements of UL Standard 1703 are compared to the requirements of UL Outline of Investigation 2703. In the current environment of a surplus of PV modules, reduced demand, and a drive to reduce the cost of PV systems, there are great variations in the construction of a various PV modules. Reducing the thickness of the aluminum frames, which may affect mounting methods, can reduce costs. Quality control is sometimes lacking due to budget constraints and the thickness of the anodizing and the thickness of any clear coating cannot be firmly established or controlled.
Legal requirements associated with the certification and listing process preclude the NRTLs from publishing and sharing information on the specific construction of PV modules. If those construction parameters were distributed and evaluated, it might be possible to determine a minimum thickness of the aluminum frames that could be evaluated for mounting and grounding purposes and a maximum thickness of anodizing and clear coating that could be evaluated for grounding purposes. However, these data are not available, and to some extent might be questionable due to frequent module construction changes and limited quality control.
It will be an AHJ call on accepting the various module mounting and grounding systems whether they are used with site built racking systems or one of the commercial racks certified/listed to UL Outline of Investigation 2703.
Inspectors and plan reviewers will be seeing increasing numbers of transformerless inverters and ungrounded arrays. They should be more familiar now. The Fire Service in some jurisdictions may be urging early adoption of the Rapid PV Shutdown System so that system may happen sooner than adoption of the 2014 NEC in some areas. And, as always, module grounding continues to be important for the long-term safety of the system and progress is sometimes slow to clarify the requirements.
FEEDBACK: The information in these articles is based on the author’s interpretation of the Code. I have received feedback that my interpretation on 705.12(D)(1) in the 2014 NEC in the “Perspectives on PV” in the Jan-Feb IAEI NEWS is incorrect. Some of the people involved in writing this section of the Code maintain that there is no significant change in the intent of this section from the 2011 NEC and that multiple backfed breakers are still allowed in the main panelboard or existing subpanels as long as the other requirements for such backfed breakers are met.
The author has retired from the Southwest Technology Development Institute at New Mexico State University, but is devoting about 25% of his time to PV activities in order to keep involved in writing these Perspectives on PV articles in the IAEI News and to stay active in the NEC and UL Standards development. He can be reached, sometimes, at: e-mail: email@example.com, Phone: 575-646-6105
The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous “Perspectives on PV” articles for easy downloading. A color copy of the latest version (1.93) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html.