ground-fault protection for pv

8/3/2019 Ground-Fault Protection for PV

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IAEI NEWSJanuary.February 2008 www.iaei.org

ground-fault protection for pv systems

Once upon a time (the 1987 Codecycle) in the

land o Quincy, a group o alchemists rom anational laboratory was elaborating on the ex-

cellence o their photovoltaic (PV) test acility in thedistant Land o Enchantment. Tey showed some se-nior reghters a picture o a burned PV module thathad been subject to a ground ault and had subsequentlymelted down. Te alchemists ailed to mention at thetime that this was a prototype, unlisted PV module,that the module was on a concrete pad, and that groundaults in PV systems were somewhat rare. Tese re-ghting pros said to themselves, PV ground aults lead

Ground-Fault

Protection for

by John Wiles


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www.iaei.org January.February 2008 IAEI NEWS


to res. Fires on the roos and in the attics o dwellings

are very hard to ght. Tey then told the PV industryto propose Section 690.5 or the 1987 NECto requirea ground-ault protection device (GFPD). Te proposal

was accepted and the requirement was established, butno hardware existed.

In 1989, I joined the PV industry as a ull-time em-ployee at the Southwest echnology Development Insti-tute. One o my rst projects was to develop prototypehardware that could be used to meet the new Section690.5 requirement. Tis eort was unded under con-tract to Salt River Project, a Phoenix, Arizona, electric

utility. In the 1987 Code, the requirements or this re-reduction device were to:

1. Detect ground aults in PV arrays mounted on theroos o dwellings

2. Interrupt the ault current3. Indicate that a ground ault had occurred4. Disconnect the aulted part o the PV array5. Crowbar or short circuit the PV arrayTe original GFPD prototype was developed in two

versions that were similar except or voltage rating. Te

basic concept was to insert a 0.5 or 1.0 amp circuit break-er in the dc system-bonding conductor connecting thegrounded circuit conductor (usually the negative) to thegrounding system (the point where equipment ground-ing conductors and grounding-electrode conductor areconnected together). Any ground-ault currents mustfow through this bond on their way rom the ground-ault point back to the driving source, the PV moduleor PV array. See gure 1. When the current in this bondexceeds 0.5 or 1.0 amp, the circuit breaker trips to theopen position. Tis action interrupts the ault current,

SystemsPVSystems


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even when the ault is many eet away on the roo o the

building and provides the indication that a ground aulthas occurred. Requirements 1, 2, and 3 are satised bythese actions.

Tis small circuit breaker is mechanical ly linked toone to our large, 100-amp circuit breakers and theyopen when the 0.5 amp circuit breaker opens. Teseadded breakers are connected in series with each othe incoming ungrounded conductors rom the PVarray and when they open, the PV array is discon-nected rom the rest o the system, thereby meetingrequirement 4.

Requirement 5 was added to reduce the PV array

voltage to zero by shorting the positive and negative con-ductors together to minimize a potential shock hazard.In the original GFPD design, this was accomplished ei-ther by using a motor-driven circuit breaker on 48-voltsystems or by using a solenoid-driven (closed) shunt-tripbreaker on the higher voltage systems. Tis th short-ing requirement was later removed rom the NECwhenit was determined that it might be possible to damage a

new technology PV module by short-circuiting it. Temodule was never produced, but the crowbar require-ment was not reintroduced even though the PV wiring

Figure 1. Ground-fault currents go t hrough the bonding conductor.

Photo 1. One-pole, ground-fault protective device for 48-volt PV system


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Photo 3. Four-pole, ground-fault protective device for 48-volt PV system

can handle the worst case short-circuit currents and isoversized by a actor o 125 percent. It is an impressivedemonstration when circuit breakers rated at 750 voltsclose and short circuit a 100-amp PV array that has anopen-circuit voltage o 600 volts.

Modern Ground Fault Protection DevicesTe early designs o the prototype GFPDs were released

Photo 2. Two-pole, ground-fault protective device for 48-volt PV system

to the PV industry in 1991. Finally, in 1997, a GFPD wasmanuactured or the 48-volt and below PV systems, and

that device used the exact design and components as theprototype. See photo 1. Other ground-ault devices orthe low-voltage systems soon ollowed as these o-grid,stand-alone systems became more common and were in-spected more requently. See photos 2 and 3.

As the higher-voltage, utility-interactive PV invertersbecame available in the late 1990s, it was more cost-e-ective to use a 0.5 or 1.0 amp use as the sensing elementand use the control electronics in the inverter to moni-tor the use, indicate that a ground ault had occurred(light or display), and shut down the inverter (eectivelydisconnecting the equipment). See photo 4.

Coming in 008NEC-2008 will require GFPDs on nearly all PV systemsincluding those mounted on commercial buildings (non-dwellings) and on the ground. Tis requirement wasadded to the NECbecause on the larger PV arrays, aground ault, i not interrupted, can continue as long asthe sun is shining, and may not be detected until signi-cant damage has been done. Te possible arc rom theground ault and the overloaded equipment groundingconductors may each pose hazards.


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Sizing equipment grounding conductors using NEC250.122 or PV systems with uses does not always re-sult in a conductor size that can withstand continuousground-ault currents. Te conductor and overcurrent

sizing requirements or PV source and output circuitsand the current-limited nature o PV module outputscannot ensure that overcurrent devices will open prop-erly in a very short time as they do on ac circuits. Tere-ore, a requirement was added to interrupt the ground-ault currents on all PV systems when they exceed thelow 0.5 or 1.0 amp value.

Beore May 2007, inverters larger than about 10kW had only partial GFPD unctionality. Tey de-tected the ground aults, indicated that the ault hadoccurred, and shut down. However, they did not in-terrupt the ault currents. Now, with a change in UL

Standard 1741 or PV inverters and the2008 NEC, allutility-interactive inverters will have ull unctionality

with respect to ground aults and will act in a mannersimilar to the smaller residential units. O grid, PVsystems with batteries operating at 48 volts nominal, orless, will have the GFPD built into the charge control-ler, or an external device will have to be added. Small,dc o-grid systems that have no dc or ac wiring insideor on a building will be exempt rom the GFPD re-quirement i the equipment grounding conductors areoversized by a actor o about two.

Te AC Ground-Fault IssueTe common alternating-current ground-ault circuitinterrupters (GFCI) are not designed to be backed. Teoutput o a utility-interactive inverter connected to the

load terminals and backeeding a receptacle or breakerGFCI, a 30-milliamp equipment protection ground-ault breaker, or even a 6001200 amp main breaker

with ground-ault elements may damage that device with no external indication o a problem. Anytime autility-interactive PV system is installed, the entire acpremises wiring system should be examined all the wayrom the PV inverter output to the service entrance toensure that there are no ground-ault devices in that cir-cuit that may be subject to backeeding. Some o thenewest ground-ault breakers in the 1000 amp and larg-er sizes are listed as suitable or backeeding, but that

inormation must be obtained on any ac ground-aultdevice that could be subject to backeeding.

Arc-Fault Circuit Interrupters Arc-ault circuit interrupters (AFCI) are, in some ways, similar to GFCIs and should not be backedby PV inverters unless listed and identied or back-eeding. Tey are being required in many locationsthereby increasing the saety o electrical systemshere in the U. S. DC arc-ault circuit interrupters arenot currently available.

Photo 4. Ground-fault fuse on high-voltage inverter


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Te Southwest echnology Development Institute web site maintains a PV Systems Inspector/InstallerChecklist and all copies o the previous Perspectiveson PV articles or easy downloading. Copies o CodeCorner written by the author and published in Home

Power Magazineover the last 10 years are also availableon this web site: http://www.nmsu.edu/~tdi/Photovol-taics/Codes-Stds/Codes-Stds.html

Te author makes 68 hour presentations on PVSystems and the NEC to groups o 40 or more inspec-tors, electricians, electrical contractors, and PV proes-sionals or a very nominal cost on an as-requested basis.

A schedule o uture presentations can be ound on theIEE/SWDI web site.

John Wiles works at the Institute for Energy andthe Environment (IEE) (formerly the Southwest

echnology Development Institute) at New MexicoState University. IEE has a contract with the US De-partment of Energy to provide engineering support tothe PV industry and to provide that industry, electri-cal contractors, electricians, and electrical inspectors

with a focal point for Code issues related to PV systems. He serves asthe secretary of the PV Industry Forum that submitted 42 proposals for

Article 690 in the 2008 NEC. He provides draft comments to NFPA for Article 690 in the NEC Handbook. As an old solar pioneer, helived for 16 years in a stand-alone PV-power home in suburbia withhis wife, two dogs, and a catpermitted and inspected, of course. Hehas just installed a new PV system on his home that is utility-interactivewith a full-house battery back up using a New Mexico HomeownersElectrical Permit.

Tis work was supported by the United States Department of Energ y underContract DE-FC 36-05-G015149

However, we know that there is some danger associ-ated with a line-to-line ault in the dc wiring o a PV ar-ray. Te PV industry and Underwriters Laboratories arestudying the issue to determine the signature o a typicaldc arc originating rom a PV system and how, i possible,

to detect, control, and extinguish that arc. Tis is not aneasy task because the electrical sources (the PV modules)in any system are widely dispersed and numerous.

SummaryTe number o PV installations is increasing at more

than 20% a year. Nearly all PV systems will soon be re-quired to have a ground-ault protective device that willminimize the possibility o res starting rom groundaults in PV arrays. Eorts are continuing to enhance thesaety o PV systems or the general public through revi-sions and additions to the National Electrical Codeand ULStandards. Te goal is to have sae, reliable, and cost-eec-tive PV systems. Te green uture must be a sae uture.

For Additional InformationI this article has raised questions, do not hesitate to

contact the author by phone or e-mail. E-mail: [email protected] Phone: 575-646-6105

A color copy o the latest version (1.7a) o the 150-page, Photovoltaic Power Systems and the 2005 NationalElectrical Code: Suggested Practices, published by SandiaNational Laboratories and written by the author, may be

downloaded rom this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/PVnecSugPract.html

Ferms Fast Finder is the leading comprehensive cross-reference index toNEC-2005. Each entry, reference and source has been carefully crafted and

examined for precise detail and ease of use. In section one, new navigationalguides are built into each page to facilitate locating information quickly. Volumetwo contains more than 186 extensive and practical rules of thumb, formulas,illustrations, tables, charts, diagrams, and examples of situations encounteredin the field. A new section contains cross-references to the 2006 InternationalResidential Code (IRC). Users of the NECwill discover how parallel these

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ground-fault protection for pv

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