Grounding
In the "Perspectives on PV" article in the
September-October 2004 issue of the IAEI News, the
subject of grounding PV systems was covered in some detail. In
the March-April 2005, IAEI News, we discussed the
changes to Article 690 that appear in the 2005 National
Electrical Code. As normally happens over the three-year
code development cycle, new thoughts and ideas come to the
forefront about how things should be done. Here are some of
those thoughts as they apply to grounding smaller PV systems
with single inverters sized below about 10 kW. Figure
1 shows the dc grounding for a PV system as spelled out in
Section 690.47 of NEC-2005 and as described in the
above-mentioned article. Inspector Russ Coombs of Bakersfield,
California, suggested that if the ac ground rod cannot be
found, then the dc grounding electrode conductor might be
spliced (irreversibly) to the ac grounding electrode
conductor. I think this is a good suggestion because in many
older buildings, the ac grounding electrode is buried in
non-accessible locations.
PV system designers, PV integrators and
installers are always looking for ways to meet the code safety
requirements, install the system at the lowest cost, and make
the system look good. The grounding system shown in figure
2 has been proposed as an alternate grounding system to
meet most of the NEC requirements for grounding these
systems. There is no dc grounding electrode (ground rod)
located at the inverter. An unspliced 8 AWG (if allowed, based
on the type of existing ac grounding electrode) bare or
insulated conductor (marked green) is routed from a grounding
terminal in the inverter along with the ac inverter output
circuit conductors to and through (no stopping) to the ac
ground rod. In this example, the 8 AWG conductor serves as
both the dc grounding electrode conductor (unspliced, minimum
size) and the ac equipment-grounding conductor. It should be
noted that all grounding terminals and lugs
(equipment-grounding and grounding electrode conductor) are
electrically connected together in the inverter and may
generally be used interchangeably depending on the size of the
conductors they will accept.
This method only works on the smaller
string inverters where the ac equipment grounding conductor is
8 AWG or less and the ac grounding electrode is not something
like a UFER (concrete-encased electrode) that may require a 4
AWG grounding electrode conductor. It is usually not
appropriate for the 10 kW and larger three-phase inverters.
Multiple, Small
String Inverters
Where multiple small inverters are installed in a single
location, it is probably best to install a 6 AWG (if allowed
based on the type of grounding electrode) bare, grounding
electrode conductor from the first inverter in the set to a dc
grounding electrode, which is then bonded to the ac grounding
electrode. As allowed by NEC-2005, this dc grounding
electrode conductor may also be routed and connected directly
to the ac grounding electrode. If the ac ground rod cannot be
found, then this conductor might be spliced (irreversibly) to
the ac grounding electrode conductor. This dc grounding
electrode conductor is routed beneath each of the other
inverters in the set. A short, 6 AWG grounding electrode
conductor is connected to a grounding terminal in each of the
other inverters and then irreversibly spliced to the dc
grounding electrode conductor running beneath each inverter
(see figure 3).
In this manner, only one dc grounding electrode conductor is
required for the entire set. This is similar to the way
multiple service disconnects are grounded in an apartment
complex as shown in Exhibit 250.28 in the 2002 NEC
Handbook.
Lightning Surge
Protection
PV installers should note that the single-inverter grounding
method runs the dc negative grounding system and the dc
equipment-grounding conductors all the way back to the ac
grounding electrode along with the ac output conductors from
the inverter. Lightning induced surges may also travel this
path and this may increase the possibility of
lightning-induced surge damage to the PV equipment with this
method of grounding the dc systems. Placing a dc grounding
electrode at the inverter (bonded to the ac grounding
electrode) may help to reduce surge damage. Also adding
supplementary equipment grounding electrodes for the PV array
mounting racks/module frames as shown in figures 1 and 2 and
not bonding them to other grounding electrodes may reduce the
potential for lightning damage (see NEC, 250.54).
Fine Stranded
Cables
Since the Perspectives on PV article on fine stranded cables
was published in the January-February issue of the IAEI
News, I have received calls from people in other
industries about connections failing where fine stranded
cables have been used improperly. These failures have been
associated with electric vehicle power cables, motor
connections, and a few other high-current applications. At
Underwriters Laboratories, the principal engineer for
Distributed Energy Resources Equipment and Systems is going to
process a bulletin and UL 1741 (PV Inverters and Charge
Controllers) revision to clarify the use of appropriate
connectors and terminals with fine stranded conductors.
If you are in another industry that uses
these conductors and associated connectors improperly, or you
inspect such equipment, notifying UL might get some additional
corrective actions taken. Inspectors can contact UL and file a
field report at the following UL web site: (https://www.ul.com/regulators/ahjprod.cfm).
Others can file a report to UL at this site: (https://www.ul.com/consumers/conproddb.cfm).
I can supply a PDF of the original article, if needed.
Germany Does It
Right
I spent ten days in Germany in early March visiting PV
equipment manufacturers, looking at PV installations (photo
1) and touring residential construction projects (photo
2). I was pleasantly surprised to find that trained
electricians are installing most PV systems in Germany.
Germany is second only to Japan in the number of PV
installations.
The electricians that I talked with were
familiar with the use of fine stranded conductors and the
equipment-production facilities I visited used them regularly.
All locations had a wide range of crimp-on wire-end ferrules
and sleeves available, and they also had the proper crimping
tools for placing these devices on fine stranded cables before
inserting them into terminals. Even the building supply stores
(equivalent to Home Depot and Lowes) had these ferrules
readily available (photo
3).
I discovered that the typical residential
and commercial wiring in Germany is accomplished with a
jacketed, sheathed, three-four conductor cable where each of
the main conductors consists of flexible, fine stranded wires
(photo 4). These
types of cables have been used for decades. Where our type NM
cables typically have solid conductors up to 10 AWG, the
German equivalents use fine stranded flexible conductors. The
German electric dryer and range cords use fine stranding like
ours do, but theirs have ferrules attached (photo
5).
It appears that the lack of familiarity
with the proper use of fine-stranded cables here in the U.S.
can possibly be traced to the fact that the typical
electrician (and home owner) rarely deals with these cables.
In Germany, where these cables are used daily, everyone seems
to know how to properly install them. I wish we could import
that knowledge base to the U.S. (along with the excellent
German rail system).
For Additional
Information
If this article has raised questions, do not hesitate to
contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu;
Phone: 505-646-6105
1 A PV Systems Inspector/Installer
Checklist will be sent via e-mail to those requesting it. A
draft copy of the 143-page, 2005 edition of the Photovoltaic
Power Systems and the National Electrical Code: Suggested
Practices, published by Sandia National Laboratories and
written by the author, may be downloaded from this web site
(http://www.nmsu.edu/~tdi/roswell-8opt.pdf.) The Southwest
Technology Development web site (http://www.nmsu.edu/~tdi)
maintains all copies of the "Code Corner Columns"
written by the author and published in Home Power Magazine over the last ten years. Copies of previous "Perspectives
on PV" are also available on this web site.
The author makes 6–8 hour presentations
on "PV Systems and the NEC" to groups of 40
or more inspectors, electricians, electrical contractors, and
PV professionals for a very nominal cost on an as-requested
basis.
John Wiles works at the Southwest
Technology Development Institute (SWTDI) at New Mexico
State University. SWTDI has a contract with the US
Department of Energy to provide engineering support to
the PV industry and to provide that industry, electrical
contractors, electricians, and electrical inspectors
with a focal point for code issues related to PV
systems. He serves as the secretary of the PV Industry
Forum that will be submitting 30+ 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, he lives in a stand-alone PV-power home in
suburbia with his wife, two dogs, and a cat—permitted
and inspected, of course.
This work was supported by the United
States Department of Energy under Contract
DE-FC04-00AL66794 |
