more grounding info [RE-wrenches]

Mon May 10 07:40:31 PDT 2004

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I found the note below from John Wiles very useful.

aloha,
marco

Grounding PV Modules

4/29/04  John Wiles    SWTDI/NMSU    jwiles at nmsu.edu

Grounding PV modules to reduce or eliminate shock and fire hazards is
necessary but difficult.  We typically use copper conductors for electrical
connections and the module frames are generally aluminum.  As many of you
know, copper and aluminum don't mix as was discovered in numerous fires in
houses wired with aluminum wiring in the 1970's.   PV modules generally have
aluminum frames.  Many have mill finish, some are clear coated, and some are
anodized for color.  The mill finish aluminum and any aluminum surface that
is scratched quickly oxidizes.  This oxidation and any clear coat or
anodizing form an insulating surface that makes for difficult long-lasting,
low resistance electrical connections (e.g. frame grounding).   The
oxidation/anodizing is not a good enough insulator to prevent electrical
shocks, but it is good enough to make good electrical connections difficult.

Underwriters Laboratories (UL) who tests and lists all PV modules sold in
the US requires very stringent mechanical connections between the various
pieces of the module frame to ensure that these frame pieces remain
mechanically and electrically connected over the life of the module.  These
low-resistance connections are required because a failure of the insulating
materials in the module could allow the frame to become energized at up to
600 volts (depending on the system design).  The National Electrical Code
(NEC) requires that any exposed metal surface be grounded if it could be
energized.  The installer of a PV system is required to ground each module
frame.  The code and UL Standard 1703 require that the module frame be
grounded at the point where a designated grounding provision has been made.
The connection must be made with the hardware provided using the
instructions supplied by the module manufacturer.

The designated point marked on the module must be used since this is the
only point tested and evaluated by UL for use as a long-term grounding
point.  UL has established that using other points such as the module
structural mounting holes coupled with typical field installation
"techniques" do not result in low-resistance, durable connections to
aluminum module frames.  If each and every possible combination of nut,
bolt, lock washer and star washer could be evaluated for electrical
properties and installation torque requirements AND the installers would all
use these components and install them according to the torque requirements
(we all have and use torque wrenches and torque screw drivers don't we?), it
might be possible to use the structural mounting holes for grounding. 

Most US PV module manufacturers are providing acceptable grounding hardware
and instructions.  Japanese module manufactures are frequently providing
less-than-adequate hardware and unclear instructions.  Future revisions of
UL 1703 should address these issues.  BP Solar is to be congratulated for
getting their module listing to include making new grounding points at other
locations than the marked points.

In the meantime, installers have to struggle with the existing hardware and
instructions, even when they are poor.  SWTDI has identified suitable
grounding hardware and provides that information when installers ask about
grounding-a frequent topic.   And yes, we are using the hardware and methods
described below to ground Sharp modules in our new inverter test facility.

For those modules that have been supplied with inadequate or unusable
hardware or no hardware at all, here is a way to meet the intent of the code
and UL Standard 1703.  

For those situations requiring an equipment-grounding conductor larger than
10 AWG, a thread-forming stainless steel 10-32 screw can be used to attach
an ILSCO GBL4 DBT lug to the module frame at or adjacent to the point marked
for grounding.   A #19 drill is required to make the proper size hole for
the 10-32 screw.   The 10-32 screw is required so that at least two threads
are cut into the aluminum (a general UL requirement for connections of this
kind).  The thread-forming screw is required so that an airtight,
oxygen-free mating is assured between the screw and the frame to prevent the
aluminum from reoxidizing.   It is not acceptable to use the hex head green
grounding screws (even when they a have 10-32 threads) because they are not
listed for outdoor exposure and will corrode eventually.    The same can be
said for other screws, lugs, and terminals that have not been listed for
outdoor applications.  Hex-head stainless steel "tech" screws and sheet
metal screws do not have sufficiently fine threads to make the necessary low
resistance, mechanically durable connection.   The only thread-forming,
10-32 stainless steel screws that have been identified so far have Phillips
heads; not the fastest for installation.   

The ILSCO GBL4 DBT lug is a lay-in lug with a stainless steel screw made of
solid copper and then tin plated.  It accepts a 4-14 AWG copper conductor.
It is listed for direct burial use (DB) and outdoor use and can be attached
to aluminum structures (the tin plate).  The much cheaper ILSCO GBL4 lug
looks identical, but is tin plated aluminum, has a plated screw and is not
listed for outdoor use.  We have not been able to identify an alternative to
the GBL4 DBT, but continue to search.

If the module grounding is to be done with a 14-10 AWG conductor, then the
ILSCO lug is not needed.  Two number 10 stainless steel flat washers would
be used on the 10-32 screw and the copper wire would be wrapped around the
screw between the two flat washers that would isolate the copper conductor
from the aluminum module frame.

What size conductor to use?  The minimum code requirement is for the
equipment-grounding conductor for PV source and output circuits be sized to
carry 1.25 times the short-circuit currents at that point.  While this may
allow a 14 AWG conductor between modules, a conductor this small would
require physical protection between the grounding points.  Some inspectors
will allow a 10 AWG bare conductor to be routed behind the modules from
grounding point-to-grounding point if the conductors are well protected from
damage as they would be in a roof-mounted array.  If needed, an 8 AWG or 6
AWG sized conductor may be required (to meet the code or to satisfy the
inspector) and then the ILSCO lugs should be used.

Yes, we would all like to use the module mounting structure for grounding.
I have encouraged UNIRAC and others to get their products listed as
field-installable grounding devices, but I suspect that they are running up
against that aluminum oxidation problem also as well as the lack of
consistency in tightening nuts and bolts in the field.

The code allows metal structures to be used for grounding and even allows
the paint or other covering to be scraped away to ensure a good electrical
contact.  We see numerous types of electrical equipment grounded with sheet
metal screws and star washers.  This works on common metals like steel, but
not on aluminum due to the oxidation.

I am also trying to interest the module manufacturers in making that
aluminum connection in the factory and giving us a copper-compatible
terminal in the j-box or on the frame as is done with the 300-watt
RWE-Schott modules.

Marco Mangelsdorf
President
ProVision Technologies, Inc.
69 Railroad Avenue, A-7
Hilo, Hawai'i 96720
(808) 969-3281, 934-7462 facsimile
www.provisiontechnologies.com

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