more grounding info [RE-wrenches]
Matt Tritt
solarone at charter.net
Mon May 10 09:08:58 PDT 2004
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Marco,
I have a project to re-locate a small (12 X BP 140?) module array at a
site in Big Sur that's about a 1/4 mile from the coast. The array was
installed in 2002 by someone who seems to have been trying for some kind
of grounding record. There are no less than three ground rods, each
protected by a concrete burial box and cover, and all inter-connected
with # 6 bare copper, as are all the modules and each frame rail of the
Unirac ground mount.
Every grounding lug installed on the frame members has turned into a
ball of exploding aluminum oxide (they are all mounted with S/S bolts to
the top module mounting slot) that provides excellent resistance to
conductivity. Every grounding lug installed on the modules is pretty
close behind the other lugs in their race to complete disintegration,
but the copper wire is in FINE condition! I should also point out that
the Unirac frame members are also oxidizing badly in this coastal
environment, and will need replacing within 10 years.
All the points of inter-connection between copper and aluminum show
extreme, rapid oxidation, which would tend to validate John W's
suggestions.
This leads me to ask once again; why use aluminum frames at all? Why NOT
use poltruded fiberglass or a similar non-conductive material? If module
frames were made this way installations would be much quicker and
cleaner and there would be no way of ever accidentally energizing
mounting frames. The last time I made this suggestion it was pointed out
(by somebody with stock in the aluminum industry no doubt) that module
manufacturers like doing it this way and the alternatives would make
modules more expensive and less weather resistant. Horse poo.
If you consider the energy consumed to produce aluminum, and the time it
takes to break even with the power produced by an individual module, a
non-metallic module frame makes all the sense in the world. And if
better frames result in better, quicker installations with no
possibility of grounding problems, any price difference in modules would
be quickly eliminated. (IMHO of course!)
Matt T
-----Original Message-----
From: Marco Mangelsdorf [mailto:marco at pvthawaii.com]
Sent: Monday, May 10, 2004 7:41 AM
To: RE-wrenches at topica.com
Subject: more grounding info [RE-wrenches]
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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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