UL 1703 is a 30 psf positive (downward) or negative (upward) load, or a
design load designated by the manufacturer. I've never seen a module
advertising a 30psf loading requirement--everything I've seen is at
least 50psf--which I take to mean that this "designated" design load is
usually spec'd by manufacturers to be 50 psf. 30 psf is pretty weak.
The UL tests themselves then actually go to 1.5 times that design load
as a safety factor. The load tests last 30 minutes. As I understand it
(based off a coworker's conversation with a Schott rep), this is why
Schott advertises that their modules withstand 75psf front and back. The
rep questioned why all manufacturers weren't advertising 75psf.
IEC 61215 also tests both upwards and downwards loading. Since it's
written by some French people, they specify a 2400 Pascal load, which is
50.1 psf, for both the front and back of the module. They do this in
three cycles of tests (50psf front, 50 psf back being one cycle).
However, "if the module is to be qualified to withstand heavy
accumulations of snow and ice," then during the third cycle they go to
5400 Pa for the FRONT loading. 5400 Pa is 112.8 psf. Each load test
lasts 1 hour. If a module manufacturer claims 113psf uplift resistance
then either they misunderstand the IEC standard or they make the Hulk of
PV (without the smash). Note that the IEC standard does not have the 1.5
safety factor.
Some test labs offer a package deal where they test modules to UL and
IEC requirements at the same time to qualify a module for worldwide
sale-- so they go through each test and pick the harder requirements
(between UL and IEC) so that passing each test means the module is
certified to meet both standards. IEC's 50 psf loading may have caused
some manufacturers to designate the 50 psf load for UL. Or, module
manufacturers just do the 50psf IEC load which corresponds to a
"designated" 33.3 psf load for UL times that 1.5 factor.
On 2011/10/6 17:03, Jamie Johnson wrote:
Rich, It is my understanding that modules are tested to 113psf downforce
or snow load and 52psf uplift for their NRTL listing (uplift is also a
load rating denoted as negative pressure) (don't have a copy of the
testing procedures in front of me however I have confirmed this with
more than one module manufacturer in the past).
It seems to make sense as many framed single glass modules appear to
have a wider lip under the glass and a narrow lip above the glass, there
may also be other reasons.
There are a few manufacturers out there that have had additional testing
done and will certify their modules to 75psf or 113psf uplift, usually
they have 2 layers of glass and/or a thicker AL frame and are also
heavier like the Schott 300 watt module.
Since all of our installs are in a High Velocity Windzone Area, when we
have a module manufacturer with single glass normal AL frame modules
that claim 113psf loads (uplift) we will always challenge them on it and
see if they will actually stand by that claim, in every single case
after researching it, they have backed away from the claim and
acknowledged that their modules will only withstand 52 or 53psf uplift.
It is important to understand that the load rating listed on the
marketing material (downforce or snow load) is usually not the same as
the uplift or negative pressure rating.
In our area of FL for a 130MPH windzone, our uplift or negative
pressures on a module can range from -32 to -49PSF although our
downforce is usually lower around +16. In a 140MPH windzone you can
easily exceed a normal module uplift rating of -52PSF, and although
adding a 3rd rail may reduce frame stress, it will do little to keep the
glass from blowing out in a hurricane.
As to the honesty question, IMHO the guilty parties are most likely the
marketing dept staff who lack technical expertise.
Hope this helps.
*Jamie Johnson
NABCEP Certified PV Technical Sales Professional
NABCEP Certified Solar PV Installer
General Manager*
*SOLAR POWER ELECTRIC*
**
**
-------- Original Message --------
Subject: [RE-wrenches] Module Load Rating
From: "Rich Nicol" <[email protected] <mailto:[email protected]>>
Date: Thu, October 06, 2011 1:41 pm
To: <[email protected]
<mailto:[email protected]>>, "'RE-wrenches'"
<[email protected]
<mailto:[email protected]>>
Hi Wrenches –
I’ve noticed discrepancies in load rating methods for various
modules and wondered if there was any insight from the installer
community. REC claims 113 PSF, Sharp claims only 50PSF. Solarworld
notes 50PSF load rating, but 113 PSF snow load rating. I’m assuming
the difference is that snow load rating is greater because its
assumed to be widely distributed, but yet its still per square foot
so perhaps not. Is a Sharp module such as the 240NUQ-240F2 as
durable as REC, Evergreen, Solarworld etc and they are just more
honest in their portrayal of load rating.
Thanks for your help,
Rich
**
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