Kent,
Sort of makes the case for the GFPs to be in the combiner boxes on
large systems, doesn't it? Obviously nearly worthless at the inverter
in this case.
Bob-O
On May 6, 2009, at 3:52 PM, Kent Osterberg wrote:
Richard,
I 100% agree with you.
The thing that concerns me most is that the last exhibit in the report
shows that the GFP scheme that our industry has relied on to provide
fire protection doesn't work very well. As PV system size gets larger
there are more and more opportunities for an undetected fault on a
grounded conductor that disables the GFP. Then a second fault, on an
ungrounded conductor, isn't cleared. There will be more fires. And
they can't lay all the blame on the contractor.
Kent Osterberg
Blue Mountain Solar
Richard L Ratico wrote:
Kent,
I'm trying not to lose sight of the forest for the trees here. I
agree that the
expansion joint is probably completely functional. It also appears
to be a well
intentioned effort by the installer to anticipate and compensate for
the
inevitable movement in the conduit run. The installer took the
initiative, after
all, since the design did not call for it.
My point is, even had the installer used the correct EMT type fitting,
functional or not, the fault would still have happened. With the EMT
restrained
by overly tight straps, it would still have pulled apart at the
loose coupling,
the nearest, weakest link.
But, had the installer followed through, using the appropriate RMC
to match the
choice of expansion coupling, the fault may well have been
prevented. Even when
very loose, threaded fittings are unlikely to pull apart completely.
Of course,
using 423' of 3" RMC instead of EMT, would have resulted in a more
expensive
job, though as a percentage of the overall cost, maybe it wouldn't
have amounted
to much.
IMHO, Monday morning quarterbacking, EMT was a poor choice for this
system,
subject to nearly 4" of movement. On a job of this scale, there are
simply too
many opportunities for a loose coupling to occur, whether initially on
installation, or over time after repeated thermal cycling. S...
happens. Kind of
analogous to why we pull an equipment grounding conductor in metal
conduit
systems, even though the conduit itself provides an equipment
ground. RMC would
provide for much better sleeping at night. PVC, nasty as it is in
some respects,
in this case, would have been significantly less likely to start a
fire. It's
harder to forget to glue a joint than to overlook wrenching an EMT
coupling. If
one does pull apart, since PVC is nonconductive, it's less likely to
cause a
fault. Better yet, as the report recommends, is to design a means to
clear the
fault, should one occur.
In a perfect world, EMT might be fine.
Dick
Richard,
I think you are right, the weakest link in the chain was a
compression fitting.
Maybe one that was not properly tightened. An EMT connector will
thread right
into a RMC coupling and it doesn't look like it would interfere with
the
expansion joint motion, so I still think the expansion joint may be
functional.
It should slip with modest hand pressure. Wouldn't the installer
notice if an
expansion joint wouldn't work? And then not use it. it just
wouldn't make any
sense to install it in if it didn't work.
Kent
Richard L Ratico wrote:
Kent,
I looked at the photo of that expansion coupling and initially
thought, "That'll
work". But.... if the conduit was RMC OR IMC all the joints would
have been
THREADED into couplings, and even if by mistake they were not wrench
tight, they
would not have pulled apart. Even if strapped too tightly, the roof
blocks would
probably pull over first. With EMT, contraction pulled apart the
weakest link in
the chain, a loose slip fitting.
Thanks William.
Dick
Solarwind Electric
--- You wrote:
2) The expansion joint fabricated from adapters for different conduit
systems probably works fine and may not have been a contributing
factor
to this fire, but it is prominently visible in the report.
--- end of quote ---
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