parallel connector danger?[RE-wrenches]

Ray Walters remotech at taosnm.com
Sat May 25 12:41:53 PDT 2002


Bill;

At what voltage reversed into a module do currents get high enough to cause
damage? I have seen plenty of overloaded module J-boxes with burned
terminal strips from paralleling short circuit currents that were 3 to 5
times the series fuse rating. Is this the same type of damage you've seen
as well? I deal with systems in really dangerous states of misdesign and
misinstallation. Many of our customers have very limited budgets. I go
after the most dangerous things first. This isn't California, its more like
the third world. I perform triage and I need to know once again: has anyone
seen failures in the feild (not the lab at 400vdc) of paralleling modules
to operating currents below their series fuse rating at under 100 vdc? 
I respect your need to maintain absolute NEC compliance for a high DC
voltage  grid intertie system with loads of rebate money on the line. You
should respect the millions of people around the world who  benefit from
non-code compliant solar power. If all those systems had had to meet full
NEC requirements, most of those people would still be sitting in the dark
or using more dangerous methods like gasoline generators, candles, or
kerosene lamps. 
By the way, I've worked on plenty of high vdc systems, both utility scale
grid intertie (3) and village scale pumping (14).  You don't need
paralleling, especially when a single string is making several Kw. On the
other hand, low voltage (<100vdc)systems have successfully paralleled
modules for decades. You just can't over do it. Different design parameters
exist for different people, places, and needs. The NEC and UL form a safe
little box, but I am regularly called to think outside of it.

Sincerely,

Ray 

>The need for fusing is very real. Just because some combiner boxes have had
>problems in the past does not mean that you can justify not following the
>code and the UL listing of the module. We ARE talking about reality here. I
>know there is a tremendous amount of experience on the wrenches list at 12
>and 24 volts, but you need to trust people who have been working with 48 and
>400 volt systems for just as long who have seen the problems that happen
>under fault conditions.
>
>It is very possible to drive current back through a PV module. That is why
>UL tests modules with a series fuse connected to it driving current in the
>reverse direction--this is a common case with a wiring fault.
>
>Bill.
>

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