Parallel Danger? [RE-wrenches]

Tue May 28 15:34:13 PDT 2002

Ray,

In order to get current flow, a voltage higher than Voc would need to be
applied--how about twice Voc--that should get the current flowing and is
highly attainable in 24 and 48 volt systems.

Leeway is a function of hundreds of thousands of hours of experience. Your
example may not have been truly UL allowable if you got into the details of
how an extension is listed for use. When we have the same level of
experience, we might be able to play that card. Until then, watch out for
moose.

Bill.

-----Original Message-----
From: Ray Walters [mailto:remotech at taosnm.com]
Sent: Tuesday, May 28, 2002 3:06 PM
To: RE-wrenches at topica.com
Subject: RE: Parallel Danger? [RE-wrenches]

Hi Bill;

I don't want to put you on the spot on the AC vs DC safety issue. I have a
24 VDC battery MIG welder that demonstrates the arc potential (and fire
hazard) of even low voltage DC. My point was that it appears the NEC is
allowing more leeway in standard residential circuit design than in array
circuit design.
You mentioned that UL tests a module at a certain current. Typically, what
voltage is required to reach the rated current level?
Also, I wasn't trying to be silly with the moose reference. One walked
right past the array a few months ago. By the size of the beasts, I'd bet
they'd give even a fully NEC compliant system a bad time.

Ray

At 10:04 AM 5/28/02 -0700, you wrote:
>Ray,
>
>I really don't want to get into comparitive danger discussions (or parallel
>universe discussions) because I'm not sure what they accomplish and they
>rarely hold water with the powers that be.
>
>Bottom line is that UL tests a PV module with a given amount of current
>flowing through it--nothing more. Whether that module can handle twice the
>current or spontaneously combusts at 135% of that rating is totally
>irrelevant. The UL listing is what is at stake. The point is that if you
>provide for more than 15-amps to a 15-amp module (two SP-75s in parallel
>plus a 15-amp fuse), you have violated the UL listing and cannot install it
>in a code-approved manner. If the system does cause a fire, UL could be
>obsolved of all liability because you installed a listed product
>improperly--even if the modules had nothing to do with it.
>
>As to whether systems in developing countries should meet our code--my
>response is that it should meet the spirit of our code, if not the letter.
I
>have designed and built many systems overseas and I try to stay very close
>to our code and use listed equipment because it gives predictable results
>and will last a whole lot longer than other stuff. A huge percentage of
>systems in developing countries are sitting off-line doing nothing, or have
>been rigged to run in an unsafe way because products failed and were not
>designed properly. What is the difference in cost between a system that
>lasts one-year and one that lasts 10 or 20 years. Is it worth an extra 20%
>in cost to last 10 times longer (last I checked the economic answer was
>yes). Overcurrent protection, disconnects, proper wire, proper mounting,
are
>all part of performance, reliability, and maintainability. It is very
>difficult to divide the issues and truly cut costs. (but people do it all
>the time with bismal results)
>
>Bill.
>
>
>-----Original Message-----
>From: Ray Walters [mailto:remotech at taosnm.com]
>Sent: Sunday, May 26, 2002 4:33 AM
>To: RE-wrenches at topica.com
>Subject: Parallel Danger? [RE-wrenches]
>
>
>Hi Bill and all;
>
>Here is another potentially dangerous situation to ponder: a 5amp UL listed
>device (a computer) is powered through 16 awg SJO cord to a 15 amp rated
>receptacle. This receptacle is PARALLELED with several other 15 amp rated
>receptacles (35cents made in China). This whole circuit is fed through a 20
>amp breaker. Coffee spills on the computer creating a short. Because of the
>time/ current performance of the breaker, over a hundred amps could pour
>into our soaked device for a few seconds and  20 amps could flow
>continuously into a device UL listed for only 5.
>Compare this scenario, with my maligned array wiring method: two array
>strings each having a short circuit output of 4.75 amps and a series fuse
>rating of 15 amps feed thru 12 awg wire  in parallel for a total of 9.5
>amps to a 15 amp breaker. A moose chews through the wire creating a short.
>Up to 15 amps continuous are allowed to backfeed into the circuit ( the
>charge controller also failed) and are boosted another 4.75 amps by the
>paralleled string. This time almost 20 amps could possibly (but not
>probably because of the high resistance) flow into a device UL listed for
>only 15.
>Why is the first condition allowed, but the second is not? Is 120 vac safer
>than 48 vdc?
>
>Sincerely again,
>
>Ray
>
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