Flexible Cables: Important Safety Issue [RE-wrenches]
Drake
solar at ecoisp.com
Thu Aug 26 07:35:43 PDT 2004
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It is hard not to question Wile's motives. Sandia Labs has long been a
Lockheed-Martin operation. Corporations don't often spend money unless
there is ultimately something in it to enhance their bottom line. Maybe we
should ask, "what does the world's largest manufacturer of WMD have to gain
from making renewable energy systems a thousand times safer than grid
connected power?"
There have been remarkably few problems with PV. Any problems that do
surface are much trumpeted, like a major forest fire that "may have been"
due to a PV system. Last year in Colorado a major forest fire that came
close to consuming the town of Jamestown was caused by a faulty power line
that ran though the mountains. No big deal was made out of that. There
was no campaign to change the way power lines run through the mountains.
Ten years ago solar was on the way up. If it had not been obfuscated, it
could have changed the way we power our society. Now it is still a very
small contributor to our energy budget. If Sandia had done a "Los Alamos"
on renewable energy and energy efficiency, we could be looking at a very
different picture, one that would be good for the people and the planet,
rather than the producers of coal, oil and radiation.
Drake
http://eagle-access.net/solar/
At 08:28 AM 8/25/2004 -0700, you wrote:
>Well, that's old JW all over isn't it? Find a situation that could be a
>problem or that is very, very rarely a problem and make Mt Everest out of
>a molehill. Since day one of his inputs to the RE industry everything he
>suggests or opines that UL/NEC requires raises the cost of a PV system.
>EVERYTHING! Ever notice that? This man comes to us from the nuke industry.
>I find it damn hard not to question his motives. He's kind of a Swift Boat
>PV Opinionater for Arcane Stuff, ain't he?
>Jesus H. Christ!
>
>
>> Oh my God.
>>
>> This isn't April 1st is it?
>>
>> So now all those inverters that we've been putting in with "fine stranded
>>wire" for the past 20 years are going to have to have their cables removed
>>and exchanged because "there have been problems"? What, fires? Explosions?
>>Electrocutions? Befuddled inspectors?
>>
>> I have to say that in all the years I've been working on battery-based
>>inverters, the ONLY connection problems I have observed have ONLY been
>>those that had stiffy wire for inverter cables! There is so much potential
>>for tension-caused breakage with this method, so many (virtually) impossible
>>wire bends and seeming non-compatablity with all the commonly used, tried
>>and true connection components, I just can't bel;ieve that this can be for
>>real.
>>
>> If this such a huge problem, why not simply tin the wire ends?! UL indeed.
>>
>> Matt T
>> ----- Original Message -----
>> From: "Allan Sindelar" <allan at positiveenergysolar.com>
>> To: "New wrenches posting" <RE-wrenches at topica.com>
>> Sent: Tuesday, August 24, 2004 3:35 PM
>> Subject: Fw: Flexible Cables: Important Safety Issue [RE-wrenches]
>> Wrenches,
>> Forwarded to this list at the request of John Wiles
>>
>> ----- Original Message -----
>> From: "John Wiles" <jwiles at nmsu.edu>
>> Sent: Tuesday, August 24, 2004 3:50 PM
>> Subject: Flexible Cables
>>
>>
>> Flexible, Fine-Stranded Cables: Incompatibilities with Set-Screw
>>Mechanical
>> Terminals and Lugs
>>
>> Reports have been received over the last several years about field-made
>> connections that have failed when flexible, fine-stranded cables have been
>> used with mechanical terminals or lugs that use a set screw to hold the
>>wire
>> in the terminal.
>>
>> These terminals are found on nearly all circuit breakers (except those
>>with
>> stud-type terminals), fuse holders, disconnects, PV inverters, charge
>> controllers, power distribution blocks, some PV modules, and many other
>> types of electrical equipment.
>>
>> Fine-stranded conductors and cables are considered as those cables having
>> stranding more numerous than Class B or C stranding. Class B stranding
>>(the
>> most common) will normally have 7 strands of wire per conductor in sizes
>> 18-2 AWG, 19 strands in sizes 1-4/0 AWG, and 37 strands in sizes 250-500
>> kcmil. Conductors having more strands than these are widely available and
>> are in different classes such as K and M used for portable power cords and
>> welding cables. Commonly used building-wire cables such as USE, THW, RHW,
>> THHN and the like are most commonly available with Class B stranding, but
>> are also readily available with higher stranding. Fine-stranded cables are
>> frequently used by PV installers to ease installation and are used in PV
>> systems for battery cables, power conductors to large utility-interactive
>> inverters and elsewhere.
>>
>> Some modules are supplied with fine-stranded interconnecting cables with
>> attached connectors. While these crimped-on connectors listed with the
>> module are suitable for use with the fine-stranded conductors, an
>> end-of-string conductor with mating connector may also be supplied with
>>the
>> fine-stranded conductor ,and the unterminated end of that conductor will
>>not
>> be compatible with mechanical terminals.
>>
>> According to UL Standard 486 A-B, a terminal/lug/connector must be listed
>> and marked for use with conductors stranded in other than Class B and C.
>> With no marking or factory literature/instructions to the contrary, the
>> terminal may only be used with conductors with the most common Class B and
>>C
>> stranded conductors. They are not suitable and should not be used with
>> fine-stranded cables. UL engineers have said that few (if any) of the
>>normal
>> screw-type mechanical terminals that the PV industry commonly uses have
>>been
>> listed for use with fine stranded wires. The terminal must be marked or
>> labeled specifically for use with fine-stranded conductors.
>>
>> UL suggests two problems, both of which have been experienced in PV
>>systems.
>> First, the turning screw tends to break the fine wire strands, reducing
>>the
>> amount of copper available to meet the listed ampacity. Secondly, the
>> initial torque setting does not hold and the fine strands continue to
>> compress after the initial tightening. Even after subsequent retorquing,
>>the
>> connection may still loosen. The loosening connection creates a
>> higher-than-normal resistance connection that heats and may eventually
>>fail.
>> Recent examples of failed mechanical terminals from a large PV system have
>> been found. The terminal had been torqued properly less than three months
>> before the failure.
>>
>> Solutions
>>
>> All electrical equipment listed to UL Standards has:
>>
>> . Terminals rated for the required current and sized to accept the proper
>> conductors
>>
>> . Sufficient wire bending space to accommodate the Class B stranded
>> conductors in a manner that meets the wire bending requirements of the NEC
>>
>> . Provisions to accept the appropriate conduit size for these conductors
>> where conduit is required.
>>
>> It is therefore unnecessary to use the fine-stranded cables except
>>possibly
>> when dealing with conductors 4/0 AWG and larger.
>>
>> In those cases where a fine-stranded cable must be used, a few
>>manufacturers
>> make a limited number of crimp-on compression lugs in various sizes that
>>are
>> suitable for use with fine-stranded cables.
>>
>> Factory-supplied markings and literature indicate which lugs are suitable.
>> An example is the ILSCO FE series of lugs in sizes 2/0 AWG and larger.
>> Burndy makes a YA series of lugs in sizes 14 AWG and up. In both cases the
>> lugs are solid copper. It should be emphasized: Most crimp-on lugs are not
>> listed for use with fine-stranded wire. Where the crimp-on compression
>>lugs
>> can be used, they must be installed using the tools recommended by the
>> manufacturer and, of course, they must be attached to a stud with a nut
>>and
>> washer.
>>
>> Burndy and others make pin adapters (a.k.a. pigtail adapters) that can be
>> crimped on fine-stranded cables. These pin adapters provide a protruding
>>pin
>> that can be inserted into a standard screw-type mechanical connector.
>>Again,
>> not all pin adapters/pigtail adapters are listed for use with
>>fine-stranded
>> conductors; some are intended for use with aluminum wire and others
>>provide
>> only a conversion to a smaller AWG size for B Class conductor.
>>
>> It is suggested that the use of fine-stranded conductors be avoided
>>wherever
>> possible. Where such cables must be used, they should only be terminated
>> with the appropriate connectors/lugs. Previously installed systems should
>>be
>> revisited and the cables replaced where possible or terminated properly.
>>
>> John Wiles SWTDI/NMSU jwiles at nmsu.edu 505-646-6105
>>
>> > If I can provide further information, please do not hesitate to call,
>> > e-mail or fax me.
>> >
>> > John
>> >
>> > John C. Wiles, Program Manager
>> > Southwest Technology Development Institute, New Mexico State University
>> > 505-646-6105 505-646-3841 (FAX) http://www.NMSU.Edu/~tdi
>> > SWTDI/NMSU
>> > Corner Research Drive and Sam Steel Way
>> > Box 30001/MSC 3 SOLAR
>> > Las Cruces, NM 88003-8001
>
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