Flexible Cables: Important Safety Issue [RE-wrenches]
Kurt Nelson
sunwise at cheqnet.net
Thu Aug 26 06:50:39 PDT 2004
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Hey Bob-O,
Regarding crimp/solder lugs, I used to do a "primitive" crimp on the
lugs and then heat the lug and fill with solder. I've heard that a
properly crimped connection actually has less resistance than a soldered
one (maybe those solder'ers aren't also crimping?). A couple years ago
I bought the big/proper crimping tool. It is really quick and easy.
Yet I often walk away from a job thinking that maybe I should have added
solder to the lug, to make it a permanent connection.
All that said, What I really wanted to say was that when it comes to
these Wrenches posts, and having recently been watching some of the
Olympics, I am a big fan of Bob-O's style and form.
Kurt Nelson
SOLutions
-----Original Message-----
From: Bob-O Schultze, Electron Connection
[mailto:econnect at snowcrest.net]
Sent: Wednesday, August 25, 2004 10:29 AM
To: RE-wrenches at topica.com
Subject: Re: Flexible Cables: Important Safety Issue [RE-wrenches]
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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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