Fw: Flexible Cables: Important Safety Issue [RE-wrenches]

Tue Aug 24 15:35:27 PDT 2004

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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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