LOW
TEMPERATURE FLEXIBILITY
The seasonal temperature range in
which a stock power cable might be installed or operated
in the continental U.S.A. during the course of one
year is fantastic. Winter temperatures in Minnesota
will go as low as -46F. Sheath temperatures can reach
140F during the summer in open pit mines in Arizona.
This is a spread of over 185F.
Most cable coverings are rated in
the range of 75-90 C (167-194 F). Generally little
difficulty is experienced because of high natural
ambient temperatures. Greater difficulty is experienced
when cables are flexed during application or installation
at very low temperatures. Low temperature, per se,
does no great harm once a cable is installed.
All polymers have a tendency to become
progressively stiffer as they are cooled. Cable difficulty
at very low temperatures occurs when two conditions
are reached:
- Cable coverings become too stiff
to be functional
- Cable coverings become brittle
or will shatter under impact
Many cables can be bent at very low
temperature but will shatter under impact at a significantly
higher temperature. PVC-covered building wire is a
good example. It can be bent, depending on conductor
size, from -22 to -40F before it will crack. The same
conductor in a coil dropped from a ladder onto a concrete
floor during installation could shatter at +14F. The
ability of a cable to withstand impact at low temperature
is a prime factor to consider during application or
cable installation.
Cable constructions over 600 volts
are usually composites of two or three different compounds:
(1) extruded strand shield; (2) insulation; (3) jacket.
The low temperature flexibility of this composite
will be no greater than that of the least resistant
of the components. A polyethyleneinsulated cable is
a good example. Specs written before extruded strand
shields were common practice required cold-bend requirement
of -55C (-67F). With extruded strand shields this
requirement has been removed and replaced with a -30C
(-22 F) requirement. The extruded strand shield was
the limiting component.
Polyethylene, XLP, EP, SB-R, and Butyl
rubber all have excellent low-temperature propertiesflexibility
without cracking at -50F. General-purpose Neoprene
and PVC compounds have good low-temperature characteristics,
these compounds pass cold-bend tests in the -22F to
-40F range.
Since most power cables are protected
with a PVC sheath low-temperature application must
be geared to the properties of this sheath. Observation
of a few general rules will be of great assistance
in successfully insulating cables at below normal
temperatures:
- Store cables in a warm warehouse
prior to installation.
- Avoid, if at all possible, installation
of cables when temperature is lower than 14 F.
- If necessary to install below 14
F, avoid all types of impact - for example: dropping
cable on the concrete floor, sharp bending or kinking.
- Avoid long pulls or those with
an excessive number of bends.
- When cable must be bent in a manhole
or cable vault, do it carefully and slowly using
a mandrel if space permits.
