Introduction
The demands of the industrial
environment are ongoing, with ever changing trends. Cables, which were once
able to sustain functional and operational integrity a decade ago, would not be
adequate to survive in the environment of a present day manufacturing site.
Everywhere, from the Renewable Energy Industry, Automotive Assembly Plants, to
the factories that manufactures small office machines and even in some
commercial buildings, the oil resistance of cables has become increasingly
important. Oils serve a dual-purpose role in industrial applications, both as a
coolant and lubricant, depending upon the requirements mandated by the end use
application. Sustaining trouble free cable operation under harsh chemical and
environmental conditions reduces costly manufacturing down time and helps to
eliminate or minimize periodic maintenance and costly cable replacement. All of
these factors mentioned play a major role that is critical to a consistent,
smoothly run manufacturing operation, which in the end, results in higher
profit margins.
Regulatory and Code
Changes
With the changes to the National
Electrical Code (NEC) in the past 10 years, protective conduit or raceway is no
longer required when running an exposed run (-ER) cable from the tray to the equipment
or device. Previously, when the cable was extended from tray to machine,
conduit or raceway was used primarily as a protection mechanism in helping to
prevent cable damage. Originally TC-ER cable (previously printed “open wiring”)
had a length limitation of 50 ft. from the tray to the equipment. The 50 Ft.
allowances resolved a large “grey” area in the industrial environment and was
initially a well-received solution by the industry. Due to the overwhelming
acceptance of the 50 ft. length allowance, the NEC
committee enacted further changes shortly thereafter, permitting unlimited
length of TC-ER under Article 336. With the advent of unlimited length, Article
336 also brought other issues, like a greater area of cable exposure and
susceptibility to the surrounding industrial environment. Under the typical
conditions of operation, consideration for factors such as ambient temperature,
a cables mechanical strength, unintended movement and constant exposure to industrial
lubricating and coolant oils must be taken into account. When exposed to these
conditions, the cable inevitably will begin to deteriorate; the overall jacket
may swell and/or crack, creating a potentially hazardous condition, along
with machine and production down time. These possible problems are undesirable
and necessitate the need to implement cable protection measures. When referring
to NFPA 79, the electrical standard for industrial machinery, Machine Tool Wire
(MTW) is one type of cable permitted. Under the standard for machine tool wire,
UL 1063, passing the Oil Res I test is required and further severe testing such
as the Oil Res II is optional. Environmental resistance tests, such as those per UL Standards
were implemented in response to the globalization of industry with the goal of
standardizing the oil resistance requirements of cables used in manufacturing industrial
machinery throughout the world.
Purpose and
Application
Why does oil cause such excessive
damage on certain types of insulations and jackets and how does this occur? All
compounds are not the same, for example, certain types of PVC have a higher
degree of flame resistance, while others have better oil resistance, and some
demonstrate improved flexibility characteristics. PVC formulations vary
greatly, depending on the desired properties and applications.
These properties can be achieved by
adjusting the formulations of a particular PVC compound. The modification or
addition of flame-retardants (iodine), stabilizers, and fillers allow the
compound to exhibit these types of enhanced characteristics. However, when
certain PVC characteristics are improved, the enhancement sometimes comes at a
cost, the cost being that other performance traits are affected or completely
lost.
The specific application will
determine if oil is used as a lubricant and/or coolant. Acting as a lubricant, oil
would be applied to a gear system driven by motors to prevent premature wear
down and insure smooth operation. Acting as a coolant, oil is applied during
the machine lathing process to keep metal from becoming too hot. In the field,
cables can be exposed to oil in a Wind Turbine nacelle, (the nacelle is the
area located on the top of the turbine) where oil is used in the gearbox. Cables
that lay in the floor of the nacelle are subjected to oil that is unavoidably
spilled. These cables are then exposed to oil for very long periods of time,
along with other extreme high and low temperatures causing the lower quality
jacket compounds of a cable to crack. There are many factors involved regarding
how oil will attack wire and cable, such as, exposure, ambient temperature and also
possible continued immersion. In general, increases in the amount of exposure,
the frequency and the ambient temperature, the faster oil will start the deterioration
process. In short, oil attacks the insulating compound, where it will become
virtually ineffective in its primary role as an effective insulator. This
action can create a possibly very hazardous situation, not only to human life,
but also to the overall function of the industrial machinery to which it is
connected. This results in very expensive downtime, costly repair and
in the worst-case scenario, entire replacement of the machine.
Step 1: When process oils
come in contact with PVC & Polyolefin compounds, the process oils are attracted to the plasticizers in the cable.
Step 2: The oils can be
absorbed by a Polyolefin material resulting in swelling and weakening of the cable
jacket.
Step 3: The oils can extract
the plasticizers from PVC materials making the cable jacket hard and lead to failures.
What Happens
All wire and cable insulations are not
created equal. Electrical, environmental, mechanical, and chemical attributes
will vary depending upon the individual compound formulations. Insulating compounds
contain a specific amount of plasticizers in their individual formulations,
which help promote flexibility and resistance to fatigue. When compounds are
exposed to lubricating and coolant processing oils the material either absorbs
the oil or the plasticizer will diffuse from the compound.
When oil is absorbed, it causes severe
swelling and softening of the compound resulting in degradation of tensile
properties. When the oil causes diffusion of the compound plasticizer, hardening
will result and all flexibility and elongation properties are lost. The
attached pictures will illustrate the effects that oil can inflict on cable
jackets and insulation:
Cracking – Caused during
exposure of the PVC to oil or other chemicals due the complete removal of
plasticizers, resulting in hardening and eventual cracking of the insulation
and jacket.
Melting – Caused during
exposure of the PVC to oil or other chemicals due to the absorption and
combination with the plasticizer, resulting in softening and the high
elasticity noted in the compound.
Swelling – Caused during exposure
of the PVC to oil or other chemicals due to migration of the oils into the
plasticizer, resulting in noticeable increases in insulation
and jacket diameter.
Discoloring – Caused during the
exposure of the PVC to oil or other chemicals due to the diffusion of the
plasticizers along with colorant from the insulation and jacket.
The preceding pictures verify the
damage caused by oil exposure is irreversible and creates hazardous conditions.
Now, in addition to cable replacement costs, there is also the expense of reinstallation
to be taken into account. To avoid these types of unwanted scenarios, the
customer must review the properties of the cables they are about to consider
for their application and determine suitability for the oil environment. There
are UL tests, which help determine how a cable will react in the industrial oil
environment. These tests are more commonly referred to as the Oil Res I and Oil
Res II tests, which involve continuous immersion of the cable samples in IRM
902 at elevated temperatures for a specified period of time. Passing results
are determined by the evaluation of mechanical properties and observations of
physical damage caused by the oil exposure. In 2000, Lapp as an innovator and leader,
approached UL about creating tougher standards which resulted in the creation
of AWM style 21098.The table below indicates the industry standard tests that
are used to evaluate wire and cable oil exposure performance:
Industry Oil Exposure
Tests
Example of Tensile
and Elongation Test Methods
Let us assume, for example, that the
cable jacket of your product is going to be tested for compliance to UL Oil Res
II. Tensile and Elongation tests must be performed both on the original
(unaged) and oil immersed (aged) test samples and must be prepared as defined
under UL Standard 2556. Die cut dumbbell specimens are taken from the jacket
and are then tested for tensile strength and elongation.
As for sample preparation, two marks
are applied approximately 1.3 inches apart from each other and equidistant from
the center of the dumbbell sample. (See diagram on next page). These marks are applied
at right angles to the direction of the pull in the testing apparatus. The
sample is then clamped on the tester with one-inch marks outside of and between
the grips. The grips are then separated at the rate of 20 inches per minute
until the sample breaks. Results are then recorded for elongation and pound force breakage; tensile strength
is calculated by dividing the pound force by the cross sectional area of the
specimen.
Die-Cut Specimen
Untested die cut samples are aged
under the UL Oil Res II requirement of 75°C for 60 days. After 60 days, the
samples are removed from the oil for a minimum of 16 hours. They are then
tested for tensile and elongation, which must retain 65% of the unaged values.
The following is an example for an Oil Res II test results:
Conclusion
The oil resistance of cables has now
become a critical performance parameter when electrical contractors, engineers,
and installers specify cables for end use application designs. The continued growing
popularity of oil resistance requirements is due to changes in standard
regulations and the increased performance characteristics that are mandated by
certain industries: Renewable Energy, Automotive Assembly Plants and other
production facilities. As time moves forward, superior oil resistant cables
will become standard, rather than the exception and the demand for this type of operating performance will only
continue to grow.
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