MARMON VITALink MC, Руководство по установке

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When cables are pulled into raceways or trays,
they are likely to be subjected to physical
stresses that they will never again be required
to endure. The prime cause of pulling forces is
the friction of the cable against the supporting
and contact surfaces. If the supporting surface
is straight and horizontal, this friction is caused
by the weight of the cable in contact with this
surface. If the surface is not horizontal, the
weight of the cable also affects the pulling load,
but is dependent upon the angle of inclination.
This angle may add to or lessen the total pulling
force, depending upon whether the pull is up or
down.
When a cable is pulled around a bend, it is in
contact against the inner arc of curvature of the
bend. If any substantial amount of pulling force
has been developed in the cable, the friction
load due to the pressure at this point will greatly
surpass that due solely to the weight of the
cable. Thus, bends in the run increase the
pulling load significantly.
Factors that shall be considered prior to
installation, to minimize the possibility of cable
damage, are as follows:
• Tensile strength of the conductors
• Method of attachment to the cable
• Sidewall pressure
• Estimated pulling tension
• Force required to pull the cable off the reel
• Coefficient of friction between the cable and
adjacent surfaces
• Percentage of raceway area filled
• Bend radius
Each of these items is discussed in the follow-
ing sections starting with tension calculations.
Two tension calculations are required for each
cable installation. The first calculation is the
“Maximum Allowable Pulling Tension” for the
particular cable to be installed. This value is
dependent upon the method of attachment to
the cable, the allowable sidewall bearing pres-
sure, and the construction of the cable.
Secondly, knowing the weight of the cable and
the details of the installation configuration, the
“Estimated Pulling Tension” that may occur
during installation can be calculated.
Maximum Allowable Pulling
Tension
The maximum allowable pulling tension on the
cable(s) is the lesser of the maximum allowable
tension based on conductor strength (Tc), the
maximum allowable tension based on sidewall
pressure (Tp), and the limit based on the
attachment method to the cable.
Conductor Tensile Strength
It is assumed that the method used to attach
the cable to the pull rope transfers all forces to
the conductor. The tensile strength of the con-
ductor then becomes a limiting factor for the
force that can be applied. Copper elongates
slightly before breaking, which changes the
resistance characteristics. A safety factor is
used to prevent this, as well as other items.
This tension is determined by the following for-
mula:
T
c
= K x F x kcmil
T
T
c
= Maximum allowable tension based
on conductor tensile strength (pounds)
K = Factor based on material strength
with a safety margin;
8 for annealed copper
F = Factor to account for possible
unequal tension distribution
kcmil
T
= The sum of the circular mil area of
all conductors in thousand circular
mils (kcmil)
4. Pulling Calculations