STABILITY
OF STRESS RELIEF LAYERS
Approximately 50 years ago, cable
engineers became aware that the simplest cable concept
of conductor/insulation/shield was voltage limited
as far as safety, efficiency and longevity were concerned.
Two problem areas were recognized and R&D efforts
to this day are still concerned with the best solution
of these problems.
- The interface between the dielectric
and conductor contained voids or pockets where the
insulation did not penetrate fully into the valleys
of the outer layer of stranded wire. Corona discharge
across the resulting air gaps would deteriorate
the insulation. Further, the small radii of the
individual wires in the outer periphery of the conductor
and especially any irregularities in their surfaces
produced points of high electrical stress, several
times that of a smooth cylinder over the conductor.
- A similar problem developed between
the insulation surface and the outer metallic shield
or sheath. The shield, of course, provided a fixed
path to ground for charging currents. This was required
for safety, electrical efficiency and to improve
the odds of cable survival.
The use of conducting components in
stress relieving layers at both conductor and shield
was a giant step toward the solution of the two problems.
Two types of material, in particular, made significant
contributions: Tapes and Extruded Materials. The tapes
were the first solution. Extruded materials are the
present day solution.
- Taped Stress Relief Layers. These
were porous fabric tapes impregnated with carbon
black particles. These tapes were helically wrapped
around the conductor, resulting in a strand screen
with the same potential as the conductor. Since
it was in intimate con- tact with the insulation
interface, there was no corona discharge, unless
gaps were left at tape edges. Helically wound tapes
over the insulation relieved the stress at the edges
of the shielding tapes, provided a bedding for the
metallic tapes, and assisted in eliminating voids
or gaps should the metallic tape buckle or deform
during bending, insulation or expansion.
- Extrudable Stress Relief Layers.
These were developed to overcome some of the difficulties
experienced with semi-con tapes, for example tape
overlaps, loose threads, buckled or wrinkled tapes,
tape splices and carbon black residues imbedded
in the insulation that had to be removed when splicing
and terminating. Extruded strand screens or shields
presented a rounder, smoother, continuous layer
completely filling the outer layer of strand interstices.
An extruded semi-conducting layer over the insulator
conductor provided more intimate contact with the
insulation. The inclusion of extruded layers for
stress relief at the conductor and metal shield
in cable concepts improved corona levels, breakdown
voltages and reliability, especially in wet environments.
Once stress-relieving components are
part of a cable design, it is a prerequisite that
the conductivity (low radial and volume resistance)
that qualified them remains adequate in a variety
of environments for the life of the cable. Present
industry standards require that conductivity remain
at a level equivalent to a volume resistivity of about
10,5ohm-cm.
These materials depend primarily for
their conductivity or close proximity between particles
of special carbon blacks. Increasing the space between
particles or oxidizing the carbon black particles
can result in a loss of conductivity. Conductivity
can be adversely affected by:
- Severe bending of fabric tapes,
which increases in the bends separation of the particles.
- Contact with solvents, such as
oil or creosote, which swell or expand the material
and increase the particle to particle separation.
- High temperatures that expand the
material and accelerate oxidation.
- The adhesive of some pressure sensitive
tapes which swell or expands the material.
