Cable shields and metallic sheath/armor
should be solidly grounded at one or more points so that they operate at or
near ground voltage at all times. For additional information see IEEE Std
575-1988.
Accidental removal of the shield ground can
cause a cable failure and a hazard to personnel. The length of cable run should
be limited by the acceptable voltage rise of the shield if the shield is
grounded at only one point. The derating of ampacity due to multiple-point
short circuited shields has a negligible effect in the following cases for
three-phase circuits:
a) Three-conductor cables encased by a
common shield or metallic sheath
b) Single-conductor shielded cables
containing 500 kcmil copper or smaller installed together in a common duct
c) Triplexed or three-conductor
individually shielded cables containing 500 kcmil copper or smaller
d) Single-conductor lead sheathed cables
containing 250 kcmil copper or smaller installed together in a common duct
Because of the frequent use of window type
or zero-sequence current transformers for ground overcurrent protection, care
must be taken in the termination of cable shield wires at the source. If the
shield wire is passed through the window-type current transformer, it should be
brought back through this current trans- former before connecting to ground in
order to give correct relay operation.
A nonmagnetic metallic material applied
over the insulation of the conductor or conductors to confine the electric
field of the cable to the insulation of the conductor or conductors.
Shielding practices
Single conductor cables rated above 2 kV
and multiconductor cables with a common overall discharge resisting jacket
rated above 5 kV should be shielded, except for special applications or cable
designs.
Multiconductor cable applications in the
operating range of 2 kV to 5 kV require careful judgment, and each installation
should be evaluated based on the existing and anticipated conditions. Shielding
can be used to monitor or test cable installation for additional assurance of
insulation integrity.
The following shielding recommendations
contained in the NEMA standards publications for the type of insulation being
utilized should be followed: NEMA WC 3-1980, NEMA WC 5-1973, NEMA WC 7-1988,
and NEMA WC 8-1976.
A shield screen material is applied
directly to the insulation and in contact with the metallic shield. It can be
semiconducting material or, in the case of at least one manufacturer, a stress
control material. At the high voltages associated with shielded cable
applications, a voltage gradient would exist across any air gap between the insulation
and shield.
The voltage gradient may be sufficient to
ionize the air, causing small electric arcs or partial discharge. These small
electric arcs burn the insulation and eventually cause the cable to fail. The
semiconducting screen allows application of a conducting material over the
insulation to eliminate air gaps between insulation and ground plane.
Various shield screen material systems
include:
a) Extruded semiconducting thermoplastic or
thermosetting polymer
b) Semiconducting woven fabric tape
c) Semiconducting coating (paint) used with
semiconducting woven fabric tape
d) Extruded high-dielectric-constant
thermoplastic or thermosetting polymer, referred to as a stress control layer
NEMA and AEIC standards require shielded
power cable to be partial-discharge or corona tested. This test evaluates the
effectiveness of the conductor and shield insulation screen materials and
application, and verifies the absence of voids within the insulation.
A cyclic aging test is required by AEIC as
a qualification of the cable design to ensure gaps do not develop between tape
layers as a result of expansion and contraction cycles.
Multiconductor cable shielding should be
considered in the 2 kV to 5 kV range where any of the following conditions
exist:
a) Transition from conducting to
nonconducting environment
b) Transition from moist to dry environment
c) Dry soil, such as in a desert
d) Damp conduits
e) Connections to overhead lines
f) Locations where the cable surface
collects conducting materials, such as soot or salt deposits
g) Electrostatic discharges are sufficient
in magnitude to interfere with control and instrumentation circuit functions
h) Safety to personnel is involved
i) Long underground cables
j) Direct earth burial
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