SHEATH BONDING AND GROUNDING
William A. Thue
This discussion provides an overview of the reasons and methods for reducing
sheath losses in large cables. While calculations are shown, 4
of the details are
not covered as completely as are in the IEEE Guide 575 [ 14-11. A very complete
set of references is included in that stan-.
The reader is urged to obtain a
copy of the latest revision of that document before designing a “single-point”
The terms sheath and shield will be used interchangeably since they have the
same function, problems, and solutions for the purpose of this chapter.
Sheath refers to a water impervious, tubular metallic
component of a cable that is applied over the insulation. Examples are
a lead sheath and a corrugated copper or aluminum sheath. A
semiconducting layer may be used under the metal to form a very
Shield refers to the conducting component of a cable that
must be grounded to confine the dielectric field to the inside of the
cable. Shields are generally composed of a metallic portion and a
conducting (or semiconducting) extruded layer. The metallic portion
can be either tape, wires, or a tube.
The cable systems that should be considered for single-point grounding are
systems with cables of 1,000 kcmils and larger and with anticipated loads of
over 500 amperes. Fifty years ago, those cables were the paper insulated
transmission circuits that always had lead sheaths. Technical papers of that era
had titles such as “Reduction of Sheath Losses in Single-Conductor Cables” [ 14-
21 and “Sheath Bonding Transformers” [14-31, hence the term “sheath” is the
preferred word rather than “shield” for this discussion.
Copyright © 1999 by Marcel Dekker, Inc.
2. CABLE IS A TRANSFORMER
Chapter 2 described how a cable is a capacitor. That is true. Now you must think
about the fact that a cable may also be a transformer.
When current flows in the “central” conductor of a cable, that current produces