FINNED COLUMNAR INSULATING SPACER FOR USE IN CONDUIT AND AERIAL CABLES

ENTREMISE AILETEE D'ISOLATION POUR CONDUITS ET CABLES AERIENS

English Abstract

ABSTRACT OF THE DISCLOSURE
A columnar insulating spacer in a conduit or aerial
cable includes one or more circumferential fins with the
portion of the spacer below each fin having a smaller
outside diameter than the portion of the leg above each fin.
The spacer leg diameter may decrease stepwise or may gradu-
ally taper toward the sheath.

Claims

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined as
follows:
1. A cable structure of the type wherein a conductor
is supported within a sheath by a columnar spacer, said
columnar spacer comprising; a spacer leg having at least
one circumferential fin dividing said leg into straight
leg segments, the diameter of a straight leg segment
disposed between said fin and said sheath being smaller
than the diameter of a straight leg segment on the side of
said fin closer to said conductor.
2. The assembly as claimed in claim 1, wherein said
at least one circumferential fin comprises a plurality of
fins and the diameter of straight leg segments on the side
of each fin toward said sheath being smaller than the dia-
meter of straight leg segments on the side of said each
fin away from said sheath.
3. The assembly as claimed in claim 1, wherein said
straight leg spacer gradually slants toward said sheath.

Description

8~
EACKGROUND OF THE INVENTION
When a conductive material is disposed within the
interior of a conduit or aerial cable filled with an in-
sulating gas at the operating voltage of the cable, an
electric field oscillation may build up in an epoxy molded
columnar insulating spacer fitted to the cable. Eventually,
the build-up reaches the conductor and a discharge breakdown
may occur between the conductor and the sheath. To prevent
this, the leg of the spacer is usually provided with one or
more circumferential fins, the dialector constant of the
fins is greater than that of the insulating gas, so that the
electric field around the spacer will be made non-uniform,
but this results in a degradation of the voltage withstanding
characteristics of the spacer.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a longitudinal sectional view of a
conduit or aerial cable having a conventional finned
insulating spacer; and
Figs. 2-4 are side views of a finned columnar in-
sulating spacer according to three different embodiments ofthe present invention.
A conduit or aerial cable with such a conventional
finned insulating spacer is shown in longitudinal section in
the accompanying Figure 1, wherein 1 indicates a conductor
pipe, 2 is a metallic sheath pipe, 3 is finned columnar
insulating spacer that ls typically made of an epoxy molding
and which supports the conductor 1 wirlin the sheath 2, 3'
is the leg of the spacer 3, and 3'' is a circumferential fin

l formed integrally around the leg 3'. The por-tion of the leg
above the fin 311 has an outside diameter a which is the
same as the outside diameter a' of that portion of the leg
below the fin, i.e. the portion of the leg between the fin
and the sheath. At the operating voltage of the cable, the
spacer has a non uniform electric field wherein the spacing
of the equipotential lines is increased toward the sheath 2
as indicated by the one-dot chain line in Fig. l, whereas
the electric field is concentrated at points F and G. As a
result, the voltage withstanding characteristics of the
spacer 3 are degraded.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
finned columnar insulating spacer for use in a conduit or
aerial cable which minimizes the degradation of the voltage
withstanding characteristics by reducing the tendancy of the
electric field to concentrate non-uniformly along the leg of the
the columnar spacer. Briefly, this is accomplished accord-
ing to the present invention by providing the spacer legwith a decreased diameter below the fin or fins so as to
provide varying dielectric constants along the leg. The
part of the leg closer to the sheath will therefore carry an
increased amount of the electric field so that the potential
gradient along the surface of the spacer becomes less steep.
DETA LED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Fig. 2, those components which are the same as in
the cable of Fig. l are designated my the same reference
-- 2 --

numerals. In -the leg 3', the portion disposed above the
fin 3'' has an outside diameter b which is larger than the
outside diameter c of the portion of tne leg below the fin,
i.e. between the fin and the sheath. In a second embodiment
shown in Fig. 3, the leg 3' is provided with two circumfer-
en-tial fins 3'' and 3''' which are each integrally formed
around the leg. The portion of the leg above the upper
fin 3S' has an outside diameter b1 which is greater than the
outside diameter cl of the leg portion between the upper and
lC 1OW2L fills, and trle diameter C1 is greater than the outside
diameter e of the portion of the leg between the lower
fin 3''' and the sheath 2. Rather than stepwise decreasing
the diameter of the leg as in the embodiments of Figs. 2
and 3, the leg may have a tappered configuration as thus
lS trated in Fig. 4. This tappered configuration may be
especially advantages in the case where more than two fins
are provided, but is also applicable to a lesser number of
fins such as shown in Fig. 4.
It a conduit Gr aerial cable having a finned colun~lar
insulating spacer of the type described above having a
reduced outside diameter below its fin or fins, at the
operating voltage of the cable the spare between the equi-
potential field line will increase toward the sheath as
indicated by the one-dot chain line in Fig. 2. However,
since the outside diameter of the portion of the spacer leg
below the fin is smaller than the outside diameter of the
portion of the leg above the fin, the equipotential lines
will tend to be pushed toward the sheath to thereby provide
a uniform density for spacing of the equipoten-tial lines
along the length of the leg. More specifically, the tapper-
ing of the leg toward the sheath provides varying dielectric

constants along the leg so that the portion of the leg
closer to the sheath can carry more of the electric field
and the actual field wlll approximate a uniform field. As a
consequence, the potential gradient along the surface of the
spacer becomes less steep and the voltage withstanding
properties of the spacer are improved. This results in a
consequent reduction in maintenance costs and irrprovement of
the overall electrical characteristics of the cable.
It should be noted that various changes and modifica-
iv lions can be rnade to the above-descrlbed arrangement withou
departing from the spirit and scope of the invention as
defined in the appended claims. For example, the spacer of
the present inventiorl i5 also applicable to a gas~insulated
bus conductor as well as a conduit or aerial cable.