A Discussion of Porcelain-Insulator Cutouts
All porcelain-insulator distribution fuse cutouts are
not equal. Although S&C uses some of the same insulator suppliers as other U.S. competitors, S&C’s porcelain cutout designs and procedures are not the same.
S&C’s cutout assembly procedure is carefully controlled to yield devices providing years of trouble-free
service.
The vast majority of cutout porcelain breakage problems which some utilities are now experiencing involve one competitor’s cutouts manufactured 10 or
more years ago . . . at a time when they and other
competitors often obtained cutout porcelain from
other sources.
A Review of the Basics
A distribution fuse cutout includes two basic components: a mounting and a fuse tube. The mounting includes the insulator, upper and lower live parts, connectors and, optionally, a crossarm mounting bracket.
Today’s cutout porcelain insulators employ cementedin upper and lower inserts, to which the live parts are
attached, as well as a cemented-in center insert to
which the mounting bracket is attached. S&C pioneered this design over 50 years ago, while others
were making cutouts with insulated steel bands to attach the live parts . . . a design which subjected the
porcelain to stress concentration and was susceptible
to damage from mishandling as well as deterioration.
differences can significantly affect field performance
and life. A summary of these design improvements is
provided in the Appendix.
S&C uses high-quality porcelain insulators. Each insulator must meet the minimum physical characteristics specified by S&C for tensile strength, compressive strength, density, puncture resistance, porosity,
and thermal-shock resistance.
The porcelain insulators used in S&C cutouts are thoroughly tested before the assembly process begins.
S&C has long required that porcelain insulator suppliers 100% proof-test each insulator for transverse load
strength. To indicate that the transverse load test has
been completed successfully, suppliers are required to
mark the bottom of each insulator with indelible ink.
Cementing of Inserts Is Carefully Controlled
As important as it is to obtain a quality porcelain insulator, the process of cementing the galvanized-steel
inserts into the insulator cavities is even more critical.
S&C goes to great lengths to ensure that the process is
carefully controlled, and that finished cutouts are
thoroughly and carefully inspected.
As a point of information, here are some of the steps
S&C takes to control the process of cementing the
inserts into the porcelain insulator, and then assembling the complete cutout:
Steel-banded cutout porcelain has virtually disappeared from the marketplace, as most cutout manufacturers have adopted the cemented insert design introduced by S&C. The industry-wide acceptance of
S&C’s design is a solid endorsement of the concept.
Insert Preparation
The porcelain insulator, the inserts, and the process by
which the inserts are cemented are all crucial to the
reliability of the cutout.
Insulator Preparation
Years of Design and Quality-Control
Enhancements Make a Difference
At a glance, all of today’s cutout porcelain insulators
would appear to be the same. But through years of
extensive mechanical testing of cutouts, S&C has
learned that even subtle design and quality control
•
•
•
•
•
Inserts are stored in a clean, dry environment
at room temperature.
Each insert is individually dipped in a special
bonding agent.
Insulators are stored in a clean, dry environment at room temperature.
Each insulator is mounted in a customdesigned fixture, along with the inserts.
At no time are the inserts allowed to make
contact with the insulator.
351-T81
-2Sulfur Cement Pour
•
•
Specially formulated sulfur cement, maintained at a specific temperature, is carefully
poured into the space between the insert and
the porcelain.
Each insert cavity is filled to the top in a series of separate pours. Care is taken to not
overfill the cavity. Chipping off of excess cement is not allowed.
Final Assembly and Inspection
•
should expect several percent of their installed base of
cutouts to break every year. While that may be true
for competitors’ cutouts, it most assuredly is not true
for S&C Type XS Fuse Cutouts. A discussion of
S&C’s cutout porcelain return history is provided in
the Appendix.
Interestingly, the normal breakage mode of cutout
porcelain is usually horizontally at an insert. But one
manufacturer’s cutouts have recently been seen to
break in an entirely different manner: vertically along
the long axis of the insulator.
Final assembly is not permitted until a specified time has elapsed since the last pour.
All cemented surfaces are given a two-part
epoxy paint coating to seal them.
S&C’s Quality Assurance personnel inspect
every completed cutout. The reclamation of
parts from rejected cutouts⎯especially the
porcelain and the inserts⎯is not allowed.
While cracked or broken cutout porcelain may only be
an inconvenience to some utilities, it may be considered a serious safety incident by others. In the final
analysis, safety should be the major driver for this
concern.
S&C has tested, and continues to test, cemented inserts to improve the process. A summary of S&C’s
design testing is provided in the Appendix.
In response to broken porcelain concerns, some utilities are now making an across-the-board move to
polymer-insulator-equipped cutouts⎯which they view
to be the ultimate solution to the problem.
•
•
Why Cementing of Inserts Is Crucial
It is S&C’s belief that the porcelain breakage problems some utilities have experienced with cutouts are
attributable to less-than-exacting processes and lack of
quality control by the cutout manufacturer.
Upon examining a variety of competitors’ cutouts,
S&C has noted many instances in which the inserts
were making contact with the inner porcelain cavity
wall. Such contact creates a stress point that can
eventually precipitate porcelain cracking under thermal shock and/or mechanical operating stresses. As
noted above, S&C’s assembly fixturing is designed to
prevent the inserts from touching the porcelain.
From the examination of competitors’ cutouts, S&C
has also concluded that the temperature of the sulfur
cement is not being closely maintained during the
pour. Excessive heat can change the physical structure of the cement; insufficient heat can result in improper material flow during the pour.
Polymer-Insulator Cutouts Are Not
Necessarily the Answer
But polymer insulators are not a panacea. The field
performance of some polymer-insulator cutouts shows
that these devices are not always trouble-free. Some
have exhibited flashover problems due to poor resistance to tracking. Others have exhibited poor mechanical handling due to a lack of torsional rigidity. A
thorough evaluation of each manufacturer’s polymer
cutout insulation system and mechanical design is
warranted before making the change.
Since polymer-insulator cutouts are more expensive,
they should be used for good reasons. S&C offers
polymer-insulator cutouts not as a substitute for
poorly produced porcelain-insulator cutouts, but rather
to meet some customers’ requirements for a lighterweight cutout and/or a cutout offering improved performance in coastal environments. Other customers
like the more forgiving nature of the polymer insulator, which won’t chip when the cutout is out of the
box and subjected to rough handling.
Porcelain Breakage Is NOT a Fact of Life
S&C Is Committed to the Manufacture of
Porcelain-Insulator Cutouts
Some manufacturers of fuse cutouts have claimed that
all cutout suppliers have a high rate of porcelain insulator breakage in the field, and that a reasonable user
Understanding the care S&C employs in making porcelain-insulator cutouts, and why the various process
-3steps are important, provides much needed insight into
the matter. S&C has no intention of getting out of the
porcelain-insulator cutout business. Over 50 years of
excellent field performance attest to the superiority of
S&C’s porcelain-insulator-equipped Type XS Fuse
Cutouts.
A variety of improvements have been made to S&C’s
Type XS Fuse Cutouts in the course of those 50 years.
Unlike other manufacturers, S&C’s policy has
been⎯and will continue to be⎯to notify all fuse cutout customers when significant design changes are
being effected.
-iAPPENDIX
S&C’s Cutout Porcelain Design Improvements
S&C has continued to manufacture its cutouts to exacting standards, at the same facility, for over 50
years. Although the cement formulation and bonding
agent remain unchanged, S&C’s carefully controlled
manufacturing process has been improved in a variety
of ways over the years. Here’s a run-down of the major changes effected to the cutout porcelain.
•
Stress reduction at center insert. In 1959,
the back of the insulator was flattened to reduce mechanical stress and to make it easier
to pour the sulfur cement.
•
•
•
Strengthening around insert holes at ends.
In 1977, the root cross-section geometry was
changed to increase insulator strength.
Stress reduction at the insert holes. In
1983, the insert hole geometry and insulator
specification were changed to reduce mechanical stress.
100% transverse load testing. This test was
implemented in 1985 and has been improved
a number of times since.
_________________________________________
S&C’s Design Testing of Cutout Porcelain
Inserts
S&C has performed the following design tests on cutout porcelain inserts:
• Insert integrity of as-poured insulators.
• Insert integrity during thermal-cycle testing⎯twelve cycles of 50°C to -40°C.
•
•
•
Insert integrity during thermal-shock testing⎯six cycles of 65°C to 4°C, with cutout
immersed in water.
Insert integrity during freeze-thaw cycle
testing⎯seven cycles to -40°C, with cutout in
vacuum-over-water tester.
Insert integrity during long-term outdoor
exposure testing⎯examination every 6
months to 1 year.
- ii S&C’s Cutout Porcelain Return History
S&C’s return rate for cutout porcelain is extraordinarily good: 0.00103% of units shipped. Nearly all returns have been from colder-climate northern states.
The very few problems noted have included:
• Drying cracks. These have occurred at the
transitions between the cavities and the main
body. They are the result of incomplete drying before firing, which can produce excessive
difference in shrinkage rates.
• Erosion of the cement. This has occurred on
cutouts located in areas with higher-thannormal leakage currents, and is generally seen
at the top and center inserts. Long-term expo-
•
sure to these leakage currents causes porosity
in the cement surface, which can create water
ingress paths into the insert cavities.
Melting of the cement. This has occurred on
cutouts on which a connector cable has not
been properly prepared or tightened, or too
small a conductor has been used. It results in
thermal runaway of the connectors at both the
top and bottom inserts.
The following bar charts summarize the conclusions
of S&C’s field return reports for porcelain insulators
used in Type XS Fuse Cutouts.
Type XS Fuse Cutout Field Return Reports (1995 - 2003)
18
16
Number Returned
14
12
10
8
6
4
2
0
New
1-5
6 - 10
11 - 15
16 - 20
Age of Insulator When Returned (Years)
21 +
Unknown
- iii -
Type XS Fuse Cutout Field Return Reports (1995 - 2003)
Number Returned
25
20
15
10
5
0
Porcelain
Defect Current
Design,
Process
Porcelain
Defect - Old
Designs &
Processes
Operational
Issue
Competitor's
Cutout
Reason for Porcelain Break
Mechanical
Damage
Unknown