A Discussion of Porcelain-Insulator Cutouts

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1 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. 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. The porcelain insulator, the inserts, and the process by which the inserts are cemented are all crucial to the reliability of the cutout. 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 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: Insert Preparation Inserts are stored in a clean, dry environment at room temperature. Each insert is individually dipped in a special bonding agent. Insulator Preparation 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

2 - 2 - Sulfur 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 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. 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. 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. Porcelain Breakage Is NOT a Fact of Life 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 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. 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. Polymer-Insulator Cutouts Are Not Necessarily the Answer 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. 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. S&C Is Committed to the Manufacture of Porcelain-Insulator Cutouts Understanding the care S&C employs in making porcelain-insulator cutouts, and why the various process

3 - 3 - steps 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.

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5 - i - APPENDIX 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.

6 - ii - S&C s Cutout Porcelain Return History S&C s return rate for cutout porcelain is extraordinarily good: % 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 exposure 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 ( ) Number Returned New Unknown Age of Insulator When Returned (Years)

7 - iii - Type XS Fuse Cutout Field Return Reports ( ) 25 Number Returned Porcelain Defect - Current Design, Process Porcelain Defect - Old Designs & Processes Operational Issue Competitor's Cutout Mechanical Damage Unknown Reason for Porcelain Break