Compression Dies: Part 2 By Waymon P. Goch and Carl R. Tamm Tel.800-269-1462 Fax.800-311-9857 2663 Lyman Drive, Clinton, OH 44216-9731 www.classicconnectors.com Classic Connectors, Inc. Last month s Newsletter covered the types of single and conventional compression dies and accessories most commonly used for full tension compression connectors on overhead transmission and distribution conductors. In this part we ll discuss some of the potential problems and provide some hints on how to avoid them. The most common cause of premature compression connector failure is improper installation: Failure to brush, clean and properly prepare the conductor immediately prior to connector installation. Failure to use the proper amount of the recommended filler compound at the proper time. Excessively worn, misaligned, or incorrect compression dies and failure to completely close. Improper or Partial Insertion of the conductor in the connector. All can result in an installed connector with higher resistance that will cause the connector to run hotter than the conductor. Increasing conductor temperatures (and resistance) may cause annealing of the connector components and result in thermal runaway and ultimately electrical and mechanical failure. Improper dispersion of filler compound will reduce the corrosion resistance by allowing moisture and contaminants to penetrate between the conductor strands and into the connector body, which is also accelerated by expansion and contraction during thermal cycling. Improper compression can damage the conductor and connector and reduce the mechanical strength. A popular and commonly employed method of detecting hot running connectors is Infrared (IR) measurement. That method is effective but it should be recognized that by the time the hot connector can be detected with IR the damage has already been done and the connector is likely well on its way to failure. (1) Following are typical problems and some recommended solutions: Reverse compression: In last month s Newsletter it was noted that no connector manufacturer recommends reverse compression nor guarantees the connector performance if it is installed in that manner. The consequences of reverse compression are many including breakage of conductor steel core and aluminum strands, as well as fracture of outer aluminum sleeves due to internal hydraulic pressure. Bowed accessories: A bow in the compressed section of an accessory does not affect the electrical or mechanical performance of that accessory provided the bow is not excessive. Bow should be considered excessive if it is more than ½ the conductor diameter in a splice or ¼ the conductor diameter in a dead end. (Note: Mr. McCoy might not agree with these limits). Bow is measured by placing the bowed fitting end to end on a flat edge or surface and measuring the distance between the flat edge or surface and the center of the fitting. Under no circumstances should straightening be attempted by applying force to the aluminum accessory. Subjecting the uncompressed region of the accessory to bending forces could possibly cause stress cracking. Page 1 of 6
It is also not possible to straighten a bowed accessory by applying tension. Occasionally the steel barrel of an eye, clevis, sleeve, etc. bows during compression to the extent that the aluminum sleeve cannot slide back over it. There is a safe way to straighten those kinds of steel components. Remove the lower die half from the press and center a steel bar, approximately 1 thick x 2 wide x 6 long in the lower die holder leaving the upper die half in the upper die holder. Place the bowed barrel or sleeve on the steel bar with the bow pointing up and centered in the upper die half. Actuate the press until the top die makes contact and straightens the bowed fitting. This will not require complete die closure, nor should it. Conditions that cause or intensify connector bowing Connector eccentricity (consistent wall thickness of the connector) Usually not a problem with transmission accessories since most are made from seamless drawn tubing with very tight tolerances. Die bite length. Excessive and non-overlapping compressions can contribute. Longer die bite lengths are used with larger presses. Die offset. Occurs when the die retainers of the press head are worn or broken and do not keep the die halves longitudinally in line. Compression die orientation. Caused by loose die holders in the press head. Conductor straightness. During compression, the connector will follow the direction of the conductor. Straighten the conductor and support it outboard of the area being compressed. Keep the connector perpendicular to the dies. Die surface condition. The compression surfaces must be clean and free of oxidation, rust, dirt, and other foreign material. Surfaces should be clean, smooth, and lightly lubricated. A better lubricant All major manufacturers recommend lightly oiling the die faces. However, there are more effective means that can be used to lubricate and provide for a cleaner, smoother compression. One is to wrap the compression fitting with several layers of thin, clear plastic film; such as Saran Wrap or similar cling wrap before beginning compression. This clear film allows one to see the information on the fitting, including the start position knurl. The plastic film in which the compression fitting was packaged can also be used. Usually as a single layer that can be held in place with tape. This film is typically thicker than cling wrap and not as clear so the start knurl position can either be marked on the film as it is applied or the film can be added after the first compression. After the compression operation is complete, the film is stripped off. Page 2 of 6
And perhaps the best part! It is dry, and won t get your gloves and clothes oily or greasy and, one more little trick. Wrap it past the end of the connector, so that when that excess inhibitor is expelled, it is captured neatly in the plastic wrap, and won t end up all over you and your tools! Other effective lubricants that have been used successfully (and are not as messy as oil) include Ivory soap and paraffin wax. Improper or Partial Insertion A number of connector failures are related to partial insertion of the conductor into the connector, or improper centering of a conventional two-die compression splice. The result is that a reduced area of stranding is available for conducting current. The number one reason this occurs is it is apparently just too time consuming to make a mark on the conductor! Or, the mark gets wiped off during the cleaning operation. Here is the solution: Use a wrap of electrical tape as the mark. Easy, Available, Stays in place! Make that mark an easy distance from the end of the connector not at the mouth where it will certainly get wiped away during that proper cleaning and brushing procedure! On a splice, chose an easy measurement, like four feet from the center. Then, when you slide the aluminum sleeve in place, simply align the center mark 4 feet from your tape! Don t have a tape measure? Use anything you have of a fixed length, like 3 brush handles or four tube lengths of inhibitor, or the length of a prybar or shotgun stick the distance does not matter, nor what units of measure are used - only that you can repeat it easily, and that your mark is a sufficient distance away so as not to be lost in the process. Birdcaging Birdcaging most frequently occurs with ACSS and similar conductors made with fully annealed aluminum strands but can occur with any type of stranded conductor. During compression, aluminum (both accessory and conductor) extrude approximately 1/8 per inch. Recall that the extrusion is in both directions during the first compression but only in the span direction on subsequent compressions. That aluminum has to go somewhere. Be sure the grip is at least 10 feet away from the compression connector. Don t attach ground clamps close to the working area. Work out any birdcages and milk them toward the span if they develop during compression. Filler compounds Use the appropriate quantity of the recommended inhibitor compound at the appropriate time. There is supposed to be an excess of inhibitor in the connector, such that a reasonable quantity is expelled during compression. Filler compounds are required to fill internal voids between steel barrels and outer sleeves, between conductor strands, and between conductor strands and outer sleeves. Compression may reduce or remove galvanizing from the steel barrel and the filler compound is necessary to protect the steel from oxidation. Completely filling the void area forces compound into the end of the conductor, filling voids between conductor strands and sealing the end from air and moisture. Filler compounds contain hard grit particles that improve mechanical holding strength. Those particles also serve to remove non-conductive oxides from the surfaces of strands and sleeve during compression to clean those surfaces and improve electrical conductivity. Per the second bullet point above, this first wave of excess filler compound has served to clean the stranding, and therefore it is full of unwanted debris and it is intended that this be expelled from the connector. The objective is to make the connection last longer, not save 25 cents worth of inhibitor compound! Page 3 of 6
The following images illustrate some of the topics discussed above Excessively bowed dead end Bowed steel dead end Lower die replaced with steel bar Straighten fitting in press Fitting after straightening Birdcage caused by stringing block not rolling freely Page 4 of 6
Installers working out birdcage Birdcage caused during compression Corrosion resulting in tensile failure of aluminum and steel strands due to excessive heating of steel and high resistance interface Dried filler compound resulting from heat and aging Corrosion failure due to dried filler compound Arcing between strands and splice body Corrosion due to little or no filler compound Page 5 of 6
Acknowledgments: The authors greatly appreciate and acknowledge the assistance of Ray McCoy, ACA Conductor Accessories, in the preparation of this article. Ray also has a great deal of experience in the design, testing and installation of these devices as well as in failure analysis. In a sense, this is a compilation of a few things three old guys have learned over many years. References: (1) Do you know the condition of your splices? Joe Renowden, Consulting Engineer, SensorLink, Sensor Note 304. Compression Accessory Concerns, Ray McCoy, ACA Conductor Accessories, presentation during the IEEE TP&C Construction Working Group meeting, July 28, 2009, Telus Convention Center, Calgary, Alberta, Canada. IEEE Std 524, IEEE Guide to the Installation of Overhead Line Conductors Page 6 of 6