Abstract: This report will focus on the reliability of
electrical terminations. We will explore
the factors which comprise high integrity connections, and those which contribute
to failures thereof. We will conduct
a complete review of the dynamics of electrical connections and analysis of
the new technology of “Spring Compression” in larger cable connectors.
Keywords: Spring Compression, Electrical Connectors, Wire Terminations, Lugs, Cabling, Cytolok, Compression Connectors, Conductor Terminations, Connectors.
I. INTRODUCTION
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No matter the complexity or magnitude of electrical circuits, the “weak link” found in these chains has proven time after time to be connection points. Numerous assessments have stated that 90% of failures of electrical circuits occur at the termination points. [1] No matter the expense and integrity of transformers, switchgear, UPS systems, etc. such an apparently insignificant part of the circuit, the connector, will bring the most elaborately designed system to a halt, usually at a critical time!
This presentation is intended to identify critical areas of potential and recurring failure points of electrical connections and/or terminations associated with any electrical gear or system.
The predominant factors which contribute to these failures will be identified and some potential solutions offered.
A. DIFFERING COEFFICIENTS OF LINEAR EXPANSION.
Quite commonly, most electrical connections are composed of a variety of materials. Usually the conductive portions, or at least the components intended as principal current carrying portions, are made of either copper or aluminum, or a combination of the two and various alloys thereof. The fastener itself, and usually washers are more often steel. Add to this a variety of plating materials often used. One can quickly realize, with all these materials having differing rates of expansion and contraction, that throughout the life of the connector, and the temperature changes it will be subject to, there will be significant movement within. These temperature changes come from both ambient variations and load induced changes from zero to full load.
This equates to a dynamic condition as opposed to the more common view of electrical connections being static in nature. Additionally, the following items, Vibration and Cold Flow, contribute significantly, further adding to the definition of “dynamic” as a descriptive term for electrical connections.
B. VIBRATION
Secondary sources of vibration which affect the electrical connection include resident vibrations in mechanical equipment, attached to or in close proximity of the connector in question.
C. CREEP OR COLDFLOW
Essentially all connections exhibit a certain degree of elasticity. The reason being that nearly all materials exhibit some degree of elastic deformation and recovery prior to exceeding the elastic limits of the material. An example is the screw on a terminal board. As the screw is tightened, a certain amount of elasticity is exhibited prior to the point plastic deformation occurs in the conductor itself. The screw becomes the predominant elastic factor, because as tension builds in the shank of the screw, it deforms elastically. This is ever so slight, but is sufficient to compensate for the contraction and expansion of the joint due to thermal changes for a period of time.[2]
D. SPRING COMPRESSION
Elasticity, this long recognized necessary component of a high integrity electrical termination, is the primary focus of this report, addressed to a new definition for large power electrical termination, Spring Compression.
Large power terminations are often accomplished utilizing a Belleville or spring washer to provide the necessary elasticity to compensate for the greater dynamic movement exhibited by such joints, as well as to counter vibration. Recognized as necessary, and commonly specified as mandatory, since the spring tension introduced to the bolt is proven reliable to combat the forces of vibration in the bolted joint portion of a connector. However it does nothing to enhance the electrical interface between the connector device and the conductor itself.
The electronics industry has brought to light, the detailed study of the dynamics of electrical connections. Due to utilization of lower and lower potentials in electronic circuitry, and the use of plug in modular components, high integrity low resistance electrical connections become an absolute necessity. Predominant factors affecting this integrity are the various components that make up this dynamic interface. One may recognize that the same forces which affect micro-circuitry also have the same pronounced effect on power connections.
The primary criterion in an electrical connection is low electrical resistance. An optimal target resistance for a connector is that it maintains a resistance equal to or less than an equivalent length of solid conductor.
Stability of this resistance is essential. Initial resistance factors are important, however it is far more important (and more difficult) to obtain resistance levels that will remain both low and stable throughout the life of the equipment.
The two primary factors necessary to produce a quality electrical contact are the contact area and the contact pressure. A good connection must maintain sufficient pressure to prevent entrance of atmosphere. This is known as a “gas tight” joint.
A general rule in the development and maintenance of a gas tight connection is maintained pressure of 2000 psi or more. Some materials require pressures up to 20000, however most common electrical conductor material will exhibit a serious degree of creep or cold flow at these pressures.
Crimp connections are typically formed under initial pressures exceeding 30,000 psi, however the residual pressures remaining after the tool is removed are only about 1000 psi. [2]
The contact area is not often apparent as it may seem. Two metallic surfaces having respective areas of one inch, when brought together, do not necessarily have a contact area of one square inch. If the clamping pressure is small, only the high spots of the surfaces actually touch. This leads to large currents passing though these small areas, and often results in melting of the metal at the high spots.
Actual areas of contact occur at microscopic asperities. The total actual contact area is determined by the contact force alone, independent of the contact area. [2]
With sufficient surface area, the initial pressures required to pierce the oxide layer or contaminant film, may reduce once the surface contact has been made. Thus the success of crimp connections. The question is how much can this be reduced before the gas tight connection is lost.
This value varies with connector styles, but can be defined as the point where vibration and handling cause partial separation of the contact areas where atmosphere may enter and corrosion begin, or the effective contact area is sufficiently reduced through dislodging microscopic particles of contact. At this point, resistance will rise and heating will occur.
Movement of the interconnecting surfaces due to thermal effects of expansion and contraction lead to many failures of electrical connections. The three most common methods of making power connections are Mechanical Connectors which typically utilize threaded fasteners in either a tension or compression mode, Compression Connectors, which utilize tools to plastically deform the connector, compressing it onto the conductor, and Welding or Brazing.
Each of these are proven methods, well accepted in the industry, however each also has it’s problems.
A. MECHANICAL CONNECTORS, of the set screw type, are the most common. While these connectors have been used for decades with a rather well known degree of success, evolution of more reliable technologies demonstrates the fact that a weakness exists. The most widely recognized failure mode with set screw connectors is backing out of the setscrew. This obviously results in a loose connection which soon enters a thermal runaway condition. Although actually inappropriate, many maintenance procedures call for periodic re-torqueing of this type connectors, usually on a semi-annual basis.[1]
The reasons for this loosening tendency are a combination of vibration, cold flow or creep of the materials, and repeated expansion and contraction of the connection due to load cycle changes, and ambient temperature changes. Unlike the previous example, there is no screw shank to provide any elastic reserve. Once the joint loosens sufficiently, the resistance rises which in turn causes the temperature to rise. This becomes a self defeating situation, which eventually results in catastrophic failure of the connection.
The areas most prone to failure with this type of connector are moving equipment, such as motors, and their related driven equipment which due to the motion are subject to mechanical shock and vibrations. Transformers and other magnetic devices are also subject to intense vibrations due to magnetostriction, and also due to the varying load factors which subject the circuit to continuous temperature variations.
B. COMPRESSION CONNECTORS: The advance of technology gave rise to the development of a more reliable permanent connection, that being compression connectors. The basis behind this technology is tooling designed to provide sufficient compression force to plastically deform a tubular connector onto a conductor. This tool must also provide a suitable degree of repeatability, as the final dimensions are critical to ensure a joint of high reliability. A variety of very complex and relatively expensive tools have been developed to perform this operation.
The most common problems associated with this method are (1) availability of the tools, (2) maintenance and calibration of the tools, (3) proper use and positioning of the tools, (4) use of the properly calibrated die for a particular connector, (5) misapplication of a tool on the wrong brand of connector, (6) awkwardness of positioning the tool in relatively inaccessible areas, and (7) using an inappropriate sized connector / conductor combination.
While this type connection has proven to provide significant improvements in the integrity of electrical terminations in comparison to mechanical connectors, similar problems occur over the operating life of this type connection as well. The bolted joint itself is subject to the same problems, and these are usually combated with a Belleville washer.
The compression joint is subject to thermal expansion and contraction, and having limited elasticity, can over a period of time, begin to loosen, just as the mechanical connector, resulting in resistance heating, and eventually to thermal runaway.
The tooling as outlined, is typically the most significant factor where inappropriately constructed joints are the problem. The tools and dies are seldom if ever interchangeable between makes, not only of tools, but also connectors. This often results in the use of the wrong tool, and is usually a known situation, as the necessity of making the joint occurs, and the correct tool is simply not available.
These tools require maintenance and periodic calibration. Manufacturers following modern Quality Assurance programs must, as all should, follow requirements of such periodic checking and calibrating procedures for their compression tools and associated dies.
With the predominant use of finer stranded cables, for their increased flexibility, oversized compression connectors are used as a necessity to accommodate the larger diameter of such cables, and then must be over compressed by using an inappropriate tool / connector combination. While this has been done successfully in many applications, it simply offers more potential for error than does an appropriately engineered and tested conductor / connector / tool combination.
C. WELDED JOINTS: A common method for making quality electrical connections within transformers is welding. While welding of similar metals makes an excellent electrical connection, it too has certain problems. Few cases have been reported of welded joints failing, whereas soldered joints are more prone to fail. Failure mode in this type of joint usually results from vibration and heating. During the welding process, oftentimes a more crystalline structure occurs. The transition area of this crystalline weld and the highly malleable annealed copper occasionally becomes a point of failure.
The major problem with this type of joint is not really the integrity as much as it is operator error. Welding, in most cases, is a manual operation, requiring highly skilled technicians. The repeatability from day to day or even weld to weld is often questionable.
It has been shown that multiple conductor welded connections often exhibit differing resistance readings from the various conductors all welded in the same joint. Cases have been found where this variance in resistance significant enough to cause imbalance in coils and thus reduce their intended design performance. Additionally, it is necessary to safeguard the equipment during welding to prevent damage from spatter and heat transfer.
Moreover, it is time consuming, requiring substantial cool down periods prior to continuing the assembly process. This brings an element of concern for worker safety, as burns become a notable injury on the safety lists. Also, significant shielding is often required to prevent inadvertent damage to underlying or adjacent electrical insulation.
D. SPRING COMPRESSION CONNECTORS: From analysis of the most common methods of electrical connections, and an understanding of the respective problems each offers, let us now look at a new method of electrical termination.
Listed by Underwriters Laboratories, under UL Standard 486A & B, a new development in electrical power connectors is defined as “Spring Compression.” This type of connector exhibits a dynamic resilient interface to the conductor being terminated and also to the termination point.
This “Spring Compression” assures that the initial pressure obtained in the self crimping process, is retained throughout the life of the connection, and compensates for thermal expansion and contraction, conductor cold flow, and vibration. The very elements previously discussed as being the major contributors to failure of electrical connections.
Example of “Spring Compression” type connector:
Present embodiments of “Spring Compression” connectors incorporate two interlocking components. [5] A conductor receiving bore exists in both components such that straight alignment of these bores is achieved when the connector is in the open position. The conductor is inserted in the bore at this time. Closing the connector, which is accomplished by simply tightening the mounting bolt, results in a pre-determined misalignment of these bores, consequently compressing the electrical interface areas into intimate contact with the conductor.
The simplicity of this design allows for these connectors to be pre-engineered to fit the associated conductor or plurality of conductors, when closed. The mounting bolt thus serves not only to provide means of attachment to the terminal pad or point, but also as the mechanism to accomplish the mechanical compression of the connector into intimate contact with the conductor. A self regulating device is thus provided, requiring only sufficient torque to close and mount the connector. Additional torque cannot in any manner damage the connection, and torque limitations are only required to protect the mounting hardware, thereby not over stressing the bolt or stud.
The unique technology of “Spring Compression” occurs due to the closure rate being pre-engineered for a particular conductor size. The nature of the design accommodates variations, typically about two conductor sizes apart, and a variety of stranding configurations. During the closure process, a point is reached where compression and plastic deformation of the conductor cease prior to full and complete closure. As the closure process continues beyond this point, elastic deformation occurs in two lever arm components of the connector, which in turn convert this resultant spring pressure into the electrical interface area and equally to the mounting bolt in the form of tension on the bolt.
The resultant pre-engineered compression rate is therefore maintained in the elastic deformation of the connector, which is available to compensate for any resultant creep or cold flow which occurs over years of service. Therefore a “Spring Compression” connector, disassembled years later, will have very nearly the equivalent mechanical pressure across the electrical interface, as it did at the time of installation.
During the service life, this constant pressure assures maintenance of a gas tight joint, thus eliminating ingress of oxygen and potential electrolytes and thereby eliminating the potential for galvanic corrosion.
When a pressure connection is subjected to high temperatures, the pressure at the joint is relaxed. Cold flow is the same process of relaxation occurring over time. One large OEM has tested Cytolokâ brand Spring Compression connectors up to 240°C with no measurable difference in resistivity. One can assess that as these extreme temperatures can be compensated for with the elastic reserve in these connectors, likewise they will compensate for creep. The soft materials of electrical conductors are subject to creep or cold flow. As creep takes place in the conductor during many years of service, it is obviously an advantage to have the loss of potential energy in the conductor compensated for by the energy stored in the connector.
The common equation for measuring elastic reserve in springs is E=1/2FD where F is the force and D is the elastic return. Cytolokâ brand Spring Compression connectors provide up to 36,000 psi to the connector interface. Where the yield point of typical copper conductors is approximately 32,000, the elastic reserve dependent on conductor size and configuration, will be from 16,000 to as much as 24,000 psi. With Aluminum conductors, having a much lower yield point, averaging around 8400 psi, the elastic reserve will fall off to approximately 4000 psi, a level high enough to maintain superior electrical properties, well above the 2000 psi of a good “gas tight” joint, yet low enough that creep or stress relaxation is retarded to a very slow rate.
Dimensional changes due to linear thermal expansion and contraction of these joints resulting from constant temperature changes which occur in most circuitry, from both varying electrical loads and ambient changes are well within the elastic range of these connectors. Therefore, loosening of the electrical interface is equally prevented, and the changes in the spring force are virtually immeasurable.
Vibration, being one of the key elements associated with failed connectors and circuitry is addressed by the spring tension on the bolt, thus eliminating the need for a spring washer. More importantly, the area not addressed by other connectors, the interface between the conductor and the connector, has equivalent spring compensation and counteracts the effects of vibration at this very common point of failure as well.
III.
CONCLUSION
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This technology has been in existence for a period of over 15 years. Our industry is one which moves relatively slow concerning adoption of new technologies. Time and testing are required to prove the integrity of new processes and devices.
A barrage of testing has been conducted on these “Spring Compression” connectors, by nearly every standards organization on the planet. This technology has not only proven itself through these tests, but has shown to be significantly superior in many instances.
Independent testing has shown this type connector to operate up to 14 degrees C cooler than mechanical or compression connectors in the same conditions. [3] A variety and series of internationally recognized test standards, involving 500 to 2000 heat aging cycles, have shown this technology to provide, extremely stable, high integrity electrical connections, without fail. [4]
The design of these connectors accurately provide high integrity, repeatable pre-engineered connections to conductors, without the use of special tooling. There is no calibration required, as the connectors themselves are specifically designed to provide the correct compression on each respective conductor. There is no need for welding apparatus, compression tools, torque wrenches, or other expensive equipment, nor is there need for highly skilled labor.
Many existing problems have been eliminated by utilizing this simple but innovative technology. This technology has proven in virtually every application to reduce labor by 50% compared to other methods of conductor termination. Thus a reduction of time and labor requirements are realized, while yielding an electrical connection of exemplary performance, integrity, and reliability.
IV.
REFERENCES
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[1] J.F. McPartland, Editorial Director, Feb. 1983 Issue, Electrical Construction & Maintenance Ó Copyright 1983 McGraw-Hill, Inc.
[2]
G.L. Ginsberg, Editor, Connectors
and Interconnections Handbook, Vol. 5
The Electronic Connector Study Group, Inc.
[3] T.L. McKoon, Project Manager
Connector Comparison Evaluation, Project C92593
January 1995
Southern Electric International, GA Power Research Ctr.
Forest Park, GA
[4] R.M.B. Adair, Technical Director
Product Information Guide
Gulf Connectors, Inc., Lehigh Acres, FL
[5] Internet Web Page, www.cytolok.com
The Author:
Carl Tamm is an Applications Engineer with Gulf Connectors, Inc. His prior experience in the electrical connector field included seven years as Design Engineer for a major firm, designing, prototyping, and specifying special application electrical connectors. He holds eight patents. Present duties include Technical Sales Manager for North America, specializing in problematic, OEM, Telecommunications, and Utility applications of electrical terminations.