R E F ERENCE O-2 | www.hubbell.com/burndy Basic Factors: The basic factors which influence the design and performance of pressure wire connections are as follows: 1. Creep 2. Surface Oxide 3. Corrosion A fourth factor, known as thermal effects, is also a consideration, but due to the technical nature and length of this topic, it will not be discussed here. At the outset it should be pointed out that these factors give rise to much more difficult problems in connections involving aluminum conductors than those encountered in copper to copper connections. Creep (Cold Flow) Creep is the cold flow of the metal under pressure and it continues until the pressure reduces to a value at which any further creep is negligible. Creep properties depend on the particular metal or alloy and on its hardness; alloys having less creep than pure metals, and harder metals have less creep than soft metals. In a typical connection, the conductors are generally of pure metal and often of soft temper and therefore, subject to considerable creep. In addition, the condition is further exaggerated when aluminum is the conductor as compared to copper, since its creep rate is many times that of copper. Effect of Creep: Figure 2 shows typical curves of total contact resistance plotted against total contact force. Curve A shows how the contact resistance continually decreases with increasing contact force. When the full contact force F1 is reached, the contact resistance reaches the low value of R1. In general, the full tightening force on a connector greatly exceeds the maximum force for which there is no appreciable creep. Therefore, the force will gradually settle down to a value after which there will be no further significant creep. Fortunately, however, the resistance does not climb back up along curve A, the tightening curve, but instead it follows a new curve B, the relaxing curve, along which the resistance changes very little until the force relaxes to a value such as F2. Admittedly, the point of “no appreciable creep” is difficult to define. For pure metals, especially in the soft state, there is always some creep, even at very low pressures at room temperature. However, we do know that the pressure required to produce the same creep rate is several times greater for copper than for aluminum. Thus, to permit the same contact force F2 for aluminum and copper, the contact area A required for aluminum can be expected to be considerably greater than that required for copper. This explains why the contact areas for connectors for aluminum must be considerably greater than for copper and why many light duty connectors for copper are entirely inadequate for aluminum, even when specially plated and when recommended compounds are used on the contact surfaces. Relaxation: Relaxation of pressure due to creep, or for any other reason, would be a much more difficult factor in a pressure connection were it not for the relationship of contact pressure to contact resistance on the relaxation curve as shown in Figure 2. It is frequently observed that some time after the bolts of a clamp type connector are tightened, the bolt tensions are relaxed appreciably. The question arises as to whether it is necessary to retighten the bolts to the original torque value. In a properly designed connector, retightening is unnecessary since the contact resistance should increase very little due to the relaxation of pressure, as shown by the relaxation curve of Figure 2. This fact is largely responsible for the successful operation of a compression connector. The application of the compression tool applies very high pressure, establishing very low contact resistance. The removal of the compression tool releases a very large proportion of this pressure, and creep further relaxes this pressure. Fortunately, the contact resistance increases very little due to this pressure relaxation. Introduction - Basic Electrical Connections Principles Introduction - Basic Electrical Connection Principles Contact Force: The previous analysis shows that the total contact force largely determines the contact resistance. Thus, to achieve the desired low value of contact resistance, the proper size and number of bolts in a clamp type connector must be supplied, and the compression tool must apply the proper force to a compression connector. In addition, the connector must be designed with sufficient structural strength, contact area, and resilience, to assure that the contact force cannot relax beyond the point where contact resistance begins to rise appreciably, as shown in Figure 2. Surface Oxide The contact of pure metallic surfaces cannot be assured in practical connections. Surface contamination must be expected, especially surface oxidation. These surface films are insulators as far as contact resistance is concerned, and they must be broken to achieve metal to metal contact to make an adequate electrical connection. The difficulty of breaking the film depends on the nature of the film, its thickness, and the metal on which it is formed. Copper oxide is generally broken down by reasonably low values of contact pressure. Unless the copper is badly oxidized, good contact can be obtained with very little or no cleaning. Silver oxide is even more easily broken down by the contact pressure; and since silver oxide forms less readily at elevated temperatures, silver contact surfaces are preferred over copper when used for high temperatures. For this reason, it is considered good practice to silver plate copper contact surfaces that must operate at temperatures over 200˚ C. On the other hand, aluminum oxide is a hard, tenacious, high resistance film that forms very rapidly on the surface of aluminum exposed to air. In fact, it is the toughness of this film that gives aluminum its good corrosion resistance. The oxide film that forms after more than a few hours is too thick and tough to permit a low resistance contact without cleaning. The aluminum oxide film is transparent so that even the bright and clean appearance of an aluminum connector is no assurance that the low contact resistance can be attained without cleaning. In addition to the necessity for cleaning the oxide from aluminum, the surface should be covered with a good connector compound to prevent the oxide from reforming. Common practice is to clean the surface with a wire brush or emery cloth. The compound should be applied immediately after cleaning, or the compound should be put on first and the surface scraped through the compound. Present practice is to scratch brush dry and to apply the compound immediately thereafter. This allows a more thorough job of cleaning the conductor. Figure 2
