BURNDY® Reference Catalog
▲Introduction - Basic Electrical Connection Principles:
■Basic Factors:
●The basic factors which influence the design and performance of pressure wire connections are as follows:
◆Creep
◆Surface Oxide
◆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 in this publication.
●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.
■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℃.
●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.
●Contact Compounds: Petrolatum or No-Oxid are good contact surface compounds for aluminum, but BURNDY® PENETROX™ A, a petroleum type compound containing zinc dust, has the additional advantage of assisting in the breaking down of the contact resistance. How this is accomplished is not certain, but it appears that the zinc particles of PENETROX™ A probably act as current bridges in the breaks in the oxide film. For more complete information about the PENETROX™ line of compounds, refer to the Accessories section of this catalog.
■Corrosion
●The electrical conductivity and mechanical strength of an electrical connection must remain stable under the deteriorating influences of the environment. This deterioration is corrosion. It is the electrolytic action of moisture and other elements of the atmosphere in conjunction with the metals of the connection. If the conductors and connectors are of copper or a corrosion resistant copper alloy, corrosion is usually a minor factor. However, it is a very vital factor if aluminum is involved.
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