TECHNICAL INFORMATION
Welding Connectors Welding, particularly of aluminum bus, has become increasing- ly popular because the materials are united in a homogeneous bond. The bus can be joined directly or through the use of weld- ments. Weldments eliminate the need for field cutting and match- ing. They also act as fixtures and help align the bus structure during erection. Tungsten inert gas (TIG) has become increasingly popular for joining of aluminum conductors. Pressure Connectors - Bolted and Compression The simplest and most widely used method of joining conductors is by means of externally applied pressure. This pressure can be developed by means of clamp type connectors using bolts and nuts or by compression connectors using special compression tools to develop the necessary forces. As two surfaces are brought together to make a joint, the micro- scopic peaks touch each other as shown in Figure 1. As force continues to be applied, the relatively few peaks flatten out into a large number of plateaus and current is transferred across the interface. The relationship is clearly shown in Figure 2 in which resistance is plotted against force. The important thing about this relationship is that once sufficient force has been applied to estab- lish a safe value of resistance, considerable relaxation can occur before the resistance starts to rise again. A well designed clamp or compression connector thus has some safety factor built into it. All clamp type connectors depend upon the thrust developed by the bolts to deliver the force necessary for a sound, stable con- nection. For a bolt to do its job, it must have adequate strength so that it can be torqued properly. It must develop correct thrust for the installation torque recommended, and above all, it must be reliable and not fail during service.
Wherever there is a conductor carrying electric current, it is necessary to make a connection. This is true whether the con- ductor is a solid or stranded wire, a rectangular bus bar or a piece of copper pipe. This is also true whether the conductor is an inch in length, a foot in length, or several hundred feet in length. Conductors are joined by several methods. The oldest methods are the fusion of conductors by means of soldering, brazing or welding, with the very oldest being the blacksmith form of welding. Joining techniques of this type require special skill and special equipment, consequently, other means have been developed which are more generally applied. These are the pressure methods which are divided between the bolted or mechanical type joint and the compression type connec- tor which uses special tools to develop the necessary forces. One other joining technique that is frequently used is soldering. However, this has its limitations, because of the possibility of the joints melting out during temporary overload conditions. Connector Materials Since most electrical conductors are either copper or alumi- num, it also has become standard of the connector industry to make connectors of copper and aluminum. The category of copper connectors includes pure copper and alloys of bronze and brass. The materials can be fabricated by casting, forging, extruding, punching, or any combination of these processes. The aluminum connectors are made of alloys used to pro- vide the best electrical conductivity. However, in some cases, a compromise material is used to give optimum combination of electrical conductivity and mechanical strength. A general practice has been to use copper connectors for copper con- ductor and aluminum connectors for aluminum conductor. In some cases where a transition from copper to aluminum con- ductors must be made, it is advantageous to use a bi-metallic connector, thus making the transition in the connector rather than in the junction of the connector and conductor.
A
Tightening
Relaxing
B
F 2
R 1
F 1
Contact Force
Figure 1. As force is increased, the surface roughness flattens out creating a multiplicity of parallel paths
Figure 2. Contact resistance curves
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