5.1 As stated in Terminology B542, contact resistance is comprised of a constriction resistance and a film resistance. When present, the latter of these is usually much greater in value and dominates the contact resistance. For a given contact spot, when the film resistance is zero or negligible the contact resistance for that spot is nearly the same as the constriction resistance and therefore, as a practical matter, has a minimum value which represents a clean metal-to-metal contact spot. As real contact surfaces exhibit varying degrees of roughness, real contacts are necessarily composed of many contact spots which are electrically parallel. In practical cases the clean metal-to-metal contact spots will carry most of the current and the total contact resistance is primarily dependent on the size and number of metallic contact spots present (see Note 1). In addition, acceptably low values of contact resistance are often obtained with true areas of contact being significantly less than the apparent contact area. This is the result of having a large number of small contact spots spread out over a relatively large apparent contact area.

Note 1: The term metallic contact as used here is intended to include the so called quasi-metallic contact spots as well. The latter case was discussed in Electric Contacts by Holm. 3

5.2 The practical evaluation and comparison of electrical connections depend in large part on their contact resistance characteristics. On the one hand, the absolute value of contact resistance is greatly dependent on the size and distribution of the metallic conducting spots within the apparent area of load-bearing contact. On the other hand, a comparison of the initial resistance to the resistance after aging indicates how stable the system is in maintaining the initial contact area. Both of these characteristics should be considered when evaluating contact systems. The criteria employed in evaluating contact resistance and stability are not a part of these test methods as they depend on specific applications and therefore, will not be quantitatively stated. However, an estimate of contact resistance3 resulting from good metallic contact can be made for a given physical situation and used as a comparison to actual measurements to determine how effective the system is in establishing stable metallic contact. Resistances measured by these methods before, during and after simulated life tests are used as a means of determining the stability of contacts within a device.

1.1 These test methods cover equipment and techniques for measuring the resistance of static electrical connections such as wire terminations or splices, friction connectors, soldered joints, and wrapped-wire connections.

1.2 Measurements under two distinct levels of electrical loading are described. These levels are: (1) dry circuit, (2) and rated current. One or both of these levels of loading may be required in specific cases.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Safety Data Sheet (SDS) for this product/material as provided by the manufacturer, to establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations prior to use.

1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.