The contact resistance is part of the transition resistance (transfer resistance), which occurs when the electric current is passed from one conductor to another. The transfer resistance is made up of the contact resistance and the resistance of the electrically conductive material.
The more secure the connection is mechanically and electrically, the lower the resistance at the contact point. It should be low in relation to the other resistances occurring in a circuit so that the function is not affected. In electronics, a poor contact can endanger an entire system. In transmission technology, contact points lead to attenuation, which must be taken into account when considering levels.
The ideal contact resistance between closed contacts is practically zero. In practice, it is a few milli-ohms. For relays, the contact resistance is less than 0.1 milli-ohms. In addition, the behavior of the contact resistance over the service life plays an important role for relays. The contact resistance increases over the service life due to oxidation and corrosion and may well be 1 to 2 ohms after several million switching cycles.
In power transmission with currents of several hundred amperes (A), the contact resistance must be extremely low so that the contact points do not heat up, which can lead to fires. Contact resistances in power electronics range from 100 micro-ohms to 1 milli-ohms. The magnitude of the contact resistance depends on the contact material, its micro- roughness, geometry, chemical layers, as well as the size of the contact area and the contact force. The larger the area, the lower the contact resistance.