Changes

This load range is characterized by the fact that the voltage is below the softening voltage of the respective contact material (< approx. 80mV) and the current stays below 10mA. Because of this low electrical load the switching occurs without any electrical discharge and also without any significant thermal stress on the contact spot. The main influences on the contact behavior are therefore chemical and mechanical in nature, such as contamination, and dust or abrasion particles for-med on the contact surfaces. The required high reliability can only be reached by using highly corrosion resistant contact materials. Since dust particle contamina-tions play a major role in determining the failure rate of these contacts, double (bifurcated) or multiple contacts are used frequently.

In this load range the voltage is between the softening and melting voltage of the contact material and the current is below 10mA. Because of the higher voltage compared to dry circuits a temperature induced softening of the contacting surface area occurs which increases the contacting area. Besides high corrosion resistance a higher hardness of the contact materials is required for this load range.

This load range is characterized by a voltage below the minimum arc voltage and a current below 300mA. In this range discharges occur between the

contacts which can electrically or thermally destroy at least partially contamination layers on the contact surfaces. At lower electrical load organic films may not be thermally destroyed completely which may lead to a steep increase in contact resistance. In DC circuits short arcs may result in material transfer. Contact materials for this load range need to be resistant against corrosion and the tendency to material transfer.

The main characteristic of this load range is the presence of stable electrical arcs. Caused by the interaction between contact material and electrical arcs the electrical life of contacts is limited by arc erosion or material transfer and in the case of higher make currents also by weld failures. For contact material selection the type of electrical load, i.e. resistive, inductive, capacitive, motor load, which determine the time function of the electrical current, is most critical.