This load range is characterized by the fact that the voltage is below thesoftening voltage of the respective contact material (< approx. 80mV) and thecurrent stays below 10mA. Because of this low electrical load the switchingoccurs without any electrical discharge and also without any significant thermalstress on the contact spot. The main influences on the contact behavior aretherefore chemical and mechanical in nature, such as contamination, and dustor abrasion particles for-med on the contact surfaces. The required highreliability can only be reached by using highly corrosion resistant contactmaterials. Since dust particle contamina-tions play a major role in determiningthe failure rate of these contacts, double (bifurcated) or multiple contacts areused frequently.

In this load range the voltage is between the softening and melting voltage of thecontact material and the current is below 10mA. Because of the higher voltagecompared to dry circuits a temperature induced softening of the contactingsurface area occurs which increases the contacting area. Besides highcorrosion resistance a higher hardness of the contact materials is required forthis load range.

This load range is characterized by a voltage below the minimum arc voltageand a current below 300mA. In this range discharges occur between thecontacts which can electrically or thermally destroy at least partiallycontamination layers on the contact surfaces. At lower electrical load organicfilms may not be thermally destroyed completely which may lead to a steepincrease in contact resistance. In DC circuits short arcs may result in materialtransfer. Contact materials for this load range need to be resistant againstcorrosion and the tendency to material transfer.

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

For lower electrical loads mainly high precious materials based on Au and Pt areused because of their high corrosion resistance, the latter materials howeverused only in limited quantities because of the high price of platinum metals. Agbased materials cover the medium load range and are alloyed with Pd forcurrents <1A and voltages > 24V, and for loads above these levels Agcomposite materials with additions of Ni, or the metal oxides SnO₂, ZnO, or CdOare used. While the Pd addition reduces the silver sulfide formation in sulfurcontaining environments, adding metal oxides increases the resistance againstwelding and arc erosion at higher make currents. At high switching currents andswitching frequency tungsten containing contacts are used, mainly as switchingpre-contacts which absorb the electrical arcs at high make and break currentswhile parallel contacts mainly produced from silver containing materials such asAgNi0.15 (Fine-Grain Silver) are employed for current carrying in the closedcondition.

Primarily the specific stresses on the contact assemblies must be consideredduring the selection of contact materials:

*During make of bouncing contacts mechanical wear, arc erosion,and material transfer occur, the latter mostly in DC switching circuits.

*In the closed condition the value and consistency of the contactresistance must be considered. Both are affected by the resistance to

*During off-switching (break) the frictional wear leads to material loss;besides this material transfer and arc erosion effect contact life.