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===<!--6.1.1 -->Low and Medium Electrical Loads===
Switching processes at low and medium electrical loads are experienced for example in relays and switches for the measuring technology, telecommunications, automotive usage, and appliances. The switching voltage ranges from μV to 400V with currents between μA and about 100A.
Guided by empirically developed arc-limiting graphs typical switching processes can be distinguished. As <xr id="fig:Arc-limiting graphsArc_limiting_graphs"/> <!--Fig. 6.1 --> illustrates, voltage and current determine if switching occurs without arcing, results in a glow discharge, short instable arcs are generated, or a fully developed electrical arc is created. The more exact current-voltage curve characte-ristics are depending on the electrical contact material used. They also depend on the contact gap and the atmosphere the switching occurs in; an ambient air atmosphere is assumed in the shown schematic example.<figure id="fig:Arc-limiting graphsArc_limiting_graphs">
[[File:Arc-limiting graphs.jpg|right|thumb|Arc-limiting graphs (schematic) 1. Arc-less switching<br /> 2. Short instable arcs <br />3. Glow discharge <br />4. Full electrical arcs]]
</figure>
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.
<xr id="fig:Application ranges switching current and voltage"/> <!--Fig. 6.2 --> gives an overview for commonly used electrical contact materials for different load ranges in switches used in the information technology up to the transition range towards power switching applications. The ranges are illustrated as a function of switching current and voltage.
<figure id="fig:Application ranges switching current and voltage">
[[File:Application ranges switching current and voltage.jpg|right|thumb|Application ranges (switching current and voltage) of contact materials for
information technology and transitioning into the power switching devices]]
</figure>
For lower electrical loads mainly high precious materials based on Au and Pt are used because of their high corrosion resistance, the latter materials however used only in limited quantities because of the high price of platinum metals. Ag based materials cover the medium load range and are alloyed with Pd for currents <1A and voltages > 24V, and for loads above these levels Ag composite materials with additions of Ni, or the metal oxides SnO<sub>2</sub>, ZnO, or CdO are used. While the Pd addition reduces the silver sulfide formation in sulfur containing environments, adding metal oxides increases the resistance against welding and arc erosion at higher make currents. At high switching currents and switching frequency tungsten containing contacts are used, mainly as switching pre-contacts which absorb the electrical arcs at high make and break currents
while parallel contacts mainly produced from silver containing materials such as AgNi0.15 (Fine-Grain Silver) are employed for current carrying in the closed condition.
==References==
[[Application Tables and Guideline Data for Use of Electrical Contact Design#References|References]]
[[de:Kleine_und_mittlere_elektrische_Last]]
