Hohe elektrische Last

Hohe elektrische Last

Bei hohen elektrischen Lasten, die überwiegend im Bereich der Energietechnik auftreten, sind die Schaltvorgänge weitgehend mit dem Auftreten von Lichtbögen verbunden. Die Beherrschung des Schaltlichtbogens ist in den meisten Anwendungen das zentrale Problem. Je nach Schaltgerätetyp stehen bestimmte Anforderungen im Vordergrund, nach denen die Wahl des Kontaktwerkstoffes erfolgt. Wie in der Informations- und Nachrichtentechnik sind dabei die Probleme zu berücksichtigen, die bei den Schaltvorgängen und der Stromführung auftreten.

Typische Anwendungsbereiche für Kontaktwerkstoffe in Schaltgeräten der Energietechnik in Abhängigkeit von Schaltstrom und -spannung

Anwendungsgebiete für Kontaktwerkstoffe in Schaltgeräten der Energietechnik in Abhängigkeit von Schaltstrom und Schaltspielzahl

• Make operation
  Make erosion caused by pre-close and bounce arcs
  Welding mainly during bounce arc
  Mechanical wear mainly through bounce and relative motion
  

• Current carrying through closed contacts
  Increased contact resistance and temperature rise
  during nominal load
  Welding through high contact resistance during overload and short circuit load
  Welding during dynamic separation of the contacts with arcing

• Break operation
  Arc erosion during opening
  Arc movement
  Arc extinguishing
  Mechanical wear

The typical application ranges for different contact materials in devices for power engineering are illustrated in Figure 1 and Figure 2. In the lower load ranges mostly silver and fine grain silver (AgNi0.15) are used because of their high electrical and thermal conductivity. With increasing currents the more arc erosion resistant AgCu alloy materials are used. For the medium current range up to 100A Ag/Ni composite materials are advantageous because of their lower and consistent contact resistance and their favorable re-solidification properties. If higher welding and at the same time arc erosion resistance are required, such as for example in motor contactors for switching currents up to 5,000A, silver – metal oxide materials are superior. In protective switches (mainly circuit breakers) which are required to handle high short circuit energies, asymmetrical contact pairings are used where the fixed contact is made from Ag/C materials and the moving ones consist, depending on the device characteristics, of Cu, Ag/Ni, or Ag/W. For UL rated and certified circuit breakers (UL = Underwriters Laboratories) which are mainly used in North American power distribution networks symmetrical pairings of Ag/W or Ag/WC are the preferred contact materials.

For very high loads in main power switches and power circuit breakers for medium and high voltage power engineering applications the most suitable materials are tungsten based infiltration materials such as W/Cu.

References

References