Main Articel: [[High Electrical Loads| High Electrical Loads]]
*Make operation
<br/>Make erosion caused by pre-close and bounce arcs
Welding mainly during bounce arc
Mechanical wear mainly through bounce and relative motion<br/>
*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 Figs. 6.3 and 6.4. 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.
Fig. 6.3:
Typical application ranges
for contact materials in
power engineering
switching devices as a
function of switching
current and voltage
Fig. 6.4:
Application ranges for
contact materials in power
engineering switching
devices as a function of
switching current and
numbers of operation
===6.2 Contact Materials and Design of Contact Components===