The word “contact“ not only describes the conductive connection ot two mechanically separate electrical conductors, but also the conductive parts (contacts) even if they are not touching.
- Non switching contacts as in connectors being opened only for service or installation (e.g. screw connections)
- Sliding contacts
- Plug contacts to carry but not to switch current
- Switching contacts as in relays, contactors and switchgear
Relay contacts are physically separate but switchable electric conductors designed to make an electrical connection, carry the load current, break the circuit and electrically isolate the load from the supply. How well the contact system actually performs is dependant on the suitability of the contact material, the contact arrangement and the mechanical design.
Unfortunately, actual relay contacts do not have these characteristics. An optimal contact material with high conductivity, resistance to oxidation or chemical reactions and resistance against wear and thermal influences during switching can only be a compromise. Design and cost clearly limits parameters such as the size of contact area, contact forces, relay sensitivity, and the need for big contact gaps for high dielectric strength.
Typical and most basic influences on electrical contacts and their respective effects are shown in the following tables.
Influence On Electrical Contacts
|Heating, melting, material migration, chemical reactions, frilling, electrical discharge, contact resistance|
|Thermal||• Arc||Melting of contact material, material migration|
|Deformation, wear, cold welding, contact resistance|
|Ambient conditions||• Dust
|Increased wear, particles, formation of chemical layers and corrosion|
|Chemical||• Oxidation||Contact resistance, inorganic and organic layers, corrosion|
Influence On Switching Contacts Depending On Load Range
|Load range||Main influences||Contact material||Considerations|
< 100mV, < 10mA
low level switching
|Contact resistance, sealed relays, wipe movement, twin contacts, outgas free and wear resistant plastic material|
|• AgNi 0.15
• AgNi 10
|Sealed relays, trilling, material transfer, contact resistance, outgassing|
|• AgNi 0.15
• AgNi 10
|Electrical life, contact welding, electrical wear, high temperatures, isolation properties, corrosion for sealed relays|
Resource: Schrack Relays