Electrical contact
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.
Contacts comprise:
- 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
| Influences | Parameters | Effect |
| Electrical | • Current • Voltage | Heating, melting, material migration, chemical reactions, frilling, electrical discharge, contact resistance |
| Thermal | • Arc | Melting of contact material, material migration |
| Mechanical | • Friction • Pressure | Deformation, wear, cold welding, contact resistance |
| Ambient conditions | • Dust • Gases | 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 |
| Dry circuit < 100mV, < 10mA low level switching <1V, <10mA | • Mechanical • Chemical | gold plated materials | Contact resistance, sealed relays, wipe movement, twin contacts, outgas free and wear resistant plastic material |
| Intermediate level <15V, <300mA | • Mechanical • Chemical • Electrical | • AgNi 0.15 • AgNi 10 • (AgSn02) • (AgCd0) | Sealed relays, trilling, material transfer, contact resistance, outgassing |
| Power contacts 10-400V, 300mA-30A | • Electrical • Chemical | • AgNi 0.15 • AgNi 10 • AgSnO2 • AgCd0 | Electrical life, contact welding, electrical wear, high temperatures, isolation properties, corrosion for sealed relays |
Resource: Schrack Relays
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
Dear sir,
Hi! I am from India. We are distributor for Hongfa relays in India.
One of our customer using HF46F/12-HS1 RELAY since 2009. Application is Fan Regulator.
Currently he is facing problem of contacts sticking even after load is nil. Load is less than 500mA while relays is 5A. They are using 3 relays in one board. There is no change in circuit. Please help what may cause of failing of relays.
Edvard,
You sound like you have lots of relay design experience. Have you ever tried to (or heard of) restoring a low contact resistance to a set of contacts that have been used under low current conditions and have increased resistance? I am about to try to restore a set of auxilliary contacts on a power relay that was not properly wetted.
thanks.
Place a capacitor across the aux contacts to provide enough whetting/fritting current on closure.
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