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Properties of Relay Contact System
Properties of Relay Contact System

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.

An ideal relay contact would consist of highly conductive metal with chemically clean surfaces (no oxidation) and a large, wear resistant, effective contact area. Open contacts would ideally have infinite dielectric strength for electrical isolation.

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

InfluencesParametersEffect
Electrical• Current
• Voltage
Heating, melting, material migration, chemical reactions, frilling, electrical discharge, contact resistance
Thermal• ArcMelting 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• OxidationContact resistance, inorganic and organic layers, corrosion

Influence On Switching Contacts Depending On Load Range

Load rangeMain influencesContact 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

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

5 Comments


  1. mandar penkar
    Mar 11, 2016

    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.


  2. David Cohen
    May 05, 2014

    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.

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  4. […] causing it to rotate.Until the rotational forces are great enough to turn the disk and bring its moving contact against the stationary contact, a spring restrains the disk motion.Close-up of an induction-type […]

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