As the most commonly used control component in non-standard automation control, it is very important to understand the materials and life of relays. Choosing ideal contact materials and relays with longer life can reduce maintenance costs and equipment failure rates.
The electrical life expectancy of general-purpose relays and power relays is usually at least 100,000 operations, while the mechanical life expectancy may be 100,000, 1 million or even 2.5 billion operations. The reason why the electrical life is so low compared to the mechanical life is that the contact life depends on the application. The electrical rating applies to the contacts that switch their rated loads. When a set of contacts switches a load less than the rated value, the contact life may be significantly longer. For example: 240A, 80V AC, 25% P.F. contacts may switch a 5A load for more than 100,000 operations. However, if these Solid silver contacts for electrical are used to switch (for example: 120A, 120VAC resistive load), the life may exceed one million operations. The rated electrical life also takes into account arcing damage to the contacts, which can be extended by using proper arc suppression.
Contact life ends when the contacts stick or weld, or when one or both Solid Silver Contact Rivets lose too much material and cannot make good electrical contact, which are the result of cumulative material transfer during continuous switching operations and material loss due to spatter.
Silver contacts for Relay are available in a variety of metals and alloys, sizes and styles, and the contact selection needs to consider the material, rating and style to meet the requirements of the specific application as accurately as possible. Failure to do so may result in contact problems and even early contact failure.
Depending on the application, contacts can be made using alloys such as palladium, platinum, gold, silver, silver nickel, tungsten, etc. Mainly silver alloy compounds, cadmium silver oxide (AgCdO) and silver tin oxide (AgSnO), silver indium tin oxide (AgInSnO), are widely used in general purpose and power relays for medium and high current switching.
Cadmium silver oxide (AgCdO) has become very popular due to its excellent corrosion and welding resistance and very high electrical and thermal conductivity. AgCdO is produced by mixing silver and cadmium oxide using powder metallurgy technology. It is a material with conductivity and contact resistance close to silver (using slightly higher contact pressure), but has excellent erosion and welding resistance due to the inherent welding resistance and arc quenching characteristics of cadmium oxide.
Typical AgCdO contact materials contain 10~15% cadmium oxide. The resistance to adhesion or welding increases with the increase of cadmium oxide content. However, due to the decrease in ductility, the conductivity decreases and the cold working characteristics decrease.
Silver cadmium oxide contacts are available in post-oxidation or pre-oxidation. Pre-oxidation materials have been internally oxidized before forming the contact point and contain more evenly distributed cadmium oxide than post-oxidation, which tends to bring the cadmium oxide closer to the contact surface. Post-oxidation contacts may cause surface cracking problems if the shape must be significantly changed after oxidation, such as: double-headed, moving blade, C-type contact rivets.
Silver Indium Tin Oxide (AgInSnO) as well as Silver Tin Oxide (AgSnO) have emerged as good alternatives to AgCdO contacts where the use of cadmium in contacts and batteries is restricted in many parts of the world. Therefore, tin oxide contacts, which are about 15% harder than AgCdO (12%), are a good choice. In addition, Silver Indium Tin Oxide contacts are suitable for high surge loads such as tungsten filament lamps where the steady state current is low. Although more resistant to soldering, AgInSn and AgSn contacts have higher volume resistance (lower conductivity) than Ag and AgCdO contacts. Due to its resistance to soldering, the Solid Silver Contact Rivet mentioned above is very popular in the automotive industry where 12VDC inductive loads often cause material transfer.
