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Electrical Contacts ApplicationsEach contact application must be analyzed on its own merits. Electrical circuit considerations, mechanical forces, environmental and service requirements (among other aspects) are relevant to the contact application. As you likely already know, similar applications often use very different materials that depend on the very specific circumstances of the application. Please call or email us with your inquiries and let CMW work for you in your search for the product, component or material that is right for your electrical contact application. For more information concerning common applications using CMW®.
Aircraft Circuit Breakers and Relays   Most common aircraft circuit breakers and relays require direct current, high-reliability circuit protection for control and/or enabling systems or indicating alarms. Heavy duty contacts include CMW Elkonite® G12, G13, G14, G17, G18, 35S, and D54®
Medium duty contacts include CMW D54® and CMW® D55 CMW® D54 and CMW® D55 Silver-Cadmium Oxide ContactsSilver-semi-refractory materials are not true alloys, but rather mixtures of silver and other elements, metalloids, or metallic oxides. These materials are usually fabricated as mixtures of constituent powders by conventional powder metallurgy techniques. Powder metallurgy techniques are used because constituents do not have compatible melting temperatures or they are not soluble in each other. However, in the case of silver-cadmium oxides and silver-magnesium oxides, it is possible to form true alloys of the elements and then convert one or more of the elements into an oxide by internal oxidation techniques. The silver-semi-refractory materials have relatively high electrical conductivity combined with other desirable features such as low stable contact resistance and excellent resistance to sticking and welding. These properties are obtained by additions of cadmium oxide, nickel, carbon, magnesium, iron, or other elements or compounds. CMW® D54 (90% Silver / 10% Cadmium Oxide) and CMW® D55 (85% Silver / 15% Cadmium Oxide) are produced as individual contacts by traditional powder metallurgy techniques (mix - press - sinter - repress to density). Normally, the powder metallurgy type materials are used in disc or rectangular shapes. The powder metallurgy process provides and economical method of producing radii, projections for welding or locating, indentations for solder traps and round or irregularly shaped contacts without material loss. While the cadmium oxide content can be readily varied, the two standard materials (10% CdO and 15% CdO) satisfy most contact requirements. The contacts can be supplied with a "backing" layer of fine silver which is recommended for attachment by brazing. If omitted, the cadmium oxide can cause a brazing alloy to develop brittle phases and ultimately a detachment of the braze. Besides standard discs and rectangles, irregular shapes typical of powder metallurgy parts can be produced economically. The addition of cadmium oxide to silver greatly improves the anti-welding characteristic. Of Ag/CdO materials, the powder metallurgy type has the least tendency to welding or sticking of all. It is the material of choice for arcing contacts of medium-low to medium-high current rating where "non-sticking" primary concern and erosion rates higher than wrought type Ag/CdO can be tolerated. It is not used in "high reliability - low current" applications (where CMW Elkonium® materials are best), nor in "high current circuit breaking" (where Elkonite® materials are superior). Generally this makes it a choice for "high-end" household and "low-end" industrial relay and contactor applications. The cost of Ag/CdO materials are directly tied to the cost of silver and the silver content of the material. As would be expected, the conductivity (and therefore resistance to temperature rise) is better with higher silver content. Light duty contacts include CMW D54X®, D54F®, CMW® D55X, D55F, Fine Silver CMW® D54X (90% Silver / 10% Cadmium Oxide) The "X" is appended to the material names to denote that the material is cast and formed as an alloy of Silver and Cadmium. The cadmium within the contact is then "post-oxidized" by heating in the presence of oxygen to convert it to cadmium oxide. Because of the kinetics of the process (which involves both migration of oxygen and cadmium) there is a resulting high concentration of fine Cadmium Oxide phase near the surface of the contact, and a lower concentration of courser Cadmium Oxide as one approaches the core of the item. Sometimes this "core" is referred to as a "depleted zone" or "silver-rich zone" due to the fact that the cadmium oxide content at this point is in much lower concentration. This should not be confused with an "un-oxidized core" (failure of sufficient oxidation time to reach cadmium at the core) or "reduced layer" (a surface condition where cadmium oxide has been re-reduced to cadmium because of contact with a "reducing" atmosphere). In the latter two cases, the cadmium is still present, but not in its "oxidized" form. The kinetics of the situation, (and processing techniques at CMW) limit the "center-to-surface distance" for this type of material to about 1/16 inch. Contacts thicker than 1/8 inch will likely have an "un-oxidized core". Of Ag/CdO materials, the "post-oxidized type" has the least tendency to for early erosion. It is the material of choice for arcing contacts of medium-low to medium-high current rating both where early "erosion" primary concern and anti-welding is also a high priority. Magnetic contactors, relays, and industrial controls are common applications. It is often chosen for its manufacturing ease and economics. Prior to oxidation, the silver cadmium alloy is readily cold worked, and can be headed from wire (to make "rivet contacts"), or rolled and formed to exotic shapes from strip with very good manufacturing economics for high volume applications. It is not used in "high reliability - low current" applications (where CMW Elkonium® materials are best), nor in "high current circuit breaking" (where Elkonite® materials are superior). Generally this makes it a choice for "high-end" household and "low-end" industrial relay and contactor applications. The cost of Ag/CdO materials are directly tied to the cost of silver and the silver content of the material. As would be expected, the conductivity (and therefore resistance to temperature rise) is better with higher silver content. CMW® D54F (90% Silver / 10% Cadmium Oxide) The "F" is appended to the material names to denote that the material is cast as a grain alloy of Silver and Cadmium. This grain is then oxidized to become Silver-Cadmium oxide grain. After oxidation the grain is subsequently hot and cold worked into wire, bar, or strip, and ultimately headed, or blanked, or coined into contacts. CMW refers to this processing as "Pre-oxidized", because conversion to cadmium oxide "precedes" formation of the contacts. Generally pre-oxidized ("F") AgCdO material is quite similar in properties to post-oxidized ("X") material with the exception of microstructure. Pre-oxidized ("F") material has a uniform distribution of fine and coarse cadmium oxide phase, while the post-oxidized ("X") material does not (see above). Because of this, there is no fear of reduced resistance to sticking as one wears down the contact and approaches its core region. Pre-oxidized Silver Cadmium Oxide ("F") materials cannot be cold worked to the degree that post-oxidized "X" type material. There are definite limitations on the heading of wire to form rivets, as well as rolling and coining. Compositions greater than 13.3% CdO are not considered to be "headable" at all. Conversely, there is no "1/8 inch thickness" limitation on contacts with "F" materials, since kinetics of oxidation at the contacts geometry are not a consideration. Thus heavier duty contacts can be blanked from thick "F" type strip. In strip form, fine silver backing is recommended as a brazing surface to eliminate brazing problems that occur if it is omitted. This backing is readily applied during the hot rolling of the strip. Of Ag/CdO materials, the "pre-oxidized type" has the superior arcing properties for everything except "anti-sticking" where the "powder metal" type is superior. It is the material of choice for arcing contacts of medium-low to medium-high current rating both where economics for a heavier and high volume application is of primary concern. Magnetic contactors, relays, and industrial controls are common applications. It is not used in "high reliability - low current" applications (where CMW Elkonium®s are best), nor in "high current circuit breaking" (where Elkonite® materials are superior). Generally this makes it a choice for "high-end" household and "low-end" industrial relay and contactor applications. The cost of Ag/CdO materials are directly tied to the cost of silver and the silver content of the material. As would be expected, the conductivity (and therefore resistance to temperature rise) is better with higher silver content. Electrical contacts for high reliability make and break cycles.Best materials include CMW D54®, D54F®, D54X®
Rotary SwitchesBest materials include CMW D54®, CMW D54X®, CMW® Fine Silver
Industrial Controls and RelaysBest materials include Elkonite® 20S, 35S, 50S, CMW Elkonite® G12, G13, G17, G18, D50, D56, D54®, D54F®, D54X® Limit SwitchesBest materials include CMW® Coin Silver
Knife SwitchesBest materials include CMW® D58, D1058 Door Closure MechanismsBest materials include CMW® Coin Silver Motor Controllers and Line StartersThe CMW® silver-cadmium oxide family of materials perform best in these high amperage switching applications. These materials include CMW D54F®, CMW® D55F, D154F and D355F Load Tap ChangersBest materials include Elkonite® 1W3, 3W3, 5W3 tips on copper for arc resistance. Best materials also include Elkonite® 40W3, 50W3 for arc resistance
Reclosers: Arcing contacts are requiredBest materials in air include Elkonite® Silver Tungsten Best materials in oil include Elkonite® Copper Tungsten Vacuum switchgear, breakers and interruptersElkonite® 40W3 and 50W3 are frequently used. In some applications, a Silver Tungsten Carbide Elkonite® such as CMW Elkonite® 1060 is used. This helps reduce the current chopping by putting some silver in the plasma arc, slowing the speed that the arc extinguishes. Diesel-Electric Locomotives and Mining Equipment ContactorsThe CMW® Silver Cadmium Oxide family of materials perform the best. Other materials include CMW D54®, D55
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