It can be easy to overlook the importance of electrical contacts; after all, they are hidden away in many products. But without effective contacts, the products we rely upon will fail.
A new resource, created by Copper Development Association (CDA), discusses the nature of the electrical contact and the material properties required for contacts across various applications.
CDA highlights the importance of electrical contacts and points to some high-profile and costly system failures. One example it gives is the F-16 fighter plane, where connector failure has been linked to the cause of engine failure and aircraft crashes costing more than $100million. Fretting corrosion, caused by vibration, led to failure of the electrical connector – which featured tin plated pins plugged into gold-plated sockets – that supplied power to the main fuel shut-off valve. This failure caused the fuel to stop flowing to the jet engines.
“Copper in Electrical Contacts is an in depth, yet easy to read resource that explains why copper is such a good contactor, its omnipresence in various contacting applications and its many alloys available,” says CDA director Angela Vessey. “It will be of value to anyone interested in specifying and using copper and copper alloys in any type of electrical contact.”
The publication has been edited by David Chapman, recently electrical programme manager for Copper Development Association and author and chief editor of the LPQI Power Quality Application Guide.
Copper in Electrical Contacts is a handy technical resource. Chapman explores a wide range of contact types and materials within the publication, as well as taking a close look at the contact interface and ways in which the interface can be affected by oxidation, corrosion and mechanical factors.
One of the great advantages of copper is the wide selection of alloys available, where the contact properties of copper are combined with other metals to provide good mechanical properties.
According to Chapman, a well designed connector will deteriorate relatively slowly, at a rate determined by the nature of a number of different processes. However, he notes that while the initial stage persists for a long time without causing any noticeable changes, when the contact resistance increases sufficiently to raise the local temperature, a self accelerating deterioration process is triggered, resulting from the interaction of thermal, chemical, mechanical and electrical factors. “Hence,” he points out, “no deterioration will be noticeable until the final stages of the connector life.”
High contact resistance, says Chapman, implies a high voltage drop across the contacts. This leads to a limitation to the current carrying capacity of the contacts. “Increasing the contact force reduces the contact resistance,” he says, “but higher force increases wear and implies a more sturdy construction that may not be appropriate in the application” Inclusions in contact metal
Contact resistance can be reduced by other means, such as the use of materials which produce good metallic contact with moderate contact force, or by designing the contact system in such a way that the contacts are wiped clean as they are brought together. The softness of copper and silver means that contacts faced with these materials need lower contact forces.
Oxidation is widely accepted as the most serious degradation mechanism occurring in mechanical connectors, says Chapman.
Corrosion, which can be chemical or electrochemical, begins at an exposed metal surface with the formation of a corrosion product layer and continues as long as reactants can diffuse through the layer and sustain the reaction.
“Oxide films and the products of corrosion on the contact surfaces reduce real contact area and increase contact resistance,” says Chapman. “The resultant increase in temperature in turn accelerates the rate of attack.”
A simplified version of the copper alloy tree. According to CDA, there are more than 400 copper alloys, each offering properties to particular applications. |
If contacts are designed to slide over each other (or wipe) when they are broughtA simplified version of the copper alloy tree. According to CDA, there are more than 400 copper alloys, each offering properties to particular applications. together or if connectors are frequently plugged and unplugged, there will be a self-cleaning action which helps to reduce contamination, but at the expense of increased wear.
Although copper oxidises very slowly in air, copper contacts are affected by oxide films formed, for example, by arcing. “A silver coating usually solves this problem,” Chapman notes, “but is subject to environmental attack, forming silver compounds, especially the sulphide, on the surface. However, these will tend to decompose under the heating action of a temporary high contact resistance and restore the contact resistance again to a low level.”
Fretting, mentioned above in connection with the F-16 fighter, is accelerated surface damage at the interface between contacts caused by small oscillations. “Experimental evidence,” Chapman points out, “shows that amplitudes of less than 100nm are sufficient to produce fretting.”
A number of mechanisms are believed to be involved in fretting. Alongside vibration, differential thermal expansion and junction heating contribute.
As Chapman notes: “[Fretting] is ineffective in clearing away wear debris and accumulated oxides.” This results in a thick, but localised, insulating layer being created, increasing contact resistance dramatically and, potentially, results in virtual open circuits.
Chapman reviews four categories of contact: arcing, which make or break under load; non arcing, which make or break while off load; fixed, contacts which may never be opened; and sliding, which maintain contact during movement under load. “Although each of these types share common characteristics,” says Chapman, “each have different requirements, according to their electrical and mechanical needs.”
An extensive review of of the contact materials used most frequently is included. The effect of combining copper with different elements is explained by Chapman in 16 alloys, ranging from silver bearing copper and silver-copper to copper-tellurium and copper-zirconium. He then considers bronzes and brasses, before considering the application of other elements as contacts.
“The range of materials used in contact and assembly design is extremely wide and includes materials selected for their excellent low
current behaviour, as well as strong, mechanically stable and durable alloys for high current contact assemblies,” he says. “Copper and copper alloys have a place in most, perhaps all, contact assemblies as conductors, contacts or as mechanical parts of the assembly.”
The publication concludes with extensive information on the physical and mechanical properties of important contact materials and a range of alloys. Chapman also provides information about the typical materials used for arcing and non arcing contacts and the potential applications.
The Copper Development Association promotes and supports the correct and efficient use of copper and copper alloys for all major applications. |
