Have you ever wondered why some devices tend to perform better and last longer?  Maybe one should analyze the makeup of the device. When we consider electrical devices we should concentrate on the conductors or contact units. High conductivity leads to higher performance and longer life.

So how do manufacturers ensure higher conductivity? It depends on the conductivity of the metal used. Predominately most metals conduct electricity, but some have a higher performance than others. This performance also depends on the purity and hardness of the metal. In some cases an alloy of two different metals are used for better results.

Electrical resistivity and conductivity is an important property for materials. Different materials have different conductivity and resistivity. Electrical conductivity is based on electrical transport properties. These can be measured with multiple techniques by using a variety of instruments. If electricity easily flows through a material, that material has high conductivity. Some materials that have high conductivity include copper and aluminum. Electrical conductivity is the measure of how easily electricity flows through a material. Forbes T. Brown (2006). Engineering System Dynamics. CRC Press. p. 43. ISBN 978-0-8493-9648-9.

Resistivity and Temperature Coefficient at 20 C*

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Types of Copper and Copper Alloys used for Contacts

Copper

Commercially pure copper has an electrical conductivity exceeded only by silver, and this, together with its ready availability in a wide variety of forms, and its low cost and plentiful supply compared with precious metals, makes it an obvious choice as a contact material. Full details of the commercially available grades and relevant British Standards, together with their mechanical, physical and electrical properties, are contained in CDA publication TN20 - Copper Data. (now superseded by TN 27).

Copper has a high melting point (1083°C) and high corrosion resistance. There are no difficulties of attachment by brazing or soldering, and its thermal conductivity is high 397 W/m°C. The disadvantage of copper for contacts is its tendency to form heavy oxide films of relatively high resistance, especially when arcing occurs. This effect is particularly disturbing at lower currents and voltages and at low contact forces. At higher currents the contact surfaces are kept clean by the arcing on make-break contacts, and high contact forces can move the friable oxide from the conducting area, while high voltages can break the oxide film down.

Copper can therefore be used for the higher current range as far as contact resistance is concerned, although the welding and erosion performance must be taken into account. When contacts remain closed for long periods, however, the growth of oxide on copper contacts can become excessive, and lead to overheating, sometimes followed by complete loss of contact. If sustained arcing at low current is initiated, runaway oxide growth can penetrate deep into the contacts. Silver facing of copper contacts prevents oxidation troubles in such a case. Such considerations explain the superior behaviour of copper in vacuum, compared with its performance in air.

The effect of surface layers can be overcome for airbreak contactors with frequent switching by suitable design, for instance by introducing a sliding and rolling action of the contacts, or by high contact force. This increases mechanical wear, and implies that pure copper contacts must be larger than necessary electrically, in order to accommodate this wear. Very important, especially for high current operation, is the erosion of material due to arcing.

The relation between rate of wear (in microgrammes/sec) and arc current (in amperes), valid between 5 amp and 800 amp, is of the form:- k.i1.6 dt dW in which k =2.4 for copper. This relation only applies at currents where bulk melting of the material and spraying of droplets does not occur. Above 800A (the "discontinuous erosion current") where bulk melting occurs, a similar power law is followed, with a numerical erosion factor (k) of 36.

In vacuum contactors special designs of copper contact are used to prevent effective current from reaching discontinuous erosion. In contactors for d.c. use, the erosion of the two contacts may be quite different. Although the transfer of material is generally from the anode, this is not the case for all values of current; in fact the transfer of metal by the arc has a number of regions dependent on the mode of arcing, and the formation of jets from anode and cathode. For copper between 10 and 300A the transfer is from cathode to anode in air, and the rate of transfer is approximately proportional to a power function of the arcing current: (rate of transfer) = k i 2.25. At approximately 300A the transfer such that the anode keeps constant weight, and above this current it increases in weight. This kind of arc transfer is a different phenomenon from the "fine transfer" occurring in relay contacts at low currents, which is due to the rupture of metallic bridges, and is generally from anode to cathode.

Static and dynamic welding of copper contacts is discouraged to a certain extent by the existence of oxide layers, dependent on the contact force. The threshold of static welding is of the order of 4 kiloamp at 100N contact force, with a weld strength of 330N/mm2, while the threshold of dynamic welding lies at about 50 amp for 25N contact force, with a weld strength of the order of grammes.Weld strength rises with current, until at 200 amperes it is approximately 100/mm2, measured by direct pull. A "knuckling" action is needed in a contactor design to assist the breaking of welds in copper contacts. A recent explanation of the lower strength of the dynamic weld of given area attributes this to the imperfections (e.g. inclusions ofair or oxide) in the welded area.

Copper Alloys

A number of copper based alloys are used for contact and contact backing manufacture, because of their particular properties which suit them for the required application. Full details of the range of copper alloys available, their composition, mechanical properties and relevant British Standards, are contained in CDA publication TN10 - Composition and Properties of Copper and Copper Alloys. Those alloys most frequently used for contact or contact backing manufacture are briefly described in the following sections.

Silver-Bearing Copper

The addition of small amounts of silver (generally below 0.12%) has the effect of raising the annealing temperature of copper without any appreciable reduction in electrical conductivity. Assemblies made from this alloy can be connected by soft soldering without loss of hardness. Commutators of high performance motors are frequently made from this alloy. Alloys with higher silver content (2 - 8%), plus in some cases up to 1.5% cadmium, have been used for anti-weld contacts in transformer load switches.

Silver-Copper

These alloys have improved hardness, wear and dynamic welding properties. They are used for contacts rather than contact backing. Well known alloys are hard silver (3% copper), standard silver (7.5%) and coin silver (10%). A material used for sliding contacts because of its hardness and resistance against mechanical wear and transfer, is silver-copper-phosphorus (2% Cu, 0.1% P).

Silver-copper is not normally used in contactor contacts, although alloys with 7.5% and 10% copper have been found to have a threshold of dynamic welding at about the same current as for copper or silver, but with a much lower weld strength up to 250 Amperes. Silver-copper oxide has a considerably improved performance in this respect, being highly resistant to dynamic welding.

The physical properties of the silver-copper alloys vary with percentage copper and with heat treatment, which can produce a tensile strength range for hard silver, for instance, from 450 to 1150 N/mm2, or for a 50% copper alloy, from 800 to 1550 N/mm2.   Fink and Beaty, Standard Handbook for Electrical Engineers 11th Edition, page 17-19.

In summary, it is always a good practice to analyze the makeup of your device to determine which material is best for one’s application. Many manufacturers use many types of metals for various reasons. Here at Joslyn Clark we use silver cadmium oxide contacts on our magnetic contactors and starters to increase reliability and longevity.