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 (now superseded by TN 27)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.
