contacts will not weld together when closed, they are
better than metal contacts for ensuring that a de-
energized contact is open. However, carbon contacts
are used only when necessary. Because the current
capacity of carbon per square inch of contact surface is
very low, the contacts made of carbon must be relatively
Silver contacts are used extensively in pilot and
control circuits, on relays, interlocks, master switches,
and so on. They are used also on smaller controllers and
on heavy-duty equipment where the contactors remain
closed for long periods of time with infrequent
operation. Silver contacts are used because they ensure
better contact than other less expensive material.
Pure silver contacts and silver-cadmium-oxide
contacts should not be replaced until they become too
worn to give good service. Their appearance will
indicate when they are worn to such an extent that they
are no longer serviceable (fig. 7-62).
ELECTRICAL AND MECHANICAL
WEAR.Normally, contacts are subjected to
electrical and mechanical wear as they establish and
interrupt electric currents. Electrical wear is usually
greater than mechanical wear. If a movable contact
assembly has no appreciable sliding action on its
associated stationary contact assemblies, mechanical
wear will be insignificant.
Electrical wear or erosion is caused by arcing when
the contacts are establishing and interrupting currents.
During arcing, a small part of each contact is melted,
vaporized, and blown away from the contact. As a pure
silver contact erodes, its arcing surface changes in color,
contour, and smoothness. Figure 7-62 shows typical
changes in contour and smoothness.
Normally, a new contact has a uniform silver color,
a regular contour, and a smooth arcing surface. As the
contact wears, discolorations usually give it a mottled
appearance, showing silver, blue, brown, and black.
The black color comes from the silver oxide formed
during arcing. Silver oxide is beneficial to the operation
of the contact.
Electrical erosion may cause uneven wear of the
contacts and consequent contour irregularity. Uneven
contact wear doesnt necessarily indicate that the
contact should be replaced Manufacturers usually
provide a total thickness of silver equal to twice the
Figure 7-62.Silver contacts.
wear allowance associated with the contact to allow for
uneven contact wear.
Melting and vaporization of contacts cause pitting
of the arcing surface. The pitted surface has high spots
which are quite small in area. Tests indicate that such a
surface is better than a surface which has not been
subjected to arcing because its circuit-making
reliability is improved.
A silver-cadmium-oxide contact shows the same
wear characteristics as a pure silver contact, except that
small black granules may be evident on the arcing
surface. These granules are cadmium oxide, a black
material which is scattered throughout the mixture that
has formed on the contacts. Silver oxide is formed
during arcing, just as with a pure silver contact. The
addition of cadmium oxide greatly improves contact
operation because it minimizes the tendency of the
contacts to weld together, retards heavy transfer of
material from one contact to the other, and inhibits
WEAR ALLOWANCE.A contact is service as
long as its wear allowance, and its associated contacts,
exceeds the minimum value specified by the
manufacturer. (Usually the minimum value is 0.015 to
0.030 inch). The wear allowance of contacts is
defined as the total thickness of contact material which