3-27
MAINTENANCE OF RELAYS
The relay is one of the most dependable electromechanical devices in use, but like any other
mechanical or electrical device, relays occasionally wear out or become inoperative. Should an inspection
determine that a relay is defective, the relay should be removed immediately and replaced with another of
the same type. You should be sure to obtain the same type relay as a replacement. Relays are rated in
voltage, amperage, type of service, number of contacts, and similar characteristics.
Relay coils usually consist of a single coil. If a relay fails to operate, the coil should be tested for
open circuit, short circuit, or short to ground. An open coil is a common cause of relay failure.
During preventive maintenance you should check for charred or burned insulation on the relay and
for darkened or charred terminal leads. Both of these indicate overheating, and the likelihood of relay
breakdown. One possible cause for overheating is that the power terminal connectors are not tight. This
would allow arcing at the connection.
The build-up of film on the contact surfaces of a relay is another cause of relay trouble. Although
film will form on the contacts by the action of atmospheric and other gases, grease film is responsible for
a lot of contact trouble. Carbon build-up which is caused by the burning of a grease film or other
substance (during arcing), also can be troublesome. Carbon forms rings on the contact surfaces and as the
carbon rings build-up, the relay contacts are held open.
When current flows in one direction through a relay, a problem called "cone and crater" may be
created at the contacts. The crater is formed by transfer of metal from one contact to the other contact, the
deposit being in the shape of a cone. This condition is shown in figure 3-25(A).
Figure 3-25.—Relay contacts.
Some relays are equipped with ball-shaped contacts and, in many applications, this type of contact is
considered superior to a flat surface. Figure 3-25(B) shows a set of ball-shaped contacts. Dust or other
substances are not as readily deposited on a ball-shaped surface. In addition, a ball-shaped contact
penetrates film more easily than a flat contact. When you clean or service ball-shaped relay contacts, be
careful to avoid flattening or otherwise altering the rounded surfaces of the contacts, YOU could damage
