Part
12470
Rev. 2 3/25/99
8
approximately 1000 amperes. Higher currents are usually available from the terminal rated 500
amperes; HOWEVER, these higher currents are available only for very short durations as illustrated
in the overload capacity table above. For example, it is possible to get 1800 amperes from this
terminal through a typical 225 ampere molded case circuit breaker for an instantaneous trip test.
SECTION III
TEST PROCEDURES
MAINTENANCE OF MOTOR OVERLOAD RELAYS
The prime function of the motor overload relay is to prevent operation of a motor for too long a
period of time when an overload condition exists.
In general, motor starters are applicable to a given horsepower range of motors. The voltage and
current requirements of the application will "size" the starter under NEMA requirements, but the
actual starting current, running current and ambient temperature will determine the overload relay
rating required to protect the motor without nuisance tripping.
Selection of the properly rated overload relay can be made by reference to tables or charts supplied
by the manufacturer of the overload relays and motors. Whenever a motor trips out, it is poor
practice to uprate the overload relay indiscriminately; the motor may actually be working under an
overload condition or the overload relay may be operating improperly. Uprating the overload relay
could permit an overload to continue, resulting in deterioration of the motor insulation and reduction
in motor life. Therefore, careful analysis should be made as to the cause of the nuisance trip before
changing the rating of the overload relay.
Operating characteristics of the motor overload relay should be verified at regular intervals. Typical
practice dictates inspection of overload relays at periods of one to two years, with an actual test of
tripping time to be made at intervals of two years. The test interval can vary with the type of service
involved and the importance of the motor to process or production.
Motor overload relays incorporate an element which actuates a set of contacts connected to the
motor control circuit. These contacts open the circuit of the holding coil in the motor starter and
interrupt the power to the motor.
In general, there are three types of motor overload relays in use:
