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2018-07-02·BA00·III·en·08
Tripping Characteristics
19
Motor protection circuit-breaker
8523/8
05947E00
Circuit-breakers (motor protection switches) for squirrel cage motors must be
so selected that the tripping time with 3-pole loading is not greater than the
warm-up time t
E
given on the machine test plate.
(The tripping time should be taken from the I
A
/I
N
ratio curve of the machine to
be protected.)
The value for tripping time t
A
relative to the ratio of operating currents I
A
/I
N
should not only guarantee safe switch-off within the warm-up time (t
A
≤
t
E
)
but also still enable the motor to run up to speed safely when the trip is at
operating temperaturte.
IEC/EN 60079-7 specifies that the warm-up time t
E
shall not be shorter than
5 seconds.