Eaton ZB12/XTOB BC1 Series, Manual

Page: 51 / 103

47
03/15 MN03407004Z-DE/EN
2 Engineering
Overload monitoring of
motors in the Ex e area The Ex e protection of motors is achieved by means of
special design measures. The motors are assigned to
temperature classes on the basis of the highest permissible
surface temperatures. The temperature rise time t
E
and the
ratio between starting current and rated operational current
I
A
/I
N
are calculated also and specified on the rating plate of
the motor.
The temperature rise time t
E
represents the time that expires
for the temperature of the motor winding to rise from its final
rated operational temperature up to the limit temperature,
at a starting current of I
A
t.
However, Ex e motors are not safe on their own. Explosion
safety can only be achieved by taking additional measures
during installation and by selecting appropriate operational
conditions (PTB testing regulations), e.g. by adding a
correctly rated and set overload protection to the circuit.
Adjusting the overload
current protection
j
Danger!
The selected current overload protection system must not
only ensure proper motor current monitoring, but also that
the seized motor is switched off within the temperature
rise time t
E
. This means that the protective device must be
rated so that the tripping time t
A
for the I
A
/I
N
ratio of the
Ex e motor, that is specified by the characteristic curve, is
not greater than the temperature rise time t
E
in order to
switch off the motor safely within this time (
a
following
example).