35
09/15 MN03407003Z-DE/EN
2
Projection
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. In addition, temperature rise time t
E
and the ratio I
A
/I
N
of the starting current I
A
to the rated
operational current I
N
are also determined and specified on
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 startup current of I
A
.
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.
Setup of the overcurrent
protection system
j
Danger!
The selected overload protection system must not only
ensure proper monitoring of the motor current, but also
that the seized motor is switched off within the tempera-
ture rise time t
E
.
In other words:
The protective device must be rated in such a way so as to
ensure that the tripping time t
A
for the ratio I
A
/I
N
of the
Ex e motor is not higher than its temperature rise time t
E
according to its characteristics curve, in order to safely
switch off the motor within that period (
a
following
example).