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2
Engineering
Overload monitoring of
Ex e motors
The Ex e protection of motors is achieved by means of
special design measures. The motors are assigned to temper-
ature classes on the basis of the highest permissible surface
temperatures. The temperature rise time t
E
and the ratio
between startup current and rated operational current I
A
/I
N
are calculated in addition 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 startup current of I
A
.
However, Ex e motors are not intrinsically safe. Explosion
safety can only be achieved by taking additional measures
during installation and by selecting appropriate operating
conditions (PTB testing regulations), e.g. by adding a
correctly rated and set overload protection to the circuit.
Setup of the overcurrent
protection system
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
. This means, that the protective device
must be rated in such a way so as to ensure that the trip-
ping 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).
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