2 Engineering
2.1 Overload monitoring of Ex e motors
PKZM0-…/XTPR…BC1NL, PKZM0-…-T/XTPT…BC1NL
01/22 MN03402003Z-DE/EN
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31
2 Engineering
2.1 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 temperature 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.
2.2 Setup of the overcurrent protection system
Example
I
A
/I
N
= 6, t
E
= 10 s
Figure 5: Tripping characteristic for motor-protective circuit-breaker
The motor is reliably protected.
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 temperature rise time t
E
.
This means, that 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).
2
5
20
50
200
1
2
5
10
20
40
1
2
5
10
20
2h
1.5 2
3
4
6
8 10
15 20
30
PKZM0-...
XTPR...BC1
Seconds
Minutes
Milliseconds
3-phase
2-phase
Rated current
Tripping time
