If an overload occurs, the electronic current limiting function of the 24 V power supply limits
the output current to a maximum value (see Section Technical data (Page 27)). The output
cables are protected against a thermal overload if they are dimensioned corresponding to
the maximum rms output current, or protected using additional components (for example,
miniature circuit breaker, fuses).
However, a load circuit that fails as a result of overload, for instance, should frequently be
reliably and quickly identified and specifically switched off before the power supply goes into
current-limiting mode (in current-limiting mode, the supply voltage would also be reduced for
all of the remaining 24 V loads).
The SITOP PSE200U selectivity module with 4 outputs (versions with adjustable output
current range for each output from 0.5 - 3 A and 3 - 10 A) is available for this purpose; this
monitors the 24 V branches for overload and short circuit (Figure 9-3 Electronic protection of
24 V loads using the SITOP PSE200U selectivity module (Page 44)). Brief current peaks,
e.g. as a result of a high inrush current, are permitted, and branches with a longer overload
are switched into a no-current condition. This is also ensured for cables in a high-ohmic
condition and for short circuits that slowly develop over time.
When an output fails, the fault is signaled using a group signal contact or as a single channel
signal, and the branch of the module involved is displayed using an LED.
For variants with single-channel signaling, function blocks for evaluation purposes are
available for SIMATIC S7-1200/1500/300/400, for STEP 7 Classic and TIA Portal at no
charge.
You can find additional information at:
Manual SITOP selectivity modules
(https://support.industry.siemens.com/cs/ww/en/view/108989004
)
Figure 9-3 Electronic protection of 24 V loads using the SITOP PSE200U selectivity module
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