Siemens SIMATIC S7-1500 Manual page 17

Switching of loads with ground if the F-module is configured PM-switching
If the following two conditions are met, F-DQ 8x24VDC/2A PPM detects a short-circuit:
● If loads that have a connection between chassis and ground are switched by the
F-DQ 8x24VDC/2A PPM (for example, to improve the EMC properties).
● If chassis and ground are connected at the power supply unit.
From the perspective of the F-module, the M-switch is bridged by the chassis-ground
connection (refer to the diagram below for an example).
Image 3-2
Remedy:
● Increase the parameters "Max. readback time dark test" and "Max. readback time switch-
on test".
● Increase the value of the resistance between chassis and ground at the load end to more
than 100 kΩ.
Or:
● Use the F-DQ 8x24VDC/2A PPM configured as a PP-switching module.
Digital output module F-DQ 8x24VDC/2A PPM (6ES7526-2BF00-0AB0)
Manual, 01/2016, A5E03858037-AA
Switching of grounded loads (resistance between chassis and ground)
Connecting
3.1 Block diagram
17