Siemens Simatic S7-1500 Manual page 89

The figure below shows the current flow upon interruption of both ground cables.
Figure 4-17
If a wire break occurs on the first and on the second ground cable from the central terminal
block to the terminals 30 and 40 of the module, a malfunction occurs on the module. Both
ground connections of the module are interrupted.
The supply current flows from the power supply 2L+ via terminal 29 to the module. In the
module, the current flows via the output driver into the parasitic diode and exits the module
via the output terminal, e.g. as shown in the figure via terminal 27. The supply current
therefore flows via the connected load. The internal supply current is typically 25 mA.
Interruption of both ground cables
If the ground terminals 30 and 40 are interrupted, the following incorrect response can
occur:
The activated outputs, which are switched to high, start to switch back and forth between
high and low. If the load connected at the output is sufficiently small, the output is
continuously activated.
CPU 1512C-1 PN (6ES7512-1CK00-0AB0)
Manual, 09/2016, A5E35306440-AB
Current flow upon interruption of both ground cables using the digital on-board I/O X11
as an example
WARNING
4.3 Terminal and block diagrams
Wiring
89