Siemens SIMATIC Operating Instructions Manual page 88

Fail-Safe Electronic Modules
8.2 4 /8 F-DI/4 F-DO DC24V/2A PROFIsafe Digital Electronic Module
Switching of Grounded Loads
If the 4/8 F-DI/4 F-DO DC24V/2A PROFIsafe electronic module switches loads that have a
connection between the chassis and ground (e.g., to improve the EMC properties) and if the
chassis and ground are connected in the power supply, a "short circuit" will be detected.
From the perspective of the F-module, the M-switch (current sinking) is bridged by the
chassis-ground connection (refer to the following figure for an example for a 4/8 F-DI/4 F-DO
DC24V/2A PROFIsafe electronic module).
Remedy:
The resistance (R) between the chassis and ground on the load side must be greater than
100 kΩ.
Figure 8-15
88
Switching Grounded Loads (Resistance Between Chassis and Ground)
ET 200pro Distributed I/O System - Fail-Safe Modules
Operating Instructions, 05/2007, A5E00394073-02