Siemens ET 200pro Operating Instructions Manual page 45

Rule
Always connect the rack to functional ground (FG). The connection to functional ground is
required to discharge interference and for EMC compatibility.
Note
Low-impedance connection to ground potential
Always make sure you have a low-impedance connection between the rack and ground
potential.
If you do not install the rack on a grounded surface, always create a low-impedance
connection to ground potential, for example, with the help of a copper braid.
ET 200pro distributed I/O system
Operating Instructions, 03/2013, A5E00335544-08
Mounting
3.2 Rack
45