Plant without ground

Dear, Eng.ahm,

First of all I should know why you don't have ground - is there a technical reason or request, e.g. like on ships the short-circuit monitoring?
Then you have to tell about your rules for screening of your signal and communication cables.
And also your environment is quite important: Do you have high frequency disturbances from radio, DC/DC converter or rectifiers, inverters or drive controls using GTOs?

You have two options with SIMATIC PLC because they use two different supplies for CPUincluding internal bus and external signal loops (power supply to I/O modules):
1. locally grounded CPU supply and separated I/O modules supply
2. all supplies ungrounded
In both cases you have to select modules that provide a high separation voltage Ucm (common mode). And you should ground the ungrounded supplies with a resistor 1 or 10 MOhmto local ground to avoid electrostatic voltages that can damage the modules. PROFIBUS cable screens and other screen of cables to areasof different power suppy or connection to ground will be grounded only on one side, the other side having a capacitor (15 - 22 nF) parallel to 1 or 10 MOhm to local ground, but disconnected in the plug of that PLC or COM-module! Youwill find this described clearly in the PROFIBUS installation guideline.
The idea is toprevent any current from difference in local potential or induction or unbalanced installation to go via your signal cables or screens.
If you use PROFIBUS for long distance in this area may be you should use some additional repeaters (6ES7972-0AA02-0XA0) because they can separate electrical segments of PROFIBUS.

If you like give some more details to get additional advice.