floating and Nonfloating I/O modules

[karkas44]

Hello everyone,

 

     I'm checking out the manual fot the S5 ET 100U due to some power supply problems in the factory I work. I've worked in projects with S7 PLCs and ET 200s, but these old PLCs and modules are still operative  in the factory. I'm attaching a picture with an excerpt of the manual. What do they mean by floating and nonfloating modules, exactly?

 

 

Thank you veyr much in advance.

[panic mode]

Floating means that output is galvanically isolated so it 'floats'. This means that it is not referenced to ground and if you measure potential difference between one of output terminals (positive or negative) and ground, just about any value is possible. 

 

Using floating output is special case in electrical standards around the world and may need special condition in order to be deemed acceptable. 

Normally this is reserved only for low power device, some instrumentation etc. and when used, double insulation is a minimum. You can see this on small consumer devices and power supplies - it is double wall rectangle. 

Generally transformer and PSU outputs are required to be referenced in order to meet safety requirements and avoid harm due certain modes of failure (leakage current). 

 

 

 

 

[DanW]

Non-floating I/O modules are the ones that have problems with ground loops due to the difference in ground potential between where the field source device is powered and where the PLC is powered.   One connects a signal with too much common mode voltage and there's an offset or the signal is driven offscale in either direction (depending on the polarity of the ground differential).   

That's why Siemens says everything must be equipotential - no ground potential differences.   Easy to print in a manual, ink is cheap and maybe there's a plant somewhere that is equipotential (the old NORAD radar sites were probably equipotential given the effort to install and maintain ground planes) but I'm not sure I've been in one.