Connection between metal enclosure and 0V in industrial system?
Most of the time you will either have a direct connection at a single point connecting the internal system ground to the chassis ground, or a ground to chassis connection with filtering like resistance, capacitance, and ferrite beads. It's up to the product designer to determine the most appropriate approach, and many times it's adjusted during EMC testing.

There's no hard-and-fast rule-- it's a complex subject and usually the design engineer can't know for sure whether the grounding scheme will meet EMC requirements until it's tested.

There are many books written on the topic, for example

http://www.amazon.com/Introduction-Electromagnetic-Compatibility-Clayton-Paul/dp/0471755001/

Of course, safety is also a factor when dealing with Earth ground, so any approach has to be compatible with those goals too.