Isolation to things like frame ground is necessary. The twisted pair ethernet network itself floats.
See IEEE 802.3at or IEEE 802.3-2008 (current revision) clause 33.4. These specifications are available for download over the internet (http://standards.ieee.org/about/get/802/802.3.html). You might also consult IEC/EN/UL 60950-1. There is a section on safety regarding TNV networks and devices.