The safety ground is the "equipment ground", (the ground pin of a standard
120V AC receptacle if you are plugging your fieldpoint in, the green or bare wire
in standard US wiring practice if it is hard-wired). If you are using a PS-5 power
supply, for example, the ground pin on the terminal block should also be connected
to this ground, as should any metal box or base the fieldpoint and its power supply
is housed in.
Unless this is crystal clear to you, I would recommend getting a qualified
individual to wire up the AC power and equipment ground for your system to
make sure it is both code-correct and (most importantly) safe. Electrical codes are
pretty specific about how equipment grounds are properly connected and wired, because
a life may depend on it.
Bob
