Troubleshooting grounding problems can be difficult at best. It was not
clear what you measured your voltage in reference to. In a floating DC
system, if you read a voltage to ground, you have a ground in your
circuit. If you do not, then your problem lies elsewhere (a problem
with your system reference?). A cross connection to a different system
can be a possible cause of this problem as can a bad power supply or a
wiring mistake. Also, make sure that you do not have AC in your DC
circuit
If you do have voltage to ground, you do have a ground in
your power system but it is unlikely that a single ground would cause
this problem (though not impossible) and a lot harder to find though
there are ways to do it. When strange voltages appear in a system, it
is commonly caused by a ground loop of some sort. One thing I would
suggest is that you use the method Phil described at the closest you can
get to your power supply upstream of any sensors and as much of your
power distribution as you can and move outward toward your sensors to
see if you can find ground loop current. This can eliminate the power
distribution being involved and sometimes find the ground loop current
path. Also note that the power supply does not necessarily have to be
involved in the ground loop but can still be affected by it. If you
still cannot find the ground loop, you can then try the sensors. Once
you have found the first ground, you have done the easy part, finding
the second ground can be more difficult and you should leave the first
ground intact till you find the second ground. Also, be careful in
disconnecting grounds as some times you do not know what they are
connected to if there is another ground in the system.
It is also
good to use your power system drawing or a system map to document as
you go because these problems can be quite complex requiring some logic
to figure out.
In : Fault Troubleshoot