Ground Loop

[kfitch42]

The other day I stopped by the local university surplus store, and, on a whim, I bought a signal generator for $5 (Krohn-Hite 5800A). When I got home I looked at it more carefully and realized it had a three-prong power plug. I don't see any switch/button/jumper or otherwise to select the ground for the output signal. So I assume it would be a bad idea to connect this to my oscilloscope (which also has a three prong plug). Is an 'isolation transformer' the only safe way to connect these two bits of kit together?

[w2aew]

There are two possibilities:
1) The output is floating, meaning either side can be safely connected to ground.
2) The output is grounded, meaning that the ground of the output must be connected to the DUT / equipment ground.

In either scenario, you should have no problem. 

The only issue would arise if the output is not floating, and you treat it like it was (neither side connected to DUT ground).

[ejeffrey]

There is no problem here.  If the FG output is grounded and you connect it to the scope you will end up with a ground loop, but it is probably not a problem and definitely not a safety issue.

In most cases you run into in a lab, ground loop potentials are at maximum a few mV and 10 mA. This is not enough to cause any safety issues.  In some cases this can cause measurement errors, but for low precision measurements (like most measurement done with a scope) you can just ignore it.

For completeness, I should add that ground loops can become more serious when talking about long distance connections, and damage equipment or cause safety hazards.  This is something you run into with e.g., ethernet cables going between buildings with separate electrical systems.  For use within a lab you don't have to worry.