The manual for my HV probe tells me to connect my ground clip to the chassis. I can see how this would be good advice for say a 12V car, because the chassis is made the ground and every component uses the same power that's hooked to ground.
However this can't be the case with say a CRT circuit. The way I see it, when I want to check voltage of a specific object (a capacitor, let's say) I need to go to the cap with both probes. I don't know if the chassis is "grounded" with a particular voltage (who cares,) but I've got a 24VDC fan motor inside this scope, and a 2,000VDC capacitor. Neither of these components can share a common ground because each is operating at a different voltage. If I wire the ground wire of a 30VDC LED and the ground wire from a 200VDC motor to a sheet of aluminum and turn it on, things are going to spark.
Best Answer
Things at different voltages often share the same ground.
When you say "If I wire the ground wire of a 30VDC LED and the ground wire from a 200VDC motor to a sheet of aluminum and turn it on, things are going to spark." you are making an assumption that is not necessarily true. This is where you are getting off track: "Neither of these components can share a common ground because each is operating at a different voltage"
Ground can be the same for all kinds of voltages in the same system, the same way the ground itself can be used as a point of reference when measuring different peoples' heights.
Remember, ground isn't a voltage, it is simply a reference point against which other potentials are weighed.