ECN Forum Forums Technical Discussion Electrical Theory and Applications Floating Ground - device being tested or scope?

I forgot to suggest a couple of things for your observations:
1.)Check your readings again with your isolation TX unplugged. It helps you see how much your setup is acting like an antenna. If the readings are non-zero(They will be.), try moving the leads around on the table or placing your hands along the wire insulation in various places.
2.) Switch your meter to read frequency if it has that feature. See if you're reading line frequency or a harmonic of same.
Joe

Yes, I have noticed the possibility of interference. For example, I will later post a couple images of the wave form shown on my oscilloscope when a nearby LED magnifying lamp is turned on and off. The wave form shape remains the same but the "lines get fuzzy".

Well better late than never...

Scope showing square wave with nearby LED lamp off:

And with LED lamp on:

The most preferred scope is to connect normally. Floating the scope is the last thing you would want as an alternative.
You are potentially putting line voltage on the case of the scope and bad things can start happening very quickly, this could impose a risk of life too.

We suggest you look at the wiring diagram and decide where the grounded conductor is and ground the scope at that point. The small amount of ripple you see will not affect your measurement. It is tied to the ground via an external plug.

If you are looking at something on the load side of a transformer you should be able to ground the scope normally. You can ground the scope to the circuit through a light bulb, voltage wise. If it sparks, you are going to have to reconsider your plan of action.

Be sure you have the right ground clips if you are looking at line voltage.
It is best to learn your scope technique with a transformer isolated circuit, preferably at low voltages, like a doorbell transformer or a wall wart.

Electrical companies can provide the necessary advise based on the requirement and concern with the floating ground.