Sometimes there are good reasons to disconnect that ground and make a measurement that is not
relative to the AC power input ground.
This link will take you to a Tektronix ?white paper? about making floating measurements.
This has, as one recommendation, using an isolation transformer. However, it is an absolute
requirement that there is ground continuity between primary and secondary of an isolation transformer.
Think about it. The scope has line, neutral and chassis connections - a three wire system. An
isolation transformer has a primary and secondary only. The only way to connect the oscilloscope (or
any connected equipment) with a chassis connection safely is with ground continuity from chassis to
safety ground.
Which means you have a non-floating scope with transformer isolated line and neutral.
I'm really surprised that Tek missed that, because it has been part of electrical codes for longer
than that note was written.
For site use of site power tools isolated line and neutral is just fine - because connected power
tools are double insulated or even intrinsically safe.
But test equipment, like the vast majority of Tek test equipment, requires a hard chassis ground
irrespective of isolation transformer.
Craig
