Ground of oscilloscope always connected to earth?

[tggzzz]

Example fatality from https://groups.io/g/TekScopes/topic/7632418#10795 with my emphasis

What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.

Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.

Excerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"
https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.html,
with my emphasis

Advantages
Although floating equipment is a method that uses existing equipment to make floating measurements and remove ground loops on lower frequency signals, it is an unsafe and dangerous practice and should never be done.

Trade-Offs
This technique is dangerous, not only from the standpoint of elevated voltage present on the oscilloscope (a shock hazard to the operator), but also due to  cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure but can lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation.

At higher frequencies, severing the ground may not break the ground loop as the line-powered instrument exhibits a large parasitic capacitance when floated above earth ground. The floating measurement can be corrupted by ringing. Floating oscilloscopes do not have balanced inputs. The reference side (the “ground” clip on the probe) has a significant capacitance to ground. Any source impedance the reference is connected to will be loaded during fast common-mode transitions, attenuating the signal. Worse yet, the high capacitance can damage some circuits. Connecting the oscilloscope common to the upper gate in an inverter may slow the gate-drive signal, preventing the device from turning off and destroying the input bridge. This failure is usually accompanied by a miniature fireworks display right on your bench.

Yet another trade-off is that only one measurement may be made at a time - remember all the input references are tied to each other. Once you have floated one input references, all input references are now floating at the same level.

[cdev]

Its true, I have an old Tek 2200 series scope and it had a battery add on and they give instructions in the manual how to float it using this external source of power. Nowadays they would never tell people to do or how to do that.

Unless they're not:
http://www.tek.com/oscilloscope/tps2000
http://en-us.fluke.com/products/portable-oscilloscopes/fluke-190-ii-portable-oscilloscope-190-204.html#fbid=Irx3Vyq4cGY?features
http://www.home.agilent.com/en/pc-1000004009%3Aepsg%3Apgr/u1600-series-handheld-oscilloscopes?nid=-536906711.0&cc=US&lc=eng
 

[vk6zgo]

Example fatality from https://groups.io/g/TekScopes/topic/7632418#10795 with my emphasis

What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone* in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.

Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.

*Surely the primary warning is, if you are working on something offering unusual dangers DO NOT WORK ALONE!!

Over 40 plus years, I have only seen this done once (floating an Oscilloscope in this manner), & this was done by very senior people, very gingerly, & with great care, AND with a staff member standing by with their hand on the handle of the main circuit breaker, ready to operate it instantly if required.
This was looking for some waveform distortion problems at the output of an ac Automatic Voltage Regulator.
In a linear system, the 'scope could be set up off line to trigger correctly on a 50Hz signal.
Modern switchmode supplies means much more fiddling with the controls would be necessary, with additional hazard.
The next time I was where someone had the occasion to look at a similar signal, they bought a Tek TH720A specially because of its isolated inputs.
 

[bitseeker]

For proper floating measurements on an oscilloscope you need a proper high voltage differential probe like this:
http://amzn.to/1iuKJSB

That looks remarkably similar to the Agilent one that is twice the price (same internals?):


edit:
https://www.eevblog.com/forum/testgear/lecroy-25mhz-hv-differential-probe-teardown-repair/

I'll be buying the BK for measuring things that can not easily be isolated.

Yes, they're made by Sapphire. Likewise with the EEVblog HVP-70 (70 MHz vs. 25 MHz, but same OEM). Good stuff.

For a more budget-friendly option, there's the Micsig DP10013. (However, note the difference in the multiplier, which may impact your use case.)

https://www.amazon.com/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3/

Even cheaper on eBay.

[tggzzz]

Unusually for me, I've snipped the context because in this case it doesn't matter.

*Surely the primary warning is, if you are working on something offering unusual dangers DO NOT WORK ALONE!!

I prefer avoiding the problem to (possibly) fixing the problem after it has occurred. In other words, know and use the right tool for the job.

Nowadays, as you say, one suitable technique would be:

The next time I was where someone had the occasion to look at a similar signal, they bought a Tek TH720A specially because of its isolated inputs.

Or you could do this: