Ground of oscilloscope always connected to earth?

[pascal_sweden]

I saw the video on potential risk of destroying your probe when connecting ground to wrong signal.

Was surprised to read that the ground of the oscilloscope is always connected to earth in most scopes.

Why is that not isolated in a scope? This would make it totally safe not?

[notsob]

This is for your own safety, in the past people used to sometimes float the scope (ie NO EARTH connection) - so especially with the traditional metal case scopes, the case becomes LIVE - - you touch the metal or the shielded end of the probe you die ( or you get booted across the room with serious burns.

Leave the earth connected and use an isolation transformer

[pascal_sweden]

So you would not connect the scope directly to mains, but put isolation transformer in between?
Which brand and type you would recommend? How expensive?

[BravoV]

Or ... use a special designed portable "isolated" scope, both channels and both grounds are fully isolated, for example like this Tektronix portable scope design here.

But remember, the devil is in the detail, NOT every "portable" scope have this kind of design, so just be careful.

[Bored@Work]

So you would not connect the scope directly to mains, but put isolation transformer in between?
Which brand and type you would recommend? How expensive?

No, NEVER isolate or float the oscilloscope. If you want to use an isolation transformer isolate the device in which you want to measure. But be aware that an isolation transformer (or an isolated oscilloscope) reduces but not eliminates the risk.

If you need an isolated oscilloscope, get isolated probes (note: differential probes not necessarily mean they are isolated, check if they are), or get one of those special isolated oscilloscopes.

Also note that a battery operated oscilloscope is not necessarily properly insulated. E.g. blank BNC connectors are typically a sign that an oscilloscope is not. But non-blank connectors still don't mean it is properly isolated.

[EEVblog]

So you would not connect the scope directly to mains, but put isolation transformer in between?

No!
You use the isolation transformer on the device under test.
One like this
http://amzn.to/1dUDopy

[EEVblog]

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

[nctnico]

An oscilloscope (like most equipment) is designed to be connected to ground. The safety rating is based on it. Don't forget a scope (and many other types of equipment) have exposed metal parts which could become live if there is a breakdown somewhere in the isolation.

edit: and don't think that never will happen. A while ago I didn't wait long enough to let a scope warm up when it came from the cold outside. The moisture and dirt started to conduct which caused a breakdown between the mains and the chassis. Because the scope was grounded the fuse blew.

[casinada]

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
 

[Nerull]

Imagine what will happen if you float scope ground to 120/240V mains and then absent mindedly reach over and connect another probe on a second channel and it's live BNC connector.

If you survive, you will know why you don't float scopes. A floating scope puts potentially lethal voltages on exposed conductors inches away from buttons you may be using while focused on the screen/DUT.

[casinada]

Isolated scopes have isolated BNC connectors. Without common sense anybody should stay away from this type of environments.

[Nerull]

BNC connectors, and depending on the scope the chassis, front panel, switches, etc.

If you have common sense you use the right tool for the job.

[rexxar]

I don't understand why the chassis would ever be at mains potential just because the ground is not connected. I've used my oscilloscope without the ground connected (before I knew any better) and I never got zapped. I also used it in the new apartment before I discovered there isn't any earth connection to the outlets, still not dead. 

[Lurch]

I don't understand why the chassis would ever be at mains potential just because the ground is not connected.

Then probably best you read up on why a scope is grounded, and how.

still not dead. 

I think that is more luck than anything as you appear to be unaware of how grounding and fault currents/paths in general work.

[ConKbot]

I don't understand why the chassis would ever be at mains potential just because the ground is not connected. I've used my oscilloscope without the ground connected (before I knew any better) and I never got zapped. I also used it in the new apartment before I discovered there isn't any earth connection to the outlets, still not dead. 

A little thought exercise.  Lets say you want to measure the gate waveform (Vgs) on a top mosfet in a full-bridge power supply.  You diligently get an isolation transformer with a ground lift to isolate your scope, run it off of that, clip the groundclip of the scope probe to your source pin of the mosfet, and clip the probe onto the gate.   Huzzah, you're getting a nice Vgs waveform on the scope now!  Lets add a second probe to measure Vds on the mosfet also.  You grab the metal BNC of the probe to plug it in, and as soon as the 2 BNC connectors touch... you get zapped by 400V@200KHz because the ground clip of the first probe is connected to the mid-point of the full bridge in the power supply, which goes between 0V, and 400V as the mosfets are switched. Ouch!

[grumpydoc]

No, NEVER isolate or float the oscilloscope. If you want to use an isolation transformer isolate the device in which you want to measure. But be aware that an isolation transformer (or an isolated oscilloscope) reduces but not eliminates the risk.

Indeed!

It's important to realise that once you connect the 'scope ground lead to some point in the (isolated) DUT you've just ground referenced it again. If the point you chose is at 400V referenced to the DUT ground then you can create a situation where the DUT case is now at -400V to earth ground.

Isolation transformers allow certain measurements which would otherwise blow you and/or your 'scope up to be made without letting the magic smoke out - but they are not a substitute for careful appraisal of what voltage goes where, nor a panacea for safety.

[BravoV]

A little thought exercise.  Lets say you want to measure the gate waveform (Vgs) on a top mosfet in a full-bridge power supply.  You diligently get an isolation transformer with a ground lift to isolate your scope, run it off of that, clip the groundclip of the scope probe to your source pin of the mosfet, and clip the probe onto the gate.   Huzzah, you're getting a nice Vgs waveform on the scope now! 
... <the Devil work starts here >.... 
Lets add a second probe to measure Vds on the mosfet also.  You grab the metal BNC of the probe to plug it in, and as soon as the 2 BNC connectors touch... you get zapped by 400V@200KHz because the ground clip of the first probe is connected to the mid-point of the full bridge in the power supply, which goes between 0V, and 400V as the mosfets are switched. Ouch!

+1 , and that "Ouch!" sometimes means .... you're dead.

[Kjelt]

I think you all forget that if the scope and the DUT are behind their own isolation transformers there is absolutely no reference to earth ground.
So there is no current flowing to earth ground period. Beware of the rule never to use both hands simultaneously, if you touch with both hands a high voltage then you have a (lethal) problem.
At our company we work for 20+ years with whole measurement benches behind a big isolation transformer just for safety. Just be aware there should nowhere a normal earth ground mains socket be used, everything should be behind the isolation transformer and the DUT should be behind a different isolation transformer so they are also seperated from eachother.
Still if measuring on dangerous high voltages you still should always use differential probes. For very high voltages there are high voltage probes.
Always be alert, think twice before measuring on dangerous voltages and with very high voltages build a dead mans switch and be sure there is someone else in the room with you that can toggle that switch (because with high AC voltages it twinges the muscles so you can not let go).

[HooRide]

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.

[csar]

The bench multimeters also have metal chassi, but the probes are floating. Why is not the same thing done with oscilloscopes?
Why is the multimeter safe, but the scope is not?

[Fungus]

The bench multimeters also have metal chassi, but the probes are floating. Why is not the same thing done with oscilloscopes?
Why is the multimeter safe, but the scope is not?

Does your multimeter have a tempting row of BNC connectors along the front that could all be connected to a live wire via. the ground clip of a probe?

[Scratch.HTF]

A Pace satellite receiver service manual had an instruction to float the ground of the oscilloscope (and not to touch it) when taking waveforms on the primary side of the power supply; high voltage differential probes (or alternatively in some cases, an isolation transformer) should be used instead.
In a Philips test equipment manual, the notice regarding grounding had the line "intentional interruption is prohibited".

[Fungus]

A Pace satellite receiver service manual had an instruction to float the ground of the oscilloscope (and not to touch it) when taking waveforms on the primary side of the power supply; high voltage differential probes (or alternatively in some cases, an isolation transformer) should be used instead.
In a Philips test equipment manual, the notice regarding grounding had the line "intentional interruption is prohibited".

I believe it was fashionable for a while in the 1970s because it makes life easier.

Then two channel 'scopes became more widely available, the bodies started piling up, and people decided that "easier" isn't always the best way.

[Gandalf_Sr]

I have a large HP isolation transformer that my scope runs through.  As in any test, you have t think about what you're doing when you work with high voltages.  I stuck a note on the front of my scope saying "GROUND IS ISOLATED!"

Take care my friends.

[tggzzz]

I don't understand why the chassis would ever be at mains potential just because the ground is not connected.

It is good that you know you don't understand. Now, what are you going to do about that?

I've used my oscilloscope without the ground connected (before I knew any better) and I never got zapped. I also used it in the new apartment before I discovered there isn't any earth connection to the outlets, still not dead.

Ah, the "I've walked across busy roads without looking and I'm still not dead" contention.

[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: