Floating Scopes

[Vtile]

PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there. 

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?

I assume your laptop is powered by a charger. There is a few possible explanations, but I think I can assume (but it should be threated as high energy fault until it is solved ) that the shock you have had so far is rather low energy discharge as you are still wondering it. It is possible to get only a mild shock from full mains voltage, that is why it should be threated as a high energy fault.

The real couse is of course almost impossible to quess without derailing this topic with more questions and answer. If you have reasonably trustworth DMM with atleast CAT II and similar level probes you can measure the voltage between the two objects. The probe placement is essential and needs to be on proper high impedance slots, typically the COM and V. In these kind of measurements DMM should start from the highest AC voltage range. You also should have a good workin position and not to touch both of the objects (except with the probe tips) while doing the measurent.

What you have come across is indicating that something is wrong in your laptop or in your homes / workplace radiator. The problem is something that should be inspected by an qualified electrician by visiting on your place and measuring (the cheaper route might be to get the laptop inspected first at local electronics repair shop and if it is OK, then the radiator). For a starters you could start a new thread to ie. repair, general chat, or a beginners section.

[tronde]

Your laptop have no ground connector at AC/DC power converter, but your laptop power converter can have capacitive coupling to AC (to phase)

It is quite normal to find a 1nF class Y1 capacitior connected from each of the mains supply phases to 0V output of the charger. This is becuase of noise reduction. You are normally allowed to have a ground leakage current of less that 0.25mA in Class-2 (double insulated [double rectangles] ) equipment. As you say, this can give a feeling of shock.

[Red Squirrel]

TBH I sometimes float my scope, if testing something that I am unsure of that is ground referenced or any time I work with mains stuff.  I actually feel safer floating it as I don't have to worry so much about where I'm placing the ground clip.  I can put it anywhere I want, and sometimes you want that anyway as you want to test between two specific points, and they may not be ground. 

Of course the proper way would be to put the device you're testing on an isolation transformer, but that's not always an option if it's a large stationary object like a ground referenced battery bank or other hard wired stationary equipment.   

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

[tautech]

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

The cheapest SAFE solution is differential measurement using 2 channels and maths or a differential probe. Period.

[tronde]

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

[serggio]

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

It would not be good and safe solution till user will not understand dangerous from touching open circuit parts of high voltage scheme/open test and measurement equipment parts connected to this this scheme or ever unprotected parts of leads this equipment.
Battery operated scope this is not panacea from potential danger of high voltage injury, because at this scope can be connected probes with open ground contact at the end.
Battery operated scope only suitable for floating measurement with no damage scope itself (no short circuit thru the scope).
But this is not safe at all.

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

So many solutions.... What you need exactly?

[tronde]

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

So many solutions.... What you need exactly?

Something that could transform a USB scope into wireless. A battery powered Bluetooth or WiFi scope could be really useful for mains work.

[timb]

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him. 

It also monitored current across the ground connection and, if it exceeded a set amount (0.5, 3.5 or 5ma), would completely sever the connection by opening the L-N-G relay. (Not unlike a GFCI.)

They were actually pretty cool little devices.

[tautech]

The first few minutes of this recently posted vid (Pt 2) properly examines the use of an isolated channel HH scope and when there is need to use some form of channel isolation or using 2 channels and differential measurement won't work.

https://youtu.be/rNElNKeDNyg

From this thread:
https://www.eevblog.com/forum/testgear/review-siglent-shs-1602-isolated-scopemeter/

[tronde]

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him. 

It also monitored current across the ground connection and, if it exceeded a set amount (0.5, 3.5 or 5ma), would completely sever the connection by opening the L-N-G relay. (Not unlike a GFCI.)

They were actually pretty cool little devices.

User manual and schematics here
http://w140.com/tekwiki/wiki/A6901

[tautech]

From another thread not far back that discussed safe measurement techniques:
https://www.eevblog.com/forum/beginners/dumb-oscope-question/

...Does that mean if I take a tube amp and plug it in without the ground plug, the amp is isolated from the mains (ie the hot and the neutral are isolated via the power transform and the ground is not hooked up)?

...

Seems like that could be dangerous too since the amp is then not grounded. But for a few tests maybe ok?... assuming I test to make sure the grounded components aren't carrying any potential with respect to ground. Thoughts?

In general, removing/disconnecting a ground pin is a risky plan.  There are some situations where it can help a lot with testing, but it's not something I'd do lightly. When you have gained some experience in electrical systems design you can make your own evaluation.

I suggest the following thought experiment:

• In the beginning, the chassis was grounded
• Maybe some internal circuitry - on the isolated side of the transformer - was grounded. Maybe the DC 0V (aka Ground).
• You disconnect (or 'lift') the chassis ground from the power plug
• As this point in time you're probably OK
• You want to measure something, so you connect your 'scope ground to DC 0V and measure.
• No problems yet
• You want to measure something else - maybe the voltage across the output transformer primary.
• You connect your scope probe to DC+ (could be well over 100V depending on amplifier).
• The '0V' rail is now forced down (because DC+ is connected to mains earth via your scope), so amplifier chassis is now live to negative(DC+) volts.
• You rest your hand on the chassis while poking around with the 'scope probe and get a shock.

When we consider the above scenario, we see that it takes 3 steps to shock yourself: steps 3, 8 and 10. Steps 3 and 8 were 'necessary' for your measurement plan.  Step 10 was a mistake. Maybe you're tired. Maybe someone distracted you. Maybe you dropped the probe and went to recover it.  What steps 3 and 8 did was put you one mistake away from danger.

And now I look like a safety nutter / the fun police... When you receive a significant electrical shock, several things can happen. You can suffer electrical burns at the point of contact (painful, not so bad).  You can fall off your chair (or ladder for extra fun times) and injure yourself. You can suffer an immediate cardiac event (e.g. ventricular fibrillation), collapse and possibly die.  Good thing you've got a buddy there keeping an eye on you, right?

You can even damage part of your heart and/or set up an irregular rhythm, feel OK, and then suffer a cardiac event when you go to sleep for the night, and your partner wakes up next to a dead body.  So if you do receive a significant shock (static discharge onto a door handle doesn't count) you should probably go see your doctor or local emergency department to get checked out.  It will likely take at least a couple of hours, or all bloody day depending on your triage level.  (Pro tip: tell them you had an electric shock as soon as you get in because a) it's important and b) it'll increase your triage priority level.) It may cost a lot of money (depending on public health care, insurance or lack thereof).

So that's why I wouldn't float the ground.

[Electro Detective]

I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder   

There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG!

'Isolation' devices are no guarantee of safety, accuracy or performance once hooked up with faulty or suspect wired DUTs and other test gear powered by mains electricity, nor do they have crystal ball capability

BANGS can cause injuries, blindness, or facilitate an inexperienced or mis-informed prodder's early funeral,
or more important, cost MONEY due to damage 


FWIW  I'm still a  'die hard'  isolation transformer fan     for many uses not just lab use,
but I have a good clue how the whole deal works,
check and and prep before I switch on, fail-safes galore,
and document working scenarios for future use, so no guessing games or 'impatience oopsies'   

[IanMacdonald]

Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)

If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.

Most decent bench irons run from 24v though,  and the question then arises, is earthing desirable or not?

Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.

The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.

That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise,  as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.

[alm]

A hard connection to ground is obviously not optimal for ESD, since it would be causing a discharge with a large peak current. However, I do not agree that a hard connection to earth is obviously not required for safety or that it is similar to a DMM fuse. What is the insulation rating of the handle of a low voltage iron? The hand holding the solder and/or tweezers? Touching a node that unexpectedly has high voltage on it could easily shock the user if the iron is not grounded.

If the iron is grounded, then a GFI or fuse should trigger, protecting the user. So I would argue that a grounded iron is essential when working on circuits with non-SELV (separated extra-low voltage) voltages on them, unless you are fully isolated from the board/solder/tools, or you can insure that there are no dangerous voltages in your circuit. So basically quite similar to scopes . Only in circuits with only low voltages but a very high current capacity (e.g. battery banks) with no up converters could you argue that a tip with a larger resistance to ground is safer.

That a careless user may kill some semiconductors is not a safety issue.

[Safar]

I have one solution else.
It's "semi-stupid" but anyway it can work. I use it for low voltage (devireg bus). Direct connection earth probe connector to any pole completely kill the signal. And I had no differential probe at this time. So I simple connected one signal pin of one channel to one pole, and second channel to another pole. And not connected earth connectors of probes to DUT. And had set math channel on scope A-B. So I get 1 differential channel from 2 scope channels.

Not sure how it safe for high voltage, but I guess that it possible to avoid danger voltage on BNC connectors if use 1:10 divider at least and not to disconnect probes. And you can still use eathed scope.

Of course I understand that HV differential probe is better.

Sent via Tapatalk

[alm]

That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.

[Safar]

Yes, CMRR is a problem in this method. Especially that is need to accurately setup signal level on both channels to avoid  ADC overload. And useful signal can be very low after math op

[David Hess]

That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.

The big problem is usually the limited common mode input voltage range for a given sensitivity.  The common mode rejection ratio is less of a problem if you have an analog oscilloscope because:

1. The vertical variable controls can be used to match the DC gain on each input for maximum common mode rejection ratio.  (1) Compensation can be used to trim the AC common mode rejection ratio.
2. Subtraction in a DSO *adds* the quantization noise of each channel and reduces the number of significant bits.  Some very early DSOs did the add and invert in the analog domain before the digitizer and do not suffer from this problem as much.

(1) It is actually a little eerie to perform this adjustment.  You can *see* the noise "null out" when the variable gain control is adjusted correctly.

[tronde]

Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)

If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.

Most decent bench irons run from 24v though,  and the question then arises, is earthing desirable or not?

Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.

The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.

That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise,  as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.

I don't consider this offtopic, because it can affect the safety on the workbench you also use for measurements.
As I wrote in reply #52 you need to be aware of the entire system. You can not just rely on a differential probe to be safe, so the grounding of the soldering iron is certainly worth looking at.

As you say, a mains supplied soldering iron will need a "hard earth" for safety, while a low voltage iron will need a "soft earth" for slow / controlled discharge of static electricity.

This is OK as long as you know what kind of earth you have. I have a Weller WD2 transformer. On that transformer you can change the resistance from the tip to earth by means of a 3.5mm plug / socket with a built in switch. No plug gives 0 ohm (hard earth). Plug only gives no  connection to earth, and a resistor installed in the plug gives you an earth resistance with that value. At first this seems to be rather smart. BUT - Weller goofed. The plug / socket is just a normal el-cheapo thing, and the plug can easily work loose. Then you will have a hard earth while you believe it is a soft earth. The plug they use is rather small, and I doubt you can find space for a resitor that will safely withstand 250V. I modified my transformer so I have a proper resistor installed inside the enclosure.

The grounding of the antistatic mat is also something to consider. With 230V mains it will normally be connected to earth by a 1Mohm resitor. If we place something with an earthed chassis on the mat it can bypass the safety resisor. A normal antistatic mat will have a rather high surface resistance, but the resistance to earth will decrease and a leakage current will increase.

[JDW]

A lot of good info in this thread, but one thing overlooked now in 2023 are newer scopes like the Rigol DHO800/900 series come floated due to having a USB-C type power plug and a power brick from Liteon that is only 2-prong at the wall socket.  Sure, they include a separate grounding wire (with banana connectors at both ends) "for safety," but the fact the scope comes this way is a recipe for unsafe practices, especially here in Japan where 3-prong wall sockets are utterly impossible to find in most homes.  Indeed, here in Japan, the only time you'll see a ground is near a wall socket in a room where there is a toilet (Japanese love their electronic bidet devices), or a refrigerator in the kitchen.  They are almost always screw terminals for connecting bare wires that hang off the appliances or bidets. Any other place, which is the most likely place you'd be using a scope, the wall sockets are 2-prong only.  But even if they were 3-prong, the design of the power adapter is such that the user isn't FORCED to use a grounded wall socket plug.  That Rigol grounding wire is "optional" in that it isn't built into the main power cord.  That is a big issue.

Tektronix offers battery powered scopes which are basically floating devices, but even Tektronix cautions you about them, properly saying not to test voltages above 30Vrms or 42Vpeak.  They also warn against the use of Isolation Transformers as being "Dangerous."

https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protection

But even if you are using less than 30Vrms (which is basically all I test, personally), grounding the scope provides a way to avoid Common Mode Noise on your measured waveforms.

Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.

[Andy Chee]

Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.

I did find this warning in the manual though!

[JDW]

I did find this warning in the manual though!

I consider that single mention inadequate.  Furthermore, it makes no mention of other benefits, such as addressing common mode noise riding on measured waveforms. Dave's video about floating scopes even shows how the noise vanishes when the ground lead is connected (not on the DHO800/900, but another scope).

Not everyone reads the full documentation anyway. Therefore, putting a warning sticker on the AC power adapter itself and a second sticker on the ground wire that Rigol includes would be prudent.  In the absence of that, I suspect many, MANY people will use it without the grounding wire.  Which again is okay if you're testing under 30Vrms or under 42V peak, as Tektronix points out (which is all I test).  Even so, proper grounding can address noise issues, as I've said.

[Andy Chee]

I consider that single mention inadequate.

I'm sure someone with a bigger test instrument history than I could comment on this, but I might guess that every documentation from every manufacturer, printed in the last 25 years, has inadequate safety warnings!

Certainly older documentation could serve double purpose as a HOWTO instructional course in instrument usage and application.  Modern documentation however implies a high level of assumed knowledge.

[madires]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

[JDW]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

That kind of statement seems to suggest this: "Go ahead and remove the ground from your scopes and then test high voltage because as long as you have an EMI suppression cap between the primary and secondary, it's not floating and therefore A-OK."

By using the term "floating" we are primarily talking about "safety from electric shock." And secondarily, we are talking about common mode noise reduction on measured waveforms too.