Floating Scopes

[mtdoc]

From the OP:

I've googled this and yes it had been discussed but I still don't understand why this is a big no no

And that is exactly the point. Any curious mind should want to know the why.

I've just looked back through this thread, and other than one trolling type post, what I see is everyone agreeing that it is never a good idea to float a scope. No one has endorsed it or suggested that it be endorsed or that it is currently endorsed by any institution. Many have provided good concrete explanations about why it is dangerous.  They have come at it from different angles but all reach the same conclusion. 

This is a good thing and how learning occurs

[tronde]

There are many things to be said about how to measure live circuits safely.

First of all: This is an internationl formum. Therefore we must consider the implications of that fact. From what I can see, it is one most important factor that has not been discussed in this thread, namely the mains power distribution system. It has only been briefely mentioned and from some comments I dare to say some should read up on it.

Within Europe you will find mostly some kind of the TN-system with protective earth (PE) and neutral connected together at least one place in the building. You will also find the IT-system with only local floating PE and no ground on the source transformer. You can also find the TT-system with grounded source transformer and local floating PE. You can find homes with some rooms defined as "dry" and no PE in the wall outlet. In the US you can find the split phase system with grounded neutral and as I understand some other systems as well. I know that other systems also exist, and is in use in different countries. It can also be that the distribution system used in an industial estate or office can differ from what is normal in the country.

This meaning of this is that what you will find in your lab or home is not necessarily the same as what the other person will find in his lab or home.

If we fail to understand this, we can end up by giving some very wrong and dangerous advices.

It is said that you should use an isolated differnetial probe to be safe. Soemtimes this is wrong. It is never wrong to use the probe, but the claim of being safe is sometimes wrong.

If you are connected to a TN-system AND have polarised plugs and wall sockets you will know which wire is neutral and therefore close to the ground potential. If you test a product with a live chassis, the chassis will most likely be at the same potential as PE and you can touch that and your scope safely.

Edit: Be aware that the wall socket known as "French" or "Type E" or CCE 7/5 is polarised by design, but the wiring of it is implemented differently in the countries that use it. This means that you need to test which wire is "live" and which is "neutral". Even if you live in a country that uses polarised sockets that is expected to be wired in a certan way, you should test the polarity to be sure.

BUT if you don't have polarised plugs and sockets you will not know which wire is live or neutral, neither on your scope or product. If you manage to connect the chassis so it is on the live wire, you are in deep shit if you touch that and your scope even if you have an isolated differential probe. The probe will save your scope, yes, but not you. We also have the possibility of touching other parts of the circuit than the chassis. If we touch something that is "live" and the  scope, no differential probe will save you.

We must remember that the normal Schuko plug is used in many countries regardless of the distribution system and it is not polarised.


Next thing is about isolating transformer for the device you test. In previous discussions on this forum some has said that an isolating transforer is a thing of the past and that is is better to rely on a Residual Current Breaker (RCB) that will trip if you get a shock.

First of all: a RCB is not intended to be a safety device to protect you from doing silly things. Its main function is to be a last resort if everyting else fails.

Then again we must understand that what you have can be something completely different from what is available to others. I understand that RCBs are differnet around the world, but in Europe we will most likely find a 30mA RCB known as "type A". We do also have a less known "type B".  There is one most important difference between "type A" and "type B". Type A will not trip reliably if there is a steady DC component in the earth current. Since we quite often test equipent with rectifiers we will most likely not be protected by the RCB used in most of Europe as of today. If you think a "type B" is something you should have, you must consult the data sheet for that particular type first. If connected in the wrong place, it can block the function of other RCBs in the installation.


I know that the reason for the suggestion of avoiding an isolation transformer is that you can still get a shock if you manage to connect youself in the return loop of the probe. This is true, but as we have seen a RCB can give a very false impression of safety too.

The least unsafe will probably be to use a proper isolating transformer (one with a grounded static shield between primary and secondary) for the device you test and an isolating differential probe. If you manage to touch both "live" and "neutral", no transformer or RCB will save you.




It seems like most people with a background in electronics are scared about mains voltage, and that it is somethiong really dangerous. Yes, it can be dangerous. Yes it can kill, but we are still talking about 240 volts and not kilovolts. Reinforced clearance distance is usually 4mm and creepage is 5mm and normal insulation is half of those values. It is not so that the voltage will jump out of the wall and bite your nose. What puzzles me is that people are afraid of 240V but consider 12V as safe. That is really ignorant. A charged car battery can cause a lot of danger too. Evaporating wires and metal splattering around for instance.


What all this boils down to is that we really must switch on our brain before we start to work on any circuit. If the available energy is large enough a low voltage circuit can be really damaging to both you and your test equipment. If you don't fully understand what can go wrong, you should stay away from testing on that product regardless of the voltage.


You will lnever be "safe" when you work on any kind of exposed enrgized circuit. You will only have different grades of "unsafe".


Edit: added info regarding the "French" polarised socket.

[Fungus]

I vote to leave that^ as the final post in this thread. It should now be locked.

[nctnico]

There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum.

[David Hess]

* What problem is solved by floating a 'scope?
and
* When might I want to do it?

If you can put up with the high common mode capacitance, floating the oscilloscope provides higher common mode rejection and lower noise than using a high voltage differential probe but see below for better options.

The only correct answers are:
a) Get a differential probe
or
b) Use AC coupling.

c) Float the device under test although this has many of the same problems as floating the oscilloscope.  I have occasionally floated the DUT and used a differential amplifier to get higher common mode rejection ratio and lower noise than a high voltage differential amplifier will provide.
d) Use an oscilloscope which has isolated inputs.
e) Use a probe isolation amplifier.

Back then in this thread referred to 1980s. By then differential amplifiers with excellent CMRR and bandwidth were available (Tek 7A13, 7A22, plus plugins from the 500 and 5000 series), as were high voltage probes (including the P6015 if you wanted to go crazy). Also 200-series battery powered isolated scopes (e.g. Tek 200 series that could float up to 700V on battery power) and A6902 isolated probes.

The 7A13 and 7A22 only have excellent common mode rejection when special adjustable attenuating probes are used and calibrated or the input attenuators are switched out and x1 probes are used.  (1) Modern high voltage differential probes never have excellent common mode rejection but they are good enough for many applications including usually off-line power supply design.

High voltage attenuating probes including the 7A13 and 7A22 when attenuation is used have another problem; they have high noise in direct proportion to their input attenuation.  An oscilloscope with floating inputs or the A6902 probe isolation amplifier solves this problem.

I attached a snippet from the Tektronix 1982 catalog to see what they wrote back then about floating scopes. I find it high unlikely that the major scope manufacturers, Tektronix and HP (who together accounted for about 93% of the scope sales, at least in the US), were endorsing floating scopes in the 1980s.

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".

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

[alm]

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".

Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.

[David Hess]

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".

Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.

Right about the time multimeters were practically required to use shielded banana jacks, the oscilloscope probe manufacturers were required to downgrade the voltage rating of their miniature and subminiature probe tips because of clearance and creepage requirements.

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.

That is definitely a problem but the same applications which will have problems dealing with high common mode capacitance are usually unbalanced enough to degrade the common mode rejection of a differential probe.

[mtdoc]

There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum.

But that is not what is occurring. The consensus in this thread and all the others I've seen on this forum regarding floating scopes is "don't do it" and by explaining why it is unsafe you are providing those who are skeptical of "proclamations from authority" the understanding needed to prevent them from doing it.  In addition, the ancillary discussions that arise from this topic are educational in their own right.

In the end, there is no cure for stupid. Those who are going to ignore all advice regardless - will do it - no matter what is discussed or not discussed on this forum.  But I've yet to see anything on this forum that would make that more likely - quite the contrary - I've seen lots of good discussion that should convince anyone why it is unsafe.   The reality is that there are also many people who need to understand why a practice is unsafe in order to fully internalize the advice.  Paternalistic and overly rote recitation of safety rules without context may work in some environments but IMO not on an electronics discussion forum where many people come to learn. Hopefully the excellent discussion that has ensued in this thread has caused the OP to now fully justify and internalize the advice not to float a scope.

[colorado.rob]

The world can always use more Darwin Award winners to serve as examples for others on what not to do. 

[serggio]

Wow, wow... 
How many words in this thread...
TN-C, TN-S, TN-C-S, IT, TT and other mains distribution schemes...
TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

All that need to know, that in 95% scopes (except of isolated channel scopes and portable scopes) ground conductor at your passive probe always connected to all open metal parts of your scope (chassis, USB shield, neighbor's BNCs and etc.)
End of tip on your passive probe have open metal part - ground connector, at least.

Always need remember about it when you try to connect your probe to high voltage source.

[PartialDischarge]

I'll add more fuel to the fire. I can't recommend anyone doing this, but I do it cause I know exactly what I'm doing so please don't waste your time critizing me, ok fellas?.
Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

[tautech]

I'll add more fuel to the fire. I can't recommend anyone doing this, but I do it cause I know exactly what I'm doing so please don't waste your time critizing me, ok fellas?.

Right. 

Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

BUT......by virtue of your tablet scope is not mains ground referenced you haven't floated it at all. 

[Vtile]

First of all I do want to say that high voltages, high current or high energy work with live or powered, but disconnected device under test is something that doesn't belong to hobbyist of young age or hobbyist of high age, but low experience (including lack of understanding of theory) nor with educated person without knowledge of what he or she is doing.

Someone did claim my wording about differential probes are not reasonable. It were, as the response did give more specific information of the type of "usually safe" differential probes. The CAT IV text in the faceplate of the instrument doesn't mean that it really fullfills the requirement (ie. with all the counterfeit product floating in the markets).

Now I have one extra question that also haven't discussed and also is a trap for anyone who doesn't think. I (and I assume many hobbyist that shouldn't do high energy work ) do have one of those handy USB scopes, well it is isolated and floating (with random electrical and mechanical values) when I attach it to my battery powered laptop running charger disconnected and no other equipment connected. Again the same hazard is there the metal parts of the USB scope and propably also on the laptop (through USB shielding wire) just became potentially life threatening to touch.

There is so many traps in this area of electronics and electrical work in general.

PS. Also your ESD cover/mat on the workbench is grounded through random impedance.
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. 

[Specmaster]

The way I look at this is that it does not matter if your scope is battery powered in other words a true portable, floated, isolated or double insulated and the same thing applies the DUT unless battery powered, there are many ways that you can still receive a fatal electric shock. Fact is that all things electrical demand respect and due care and diligence and in our game, especially if you're in the repair game, then live working is unavoidable so common sense must prevail.

In the real world, we would all love to have the very best and finest of equipment, especially if cost was not a factor but sadly it so often is so we each and everyone of us has to weigh up the risk with everything we do in life and the question of safety in the work area / bench is no different. If you don't feel confident working on live equipment, or if uncertain about the suitability of a piece of test equipment etc, then don't do it.

[JohnG]

Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

I have to ask: Were you measuring Vgs of a high side MOSFET in a half-bridge, where the common mode voltage is a giant square wave or similar? Did you get a decent waveform? Is it believable? If so, could you send me a picture of it? 100 MHz is likely too slow for me, but I have only been able to do what I want with a borrowed Tek IsoVu (which works very well for what it does).

John

[WastelandTek]

so let me get this straight

say I have a 5A21N here, I can hook a conventional probe to each input and then probe 2 points in a circuit that are BOTH above (or below) ground potential safely right?

I have the diff amp, so I don't need the diff probe, or am I missing something?

[alm]

Yes, assuming that you observe the limits of both the probes and the plugin. And be careful where, and if, you connect the ground leads.

Keep in mind that attenuating probes, e.g. 1:10 or 1:100 probes, may degrade your common mode rejection ratio (CMRR). A high CMRR relies on a close matching of the attenuation of both channels. If your probes have a 1% mismatch in attenuation ratio, your CMRR will be limited to 100:1. Tektronix made some probes with adjustable DC attenuation for that (like the P6055 and P6009), so you could adjust one probe to match the other. Obviously 1x probes will have a very good match in attenuation .

[HighVoltage]

For the young players:

When I was 16 years old, I had to learn a rule when measuring any dangerous voltage:
"Keep one hand in the pocket"

Do I follow this rule these days?
No!

But it made me aware of what I am doing every time I measure dangerous voltage, especially high voltage to 80.000 Volt.

[tronde]

TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

Yes, you can find som older buildings wired that way in Europe. But, you should not find any legally installed wall socket with a disconnected earth contact. If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket. This is not the same as floating the scope, because PE will always be connected to neutral at least one place in the building if you have any kind of a TN-system.

As I mentioned in my previous post, you can find some places using wall sockets without any earth contact. They have been used regardless of the distribution system, and you will of course not have any PE connected to your scope so it will be floating in that case.

[PartialDischarge]

I have to ask: Were you measuring Vgs of a high side MOSFET in a half-bridge, where the common mode voltage is a giant square wave or similar? Did you get a decent waveform? Is it believable?

Exactly. My scenario is pretty specific, but I'd say it is accurate and shows the Vgs gate charge 'waveform', Also the peak Vgs which was what worried me, is within calculations and specs.

[serggio]

If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!

I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors. 

[exe]

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?

[tronde]

If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!





I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors. 

You must read the post I answered to.

TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

What I say, and this IS true, is that if you find TN-C in a wall socket, it will be wired with PEN connected to both the earth connector AND the neutral conector.

"serggio" gave the impression that TN-C in a wall socket would give you a floating scope. This is wrong, because the earth connector of the socket will be connected to the neutral (which will be the PEN-wire in that particular instance).

It is correct that this is bad practice (and I guess illegal in most countries by today), and that is also stated by "serggio" when he called the practice "obsoleted".

We must understand that you will find a lot of different wiring around the world, and it was in fact legal to wire TN-C in wall outlets in some European countries long time ago.

This is why I said in my first post that we must know what is present in our home or lab. You can not rely on what other people will have.

[IanMacdonald]

"Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO)."

I would disagree. In those days a lot of equipment was live chassis, and bench engineers took their own safety very seriously.  More so than today, when people have gotten complacent thanks to much safer gear. In those days it was projectors with 35kV on the anodes, or Marshall guitar amps with a 1kV supply that could deliver 250mA. Either could kill you stone dead if you weren't careful enough.

The insistence of manufacturers on earthing 'scope inputs arises, I suspect, from a misunderstanding of the purpose of a protective  earth conductor, which is to form a Faraday cage around and enclosing live connections. However, if the live connections are not inside the Faraday cage, then no protection is given. In that case, an earthed object which is liable to be touched by the operator is actually a safety hazard in its own right, and the interests of safety would be best served by eliminating that object from the testbench. 

Building site practices have clearly identified that hand tools with insulating cases and NO earth have a much better safety record than earthed metal tools. If the case has to be metal then earthing is the lesser of two evils compared to a floating case, but the least hazard is achieved by eliminating the touchable metal. 

I mean, we have DMMs with fully isolated data ports. That is to prevent hazardous situations arising though the test prods being earthed via the data cable.

I daresay that isolated scope inputs might be a little harder to design since they handle much higher frequencies than meter prods, but it would be a really big safety advantage if they were isolated from earth, and from each other.  Although, with modern opto-isolators and PSU modules I really don't think it would be that hard to provide post-preamp earth isolation.

-How many lives does it have to save, for the design effort to be worth it?

"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. 

Might add that the practice of earthing 'scope inputs dates from the valve era when 'scopes themselves contained some truly eye-watering voltages. Even the preamps would have run from maybe 150v, so not earthing the input on that kind of beast would have invited having an internal fault put a dangerous voltage onto the probe. That is long-gone history though. The modern LCD scope is basically no different from a bench DMM as regards operator safety.  It would be better if the same safety principles were applied.

[serggio]

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?

Equilibrium potential. It's correct to say - no equilibrium potential. Your radiator probably have different potential from you laptop case. Your laptop have no ground connector at AC/DC power converter, but your laptop power converter can have capacitive coupling to AC (to phase), same time your radiator can be grounded thru the building construction. 
Same time your body can have resistance less that 100 kOhms (it's completely depended from many factors: external humidity, your own skin conductance and etc, etc..), so you feeling electrical shock.

That is, second thumb rule, why you should never have ground contact to neutral contact connection in wall socket at wet room (bathroom e.g.) if you have TN-C protective scheme.