Oscilloscope and grounding

[steve dove]

i have used scopes with lifted ground point for many years,never blown one up But the factor
of the power tx potential being increased,has got me considering matters,and of course it is
quite clear the method i,and many others have used over the years,is not acceptable in the real
world,i worked with engineers,as young trainee in the tv video business,and they had the ground
disconnected,was a lot of switched mode work,so going in grounded,well it would of been fireworks.
i would think buying a TOP quality differential probe is how to go about it,i have just lost a tx winding
in a very good quality four channel Hitachi scope,and reading what has been put out,by some very
knowledgeable types,am even after all the years doing this electronics lark i have,is time i took on board
some of what is said,i deffo wont ground again,certainly not with the nice replacement tek scope i have
just aquired,a lot of scopes are available on ebay etc,and it does encourage the dabblers to have a go,its up
to us that have been down the road,and very likely back again,to advise these very likely enthusiastic would
be DIY fixers,and help them avoid killing themselves,thats the bottom line because we all know its what can
result,i use mine mostly for audio work valve/transistor/mosfet etc,very rarely switch mode these days.
there are a lot of varying opinions of how to/how not,would be nice if one of the very clued up fella's could lay
it down clear as day,a lot of what is i am sure confuses,plain simple and to the point,what we do is not for
the uninitiated,i am no expert on everything these great pieces of test gear are capable,it does all i need,but
still i learn. S

 

[schmitt trigger]

If you really, really want to use a floating scope, get a battery powered one with a CAT-III or CAT-IV rated case isolation.
Most major scope manufacturers offer them, and lately they are coming with large color LCDs, with a full feature set.

Additionally, these scopes are great for vehicle troubleshooting or any field servicing where finding an AC outlet is not convenient.

 

[Reloadron]

This thread is over 10 years old. Sort of beating a dead horse.

Ron

 

[Mosaic]

I guess it still has value.
I use Differential probes now, having blown enough fuses among other things while working on PSUs. Don't fancy removing the earth....from the scope. Line noise filtering is compromised.

 

[steve dove]

Good to read the responses coming through,i certainly know,and did from the get go,not any kind of
sensible action to ground lift,but with extreme care,and indeed knowing exactly what you are dealing
with,well let me say you can get away with it,i did for years as mentioned prior,but in the health and
safety concious world as is now,lets view the practice as abs no no,and go about the art of using this
eqpt,in the way the manufacturers state we should,i would be interested to know,the other aspect of
this,the stress on transformer,how many here think,that maybe theirs failed as result of ground lift.
i wait your input.
steve

 

[JimB]

An extreme example of "floating an oscilloscope".
I once assisted in diagnosing a problem in a motor starter which had 415 volts or so inside it.
We used a battery powered 'scope and sat it on a thick wad of paper before connecting into the starter.

This took place on board a submarine which was under construction, so everything in the area was metal and so earthed.
We also had to wait until after 17:00 hrs when most people had gone home before we attempted this task.

JimB

 

[crutschow]

Personally I consider an earthed scope EXTREMELY DANGEROUS, I disconnect the earth lead on ALL my scopes, and mark them accordingly. This is common practice in the service trade, not only by engineers, but also by large multi-national electronics companies.

It may be common practice in the service trade and others but was a definite no-no in the electronics lab at Aerojet where I worked for 40 years.
But of course we didn't work on unisolated equipment.

 

[Nigel Goodwin]

It may be common practice in the service trade and others but was a definite no-no in the electronics lab at Aerojet where I worked for 40 years.
But of course we didn't work on unisolated equipment.

Again, I suggest you look at understanding the implications of either method - and why you imagine that one method might be safer than the other.

 

[Tony Stewart]

I have done all sorts of HV testing in my career with both grounded and isolated ungrounded scopes. As an R&D INstrument designer and large company Test Engineer/Mgr over a span of 40yrs, my perspective may be different or similar to yours.

To appreciate what I say, you may need to know EMC, (equipment and personal) safety, HIPOT tests, ESD fault injection effects, Partial Discharge(PD) causes from HV, stray leakage and partial insulation failure and BreakDown Voltage (BDV) limits as well as sensitivity or resolution of the signal you wish to capture.

My opinion is that the safest test uses two 10:1 balanced probes in A-B mode to get differential readings taking note on the max scope input but do not connect the probe grounds until you are sure they are low V potential difference with respect to scope probe ground.

It is sometimes better to isolate the signal with a suitable transformer than isolate your scope or design a differential adapter or get appropriate accessories for the scope.
For HVAC commercial CAP dividers are used instead of resistor dividers for safety reasons as they are less prone to creapage or leakage failures and BDV faults.

You can get false 50Vac readings on a 10M scope probe just by touching with your finger and not ground as your body acts as an antenna.

If you get a false reading >50V connect a plastic cap or long 10K resistor to load the probe . This will measure the current flow so it gives an indication of the source conductance.

If you intend to scope a 10kA busbar, make sure you know about and wear flash protection. This means a tiny ESD incident can trigger an arc that can burn or vaporize humans! and there is tons of safety gear and forums on this topic.

- It is not necessary to modify the safety ground unless you know what the effect on insulation, noise injection , insulation reduction, and effects on signal integrity and it is well labelled to be unsafe as ungrounded. Nevertheless if you understand the internal workings and the above , sometimes a cheater cord or additional safety insulation transformer line and ungrounded device may in rare situations be useful. But I say rarely as the risks exceed the benefits.

- All equipment is factory HIPOT tested (by law) . It may exceed this or be degraded due to dust and contaminants.
- AC HIPOT test= 2x line V+1kV 1 minute or reduced to 1 second at 2.2x line V +1kV or e.g. 120V/0.5mA = 240k
- This AC test conducts common mode current when a line filter is often used so a DC at peak line voltage above instead of RMS
- so for 1 second it becomes 2.2*1.414U+1kV for HIPOT testing with a limit of 100uA after slow ramp charging.


- Realize that the scope chassis is connected to both line and neutral via plastic caps from filtered Line to chassis earth ground and Line to Neutral.
There is a great variation in design choices and the dual caps to chassis may not be matched over its life span due to aging thus the scope chassis will be at mid line voltage more or less depending on balance.

EN 60384-14 defines sub-classifications for both types. X1 capacitors are used for high pulse applications, while X2 and X3 types are used for general-purpose applications with different peak pulse operating voltages and peak value surge voltages. Y-capacitors, which are used to bridge operational insulation, are classified as Y1, Y2, Y3 or Y4 according to type of bridged insulation as well as AC and peak voltage ratings. Y1 class capacitors are rated up to 500 Vac, with a peak test voltage of 8 kV. Y2 capacitors have 150 to 300 Vac ratings and a peak test voltage of 5 kV. Y3 capacitors are rated to 250 Vac with no peak test voltage specified. Y4 capacitors are rated to 150 Vac with a peak test voltage of 2.5 kV.

What important tests are related to IEC/EN 60384-14?
These include impulse voltage, endurance and active flammability tests. Application and parameters for these tests depend on the capacitor classification and sub-classification.

What capacitor types are used in line filters?
Two common types are metallised paper/film and ceramic. For Y capacitors, ceramic types are less expensive than metallised film, but unstable over time and temperature and less mechanically stable. Ceramic failure mode also tends toward short circuit, whereas metallised paper and film types tend towards open circuit.

If you don't understand these risks, go ahead and unground your scope.
you may become eligible for the Darwin Awards.