Author Topic: Basic Oscilloscope Safety  (Read 12253 times)

0 Members and 1 Guest are viewing this topic.

Offline grouchobyte

  • Regular Contributor
  • *
  • Posts: 230
  • Country: ca
Re: Basic Oscilloscope Safety
« Reply #25 on: August 30, 2016, 05:25:12 pm »
the problem is that if you croc clip ends up at any significant voltage the whole chasis can be live, so that means all the other BNC connectors on the scope or the croc clips on other probes suddenly become live. You just don't do this unless you know the voltage you are connecting to and with the mains that is not a good idea.

Simon:

Yes, what you say is TRUE. That was implied in my post. Good discussion IMHO. Are you less experienced EEs reading this?

@grouchobyte
« Last Edit: August 30, 2016, 05:29:06 pm by grouchobyte »
 

Offline Simon

  • Global Moderator
  • *****
  • Posts: 14766
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Basic Oscilloscope Safety
« Reply #26 on: August 30, 2016, 05:27:03 pm »
It's not a case of noobs, I'd never ever do it not unless it was the only way and the resultant measurement held imeasurable value. As it is i cautiously use my two channel as a diff measurement system.
 

Offline WackyGerman

  • Regular Contributor
  • *
  • Posts: 210
  • Country: de
Re: Basic Oscilloscope Safety
« Reply #27 on: August 30, 2016, 05:29:58 pm »
An oscilloscope ist not suitable to measure mains with it . First the grounding of the scope as described before , then it doesn t have the CAT rating for it ( for an outlet you need CAT III minimum  ) and it is not permitted for the mains voltage . Bad trap for beginners . The voltage marked on the scope is not the RMS value . Either it is Vpp like on the Siglent models for example or Vp on Rohde Schwarz models i.e. . Here in Europe we have 644 Vpp ( 320Vp) . So you overload your scope even it is 400 V rated  :-BROKE .
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 10963
  • Country: us
  • DavidH
Re: Basic Oscilloscope Safety
« Reply #28 on: August 30, 2016, 07:50:35 pm »
...

The voltage marked on the scope is not the RMS value . Either it is Vpp like on the Siglent models for example or Vp on Rohde Schwarz models i.e. . Here in Europe we have 644 Vpp ( 320Vp) . So you overload your scope even it is 400 V rated  :-BROKE .

It is actually worse than you describe.

For just the case of signal measurement, someone might think that a x10 probe will increase the peak input voltage rating.  However this only applies with DC coupling; AC coupling allows the capacitor in series with the input to charge to the average DC voltage which would happen if you wanted to see the AC ripple at a high DC potential.  They make special x10 and high voltage probes with built in internal shunts to handle this situation but the ones lacking this sure do not advertise it; at most their instructions say something about not using the probe with AC coupling and I've never seen a standard x10 probe with that warning anyway.  Now you know why the maximum input voltage for a standard x10 probe is the lower of the probe and oscilloscope input voltage ratings and why a standard x100 "high voltage" probe may only be good to the oscilloscope input rating of 400 volts peak or whatever.

And speaking of input coupling, why did all of those old oscilloscopes include ground coupling anyway and why implement it at the input instead of later in the signal chain where it would be easier?  Maybe Rigol did it the right way?  Ground coupling precharges the input AC coupling capacitor ... but not on a Rigol.  And without precharging, a 400 volt peak input can momentarily become 800 volts peak which is a great test to see if the oscilloscope's input protection circuitry really works ... or not.  Have there been any mysterious Rigol input failures?

For floating measurements things are even worse.  What is the maximum float voltage?  The designed breakdown voltage between the AC line and ground can be surprisingly small.  Obviously the oscilloscope has to handle the situation where hot and neutral are reversed but the rating in addition to this can be surprisingly small.

Old oscilloscopes included this specification because the manufacturer's knew oscilloscopes would be used with a floating ground and may even have recommended it for certain measurements.  Today there is no reason to include it because nobody will go against the recommendations not to float the oscilloscope's ground, right?  RIGHT?

Now where are those included probe schematics so I can see if they use an internal shunt.  And where are those oscilloscope schematics so I can see how ground coupling works and what limitations there are on floating measurements?  The results are in; the lawyers won and the engineers lost.
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 18107
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Basic Oscilloscope Safety
« Reply #29 on: August 30, 2016, 10:00:31 pm »
its nothing to do with theaudience. Allowing you chasis to be connected to an unknown voltage that is associated with the mains is nuts!
And this where you should be challenged.
You are looking at the forum as a single board when in fact as you know it is divided into sub boards and this thread is in the Beginners board where the primary audience is in fact beginners.
Those of us that wish to assist/ensure that the less experienced don't use unsafe practices often keep a overview on such threads as these.

Such practices that member grouchobyte has offered are old and widely used by those that really know what they're doing but even the knowledge to accomplish such practices will never make them safe however frequently and familiar one might be with them.
I was "hands on" instructed on how to do these tricks by a vastly experienced EE but I will never pass that info on for fear of those attempting such practices to only do so ONCE.  :scared:
Avid Rabid Hobbyist
 
The following users thanked this post: grouchobyte

Online tautech

  • Super Contributor
  • ***
  • Posts: 18107
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Basic Oscilloscope Safety
« Reply #30 on: August 30, 2016, 10:05:58 pm »
An oscilloscope is not suitable to measure mains with it .
:bullshit:
Tell us now to match the gate timing of a pair of SCR's doing mains phase control without a scope?  :-//
Avid Rabid Hobbyist
 

Offline grouchobyte

  • Regular Contributor
  • *
  • Posts: 230
  • Country: ca
Re: Basic Oscilloscope Safety
« Reply #31 on: August 30, 2016, 11:51:09 pm »
The comments in this thread are revealing in more ways then one. Interestingly, my cavalier comment, although well-intentioned, sparked a few flames..... and thats a good thing. It also means we have great moderation and oversight in this forum and in other forums. I am gaining respect for and from many members here and I like getting this kind of passionate feedback from time to time.

I built my first oscilloscope from scratch (first principles) and used up all my allowance when I was 13 years old. That was 51 years ago and have since earned my credentials and spent decades working as a design engineer for both Tektronix and HP as well as many other companies

So scopes are nothing new to me (I own and use 9) That said, I should probably apologize to any member who read my comments and took it to imply that floating a scope is safe and a no brainer. It isn't....so dont do it unless you know exactly what you are doing and have exhausted all the safe measurement options first!

I should mention that over the years,  I have also mentored engineering students at the graduate level and they all thank me for my candor, humor and guidance. I hope some of you glean that from my occasional posts


@grouchobyte



 

 

Offline Simon

  • Global Moderator
  • *****
  • Posts: 14766
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Basic Oscilloscope Safety
« Reply #32 on: August 31, 2016, 05:35:11 am »
The problem when replying here is that we don't truly know the person asking the question. As Tautech said particularly in the begginer section appropriate advice is best given. I would never tell anyone to do anything unsafe over the internet having never met them. I myself with attention deficit cringe at working on the mains as it only takes one slip up but I know the risks should I deam it of sufficient important to carry out the work and am prepared to weigh the risks and benefits up.
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 8484
Re: Basic Oscilloscope Safety
« Reply #33 on: August 31, 2016, 06:04:02 am »
There are no particular problems and the risks are manageable for a competent engineer to float a scope within a guaranteed +/-50V (peak) of true ground, provided the controls don't have any exposed metalwork, there are no connections to other test equipment or PCs, and it has a non-metallic case.   However, the days of floating a scope on mains Neutral (or worse Live) should be far behind us. 

If the D.U.T. can be run from a wall socket, simply use an appropriate isolating transformer to allow you to safely ground the point in the circuit you want to use as a reference. If it cant, it must be heavy plant, and in a work environment OSHA/HSE will hang your butt out to dry if there is an accident and you aren't using a Cat III differential probe or Cat III battery powered scopemeter, or other appropriate isolation.

I *MIGHT* consider floating a USB scope via a USB bus isolator or a wireless USB hub as there are few or no controls on the scope pod that will need adjustment while in use, and there wont be too many problems putting it in a touch-proof insulating enclosure, but I'd want a better isolated PSU for it than an ordinary wall-wart.  The problem would be finding a USB scope with software that isn't too s---ty and that doesn't have issues running on a full speed USB isolator.
« Last Edit: August 31, 2016, 06:06:25 am by Ian.M »
 

Offline Simon

  • Global Moderator
  • *****
  • Posts: 14766
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Basic Oscilloscope Safety
« Reply #34 on: August 31, 2016, 06:30:30 am »
The problem is knowing what the chasis will be flaoted to before you make the measurement. A fully isolated hand held scope will be fine if it can take the voltage level.

for example at work we just got a couple of lab supplies, they go up to 60V so the terminals are fully isolated and it states on the decal that the putput should not be floated more than 300V above earth. So they know you may want to do strange things with it, maybe connect it in series with other supplies and it has a specified limit for this. The limit on your exposed scope metalwork is a big fat 0.
 

Online Ice-Tea

  • Super Contributor
  • ***
  • Posts: 1797
  • Country: be
    • Freelance Hardware Engineer
Re: Basic Oscilloscope Safety
« Reply #35 on: August 31, 2016, 06:38:21 am »
... The limit on your exposed scope metalwork is a big fat 0.

QFT.
 
The following users thanked this post: Ian.M

Offline Simon

  • Global Moderator
  • *****
  • Posts: 14766
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Basic Oscilloscope Safety
« Reply #36 on: August 31, 2016, 06:45:56 am »
QFT ??
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 11407
  • Country: gb
    • Having fun doing more, with less
Re: Basic Oscilloscope Safety
« Reply #37 on: August 31, 2016, 08:57:02 am »
I was "hands on" instructed on how to do these tricks by a vastly experienced EE but I will never pass that info on for fear of those attempting such practices to only do so ONCE.  :scared:

Just so.

The "hands on" aspect is absolutely vital: you have to be able to show and watch a person, because they will innocently do things in a different way or do something you forgot to tell them they shouldn't. Hence I won't tell people what they should do, but I'll give indications of some of the things they shouldn't do and then refer them to standard literature.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: tautech

Offline Brumby

  • Supporter
  • ****
  • Posts: 9952
  • Country: au
Re: Basic Oscilloscope Safety
« Reply #38 on: August 31, 2016, 08:57:27 am »
QFT

Quoted For Truth. Generaly used on internet forumafter quoting someone to make sure they cannot go back and change what they've already posted. Sometimes used to express you agree with the opinion.


http://www.urbandictionary.com/define.php?term=QFT
 

Online Ice-Tea

  • Super Contributor
  • ***
  • Posts: 1797
  • Country: be
    • Freelance Hardware Engineer
Re: Basic Oscilloscope Safety
« Reply #39 on: August 31, 2016, 12:01:29 pm »
Sometimes used to express you agree with the opinion.

That part ;)


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf