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My professor always insists that I provide power to an oscilloscope thorough an isolation transformer. What is the necessity of this? What is the risk if I don't connect it? enter image description here

noufal
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    Related: http://electronics.stackexchange.com/questions/37143/isolation-transformer-and-scope-when-troubleshooting-smps?rq=1 and http://electronics.stackexchange.com/questions/17496/how-is-using-a-transformer-for-isolation-safer-than-just-playing-with-it – Adam Lawrence Jun 25 '13 at 12:51
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    Your professor might want to review their understanding of earthing test gear. It's messy trying to convey this to a professor, I realize, but it's that or the possibility of toasted scope / roasted student. – Anindo Ghosh Jun 25 '13 at 13:03
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    i think they are more cautious about the oscilloscope getting damaged (it costs around $12,000). – noufal Jun 25 '13 at 14:44
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    It's a ridiculous assumption that floating the scope will somehow protect it, when user life is at risk and plenty of ways to kill the scope still exist. Isolated differential probes will keep both the scope AND the users safe. What short-sighted fools. – Adam Lawrence Jun 25 '13 at 14:48
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    I think any scope that is so easily destroyed (beyond blowing a fuse) should cost a good deal less than $12000, or should be thrown in the trash. – Phil Frost Jun 25 '13 at 15:12
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    In a beginning electronics lab, this has to be a common and frustrating problem. Students are virtually certain to clip the ground lead to somewhere live at some point. What can be done? Put the Device Under Test on ground fault, perhaps? – Bobbi Bennett Jun 25 '13 at 17:40
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    They can use isolated differential probes! – Adam Lawrence Jun 25 '13 at 19:11
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    Before the days of interconnected instruments (Analog Scopes etc) I know it was common to float scopes. This is what you are fighting when it comes to the "Old Timers". Back in the day it was easy to float the scope rather than the unit under test. Madmanguruman is so right. I've seen minor stuff go bang because the grounding was not considered. – Spoon Jun 26 '13 at 07:59
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    Any bench setup dealing with mains-powered equipment under test simply must have an isolation transformer for the equipment under test. Lack of an isolation transformer in my opinion is grounds to refuse work due to unsafe conditions, if you are expected to take live measurements (i.e. probe the equipment while it's operating). – Adam Lawrence Jun 26 '13 at 13:04
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    You might want to view this video: EEVblog #279 - How NOT To Blow Up Your Oscilloscope! https://youtu.be/xaELqAo4kkQ – Richard Crowley Jun 14 '16 at 05:01
  • Well, I didn't flag it, but this is hardly an answer, is it? You should move this to a comment. – dim Jun 14 '16 at 06:14
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    @Richard I undeleted your 4 year old answer and made it a comment. The video is still there :-). – Russell McMahon Aug 18 '20 at 07:11

8 Answers8

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You should never float a scope with an isolation transformer! This is reckless and dangerous advice from your professor, and he/she needs a reality check.

The accepted procedure for doing work that requires isolation is to ISOLATE THE UNIT UNDER TEST, NOT THE TEST EQUIPMENT.

Why?

  • It's much easier to remember that the unit under test is what's unsafe and needs cautious handling, not your oscilloscope
  • If you hook a communication cable up to your floating scope (USB, GPIB, RS232), guess what - it's NO LONGER FLOATING. (All of these cables have earth-referenced returns)
  • As soon as you connect that floating scope return to a potential, all of the exposed metal on the scope is now at that potential. Major shock hazard.

If you cannot float the unit under test, use an isolated differential probe to do your measurements, and keep both the UUT and scope earthed. No measurement is worth the safety risk.

A battery-operated scope may seem like a good idea in this circumstance, but only if it has dedicated isolated inputs. A battery-operated ordinary scope with non-isolated inputs will still suffer the problem of the exposed metal floating up to whatever potential you connect the ground to. That's why all of the manuals for the battery operated scopes clearly say "This scope must always be earthed, even if you're running off the battery" - if you choose to ignore this, it's at your own risk. A scope with dedicated isolated inputs should still be earthed as a good practice. It's essentially the equivalent of using external isolated differential probes with an ordinary scope.

I work full-time in power electronics and have tens of thousands of dollars of lab equipment at my bench. If anyone is caught floating their scope, the float is immediately corrected by the test engineering team, the means of float is seized (most often this is a line cord with the ground prong removed) - disciplinary action is a possibility. Numerous senior/principal engineers have fried their PCs and their entire set of GPIB-connected bench instruments by trying to float the test equipment and forgetting about the GPIB interface. (No one has died yet - thankfully)

Adam Lawrence
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    What about the battery operated scopes? – abdullah kahraman Jun 25 '13 at 13:09
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    I'll add this to the body of my answer. – Adam Lawrence Jun 25 '13 at 13:45
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    i think they are more cautious about the oscilloscope getting damaged (it costs around $12,000). – noufal Jun 25 '13 at 14:45
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    @noufal how much is your life worth to you? Your school probably only values you in terms of the additional money they'll get for your completing your degree; I asume your self worth is considerably higher. – Dan Is Fiddling By Firelight Jun 25 '13 at 15:27
  • Many words about things, that engineers must not do, but no words about why... –  Aug 09 '13 at 18:43
  • If one wishes to use two scopes to measure different parts of a circuit which are both safely isolated from earth ground, but are at different potentials relative to each other, how could one accomplish that without floating at least one of the scopes? – supercat Jan 24 '15 at 23:23
  • Isolated differential probes would do the trick – Adam Lawrence Jan 24 '15 at 23:37
  • @AdamLawrence: True, if one has them handy. A lot of shops seem to have a lot more scopes than differential probes, however. For this kind of application, one wouldn't even need an isolated differential probe since the common mode voltage would be relatively low, but I don't know that I've seen differential probes with moderate common-mode voltage specs even though if one didn't need great frequency response and were willing to require a "zeroing" step, such a thing should be easy to produce. – supercat Jan 25 '15 at 19:01
  • @AdamLawrence I understand most part of your answer.i have 2 questions. 1)Let's say I use an isolated scope(insisted by my senior), and I measure a DUT which is floating,and I make sure I don't connect any USB or RS232 with isolation(I use USB Isolator), is there something else I need to worry about? – seetharaman Mar 22 '16 at 17:37
  • Lets say I use a Battery operated Scope, and measure a potential that's at a higher potential than ground, my concern would be that my alligator clip(black probe wire) is connected to a place which is at a higher potential than ground, which would've harmful if I accidentally touch any exposed pins in the scope. In which case it is necessary to use isolated differential probe. Am I right?
    • – seetharaman Mar 22 '16 at 17:38
  • You should pose these as new questions. – Adam Lawrence Mar 22 '16 at 19:17