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I am working with an instrument which has a supply of 230V AC or 400V DC. AC source is isolated at the input via 1:1 transformer and DC source is a floating supply.

I have a Yokogowa DL1620 oscilloscope. The power cord of scope does not have an earth pin. So my question is: I know unit under test shall be isolated always as I done however, can I work on an isolated unit with unearthed scope?

Also if scope is earthed then "ground" of unit under test will be at 0V. Is it desirable?

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  • \$\begingroup\$ Is what desirable? \$\endgroup\$ – Andy aka Aug 6 '14 at 9:54
  • \$\begingroup\$ Somewhat confusing question but do you mean - "What are the problems associated with isolation transformers and earthed vs non-earthed scopes"? \$\endgroup\$ – carveone Aug 6 '14 at 10:23
  • \$\begingroup\$ The answer at electronics.stackexchange.com/a/73994/11861 is very explicit! DO NOT DO THAT. It is very dangerous having everything floating. Replace the defective plug on the Scope. You will also need to have at least one point establish as a ground for when your scope is not plugged it. \$\endgroup\$ – placeholder Aug 6 '14 at 14:38
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I think we can sum up your question as:

Can I work on an transformer isolated "unit under test" with an unearthed scope? Is it desirable to earth the scope given that the ground of the unit under test (when the scope return is connected) will then be the that of the scope?

Rather than answer that myself I'll redirect to great previous answers from others which will help you more than I can!

Why do we need an isolation transformer to connect an oscilloscope?

Isolation transformer and scope when troubleshooting SMPS

How is using a transformer for isolation safer than directly connecting to the power grid?

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  • \$\begingroup\$ Hi carveone .. thank you.... you got it right but i already gone through above links before asking this question.I din not get any firm explanation. Is it OK to work with isolated unit under test with an unearthed Digital storage scope? \$\endgroup\$ – chatty85 Aug 6 '14 at 11:55
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    \$\begingroup\$ chatty85 - It seems to me that those answers (particularly the first) are absolutely explicit and have concrete explanations attached. To repeat placeholders comment above - DO NOT DO THAT unless you want to fry yourself when touching your scope (which you will invariably do without thought at some point) since it will float up to a high potential. Reread the answers pointed to by @carveone. If you do not understand them, than you had better put some time into figuring out why you don't because you are exposing yourself to a lethal danger. \$\endgroup\$ – Jon Custer Aug 6 '14 at 16:28
  • \$\begingroup\$ Jon Custer- OK. But the neutral of mains is also connected to protective earth. Does that mean my scope is also earthed? I have checked voltage between neutral and earth it is around 3 V rms. \$\endgroup\$ – chatty85 Aug 6 '14 at 16:56
  • \$\begingroup\$ @chatty85 You sure about that? If mains neutral is just as good as protective earth, why don't we just skip the earth connection and cut our copper costs by 1/3? Hint: faults happen. \$\endgroup\$ – Phil Frost Dec 5 '14 at 3:02
  • \$\begingroup\$ Had a similar issue recently. Would be glad if any of you can comment electronics.stackexchange.com/questions/236087/… \$\endgroup\$ – user16307 May 27 '16 at 10:34
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I wish to add some links that could be useful for better understandement.

  1. Tektronix: Fundamentals of Floating Measurements and Isolated Input Oscilloscopes
  2. Tektronix: Floating Oscilloscope Measurements... And Operator Protection
  3. Oscilloscope techniques and cheater plug, from "Analog Circuit Design, Volume 2"

In summary: use an unearthed oscilloscope can be dangerous for the health of the operator and detrimental to the long term health of the oscilloscope. But it is a common practice. Not a good practice, but very common, if you really know what you do.

We need to emphasize: do it at your own risk!

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