Unearthed CRO

[Electrix]

It is a common practice in Power Electronics to use a PowerScope for observing waveforms. It is common to use a CRO whose Earth Pin has been removed.
What are the reasons behind using such an unearthed CRO ?

 

[Nigel Goodwin]

It is a common practice in Power Electronics to use a PowerScope for observing waveforms. It is common to use a CRO whose Earth Pin has been removed.
What are the reasons behind using such an unearthed CRO ?

I suggest you search back through the forums!, this has been a hotly debated subject on a number of occasions :lol:

 

[stevez]

This may not answer your question but on older equipment it was possible for the chassis, ground or earth to be different than the test equipment earth or ground. If the wiring were reversed you could have line voltage potential between the earth/ground of the equipment and the earth/ground of the equipment being tested.

 

[Electrix]

It is a common practice in Power Electronics to use a PowerScope for observing waveforms. It is common to use a CRO whose Earth Pin has been removed.
What are the reasons behind using such an unearthed CRO ?

I suggest you search back through the forums!, this has been a hotly debated subject on a number of occasions :lol:

Hmm...had a good look at the previous threads, seems to be alternating cycle of good/not good.
Please excuse my lack of experience in dealing with high voltage circuits, but why would anyone want to keep a CRO Body at floating potential ?

PS: I interested in knowing Why should it be unearthed and not whether it should/should not.

 

[Nigel Goodwin]

As I mentioned, this has been argued often :lol:

Personally I think having a large earthed metal object is very hazardous, and having an earthed scope prevents you working on live equipment (for example the primary of a SMPS).

Imagine! - earthed crocodile clip from scope in one hand, reach across to clip it to your circuit, holding the circuit with your other hand (not an uncommon thing to do?). The circuit though is live, you've now got earth in one hand, live mains in the other - direct path across your heart!.

The other side of the discussion consists of feeding the live equipment through a mains isolation transformer, and having EVERYTHING earthed.

So basically two choices:

1) Nothing earthed.
2) Everything earthed.

Both methods are safe in some respects, and dangerous in others, but in BOTH cases you MUST understand exactly what you are doing any WHY.

 

[Dean Huster]

Dang! We Yanks may win you over yet, Nigel! :lol:

Nigel is quite correct about there being pros and cons with either philosophy. A search for those "hot" topics would be very educational.

Just remember that in those threads, Nigel's wrong. :lol:

Dean

 

[Nigel Goodwin]

Just remember that in those threads, Nigel's wrong. :lol:

Those who can - use un-earthed scopes.

Those who can't - use earthed scopes. :lol:

Rather a play on the old rhyme about teachers, but quite accurate as well!, in the 'real world' there are probably more un-earthed scopes than earthed ones!.

 

[Electrix]

Just remember that in those threads, Nigel's wrong. :lol:

Those who can - use un-earthed scopes.

Those who can't - use earthed scopes. :lol:

Rather a play on the old rhyme about teachers, but quite accurate as well!, in the 'real world' there are probably more un-earthed scopes than earthed ones!.

I think you are right, I checked up my power lab in college today, 90% of all the CROs were unearthed type and I check up the rest, the lab assistant says that during all Power Elect Expts, it is mandatory to remove the earthing pin.