Products > Test Equipment
Floating Scopes
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alm:

--- Quote from: David Hess on June 26, 2017, 10:09:48 am ---Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of "low voltage".

--- End quote ---
Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.


--- Quote from: David Hess on June 26, 2017, 10:09:48 am ---(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

--- End quote ---
A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.
David Hess:

--- Quote from: alm on June 26, 2017, 12:25:30 pm ---
--- Quote from: David Hess on June 26, 2017, 10:09:48 am ---Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of "low voltage".
--- End quote ---

Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.
--- End quote ---

Right about the time multimeters were practically required to use shielded banana jacks, the oscilloscope probe manufacturers were required to downgrade the voltage rating of their miniature and subminiature probe tips because of clearance and creepage requirements.


--- Quote ---
--- Quote from: David Hess on June 26, 2017, 10:09:48 am ---(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.
--- End quote ---

A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.
--- End quote ---

That is definitely a problem but the same applications which will have problems dealing with high common mode capacitance are usually unbalanced enough to degrade the common mode rejection of a differential probe.
mtdoc:

--- Quote from: nctnico on June 26, 2017, 08:40:25 am ---
--- Quote from: mtdoc on June 25, 2017, 10:44:46 pm ---There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

--- End quote ---
The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum.

--- End quote ---

But that is not what is occurring. The consensus in this thread and all the others I've seen on this forum regarding floating scopes is "don't do it" and by explaining why it is unsafe you are providing those who are skeptical of "proclamations from authority" the understanding needed to prevent them from doing it.  In addition, the ancillary discussions that arise from this topic are educational in their own right.

In the end, there is no cure for stupid. Those who are going to ignore all advice regardless - will do it - no matter what is discussed or not discussed on this forum.  But I've yet to see anything on this forum that would make that more likely - quite the contrary - I've seen lots of good discussion that should convince anyone why it is unsafe.   The reality is that there are also many people who need to understand why a practice is unsafe in order to fully internalize the advice.  Paternalistic and overly rote recitation of safety rules without context may work in some environments but IMO not on an electronics discussion forum where many people come to learn. Hopefully the excellent discussion that has ensued in this thread has caused the OP to now fully justify and internalize the advice not to float a scope.
colorado.rob:
The world can always use more Darwin Award winners to serve as examples for others on what not to do.  :popcorn:
serggio:
Wow, wow...  :clap:
How many words in this thread...
TN-C, TN-S, TN-C-S, IT, TT and other mains distribution schemes...
TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

All that need to know, that in 95% scopes (except of isolated channel scopes and portable scopes) ground conductor at your passive probe always connected to all open metal parts of your scope (chassis, USB shield, neighbor's BNCs and etc.)
End of tip on your passive probe have open metal part - ground connector, at least.

Always need remember about it when you try to connect your probe to high voltage source.
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