Scope on floating ground???

Discussion in 'General Electronics Chat' started by Man_in_UK, Feb 11, 2014.

  1. Man_in_UK

    Thread Starter AAC Fanatic!

    May 13, 2008
    Am I right in thinking that I can use a digital scope on a floating ground, as long I do not ground the scope via its power cable?

    It an Agiliant MSO2022A.

    Thanks in advance.
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    Then you say floating ground. Do you mean something like battery powered equipment, or other equipment that is galvanic isolated from the mains. Say by using an isolated transformer.
  3. MrChips


    Oct 2, 2009
    "Floating Ground" is an oxymoron but we know what you mean, i.e. you are attempting to disconnect the scope's common from the AC mains grounding pin.

    To do this safely depends on how the power supply of the scope has been designed.
    The proper way to do this is with the use of an isolation transformer.

    One has to question the reasons for wanting to defeat the ground of the oscilloscope. The most common reason is when one wants to monitor the potential drop across a component especially on the high side of a power supply, for example.

    Rather than defeat the ground of the scope there are two preferred methods of making a differential voltage measurement.

    1) Isolate the device under test using an isolation transformer.

    2) Use the dual channel capability of the scope and measure the voltage difference of the two channels by using the MATH CH1-CH2 function.

    Another way of making a floating measurement is with the use of a portable scope operating on internal batteries. I keep a Fluke 123 Scopemeter handy for this purpose.

    Simply disconnecting the GROUND pin of the power cord is NOT recommended.
  4. coolreed

    New Member

    Jun 29, 2012
    I am an old timer so I think you are asking about using a Scope probe to measure inside a circuit. Some of the older scopes have the chassis grounded so when you are using a three pronged AC plug to power your scope and insert a probe into a circuit to take a measurement and use the probe ground you can be placing a ground in the middle of the circuit and short out the circuit and cause damage. Us old timers would always use a three pronged to two prong adapter to power the scope. That way the scope chassis was isolated from ground and therefore when you used the scope probe and probe ground it would not short out the circuit.

    Many of the new scopes do not have this issue. But how do you know? Are you willing to take the chance? You must be familiar with your test equipment.

    I hope this is what you were referring to.

    Be safe.
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    a good reason for using a floating ground (ungrounded scope) even if it is a bit unsafe,-- the 1352 allen bradley vfd uses -600 vdc for its ground plane on the circuit board. several people have been suprised by this. I dont know of any scopes that were designed for troubleshooting at this voltage safely. I have run into a few others with rather high voltages on what seems to be the ground plane on circuits. the primary side of smps is at line potential, and anything connected to ground will not read properly.
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Do tht and you might complete the ground when you touch the scope.

    And .....Pooof ! u dunno what just happened.
  7. Man_in_UK

    Thread Starter AAC Fanatic!

    May 13, 2008
    Thanks all for replying. It seems that we are all talking about the same thing (in a way)
    The circuit in question has ground at -90VDC referenced to true ground. I will be safe!
  8. t06afre

    AAC Fanatic!

    May 11, 2009
    THE_RB likes this.
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    What is a cow with no legs called?

    Ground beef.

    A cow with two legs?

    Lean beef
  10. Austin Clark

    Active Member

    Dec 28, 2011
    Could the OP not also use an isolation transformer on his oscilloscope itself? That seems more preferable to me, because you wouldn't need an isolation transformer that supplies a ton of power, and you don't worry about inaccurate amounts of voltage drop and such.
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The preferred solution is to use two scope probes, invert one and add them together. My old school analog scope could do that, I assume the newer digital imitators can do the same function.

    Power ground be damned!
  12. tindel

    Well-Known Member

    Sep 16, 2012

    This is a great question - one I don't know the answer to. I know not to float a scope - primarily for the death factor, but what about putting the scope on a isolation transformer? I've done both at work a few times. I'm getting to the point where I start to yell at anyone I see floating a scope. I've also got an isolation transformer that pulls the earth ground through, I'm not sure how that helps or hurts the situation though.

    Also are isolation transformers all that much safer? I figure if you get electrocuted... you still have the power from the main providing magnetic flux to the transformer and will continue getting electrocuted.