Hot chassis & scope grounding

Discussion in 'Homework Help' started by Steve1992, Oct 23, 2010.

  1. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
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    With a hot chassis, always plug the hot chassis item into an isolation transformer. Then you are free to connect an earthed oscilloscope, probe gnd. clip to the chassis without fear of destroying something.

    Is this statement correct?
    I think it is because the isolation transformer gives you galvanic isolation?
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Correct.

    Ken
     
  3. Georacer

    Moderator

    Nov 25, 2009
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    Can someone explain the above problem?

    I understand that hot chassis refers to the metallic case of a device that doesn't ground its case, right?

    Why would that be a problem to the oscilloscope?
    How does the transformer help?
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The ground lead of an oscilliscope is 'ground'. Earth ground. By connection through the power supply of the O'scope the probe is connected with the green wire of the power outlet in your house wall.

    By lifting the ground from the power input, (Running the O'scope from an isolation transformer), you can use the probe in circuits that are not grounded at either extreme of their power supply.

    Make the mistake sometime and you will get a much more insistent form of education. :) Stuff will explode(capacitors) and start smoking (resistors and transistors). Makes for a colorful and exciting education.

    I recommend having the window open and some fans on if you want to have this thrilling educational experience.

    My personal favorite from years past was the discovery that no amount of rubber soul shoes and rubber mats will keep you from shocking the $&(@ out of yourself if you have both hands "in the circuit".

    Highly recommend
     
  5. Georacer

    Moderator

    Nov 25, 2009
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    Electrocution isn't a stranger. Once I almost burnt a hole in my finger with an electric arc.

    But this is useful information and I 'd like to clarify it.
    So, we put a 1:1 transformer between the mains and the O-Scope, and then we can raise or lower the level of the measured ground as much as we want?
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    YES

    We have a downhole tool that transmitts pressure and position data. Haven't sold any in a while but thats another story. The circuit uses 110V and -250V DC, and even a spot with -750V DC is present. O'scope is run from twenty pound fully encased transformer and the scope itself sits on a cart that has rubber wheels. It's 'isolated'.

    The tool sits on rubber covered wooden two by fours that have been cut to fit it.

    Even after complete inspection and wiring verification, the first time we hit that puppy with high voltage is ALWAYS a puckering experience ;)

    Just to clear up the picture. the transformer is not connected to earth ground on the primary or the secondary, the transformer is electrically isolated from its metal shell outer case, which IS connected to earth ground. The inner metal E-core of the transformer is NOT connected to earth. The scope has a three prong to 2 prong adapter so it can plug in to the two prong outlet on the isolation transformer.
     
  7. someonesdad

    Senior Member

    Jul 7, 2009
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    Back in the 1970's, a guy I occasionally see while shaving wanted to measure an AC current in a line voltage circuit with a shunt. He naively hooked everything up and proceeded to connect the scope probe on one side of the shunt and the probe's ground lead to the other side of the shunt. He received Kermit2's insistent form of education. :p It's a lesson you never forget.

    While the isolation transformer can isolate the scope, realize that if you connect a scope probe's "ground" wire to a voltage above earth ground, the scope's chassis and metal parts on the scope will be at this potential. If you are grounded and you touch the scope's metal parts, you could be in for a shock. While I've made (high side) AC current measurements this way using a shunt and isolation transformer, it can still be dangerous! If you have to do such things, it's much better to use a properly-isolated differential amplifier to make the measurements. Even better, use a Hall-effect probe or current transformer (make sure you know how to use a current transformer safely though).

    If you fool around with electrical measurements, you should get yourself an isolation transformer. I found a 1 kW medical grade one on ebay for $60 a number of years ago, so you don't have to spend a horrendous amount of money. Such things shouldn't give you a feeling of invulnerability either -- they just remove one potential (no pun) hazard.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
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    Not exactly.

    Hot chassis refer to the case where the metal casing of an appliance(e.g. TV from the 60's and 70's) is actually connected to the LIVE connection of the mains AC supply.

    It happens because the power cable of these TVs has only two wires and there is no ground connection required. They have no transformer inside for power supply but just simple rectifier and capacitor for HV.

    Depends on how the user plugging the power plug into the mains, the LIVE wire could be connected to the metal chassis, with half the chance.

    Of course, the manufacturer had made sure that there will be no metal parts that can be touched by user externally, except the TV aerial connector, where they have use series 2KV rated capacitors to prevent any electrical shock.

    Now what happens when you want to test some signal on the TV circuit? As the CRO is usually earthed, one can imagine the result of connecting the grounded clip of the probe to the appliance chassis, which is LIVE.
     
  9. Georacer

    Moderator

    Nov 25, 2009
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    Thanks for all the replies guys! That was useful information.
     
  10. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
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    Thanks all.

    But, just to clarify: that if the hot chassis device was isolated (through an isolating transformer) there would be no need to also isolate the scope - I thought it was bad practice to isolate an oscilloscope from its mains input.
     
  11. eblc1388

    Senior Member

    Nov 28, 2008
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    Correct.

    Extremely bad.

    Doing so would result in electrocution as now there is a chance that the metal casing of the scope becomes HOT chassis in itself, through the connection of the incoming signal common to a live point inside the test circuit.
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A male banana plug terminal will fit a ground terminal hole (US style) in most 110V outlets. Get in the habit of putting your volt meter between that and the chassis of the equipment you are working on. A surprising number of chassis's are floating at the level of the neutral AC line; supposedly hooked to ground at the supply somewhere, but it sometimes has a considerable amount of 'juice' on it. While you won't make a big impression like you would hooking up to a 'Hot' side chassis. If the Neutral line is carrying considerable voltage at that point in the supply circuit(10-60VAC), you will cause lots more current to start flowing in ground referenced circuits and a slowmotion burn out of resistors and transistors can happen. STuff just starts smoking after you hook up and start probing around, usually right under your nose.

    Do you smell something burning? :)
     
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