“Hot Chassis”

Discussion in 'General Electronics Chat' started by kcneoh, Mar 3, 2010.

  1. kcneoh

    Thread Starter New Member

    Oct 10, 2008
    I read the Oscilloscope Manual and come across the word "Hot Chassis" as read below :-

    "Be certain the device to which you connect the scope is transformer operated. Do NOT connect the oscilloscope or any other test equipment to “AC.DC”, “hot chassis”, or “transformerless” devices.​

    What is HOT CHASSIS ?

    Please give some example or case studies .

    Thanks .
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    A hot chassis is not explicitly grounded through the power cord ground. Without an explicit ground, any leakage can result in the chassis taking on a charge from the line.

    In past years, the power cord could be plugged in either way, so it was possible for the chassis of a TV or radio to be at line potential.
  3. kcneoh

    Thread Starter New Member

    Oct 10, 2008
    Hi beenthere ,

    Need to confirm the "line potential" meaning . Is it refer to only + & - but does not has ground ?

    So, any device , or, electronic circuit if never grounded and only supply by + & - potential is considered as "hot chassic" ??

    And for cheap OS , the OS chassis itself is a "hot chassis" because the OS itself never grounded ??

    Please correct me .
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    Line potential means the power at the 'lines' or the wall socket.

    In a transformer, there is a gap between the primary and secondary windings. So there is a PHYSICAL seperation from the "mains" or "lines" and the device.

    In a car, there is a hot-chassis because the chassis itself is used as a "wire" for routing the "-".

    Back when there were only 2 prong plugs in homes, there was no "ground" wire to rid the device from shorts, so any voltage at the mains could be at the chassis itself. In cases where the metal box was used as a ground and not the wall outlets ground plug.
  5. kcneoh

    Thread Starter New Member

    Oct 10, 2008
    Hi ,

    Taking example TV set as a chassis . TV set is connect to wall outlet "A" .
    Now we use Oscilloscope to test TV's PBC . The Osicilloscope ground probe is connect to TV's PCB ground .

    The Oscilloscope is connected to wall outlet"B" .

    If we use Oscilloscope's probe to probe the TV's PCB , would it cause the short circuit ,or, burn etc ???==> Please confirm .

    If it will cause the damage to TV set , how this thing happen ?? I heard about ground loop , anything to do with this ??? Maybe explain the ground loop on this case ???

    I try to link hot chassis , ground loop , short circuit as a whole picture . Please help to explain ...
  6. Mike33

    AAC Fanatic!

    Feb 4, 2005
    To the best of my knowledge, if outlet A and B are PROPERLY WIRED AND GROUNDED to the same point (bonded), and you ground your Oscope to the actual ground of the TV (Zero Volts with respect to both of the outlets' hot lines), you are in no trouble at all. If they are not the same, you can actually inject a current between components in the TV and overload them, that sort of thing.

    The trouble arises if you have a difference in potential between that something called 'ground' for the TV and the ground for the scope. Then the grounds are no longer equal, and have a potential between them - they would be 'floating' with respect to each other.

    To the main issue: "Hot Chassis" is commonly taken as an ungrounded, conductive enclosure becoming HOT, because there is no ground wire, and a wire/component has come into contact with the chassis either by accident or wiring error, causing it to be at at LINE POTENTIAL (=death). If the ground was there, it would shunt the short circuit energy to ground, blowing a circuit breaker/fuse and rendering the chassis un-powered. An unpolarized 2-prong plug connected backwards to an outlet can cause this, as can a polarized plug at times (think, if the outlet was wired backwards....).

    Most electronics books deal with this in some detail (especially the ham radio ARRL books), and are worth reading up on!

    If those concepts are a little unclear, perhaps NOT probing a TV until they are clear might be a good idea? There are some pretty high (lethal) voltages inside...just want to be sure you're not diving in without a clear understanding here! ;o)
  7. someonesdad

    Senior Member

    Jul 7, 2009
    kcneoh, your questions lead me to believe you're new to electronics and want to start using an oscilloscope. You came across some terms in the scope manual that you didn't understand and you wisely asked questions about the material.

    This is good. It's a start to learn more about making electrical measurements. However, there are situations like "hot chassis", "transformerless equipment", and fault conditions that can make working on line-powered equipment lethal. You need to be properly educated about these potential hazards (no pun) before you choose to work on such equipment.

    I recommend you learn about these things from a knowledgeable person or a school program. The proper, safe way to work on such things is not obvious to a beginner, simply because he won't have the background to understand all the things that can go wrong and probably won't have the discipline to do all the things necessary to work on a piece of live equipment. I recommend you stay away from such things until you've demonstrated to an expert that you know what you're doing.

    Alas, I can't point you to a definitive reference because I don't know of one (others on this board might). In fact, I've been working on a document called "AC Safety" for technicians and engineers and it's aimed at helping technical people understand AC safety issues and how to ensure they are doing things safely. I have the attitude that my electrical safety is ultimately my responsibility, regardless of how much I trust my employer or other workers.

    You can do quite a bit of research on the web about these topics, but it will take you a bit of time. Some things to learn about are:

    Hot chassis
    Why grounding is so important to safety
    Ground faults and ground fault interrupter circuits
    Transformerless equipment
    AC leakage and how to measure it
    What levels of leakage currents are considered safe and what levels aren't -- and why
    Hipot testing
    Isolation transformer
    Differential amplifiers for scopes
    High voltage probes

    Let me explain what may happen one day if you choose to ignore the above advice. You'll be probing a circuit board and looking at waveforms on the scope. You decide you need to connect the ground lead of the probe to get a better signal, so you do so. Bang! You get a snap and a spark and a scare. And you're not really sure what happened. Hopefully, like I did (this scenario did happen to me years ago), you'll stop immediately and figure out what went wrong, as you know you just did something stupid -- with a slightly different bit of luck, you could have been electrocuted.
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    I can put in simple terms being in the TV repair field.
    A hot chassis is particularly concerned in TV's, the SMPS uses a neutral ground in older TV's. And this ground is sometimes the TV ground too. The plug used in TV's have two pins, which result in sometimes plugged in reversely making the ground "LIVE" :eek:. in this case the chassis will have a labeled "HOT CHASSIS"
    In Scopes ground is connected to main earth.
    In order to take a reading the user will connect the scope ground to TV ground, now if the TV's chassis is live by any chance, this will create a ground loop causing the earth leakage breaker to cutoff.
    That's why it's always advised to use an isolation transformer with the scope to avoid this accident
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    Another thing to consider. You may decide that working on a circuit LIVE is a bad idea. Good.

    So you decide to unplug it.


    BUT, TVs can contain capacitors that will stay charged with LETHAL voltages and current for months or years after the set is unplugged. If you have no knowledge of how to identify these parts to avoid them, and to realize that the traces or wires on the board can be connected to these capacitors, unintentionally killing you or starting a fire.

    Learning is a very rewarding experience. Death sucks.
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    The older 'live (hot) chassis' sets in the UK sometimes had a DC supply taken from a bridge rectifier connected straight to the incoming AC supply. I suppose it was a cheap and simple way of getting around 350V DC for the valves (tubes).

    It would not matter which way the AC was connected, the internal '0V' would ALWAYS be live.

    Any set too old to have AV inputs on it is best kept fully boxed up or simply scrapped, unless you have very good knowlege of vintage and high-voltage electronics.

    (Anything with external AV inputs must have an isolated supply - but any CRT based TV and most flat panels will still use dangerously high voltages, not counting the AC supply, so they are not a good subject to experiment with as a novice...)