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Earth/Ground leakage detectors

Discussion in 'Question of the Day' started by Mayhem, Feb 20, 2006.

  1. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Let’s say that you are working on a piece of equipment and you accidentally forget to turn of the power coming into the unit.

    Not to worry though, you were smart enough to plug the unit into a GFCI / RCD (depending on where you live) as we all know that we should test equipment on a protected circuit - but that is a different post!

    The unit is in an ABS plastic box and as you are poking around, you place one hand on the unit to stabilize it. What you don’t know (yet) is that your hand is touching the neutral lead coming into the unit. A few moments later, you touch an active connection with your other hand.

    Given that you are touching both an active and neutral connection on a live piece of equipment that is in a plastic enclosure (i.e., neither you, nor the active is touching the unit ground), what happens next?
     
    Last edited: Feb 22, 2006
  2. mbenonis

    mbenonis Wireless Guy Administrator

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    Well, in theory, the GFI will detect the very very very small current flowing through you and trip, thus preventing a life-threatening current from flowing through you. This assumes that the hot wire is touching you only, and not the shell of the unit. However, I would not want to test this myself.
     
    Last edited by a moderator: Feb 22, 2006
  3. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Yes - the hot wire is touching you and you are also touching the neutral. I am also assuming that you are not actually touching the chassis of the unit. Something that I should have spelled out more clearly in this scenario, as it has a significant bearing on the outcome. I’ll add that in to the original post, in case anyone else answers it without reading this.
     
  4. mbenonis

    mbenonis Wireless Guy Administrator

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    So, was my answer correct?
     
  5. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    I'm not going to give it away with only one person having a go at answering the question. Correct or incorrect in these questions doesn't matter. What matters is that you have a go (a tick there for you).

    The aim is that we all learn something through the process and discussion and I challenge those who sit back and read but are worried that they will be laughed at to throw caution to the wind and have a go.

    Two partially correct answers or even incorrect answers are better than none at all. Those of us how post such questions do so to highlight certain points or create discussion around a topic.

    They are learning exercise not exams. There are no black marks for wrong answers. In fact, you will earn more respect through responding with a wrong answer than sitting quiet with the correct one.

    Also – (as I said before) we are not going to give up the answers without some discussion.

    Mike – the bulk of this post is not aimed at you, as you stood up and put forth your answer. Just didn’t see the point of posting a separate one.

    BTW – this is a question to be looked at and pondered in a theoretical sense. Please do not try it as a practical experiment!
     
  6. Diarmuid

    Diarmuid Active Member

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    Ok, according to a very bad knowledge of electronics, RCD's only trip, when they detect a leakage to the earth. If you are touching both the positive and negative wires, then surely the current wouldn't go to the earth, as it would not be joined to that.
    I think that it would basically go straight through the person, and if the current was strong enough, fry them.

    Also, on a side note, is there an easy way to get to the question of the day forum, because I can't find any links anywhere on the site, I only got here by following the link in another thread.
     
  7. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Well, if you are touching both the hot and the neutral, you will be seen by the supply as being another load on the circuit. That is assuming that the path between the two points where you are contacting the circuit have a lower resistance between them than the hot wire and the ground through you. If the resistance to ground is less, you will act as a path to ground, get a shock, cause an imbalance in neutral / hot currents and trip the RCD. If however, that path is of higher resistance, the flow to ground will not occur as readily and you will still get a shock between the points of contact but since the current is going out the hot and back in the neutral, there is not imbalance and so the RCD will not trip. If that resistance is low enough and enough current flows through your body, a fuse or circuit breaker should eventually cut the power, but the chances of damage to your body are high.

    So it comes back to not relying entirely on a safety device. If you want to work on something, disconnect it and verify (with a meter or the like) that there is no voltage across it. Safety devices are designed to protect for a particular fault and an RCD is not likely to be effective here.
     
  8. MHSTech

    MHSTech Active Member

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    Nothing noticable will happen.
     
  9. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Diarmuid and Chris15 get the points for this one!

    Well done – a RCD or GFCI monitors the current flowing out of the active and what is returning down the neutral. If there is a discrepancy, it assumes that some current is flowing to ground. Now, as you could be the ground, it will disconnect the power and save you from becoming crispy.

    Neither of these devices will protect against overloads, so a direct short across the active and neutral will not trip the RCD or GFCI as it simply looks for a difference, not the level.

    Therefore, if you were unfortunate enough to touch the active and neutral without touching the ground you would have a very bad day.

    It is important to know that despite having protection on a circuit, there is no guarantee that you will be protected in every situation and must therefore be very careful in what you do.

    MHSTech – I hope that you were referring to the RCD/GFCI when you said “Nothing noticable will happen”
     
  10. MircleWorker

    MircleWorker Member

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    Great Question I hope this is right, If you are touching the common (Black, US) and the neutral (White, US) and touching the plastic fixture, you would get shocked. But, Only in your hand since that is the direct path of the electric flow is in your hand, You might get burned. The GFCI only would trip the circuit off if it detects current back through the earth ground. I don't think this would be a fatal shock since the electrical path is not across the heart. Thats my final answer!
     
  11. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Unless you have the active / live / hot / whatever it is in your part of the world in one hand and the neutral in the other, in which case it could quite easily cause a fatal shock across the heart.
     
  12. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Whether the shock would be fatal or not is not the question here and lets assume that all shocks are potentially fatal and should be avoided wherever possible.

    What is important here is to see that there are situations where earth/ground leakage detectors will not shut off the circuit.

    In a situation where you are in contact with both the active/hot and the neutral/common BUT NOT in contact with the earth/ground, the RCD/CGFI will let you fry because it doesn’t see any difference in the current travelling out thought the circuit and returning from the circuit. In this scenario, it assumes that all is well.

    It is at this point that you hope that the circuit breaker trips the circuit. However, because it may take time for the breaker to trip (or a fuse to blow) you may have suffered a fatal shock during that time. This is one of the reasons that we use RCD/CGFI.

    So – the aim of this question was make you aware that under certain circumstances, protection devices may NOT save your life. Therefore, we should not rely upon them and take every precaution to ensure that we make every effort to make our working environment safe and not simply rely upon the protection.
     
  13. Chris15

    Chris15 CBMod CB Mods Premium Member

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    I guess it comes down to protection devices are good insurance but you should assume that they will fail and thus take necessary precautions. It is better to assume the worst and end up alive than to expect a safety device to save you. Don't put all your eggs in one basket.
     
  14. MircleWorker

    MircleWorker Member

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    I agree with that. still a good discussion. great to get people discussing situations dealing with safety.
     
    Last edited: Mar 21, 2006

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