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Why does grounding protect you from electricity?

  1. Sep 18, 2011 #1
    Let's saying you're working with something that has electricity running through it. If you ground yourself by touch something metal, aren't you making it easier for charge to flow through you through the metal, so a greater charge would flow through you? And if you didn't touch something metal, charge would go through more of your body but there would be less of it?
     
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  3. Sep 18, 2011 #2

    sophiecentaur

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    If you are a bird, sitting on a power line, you don't get a shock. (Lots of volts but no current can flow)
    If you are on the ground (at ground potential) and you touch a live wire, with a potential very different from ground potential. (Lots of volts and lots of available current).

    So what's this Earthing business? IF you connect all exposed metal parts of electrical equipment to Earth then they will all be at Earth potential. That saves you from ever getting a shock because nothing you touch can be at a high voltage. What's the catch? None. Only inside the equipment any fault which directly connects a live wire, internally, to the case will cause lots of current to flow and a fuse will blow. Other faults can cause a small current to flow from the live wire (Line) to Earth. These may not interfere with the operation of the equipment at all (there are many houses and electrical devices that are wrongly connected but no shocks have been delivered) but could cause a shock if the Earth connection were, somehow removed. It is possible to detect this Earth current, however, and to turn of the power (Power Breaker / Residual Current Breaker etc. etc are the names)

    By the way, you don't need "electricity running through" something to get a shock. All you need, for a lethal shock, is high enough volts there and a few tens of milliAmps to be available to flow through you. There is a difference between Voltage and Current.
     
  4. Sep 18, 2011 #3

    russ_watters

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    You don't ground yourself, you ground the electrical device you are working on.
     
  5. Sep 18, 2011 #4

    Drakkith

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    To expand on Russ's statement: You ground the equipment so YOU don't become the ground in case of a fault!
     
  6. Sep 18, 2011 #5
    But I recently saw someone attach a wristband to their hand which was connected to a conducting pad which was connected to ground. After doing this he said it was safe to work with an electronic component sitting on the pad. So if only the electronic component needed to be grounded, he could have left it on the conducting pad and not attached the wristband.
     
  7. Sep 18, 2011 #6

    sophiecentaur

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    That would be to protect some equipment (chip) from a damaging charge which a person's body could build up. Common practice in electronics assembly.
     
  8. Sep 18, 2011 #7

    Drakkith

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    We have to do this at work to avoid static electricity building up in our bodies and damaging the component through a shock. You can easily damage an electronic component as the threshold for feeling a static discharge from your hand is much higher than the amount of static that can harm the component, meaning that you can shock it and never feel the shock.
     
  9. Sep 18, 2011 #8

    rcgldr

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    As mentioned people ground themselves to avoid static with working with sensitive components to protect the components.

    When working with high voltage lines from a helicopter, the person wears a Faraday cage suit to protect the person. With the Faraday cage suit, any electricity flows around the person instead of through the person. During transitions, the person first uses a probe and then a cable to connect the helicopter to the high voltage line bring the helicopter and worker up to the same potential as the high voltage line to protect the person and the helicopter. The person also uses the probe as the helicopter flies away, probably to control where any sparking would occur as the helicopter leaves. Youtube video:

     
    Last edited by a moderator: Sep 25, 2014
  10. Sep 18, 2011 #9

    russ_watters

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    You're mixing together two separate things there:

    1. An anti-static wrist-band (as said) is there to protect the device being worked on, not the person. It provides a path for built-up static electricity on the person to be discharged to something else besides the electronics being worked on.

    2. A Faraday cage around a person working on a high voltage line provides a path for electricity to flow through instead of flowing through the person. It protects the person, not the power line.
     
    Last edited by a moderator: Sep 25, 2014
  11. Sep 18, 2011 #10

    rcgldr

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    I thought I was explaining two different situations, the first to protect equipment, the second to protect the person. I editted my previous post, hopefully it's clearer now.
     
  12. Sep 18, 2011 #11
    Why the narrow view of earthing?

    Most of the metal that is earthed is not ever intended to be part of electrical apparatus or an electric circuit.
     
  13. Sep 18, 2011 #12
    An important point about the anti-static-wrist-band: It does not provide a direct connection to ground. There is a series connected resistor, usually 1meg ohm, between the wrist band and the ground connection. This provides enough current to pass for discharging static electricity but not enough to be a safety hazard for the person wearing it.
     
  14. Sep 18, 2011 #13

    rcgldr

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    Back to the original post ...

    As mentioned above, you don't want to be grounded when touching something that has electricity running through it. In the example with the helicopter and high voltage power lines, the idea was the opposite, to isolate the person and helicopter from any actual earth ground (the helicopter is hovering in the air, which is a good insulator), and instead to end up with the same potential (voltage) as the power line by using a probe and then connecting a cable to the power line, while wearing a Faraday cage suit so that any electicity encountered flows through the suit and not the person.

    In some appliances and electrical outlets in a household, there are components that detect that the current from the hot termianl equals the current to the neutral or opposite terminal. If the currents aren't equal, it's because some of that current is leaking to earth ground, possibly through a person, so the components will quickly disconnect the current to prevent a person from being shocked. Wiki article:

    http://en.wikipedia.org/wiki/Residual-current_device

    To protect sensitive components (no electricity flowing through these) from static, a workbench with an anti static mat and wrist band are used. The component is kept on the mat or in the person's hands with the wrist strap (or ankle strap) hands to avoid any static from damaging the component. During transfer, the component is kept in an anti static bag, the equivalent of a Faraday cage suit to protect the component. Both the anti-static mat and wrist (or ankle) band are electrically connected to each other and grounded via a very high resistance so any accidental touching of something with electricity in it won't produce significant current to avoid any electrical shock of the person.
     
    Last edited: Sep 18, 2011
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