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How do Antistatic Wrist Straps work?

  1. Apr 10, 2015 #1
    My understanding is that wearing a wrist strap will help reduce sparking between the conductors on the electronic equipment (assuming they are connected to the chassis that the wrist strap is also connected to) and the person. This is because there is a low resistance conductive path (not through the air) between the conductors and the person. However, a wrist strap won't necessarily help reduce sparking between the non-conductors (which have high resistance) on the electronic equipment and the person.

    Most of the material around electronic equipment is non-conductive, so why are antistatic wrist straps even used? How is static prevented between the non-conductive material and the person?
     
    Last edited: Apr 10, 2015
  2. jcsd
  3. Apr 10, 2015 #2
    A few thoughts:

    Yes, when you connect the human body to the chassis & the conducting elements, the "packaging" of the components could still be at lots of kV's. But: (1) the charge will leak away and the conduction path is shorter than the thickness of your shoes (I hope!) - I've no real idea of the time scale here; (2) the components are much smaller than a body and so are likely to carry a smaller charge and produce a smaller current when discharged; (3) the discharge current will not flow though anything sensitive (even your finger won't feel it!).
     
  4. Apr 10, 2015 #3

    davenn

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    incorrect idea of their use

    You both misunderstand the use of antistatic wrist straps

    They are for discharging YOU before YOU touch static sensitive circuits and components. Your body can build up very high static voltages by a number of ways
    ... walking across the carpet, peeling sticky tape off a dispenser etc
    Putting on your antistatic strap when you get to your workbench discharges any high voltages via the strap and curly connecting cord to a earth grounded point
    on the work bench. The strap and curly cord resistance would normally be several MegOhms. This is good for grounding the EHT static but wont cause you to get a mains electric shock via the strap and cord. It would be standard to also have a antistatic mat on the bench also connected to this ground point

    The mat on your workbench top has already grounded the equip you are working on, so there is no issue of discharges between different device chassis

    cheers
    Dave
     
  5. Apr 10, 2015 #4
    OK this makes more sense.

    Again, this will only ground the conductors but not the non-conductors? This only works if the person is at a high floating potential (due to walking etc.) and not the equipment?
     
  6. Apr 10, 2015 #5

    davenn

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    did you read the last line in my previous post ?

    here it is again .....

     
  7. Apr 11, 2015 #6
    Yes, and you don't understand what I am asking. I know there is no issue of discharges between the chassis of each device. I am asking about the material which is not the chassis, which will not discharge through the mat easily because it is not conductive. If this material is at a high floating potential, the charges will not leak to ground (large time constant). Simply putting a device onto a mat will not ground the entire device.
     
  8. Apr 11, 2015 #7
    No insulator is perfect. Even the "non-conductive" materials in the device will conduct. They have very high resistance, so they will conduct current very slowly. However, since the conductive parts of the device, the mat, the bench, and the technician are all grounded, the amount of material which may develop a charge is pretty small. This means that the amount of charge that could potentially build up is small, and will usually have no trouble draining through the "non-conductive" materials.

    Is there still a possibility of damaging electrical discharge? Probably. No system is perfect. However, the grounding mat and strap system is pretty good and reduces the risk to a very low, acceptable level.
     
  9. Apr 12, 2015 #8

    davenn

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    Thanks for answering in my absence mrspeedybob :smile:
    spot on

    it will ground everything directly and indirectly touching the mat
     
  10. Apr 14, 2015 #9
    If this were true, humans would also be considered grounded because they're indirectly touching the ground through their shoes. The time to discharge might still be impractically long. Are circuit boards made from materials with this in consideration (to shorten the discharge time)? But ok, maybe this is all that can be done.
     
  11. Apr 14, 2015 #10

    davenn

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    and the hilited bit is the problem and is the reason why we use mats and wrist straps as the discharge, for all practical purposes, is instantaneous

    Dave
     
  12. Apr 14, 2015 #11

    rcgldr

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    Anti static bags are used to deal with that situation. The components are kept in the anti static bags which are conductive, which allows the components to get discharged over a longer period of time. The anti static bags with components should also be placed on the conductive mat for a while before removing the components and working with them. Wiki link:

    http://en.wikipedia.org/wiki/Antistatic_bag

    In addition to the bags mentioned in the wiki article, there are also pink plastic anti-static bags. I've seen these used at work stations, but not for shipping products.
     
    Last edited: Apr 14, 2015
  13. Apr 15, 2015 #12

    Drakkith

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    Unfortunately the subject isn't as simple as this. There are multiple ways a person walking around can accrue a charge, despite the fact that shoes aren't perfect insulators. One other thing to keep in mind is that an electrostatic mat is connected to a ground, which is in turn connected to the Earth. This allows for a path for any built up charge to drain from an object directly to the Earth. Walking around on a floor may or may not help in discharging any charge built up on a person. The floor itself is rarely a good conductor and is also subject to various effects that can cause a buildup of charge as well. (Like people walking on it)
     
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