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I use a wrist strap with 1 MOhm resistance connected to a ground Plug Adapter with 1.5 MOhm, so total series resistance is 2.5 MOhm. Is it OK for ESD protection or would it be better to connect the wrist strap directly to ground?

I know that this resistance is to protect the operator. However, is there an upper limit on the value of the resistor so that it doesn't affect ESD protection?

Thanks

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  • \$\begingroup\$ The resistance is not for protecting the operator. It is to protect the ESD sensitive device. \$\endgroup\$ – d3L Jan 17 '16 at 0:42
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    \$\begingroup\$ @d3l: Incorrect. It must be high in order to help protect the operator against shock in case they accidentally contact mains voltage. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 17 '16 at 0:44
  • \$\begingroup\$ @IgnacioVazquez-Abrams Are you talking about the ground plug adapter? I've never seen one with integrated resistance \$\endgroup\$ – d3L Jan 17 '16 at 0:46
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    \$\begingroup\$ @d3l: Contacting mains voltage with the opposite hand could be fatal if there were no resistance to ground. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 17 '16 at 0:52
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    \$\begingroup\$ AFAIK, it's also to do with protecting the device being worked on as well as the person. If you think about it, the person wearing the strap is grounded, so will not have a static charge (hence why it is worn). But the device/pcb/whatever may have a built up charge. Without the resistor if the person touches the device, all of that charge will discharge rapidly though the wristband to ground, in effect giving the device a massive shock (= bad). With the resistor, the charge dissipates slowly (1Meg ~= 1second discharge time) protecting the device from the ESD. \$\endgroup\$ – Tom Carpenter Jan 17 '16 at 3:04
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Either is ok, The idea of an ESD band is to slowly remove charge from your body and equipment you contact. If there was no resistance, there would be two problems:

  1. Danger of electrocution if you touch a high voltage with respect to the ESD strap ground.
  2. You could have rapid voltage shifts causing ESD like scenarios.

If we look at the 8kV Human body model (180pF of capacitance) and discharge it through some large resistors, you see that the charge is gone in milliseconds typically.

enter image description here

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  • \$\begingroup\$ Can you explain the second point, please? \$\endgroup\$ – Whiskeyjack Jan 17 '16 at 8:37
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    \$\begingroup\$ Say if the board had static or stored energy and you had a low resistance to ground, you could create a high current short circuit situation. \$\endgroup\$ – MadHatter Jan 17 '16 at 19:07
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ESD (Electrostatic Discharge) protection prevents the buildup of static charges. Now, an antistatic wrist-strap (otherwise known as an ESD wrist strap or ground bracelet) is essential to safely ground a person in order to prevent the buildup of static electricity.

The higher the resistivity of your antistatic wrist-strap the less susceptible it is to higher voltages. The extra resistance prevents excessive currents passing through your body. A good rule of thumb is about 0.75 MΩ per 250 V.

Furthermore, the higher the resistance of your antistatic wrist-strap the longer it will take to dissipate the static charge without having it "zap" you.

Thus, there is no upper limit as far as I can think of for the resistance of the antistatic wrist-band.

I'm assuming that the resistance you measured is not that of the ground adapter but of the antistatic mat. That resistance is also essential to slow down the rate of static discharge for safety reasons.

There is no fixed upper limit on resistance for the antistatic equipment. It depends on the voltage level that you are testing your device with.

References: How to know that I am grounded with an anti-static wrist strap?

https://en.wikipedia.org/wiki/Antistatic_wrist_strap

https://www.ifixit.com/Answers/View/14633/How+do+I+ground+my+ESD+mat+to+avoid+shocking+computers

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It sounds to me as though

  • the wrist-strap has a built-in resistance so that it can be connected directly to ground.
  • the plug adaptor has a built-in resistance so that any wrist-strap can be connected to it whether or not the wrist-strap has built-in resistance.

As others have said the resistance protects the operator against shock from mains and static discharge. Changing from 1 to 2.5 MΩ will have no noticeable affect.

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