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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

Luxii
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D. Arius
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  • The resistance is not for protecting the operator. It is to protect the ESD sensitive device. – Marco Jan 17 '16 at 00:42
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    @d3l: Incorrect. It must be high in order to help protect the operator against shock in case they accidentally contact mains voltage. – Ignacio Vazquez-Abrams Jan 17 '16 at 00:44
  • @IgnacioVazquez-Abrams Are you talking about the ground plug adapter? I've never seen one with integrated resistance – Marco Jan 17 '16 at 00:46
  • @d3l: Nor have I. I didn't realize you were talking only about the adapter. – Ignacio Vazquez-Abrams Jan 17 '16 at 00:47
  • @IgnacioVazquez-Abrams Now you got me confused, where is a resistor used for safety? – Marco Jan 17 '16 at 00:49
  • @d3l This ground plug adapter for example has 1 MOhm internal resistance: http://i.stack.imgur.com/66c8Gl.jpg – D. Arius Jan 17 '16 at 00:50
  • @D.Arius but is this really used for protecting the operator? – Marco Jan 17 '16 at 00:51
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    @d3l: Contacting mains voltage with the opposite hand could be fatal if there were no resistance to ground. – Ignacio Vazquez-Abrams Jan 17 '16 at 00:52
  • @IgnacioVazquez-Abrams I see, but touching mains voltage is most of the time fatal anyway. It seems silly because metal housings are grounded as well (almost zero resistance). I guess the extra security is because you are always connected to earth with the wrist strap? – Marco Jan 17 '16 at 00:54
  • @d3l But without wrist strap resistance from body to ground will have always a significant value. Is difficult to touch a grounded metal and mains voltage at the same time, and if it it occurs the resistance to ground will not be so low as to have a wrist strap directly to ground, and that you can't get out fastly. – D. Arius Jan 17 '16 at 01:03
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    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. – Tom Carpenter Jan 17 '16 at 03:04
  • @marco-a Grounded metal housings provide safety under the assumption that the housing is properly assembled, closed an non-damaged. That way, the only conductive thing you could contact with your body is the housing, which could become live as a result of internal damage (a loose wire, a leak...), but it won't, because it's grounded. The resistance here should be minimized in hopes of triggering the fuse or a circuit breaker. – ksadowski Jun 24 '20 at 18:09
  • @marco-a With a wrist strap, it's a different story - you're working with exposed electronics and it's possible to accidentally contact something at a high voltage without anything in there to divert the current before it can find its path through your body. It could be a live wire, or it could be an ostensibly low-voltage output of a badly designed or damaged power supply. That's why your protection needs to be in series with your body rather than in parallel (as it would be with a metal housing). Here, instead of the current being diverted, it's being limited by a series resistance. – ksadowski Jun 24 '20 at 18:16

3 Answers3

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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

MadHatter
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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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Luxii
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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.

Transistor
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