ESD wrist wrap resistance

Question

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

Answer 1

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.

Answer 2

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

Answer 3

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.