ESD Chair Testing

Dear Experts,
have questions regarding testing of chair. is it necessary to measure the backrest of chair or we can skip as long as we check the seating surface. referring to TR53, this was also not mentioned to check. Can you advise.

thanks in advance.

rose25,

The only requirement in ANSI/ESD S20.20 to test the back of the chair comes during the product qualification usually done by the manufacturer to ANSI/ESD STM12.1. In this standard test method there are a series of different resistance test points on the chair including the seat, backrest, armrests and foot rest. The standard test method actually prescribes 5 different resistance measurement points on the front of the back rest and 3 on the back of the back rest.
So if the manufacturer can provide evidence that they have tested the chair to ANSI/ESD STM12.1 then you should not need to do any testing of the backrest as TR53 only requires the testing of one point on the seat. Note that for facility certification of new chairs after adoption of ANSI/ESD S20.20 - 2021, the evidence of product qualification must include all the applicable data required by ANSI/ESD STM12.1, not just a statement that the chair complies.
If you plan to do the product qualification of the chair yourself you would need to follow all the requirements in ANSI/ESD STM12.1 including measuring the resistance of the backrest and be able to provide the evidence mentioned above

Dear Andy,
thank you for the clarification and additional information. Just to clarify also that if this is the first time that we adopt the S20.20 and we have no evidence that the chair that we used was not qualify as per STM12.1 and the only evidence that we can provide is the compliance verification for more than 1yr. will this be sufficient or we need to conduct again the qualification as per STM12.1 since the chair that we used was more than 1yr old.

please advise.
thank you.

rose25,

ANSI/ESD S20.20 states “Organizations that can verify the use of ESD control items before adopting this standard to certify their ESD control program can use compliance verification records to meet product qualification requirements. These records shall cover a minimum of one year and reflect a timeframe immediately before using as product qualification records. These records shall reflect test results that meet the compliance verification test limits identified in Tables 2, 3, and 4 of this standard.”

So if you have compliance verification records for the ESD chairs that go back at least in the last year and show resistance values less than the standard (in this case < 10^9 ohms point to ground) you can use this for product qualification. After you adopt the standard, any new chairs you bring into the ESD control program would need to follow the requirements for product qualification (Test record evidence from ANSI/ESD STM12.1)

Also note that seating is an optional ESD control item. If you have chairs in your EPA, you are not required to put them into your ESD Control Program Plan. Organizations that include seating in their ESD Control Program Plan usually use them as a back-up means of ESD protection as the wrist strap is the required primary means for personal grounding for seated operations. If you don’t use seating in your ESD Control Program Plan you would still need to consider them as potential insulators or isolated conductors in those sections of the standard.

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Thanks Andy. meaning compliance verification record of one point measurement only is sufficient and for the succeeding purchase (only for new material type), we need to conduct product qual as per ANSI/ ESD S12.1. Understood.

My next question is that, if we declare that we do not use the wrist strap as primary grounding system for our personnel and we utilize only the chair as our grounding system, would tailoring be necessary?

sorry for so many questions.

thank you.

rose25,

Yes, if you do not use the wrist strap system for primary grounding of personnel but instead use seating in combination with ESD flooring you would need a tailoring statement. You would also need to provide evidence that the seating/flooring system resistance and body voltage can be maintained below the similar limits of a footwear/flooring system, in other words, < 10^9 ohms and under 100 Volts. The evidence would not only have to show the measured values but would need to explain the set-up since there are no standards that specifically cover a seating/flooring system. The resistance measurement would be rather straight forward (a person holding a hand-held electrode, seated on the chair, and then measuring the resistance to ground), but the voltage measurement would require you to consider the full range of motion for operators in the seated position. You would also need to perform these tests in the lowest relative humidity the facility sees in a year. I know some facilities have done this but it does take quite a bit of effort.

Another solution, if you don’t like wrist straps, is the groundable static control garment system. This at least frees up the operators hands although they are still attached with a strap at the waist.

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Dear Andy,

I see. thank you very much.
appreciate you sharing this how to’s and other considerations.
very very much helpful.

thank you.