Our current practice is that grounding through chairs is not a requirement, and as such, it is not a requirement of our ESD plan that chairs at an ESD workstation be ESD safe. Our lab has ordered a new chair that is not designated as ESD safe (all of our other chairs are ESD safe, regardless of that not being a requirement). Grounding is not a concern as we use wrist straps and conductive flooring as our primary grounding methods.
That being said, recently I measured the charge on the surface of this new chair after someone had sat in it, using a static field meter, and it measured well over 10kV…
I believe this violates the requirement for insulators within 300mm to be below the threshold of 2000V, but it being a chair kind of puts it on the edge of this requirement (Insulators 8.3.1). What are some other opinions?
The obvious answer is to replace the chair, and it is the path I am heading down - better safe than sorry - but I’m curious about any counter arguments that could be made for keeping the chair.
I suggest you use the first option for measuring insulators given in ANSI/ESD S20.20 section 8.3.1 which says “Measure the field at the location where the ESDS item is handled. The electrostatic field shall be less than 5000 volts/meter (125 volts/inch).” The method to do this is described in ESD TR53 section 7.3.1.1.
If you measure the field with your ESDS item as it is worked on in the process, the person seated in the chair will likely suppress most of the field. If there is a chance the chair is pushed in and a person stands over the area to work or if the person sits in the chair so the back of the chair is against the worksurface, the back of the chair may be able to give off a field that would be in excess of the 125 volts/inch. Then it is dependent on how much risk you are willing to accept. Generally, I think you would be fine if your measurements are within the limits above and your people are using the chairs as intended.
Though not our method of grounding either, we have a rule that all chairs in an ESD must be ESD safe, due to that reason exactly. Our operators do push their chairs in neatly up-to the workbench and often have ESDS within 30cm of both trolleys and other worksurfaces. Chairs are tested during all ESD audits