I am running into a stubborn contact resistance issue during a new facility qualification and would appreciate any insights from those who have dealt with hard casters on newly cured epoxy.
We recently installed a new ESD epoxy floor. The floor itself passes all RTG and RTT qualifications perfectly when tested with standard 5lb compliant rubber electrodes. However, our ESD seating systems with drag chain are completely failing to ground to the floor, even though all individual chair components test well within spec.
Here is the breakdown of the failure analysis matrix we conducted:
Floor RTG/RTT (5lb Electrode): PASS (E+05 to E+06 ohms)
Chair RTG (on Epoxy floor): FAIL (8.50E+09 ohms)
Chair RTT (on Epoxy floor): PASS (6.53E+04 ohms)
Wheel Surface Material RTT: PASS (3.05E+03 ohms)
RTG Test Under Load (Operator sitting, feet elevated): FAIL (1.87E+09 ohms)
RTG Test Under Load (Operator sitting, feet on floor): PASS (9.32E+07 ohms)
Chair Product Qualification STM12.1 under 12% RH: PASS
My question:
S20.20 Certification Impact: Can our facility still pass an ANSI/ESD S20.20 certification audit if our seating systems fail RTG test(Compliance Verification), assuming our operators are strictly grounded via wrist straps or ESD footwear as their primary ground path? How will an auditor view the chairs in this state?
How to Overcome This: What is the most reliable, cost-effective method to resolve this wheel-to-epoxy interface issue long-term?
Has anyone successfully resolved this simply by swapping to a lower-durometer (softer) conductive polyurethane caster to mimic the compliant nature of the 5lb test electrode?
Sorry to hear of the issue. It is not uncommon for the chair/floor interface to have connection issues. There are a couple of solutions. First, chairs are an optional ESD control item. If you don’t have them in your control plan, the assessor should not require any information. If that is the way you wish to pursue the issue, then treat them as required process insulators and ensure that the chair poses no threat to the ESDS item.
Second, write a tailoring statement that the chairs make intermittent contact with the floor to keep the charges decreased. As part of that, include a plan to try to bring all the chairs into specifications. Disclose that to the assessor with the information that primary grounding of personnel is done with a wrist strap system.
Greetings of the day from India to all the members on this forum and a warm welcome to Mr. Nizam
You say that the Floor has passed and meeting the requirements and telling that your ESD seating systems with drag chain are completely failing to ground to the floor but, Chair has passed the PQ test per STM 12.1 at 12% Rh. Correct ?
May I know; 1. Is the PQ report justified with a detailed reportor just an assurance verbally or
a single line Compliance report from the supplier?
2. Are all the 5 wheels ESD safe or only one or two? Are the Rgp with respect to
all the wheels (provided that all 5 wheels are ESD safe) found ok ?
3. Does the chair has a default grounding chain also and is it touching the floor in
full contact?
4. Can you please share the photo of RTG Test Under Load (Operator sitting, feet
elevated and RTG Test Under Load (Operator sitting, feet on floor).
However, my few lines of opinion are as under for the reason you have mentioned that RTG Test under Load (Operator sitting, feet on floor) is meeting the requirement per ANSI ESD S 20 20.
Wrist straps are mandatory and considered as first line of defense in ESD control
for sitting operators irrespective of whether they use ESD safe chair or a normal
chair. However, in the event Wrist straps are not used but seating-footwear
and flooring systems are used in combination, ( ie; with personnel) then
perhaps you need to included a Tailoring statement in your ESD control
Program plan and follow the System resistance & body voltage w.rt. ANSI ESD
STM 97.1 & 97.2 ( Perhaps our ESDA experts would clear this point ) to
maintain at < 1 x 10^9 ohms and under < 100 Volts. As on date there is no ANSI
ESD or IEC standards on seating-footwear-flooring system measurement to
validate it but, technically and logically it is justified as the grounding
effectiveness is required when a person is seated on an ESD chair with ESD
grounded floor as a path way for dissipation. But, here as the seating system is
of main consideration and the STMs referred above are for Footwear –flooring
system the required measurements (both system resistance with personnel
and voltage in combination with personnel are recorded in sitting posture and
chair movement)
2. Further, on the failure of Rtg of the chair with epoxy floor, it is likely at times
because ESD epoxy floors are emulsion based and surface coated, consistency
in floor surface resistance in dissipative or conductive range is unlikely many a
times which is my practical observations on many of the epoxy floors.
3. Compared to the surface contact by compression through a rubber material
(similar to one used below the 5 lbs probes) the surface contact with chain as
well as the light weight plastic castors -though they are ESD safe) is not perfect at times
and thus testing with personnel seated is technically suggested and perhaps
these views may please be considered by the ESDA’s Standards Working Committee in
the next revision of ANSI ESD STM 12.1 bridging a link with ANSI ESD STM 97.1
& 97.2 .
I usually recommend to factories as Mr. John mentioned above.
In the case you still want to have a good Rtg, please check Rpp between 2 adjacent parts of your chair. You can found a lot of weak electric-connections from seating surface to floor surface. The gas-lift outer and inner can be isolated by grease, the wheel/caster can not keep good Rpp after few months, etc. And especially the drag chain. That is very bad idea to ground for chair.
For reference: Instead of drag chain, you can try a soft/flexible-steel rope, with an small but heavy-enough metal block sliding on the floor. Sliding surface shall be fine enough not to make scratch to your floor. I am testing it here in few month, and result is good so far.
Dear NIZAM
First. The result Chair RTG (on Epoxy floor): FAIL (8.50E+09 ohms).
Floor does not work with the wheels as system. That is wheels to exchange or metal chains to add. And would be OK.
Second. RTG Test Under Load (Operator sitting, feet elevated): FAIL (1.87E+09 ohms).
Incredible. What is the idea? Operator, when she/he is sitting has to use wrist strap. That is rule of S20.20.
I cannot understand the problem you have
Regards
