Question & Answer
We are trying to identify the correct resistance range for flooring installed in FAA flight control towers and flight areas. An ESD consultant recently told us to specify conductive flooring because it discharges faster. Since this flooring will be used in areas with energized equipment we are questioning the logic of this recommendation. What do you recommend and how would we support a different recommendation?
Your first order of business should be to download the latest FAA standard and review the sections on flooring for yourself. Here is the link: https://faaco.faa.gov/attachments/STD-019e2.pdf In all liklihood the consultant is unfamiliar with the standard.
As you will see, the ohms resistance range of flooring is mentioned 6 different times between section 126.96.36.199.2 and section section 188.8.131.52.9. In all but one case, the document states that the flooring shall measure in the static dissipative range. The only exception to this requirement involves what amounts to extraordinary measures and special permission:
1. It is part of a designed approach for ESD control for the equipment approved by the OPR of this document.
2. This designed approach shall include all steps required to produce an electrically safe working environment.
Testing to prove an electrically safe working environment is not the same as performing a simple set of resistance tests. Definitively stating that an environment is "electrically safe" requires special licensing, extensive research and the testing would need to be performed after the flooring has been installed.
Keep in mind that the people working in flight control areas do not test resistance of footwear or flooring, they do not wear heel straps or ESD controlled footwear. Conductive flooring will not prevent static any better than static dissipative flooring in this scenario. Therefore, you would gain nothing from installing a floor that is more conductive than the ohms range required in the other 5 sections of the FAA document. It is a mathematical fact that the electrical current that can travel through a floor increases as the resistance is lowered. This becomes problematic in areas where energized equipment is used. This point was probably lost on the ESD consultant. Some of these consultants are accustomed to designing static free areas for ANSI/ESD S20.20 electronics manufacturing areas where special ESD footwear is also the norm. Only a few ESD consultants have experience with the requirements of mission critical environments like data centers, 9-1-1 dispatcher areas and FAA flight control areas.
Number One: 184.108.40.206.2 Static Conductive Materials
Those materials with a surface resistivity less than 1.0 x 105 ohms/square when tested per ANSI/ ESD S11.11 shall be considered conductive. Conductive ESD control materials shall not be used for ESD control work surfaces, tabletop mats, floor mats, flooring, or carpeting where the threat of personnel contact with energized electrical or electronic equipment exists.
Number Two: 220.127.116.11.3.5 Static Dissipative ESD Control Floor Coverings
Static dissipative ESD control floor coverings shall include static dissipative tile, carpeting, static limiting floor finishes, and floor mats. Floor coverings in ESD special protection areas shall have a point-to-point resistance and surface-to-ground resistance of greater than 1.0 x 106 ohms and less than 1.0 x 109 ohms (ANSI/ESD S7.1). These floor coverings shall be bonded to any SRS in the area served in accordance with paragraphs 18.104.22.168.3.1 and 22.214.171.124.8 – except to a single point ground system. In circumstances involving extremely static sensitive equipment a static conductive floor covering with a lower resistance limit of 2.5 x 104 ohms (UL779) shall be implemented when it is part of a designed approach for ESD control for the equipment approved by the OPR of this document. This designed approach shall include all steps required to produce an electrically safe working environment.
Number Three: 126.96.36.199.8 ESD Control Flooring and Floor Coverings
All ESD control floors and floor coverings shall have a point to point resistance and a surface to ground resistance of greater than 1.0 x 106 ohms and less than 1.0 x 109 ohms (ESD STM7.1). ESD control flooring and floor coverings include vinyl tile, vinyl sheet, carpet, carpet tile, carpet tile with positioning buttons and others but not to include applied coatings.
Number Four: 188.8.131.52.8.1 Surface Resistance (Rtt)
Surface resistance (Rtt - Resistance top-to-top or surface-to-surface) of ESD control floors, carpets or floor mats shall be greater than 1.0 x 106 ohms and less than 1.0 x 109 ohms (ANSI/ESD S7.1). A minimum of five readings shall be taken at different locations on the floor surface and averaged together for each 500 square feet (or fraction thereof) of floor surface. These readings shall be recorded in the FRDF.
Number Five: 184.108.40.206.9 ESD Requirements for Raised Floors
220.127.116.11.9.1 Resistance from Carpet Surface to Pedestal Understructure
Carpet tiles shall have a resistance from the carpeted surface of the raised floor to the pedestal greater than 1.0 x 106 ohms and less than 1.0 x 109 ohms.