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The Safety Zone

Essential information about flooring, electrical safety and grounding

Inside Edition news segment addressing the hidden dangers of stray voltage. (June, 2008)

ESD Grade Carpet Tile Can be Too Conductive for Mission Critical Spaces

I was recently hired by a client to perform resistance measurements on some previously installed ESD grade carpet tile. This particular carpet tile was 3 years old. It was made with 44 denier conductive fibers inserted into every yarn bundle and backed with a black recycled PVC-free conductive thermoplastic. In the process of making the tests I received a painful electrical shock that caused muscle cramping in my chest; the flooring ground path lacked an adequate amount of electrical resistance to prevent the type of shock I received. This is disturbing because this type of ESD grade carpet tile is often chosen for mission critical spaces where there is no awareness of the potential hazards presented by operational computer equipment in combination with flooring materials with highly conductive properties. Based on this experience I have been polling industry colleagues to gain a perspective on safety and grounding. Armed with this information we offer several conclusions:

Safety recommendations before specifying any ESD grade floor

  1. ESD grade (very conductive) carpet tile could pose an unnecessary safety threat to people working in mission critical spaces around energized computer equipment. ESD grade carpet tile was not originally conceived for use in server rooms, data centers and spaces where people do not test the electrical properties of their footwear before entering the space. ESD grade carpet was designed for use in controlled environments like clean rooms, micro-electronics fabrication and electronics manufacturing facilities; in these types of spaces, the electrical properties of flooring and footwear are constantly monitored for safety, static mitigation and electrical compatibility.
  2. It may be advisable to require ground fault circuit interrupters (GFCIs) in places where a floor's resistance to ground measures below 80,000 ohms. (80,000 ohms is the same as 8.0 X 10 E4. Keep in mind many "ESD grade" floors are specified as having resistances as low as 25,000 ohms)
  3. All ESD grade carpet tile installations should be tested and certified using an ohm meter before the space is turned over for use.
  4. An ohm's law calculation should be performed to determine if the floor might present a safety hazard.

    Amps of current = Volts/Ohms Example: if a carpet resistance measures 10,000 ohms in a room where equipment operates at 120 volts then it is possible for a person to be exposed to a current of 12 milli-amps.

    Here is a web site with a simple plug and play Ohm's Law calculator.

How much current is dangerous?

At this web site, http://www.elec-toolbox.com/Safety/safety.htm you can easily determine the danger levels of electrical currents using Ohm's Law. In the above scenario, a shock at 12 milliamps would generate a painful experience but a person would still have the ability to let go of the charged cord or object. there is little margin for error. At 15 milli-amps a person would no longer have the ability to let go. This is why we are suggesting a higher threshold for resistance to ground measurements. in the same scenario, a person would only be exposed to 1.5 milli-amps of current of the floor had at least 80,000 ohms of resistance.


Choosing a safe floor

The graph below is a conservative recommendation for flooring resistance ranges for use in any space where it is possible a person could contact an ESD grade carpet tile with bare skin or damp footwear. if you are concerned about existing flooring, contact your Staticworx® technical department to arrange a free electrical audit of your facility floor. All resistance tests will be performed using ANSI/ESD S 7.1-2005 test methodology.

Conclusions

Many static control floors offer a static free environment without any safety compromises. There are numerous carpet tile, rubber tile and vinyl tile options. Make sure you understand the relationship between ohms, volts and electrical current before accepting any static control floor. The term "ESD grade" is merely descriptive; it is not a quantifiable. You need to specify an ohms range for static control flooring. If you are confused by technical jargon visit the Staticworx® Knowledge Center

Picture of ohm meter measuring 15,000 ohms resistance of highly conductive ESD grade carpet tile with conductive backing. This carpet tile would expose workers to an unnecessarily high amount of electrical current.



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