Safety question about grounding with resistive flooring

Discussion in 'General Electronics Chat' started by Staticworx, Mar 31, 2012.

  1. Staticworx

    Thread Starter New Member

    Mar 31, 2012
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    Does anyone have the ability to put together an electrical model explaining the theory behind identifying a safe minimum electrical resistance for grounded flooring used around energized equipment?
    The model would need to explain what would happen if a person touched 120 or 220 at the same time as they were making electrical contact with the conductive flooring.
    I am interested in putting a chart together that would identify the range of electrical currents a human body could experience based on decreasing resistance levels in the unlikely event of contact with AC voltages while touching grounded flooring. Many grounding standards like ATIS 0600321, FAA STD 019e and Motorola R56 recommend a minimum resistance to ground of 1 million ohms for static control flooring.
    The flooring is used to prevent static electricity discharges in equipment rooms.
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    I don't really understand your question? The very fact that you have a minimum of 1 million ohms to ground is very close to being an insulator. (I would imagine the standards require testing this resistance to ground on a very regular basis). The current flow if anyone was to come into contact with mains AC voltage would be no more than a couple of hundred microamps when several tens of milliamps are needed to "stop" the heart. Is it really a problem?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Concrete is considered conductive in the world of ESD. This means heel straps are a good static control method (not to replace wrist straps however).

    ESD and AC grounding have very little to do with each other though, so do not get them confused. Concrete is valued as several megohms / in².
     
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  4. paulktreg

    Distinguished Member

    Jun 2, 2008
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    One presumes then if you are stood directly on dry concrete and touch the live connector of a 117/230VAC installation it shouldn't be a problem.
     
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  5. #12

    Expert

    Nov 30, 2010
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    I think I have a better way. Stand on dry concrete and measure the resistance from you to a grounded water pipe. It is safer than grabbing a live wire and measuring the current through you.
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    That may or may not kill you, but it's likely to be highly unpleasant at best. :rolleyes:
     
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  7. Staticworx

    Thread Starter New Member

    Mar 31, 2012
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    Thanks for the responses. I should probably elaborate on the purpose of the question.
    Grounded flooring is used to minimize static generation in 1000s of data centers, flight control towers, control rooms, communication centers and other mission critical spaces. Most grounding documents recommend that these floors have a minimum resistance to ground of 1,000,000 ohms. In the ESD world, we know that a floor can have a resistance as high as 1 billion ohms and still adequately bleed static faster than a person can generate and accumulate new charges. However, the resistance needs to be inherent - not achieved with a resistor at the ground connection. FYI, 1,000,000 ohms is the same amount of resistance found in the grounded wrist straps worn by assemblers of PC boards. It is also the typical sheet resistance of dissipative table coverings used in electronics assembly.

    My question is:
    How would one calculate the quantity of milliamps in an AC circuit that would include a person, a 110 or 220 60 hz short circuit and direct skin contact with a grounded floor measuring less than 1,000,000 ohms. for example, how much current would be available if the floor measured 25,000 ohms RTG in a scenario involving 220 volts or even higher in the case of mainframe computers? This calculation is more complex than the simple Ohms law DC circuit formula.

    The person could be grounded to the floor while in contact with the voltage source. They could also be grounded by contacting a grounded chassis while standing on the floor at the same time as a frayed live wire is contacting the floor. These are two different models. It would help people if we could construct a chart that correlates the resistance of the floor with some range of potential current. This chart from OSHA http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html .html states that currents over 6 milliamps could prevent letting go. I am concerned that people do not follow recommended grounding guidelines when purchasing and installing this type of flooring. this is often because they have little or no knowledge of electricity. they merely fulfill the task of specifying a grounded floor as part of their duties as an interior designer because they don't have the technical resources and their flooring suppliers don't realize or care about the dangers either.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    The danger of electrocution by contact to 110 or 220 VAC always exists regardless of the conductive properties of the flooring. All personnel working with potentially lethal voltages must be properly trained and must take proper precautions.

    Grounded flooring is installed to control ESD and there is no relationship to personal safety.
     
  9. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    I found that the Air Force was very confused about flooring in the radar systems.

    At one point they had rubber mats that were good for 40kv. The theory was if you touched HV and you were standing on this mat you would be ok as long as you were not touching anything else.

    Then came ESD mats and the HV mat came out and the conductive mats went in. We also had wrist straps and had high voltage gloves that were to be used at the same time. Never made any sense.

    I never did use the wrist straps and as far HV I turned it off when I could there were a few occasions during alignments it had to be on. I didn’t trust the gloves but they did work. Just that initial first time grabbing HV was nerve racking.
     
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