Type: Lessons Learned
Publisher: Lawrence Livermore National Laboratory, Livermore, CA
The extinguishers static charge increased until it could discharge itself to ground via the employee. The excellent electrical insulation of the high-top boots could have allowed up to 50kV to build up on the employees body over a few seconds of extinguisher operation. At that point, surface flashover across the outer surface of one or both boots is postulated as the mechanism of discharge. This would have dissipated the excess charge in less than a microsecond. Based on the human bodys capacitance, a painful shock would have been experienced across both arms (equally) and the trunk of the body as stored-up energy was dissipated. Although the voltage involved is high, as is the instantaneous current at breakdown, the current flows through and across the body for a very brief time and is not enough to endanger the heart or be life-threatening. In principle, this is similar to static shocks experienced from lower-current events, such as shoes shuffling across carpet. NOTE: Dry chemical fire extinguishers can experience the same charging while being discharged, but at a reduced rate compared to CO2 fire extinguishers.
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