I was asked to participate in a serious flash fire accident cause and origin investigation earlier this year. The worker received severe injuries from the flash fire; they survived with life-altering injuries to their lower extremities. The flash fire involved Acetone vapors from a manufacturing cell, an office chair, and a plastic chair mat. Can you guess the ignition source? Here are a few of the critical details…
- The manufacturing cell, 25’x25′, had walls that sat directly on the concrete floor.
- The only opening in the walls was the only door into/out of the cell. The door was normally closed as the worker did not like all the noise from the rest of the facility, especially the choice of music coming from another nearby cell.
- There was no mechanical exhaust ventilation for the cell.
- The cell used ~ 35 gallons of acetone per shift.
- The cell contained an “office set up” that included a desk, phone, and chair.
- The floor of the cell had many defects from age and all the rearranging of “cells” over the years.
- The cell was NOT considered a Hazardous Location, so there were NO controls on the quantity of Acetone allowed and no special requirements for electrical or static generating equipment.
One day, the worker, on their own accord and using their money, bought a plastic floor mat at a neighborhood yard sale for $5. They brought it to work and placed it at the desk inside the cell. The desk was in the corner furthest from the doorway/opening in the cell. The plastic floor mat and office chair with plastic wheels were in place for ~ 7 months without any issues. It should be noted that none of the monthly safety audits conducted in the cell recognized the static generation hazard with the plastic mat and office chair.
But one day, as a worker was working on the cell, the supervisor used the desk inside the cell simply for convenience. Normally the cell has only the worker at the bench, but on this day, there was the worker at the bench and the supervisor at the desk. At some point, the worker at the bench knocked over a 2.5-gallon container of Acetone. As was seen on CCTV, both workers responded by soaking up the acetone with shop rags and leaving the shop rags sitting on the floor ~2-3′ away from the plastic floor mat. The workers left the cell to wash their hands and to report the spill to the EHS group. Approximately 10 minutes later, the supervisor returns, sits down in the office chair, and scoots up to the desk. Less than a minute later, they scoot across the mat to a filing cabinet; they hold that position for several seconds and then begin to scoot back across the mat to the desk. They do this three times (i.e., back and forth between the desk and the METAL filling cabinet sitting on the concrete floor). On the 4th time, when they reach out to the metal filling cabinet, the acetone vapors are ignited.
Can you guess the ignition source?
Static electricity is generated by the plastic wheels on the office chair rolling across the plastic floor mat. This static charge continued to increase as it had nowhere to go. But as soon as the worker reached out to the metal filing cabinet, which sat on the concrete floor (i.e. was grounded), the static charge found its path to the ground and was enough to ignite the acetone vapors that had accumulated on the floor of the cell due to lack of ventilation within the cell and the lack of controls on how much acetone could be safely allowed in the non-ventilated space.