CTA Report, Draft 1, ISP Review - US Chemical Safety and Hazard ...

B.4 Propagating Brush Discharge Ignition Test
During the handling of bulk powders, a propagating brush discharge may ignite dust. A sufficient charge
built up on the powder surface can break down in the form of a propagating brush discharge. Unlike a
spark discharge, which is localized between two conductive points, a propagating brush discharge is a
surface phenomenon. Consequently, it poses the potential hazard of being a delocalized multipoint
ignition source. The force of the charge breakdown is sufficient to disperse dust deposited on the surface
into the air.
B.4.1 Test Setup and Procedure
The test apparatus consisted of a high-voltage power supply (Hipotronics Model 250D) connected to a
multipoint corona generator. The base of the apparatus was an 8.5-inch sheet metal disk, which is
connected to the ground. An 8.5- by 11-inch plastic sheet, with a thickness of 0.0075 inch, was placed on
top of the disk.
A grounded 2-inch-diameter ball electrode was used to initiate the propagating brush discharge. To
estimate the energy of the discharge, a Monroe Electronics Model 282 electrostatic fieldmeter was used to
measure the change in electrostatic field (i.e., electrical potential of the surface by extension) before and
after the discharge.
To produce a charge on the surface of the plastic sheet, 30 to 50 kilovolts were supplied to the multipoint
corona generator centered over the sheet metal disk surface at a height of 25.4 millimeters. After a
charging time of 60 seconds, the generator was moved away from the surface of the plastic sheet and the
power supply was turned off.
The field meter was positioned above the surface of the plastic sheet to measure the electrostatic field.
The spherical grounding electrode was then lowered to the plastic surface. A propagating brush discharge
was produced as the electrode was lowered but before it touched the surface.
107