2024 Volume 17 Issue 2 Pages 113-118
In this paper, the incendivity of abnormal discharges occurring from an electrostatic powder coating gun was experimentally examined using two methods (i.e., transferred charge amount measurement test and explosion test). In the whole experiments, the discharge gap (the distance between the nozzle tip and the grounded electrode) was kept constant at 10 mm, and a methane-air mixture gas (12.0%) was used as the test gas in the explosion test. As for the results, under all experimental conditions, abnormal discharge did not occur when the Over Current Limit (OCL), which is the safety device of the electrostatic spray gun, was enabled. When the OCL was released, the absolute value of the transferred charge amount was 70 nC or less when the setting value of the current control device was -80 μA and -110 μA. There were no ignitions under these conditions. On the other hand, when the set value was -120 μA, -130 μA and -140 μA , the absolute value of the transferred charge amount was 1,500 nC or more. There were ignitions because the transferred charge amount greatly exceeded the ignition risk threshold value of 200 nC (absolute value) mentioned in EN50050-2. As described above, since a clear correlation was confirmed between the transferred charge amount measurement test and the explosion test, the ignition risk can be evaluated by grasping the transferred charge amount in advance.
