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The isolated hose was estimated to have a capacitance of 30 pico-farads and a modest value for the streaming current generated by the transfer of the powder aluminium was estimated to be in the region of 10 micro-amps. The resistance to ground of the isolated hose’s flange, resting on the concrete floor, fell into a range of between 1010 ohms and 109 ohms. The voltage (V=RI) on the isolated hose, based on the upper and lower limits of resistance, was estimated to fall between 100 kilo-volts and 10 kilo-volts.

The energy, W, from a spark discharge based on the capacitance and voltage on the isolated hose (W = 0.5 CV2), would have been in the range of 150 milli-joules to 1.5 milli-joules. Hence a spark from the isolated hose would have been likely to have had the capability to ignite an explosible concentration of the aluminium dust.

As the operators needed to improvise through this non-standard scenario, it is all too easy to criticise the lack of static grounding precautions that were adhered to. Electrostatic hazards are not an easy concept to grasp, especially when improvisation is required. One standard operating procedure that may have re-enforced good practice was to have the truck driver trained to test the electrical continuity of the hoses back to the grounded truck. This would enable the driver to ensure a transfer could not take place unless all components in contact with the charged powder were grounded. One means of achieving this is for the driver to carry out a resistance check with an ATEX/HAZLOC certified ohmmeter (or multi-meter) to ensure the combined resistance of the hoses does not exceed unsafe levels.

If there are concerns about the correct use of the meter an even simpler method is to utilize a device like an ATEX/HAZLOC certified OhmGuard® tester which provides the driver/operator with a visual indication of whether or not the hoses assembled together exceed safe resistance levels. By simply attaching one end of the OhmGuard to the truck and the other end, which houses the LED indicator, to the end of the interconnected hoses, the tester will automatically determine if the resistance through the hoses is higher than recommended levels. Following a “GO/NO GO” principle of operation, the pulsing green LED indicator visually confirms that the resistance of the interconnected hoses is low enough to dissipate electrostatic charge from the hose, preventing ignitable sparks being discharged into combustible atmospheres.

 

Mike O'Brien

Author Details:
Mike O’Brien, Managing Director for Newson Gale

If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.


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