Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 144 of 331

ground because it was set on a concrete floor and humidity from the concrete made the inside of the leg conductive. That particular short took a full week of troubleshooting to find and correct. Besides being inefficient, isolated systems can be dangerous because they can store too much electrical energy. All the equipment that gets wetted with elec- trically charged coating material stores electrical energy, much like a giant ca- pacitor. All that stored energy gets discharged if the system gets shorted out. If the system is big enough, and stores enough electrical energy, an operator can get injured by shorting it out accidentally by touching a charged hose or atomizer. It is impossible to draw a definite line that says, "A system this small is safe, and a system that big is dangerous." Trying to define a safe electrical shock is like trying to define a safe height from which to fall. For example, a shock itself might only be annoying, but the victim might be so startled by it that it results in a bump on the head or an injury in some other way. Although a safe system, with regard to storage of electrical energy, may be a contradiction in terms, some guidance regarding the size of a "probably unsafe" system would be useful. Un- fortunately, no regulations directly applicable to electrically charged waterborne systems are available. By making some assumptions about the meager data that is published, ex- trapolating to the 70,000\100,000 V range used for electrostatics, and plugging the resulting voltage and capacitance values into the standard equation for stor- ing electrical energy in a capacitor, the following can be developed: Maximum\Energy = 3.5 Joules = CV2 where C=Capacitance (farads) Rearranging: CMAX =7/V2 where the voltage is the maximum available from the power pack. This equation can be used as an indicator of the potential for a given isolat- ed system to pose a serious shock hazard. The capacitance of the system, as mea- sured with a capacitance bridge or a suitable capacitance meter, must be less than the value of CMAX if there is a possibility of accidental human contact, which could result in an electrical shock. For example, if a 100,000-V electrostatic paint system has a capacitance G700 picofarads, caging and interlocks should be con- sidered for operator protection. For comparison purposes, a single 55-gal. drum and 200 ft of 3/8-in. inner di- ameter hose, all set 12 in. above a ground, can have between 450 and 900 pico- farads of capacitance. This means that a typical paint system, which has much more hardware, would almost certainly exceed the maximum capacitance value and could store potentially dangerous levels of electrical energy. The storage of electrical energy can be reduced by lowering the electrostatic volt- age. The voltage term is squared in the equation for energy storage in a capacitor. This means that a given system at 100,000 V will store four times the energy that it would at 50,000 V. At the lower voltage, not only will the system be safer, but guns and cables will last much longer before breaking down electrically. Perhaps an even more compelling reason for lowering the voltage is to maximize TE. The maximum TE for most waterborne coating materials occurs between 40,000 and 60,000 V. By comparison, the maximum TE for a less conductive solvent-based material can be 90,000 V or more. Handguns present a special problem when a coating applica- tion system is converted from solvent-based materials to waterbornes. 143

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