Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 205 of 843

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. Unfortunately, no regulations directly applicable to electrically charged waterborne systems are available. By making some assumptions about the meager data that is published, extrapolating to the 70,000\100,000 V range used for electrostatics, and plugging the resulting voltage and capacitance values into the standard equation for storing 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 isolated system to pose a serious shock hazard. The capacitance of the system, as measured 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 considered for operator protection. For comparison purposes, a single 55-gal. drum and 200 ft of 3/8-in. inner diameter hose, all set 12 in. above a ground, can have between 450 and 900 picofarads 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 voltage. 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 application system is converted from solvent-based materials to waterbornes. An isolated electrostatic system for waterbornes can have multiple automatic atomizers or it can have a single handgun. It cannot have both, nor can it have more than one handgun. National Fire Prevention Association (NFPA) regulations dictate that the electrostatic voltage to any handgun must turn off when the trigger is released. Since all the atomizers in a waterborne system are connected electrically by their fluid hoses, the voltage remains "on" to an idle handgun as long as it is "on" to any atomizer in the system. This means that a handgun cannot be used with electrostatics if there are other atomizers in the system, and without electrostatics it is impossible to achieve the maximum TE. To summarize, completely isolated systems have the potential to allow the maximum TE for a given application because they allow the coating material 200

Articles in this issue

view archives of Metal Finishing Guide Book - 2013