During the discharge, these machines apply high voltage to the body for only a millionth of a second or less. So a low current is applied for a very short time, and the number of electrons involved is very small. Tesla coils Despite Tesla coils superficially appearing similar to Van de Graaff generators, they are not electrostatic machines and can produce significant radio frequency currents continuously. The current supplied to a human body will be relatively constant as long as contact is maintained, unlike with electrostatic machines which generally take longer to build up charges, and the voltage will be much higher than the break-down voltage of human skin. As a consequence, the output of a Tesla coil can be dangerous or even fatal. Arc flash hazard Depending on the prospective short-circuit current available at a switchgear line-up, a hazard is presented to maintenance and operating personnel due to the possibility of a high-intensity electric arc. Maximum temperature of an arc can exceed 10,000 kelvins, and the radiant heat, expanding hot air, and explosive vaporization of metal and insulation material can cause severe injury to unprotected workers. Such switchgear line-ups and high-energy arc sources are commonly present in electric power utility substations and generating stations, industrial plants and large commercial buildings. In the United States, the National Fire Protection Association, has published a guideline standard NFPA 70E for evaluating and calculating arc flash hazard, and provides standards for the protective clothing required for electrical workers exposed to such hazards in the workplace. Explosion hazard Even voltages insufficient to break down air can supply enough energy to ignite atmospheres containing flammable gases or vapours, or suspended dust. For example, hydrogen gas, natural gas, or petrol/gasoline vapor mixed with air can be ignited by sparks produced by electrical apparatus. Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants, grain elevators, and coal mines. Measures taken to prevent such explosions include: Intrinsic safety by the use of apparatus designed not to accumulate enough stored electrical energy to trigger an explosion Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent sparks Explosion-proof (flame-proof) enclosures, which are designed so that an explosion within the enclosure cannot escape and ignite a surrounding explosive atmosphere (this designation does not imply that the apparatus can survive an internal or external explosion) In recent years, standards for explosion hazard protection have