become more uniform between European and North American practice. The "zone" system of classification is now used in modified form in U.S. National Electrical Code and in the Canadian Electrical Code. Intrinsic safety apparatus is now approved for use in North American applications. Toxic gases Electrical discharges, including partial discharge and corona, can produce small quantities of toxic gases, which in a confined space can be a health hazard. These gases include oxidizers such as ozone and various oxides of nitrogen. They are readily identified by their characteristic odor or color, and thus contact time can be minimized. Nitric oxide is invisible but has a sweet odor. It oxidizes to nitrogen dioxide within a few minutes, which has a yellow or reddish-brown color depending on concentration and smells of chlorine gas like a swimming pool. Ozone is invisible but has a pungent smell like that of the air after a lightning storm. It is a short-lived species and half of it breaks down into within a day at normal temperatures and atmospheric pressure. Lightning Hazards due to lightning obviously include a direct strike on persons or property. However, lightning can also create dangerous voltage gradients in the earth, as well as an electromagnetic pulse, and can charge extended metal objects such as telephone cables, fences, and pipelines to dangerous voltages that can be carried many miles from the site of the strike. Although many of these objects are not normally conductive, very high voltage can cause the electrical breakdown of such insulators, causing them to act as conductors. These transferred potentials are dangerous to people, livestock, and electronic apparatus. Lightning strikes also start fires and explosions, which result in fatalities, injuries, and property damage. For example, each year in North America, thousands of forest fires are started by lightning strikes. Measures to control lightning can mitigate the hazard; these include lightning rods, shielding wires, and bonding of electrical and structural parts of buildings to form a continuous enclosure. See also Capacitor voltage transformer Charging station Electrical engineering Electric power transmission (includes a 'Health concerns' section) High-voltage direct current Low voltage Orders of magnitude (voltage) Tesla coil Spark gap References External links NFPA 70E: Electrical Safety in the Workplace, USA USA Department of Energy electrical safety handbook Electrical Safety chapter from Lessons In Electric Circuits Vol 1 DC book and series. Electricity Voltage Electrical safety Technology hazards