Because most conduit and cabinets are made from metal, it is common for these components to have sharp metal edges due to the manufacturing processes. The NEC specifies a number of protective measures to help protect wire insulation from being cut or damaged by these edges both during installation and later when in actual use. Insulated cables may not be inserted directly through knockouts, for example, due to the sharp edge around nearly all knockout holes. Clamping and other wire protection is often not required for plastic conduit parts, since the plastic is not likely to damage insulation in contact with it.
In potentially hazardous locations, more robust cable protection may be necessary. Common conduit and ductwork protects against direct physical abuse, but is neither airtight nor watertight. In wet locations, conduit may resemble standard threaded pipe in appearance, with sealed gasketed box openings to keep moisture out. Areas with potentially explosive gases need further protection to prevent electrical sparks from igniting the gases, and internal conduit gas-tight barriers to prevent potentially ignited gases from traveling inside the conduit to other parts of the building.
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