Final answer:
The most commonly used protective enclosure is NEMA 1, which is a drip-proof enclosure designed for indoor use to protect against contact with enclosed equipment and limited falling dirt.
Step-by-step explanation:
The type of protective enclosure that is most commonly used is NEMA 1, which is a drip-proof enclosure. The National Electrical Manufacturers Association (NEMA) defines this type of enclosure as suitable for indoor use, primarily providing protection against contact with the enclosed equipment and against a limited amount of falling dirt. However, when applications require more stringent protection against water and dust, a different NEMA rating may be selected, such as NEMA 4 which is water-tight, or NEMA 12 which is dust-tight and drip-proof. NEMA 7 is specified for hazardous locations and is explosion-proof. The choice of enclosure depends on the environmental conditions where the electrical equipment will be installed.