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Open Machine: An open machine is one having ventilating openings which permit passage of external cooling air over and around the windings of the machine. The term "open machine," when applied to large apparatus without qualification, designates a machine having no restriction to ventilation other than that necessitated by mechanical construction.

Dripproof Machine: A dripproof machine is an open machine in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0° to 15° downward from the vertical.

Dripproof Guarded: A guarded machine is an open machine in which all openings giving direct access to live metal or rotating parts (except smooth rotating surfaces) are limited in size by the structural parts or by screens, baffles, grilles, expanded metal, or other means to prevent accidental contact with hazardous parts. Openings giving direct access to such live or rotating parts shall not permit the passage of a cylindrical rod 3/4 inch in diameter.

Weather Protected Machines:

TYPE I – A Weather Protected Type I machine is an open machine with its ventilating passages so constructed as to minimize the entrance of rain, snow, and airborne particles to the electric parts and having its ventilated openings so constructed as to prevent the passage of a cylindrical rod 3/4 inch in diameter. (Standard on USEM "P" base verticals).

TYPE II – A Weather Protected Type II machine shall have (in addition to the enclosure defined for a Weather Protected Type I machine) its ventilating passages at both intake and discharge so arranged that high velocity air and airborne particles blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages leading directly to the electric parts of the machine itself. The normal path of the ventilating air which enters the electric parts of the machine shall be so arranged by baffling or separate housings as to provide at least three abrupt changes in direction; none of which shall be less than 90°. In addition, an area of low velocity not exceeding 600 feet per minute shall be provided in the intake air path to minimize the possibility of moisture or dirt being carried into the electric parts of the machine.

Totally Enclosed Machines: A Totally Enclosed machine is one so enclosed as to prevent the free exchange of air between the inside and the outside of the case

but not sufficiently enclosed to be termed airtight. A Totally Enclosed non-ventilated machine is a Totally Enclosed machine which is not equipped for cooling by means external to the enclosing parts. A Totally Enclosed fan cooled machine is a Totally Enclosed machine equipped for exterior cooling by means of a fan or fans integral with the machine but external to the enclosing parts.

Totally Enclosed Fan Cooled Guarded: A Totally Enclosed fan cooled guarded machine is a Totally Enclosed fan cooled machine in which all openings giving direct access to the fan are limited in size by the design of the structural parts or by screens, grilles, expanded metal, etc., to prevent accidental contact with the fan.

Totally Enclosed Fan Cooled Waterproof Machine: A waterproof machine is a Totally Enclosed machine so constructed that it will exclude water applied in the form of a stream from a hose, except that leakage may occur around the shaft provided it is prevented from entering the oil reservoir and provision is made for automatically draining the machine. The means for automatic draining may be a check valve or a tapped hole at the lowest part of the frame which will serve for application of a drain pipe.