Machine Tool, Metal Cutting - A power driven device, not portable by hand, used for the purpose of removing metal in the form of chips.

Machine Tool, Metal Forming - A power driven machine, not portable by hand, used to press, forge, emboss, hammer, blank, or shear metals.

Margin of Protection - The difference between arrester characteristics and equipment withstand level at any given instant of time. This is a factor of safety when considering equipment protection. The margin of protection accounts for various unknowns, such as unusually high magnitudes of lightning current, separations of transformer and equipment, and voltage withstand reduction caused by deterioration of old equipment.

Maximum Continuous Operating Voltage - The maximum designated root-square (rms) value of power frequency voltage that may be applied continuously between the terminals of the arrester.

Maximum Time Current Curve - A curve plotted on log-log paper that describes the time to complete interruption at a particular current for a specific device; a complete set of points (curve) containing the absolute maximum time to circuit interruption at a specific current for a particular device.

MCM - Thousand circular mils-conductor sizing above 4/0.

MCOV - Refer to Maximum Continuous Operating Voltage.

Metal Enclosed - Surrounded by a metal case of housing, usually grounded.

Metalclad - Devices in which the conducting parts are entirely enclosed in a metal casing.

Mineral-Insulated Cable - A cable having bare solid conductor(s) supported and insulated by a highly compressed refractory material enclosed in a liquid- and gas-tight metal tube sheathing; the term includes both the regular type (MI) and the light-weight type (LWMI) unless otherwise qualified.

Minimum Melt Time Current Curve - A curve plotted on log-log paper that describes the time to complete melting for a specific interrupter; a complete set of points (curve) containing the absolute minimum time to melt at a specific current for a particular device.

Mobile Home - A transportable, single- or multiple-section single family dwelling. It is ready for occupancy upon completion of set-up in accordance with required factory-recommended installation instructions.

Mobile Industrial or Commercial Structure - A transportable structure, other than a mobile home, constructed to be towed on its own chassis. Such structures are built specifically for commercial or industrial use, such as construction offices, bunk houses, wash houses, kitchen and dining units, libraries, TV units, industrial display units, laboratory units, and medical clinics.

Motors - special terminology and types:

Hermetic Refrigerant Motor-Compressor - a compressor unit in which the compressor and motor are housed within a single container structure with no external shaft or shaft seals, or the motor is housed within a container structure integral with the compressor structure, so that the motor windings operate within a refrigerant atmosphere.

Locked Rotor Current Rating - a current rating marked on electric equipment or, where not marked, is deemed to be equal to six times the full load current rating from the nameplate of the equipment.

Non-continuous Duty Motor - a motor having characteristics or ratings defined as Short-time duty, Intermittent duty, Periodic duty, and Varying duty (see definition for Duty).

Rated Load Current (for a hermetic refrigerant motor-compressor) - a value marked on a hermetic motorcompressor intended for use where applicable to ascertain wiring, protection, and control for the unit.

Service, Continuous Duty - any application of a motor where the motor can operate continuously with load under any normal or abnormal condition of use.

Service, Non-continuous Duty - an application of a motor where the apparatus driven by the motor has the characteristics defined as Short-time duty, Intermittent duty, Periodic duty, and Varying duty (see definition for Duty).

Service Factor - a multiplier that, when applied to the rated horsepower of an AC motor, to the rated armature current of a DC motor, or to the rated output of a generator, indicates an allowed loading that may be carried continuously at rated voltage and frequency.

Motor Controller — the operative units of the control system comprising the starter device(s) and power conversion equipment used to drive an electric motor, or the pumping unit used to power hydraulic control equipment.

Operating Device - the car switch, push buttons, key or toggle switch(es), or other devices used to activate the controller.

Signal Equipment - audible and visual equipment such as chimes, gongs, lights, and displays that convey information to the user.

Motor-Circuit Switch - A switch rated in horsepower.

MSG - The Manufacturer's Standard Gauge for uncoated steel.

Multi-Outlet Assembly - A surface or flush enclosure carrying conductors for extending one 2-wire or multi-wire branch circuit to two or more receptacles of the grounding type which are attached to the enclosure.

Multiple Section Mobile Unit - A single structure composed of separate mobile units, each towable on its own chassis, which, when towed to the site, are coupled together mechanically and electrically to form a single structure.

Multi-Winding Motor - A motor having multiple and/or tapped windings, intended to be connected or re-connected in two or more configurations, for operation at any one of two or more speeds and/or voltages.

Multi-Wire Branch Circuit - A branch circuit consisting of two or more ungrounded conductors having a voltage difference between them and an identified grounded conductor having equal voltage between it and each ungrounded conductor, with this grounded conductor connected to the neutral conductor.