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
MCOV - Refer to Maximum Continuous Operating
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
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,
wash houses, kitchen and dining units, libraries, TV units, industrial
display units, laboratory units, and medical
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
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.
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
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
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.