Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

P
commonly used symbol for power in watts or milliwatts.

P<sub>DC</sub>
common symbol for DC power in watts.

P<sub>input</sub>
common symbol for power input to a device in watts.

P<sub>load</sub>
common symbol for power delivered to the load.

P<sub>ref</sub>
common symbol for power reference level in watts or milliwatts.

pad
(1) a device (network) that impedance matches and/or attenuates. Typically used to refer to a coax attenuator.

(2) a concrete foundation, usually prefabricated and used to support power transformers in underground residential distribution work.

pad-mount transformer
a heavily-enclosed distribution transformer mounted at grade level upon a concrete slab or pad.

panelboard
an assembly of one or more panel units containing power buses, automatic overcurrent protective devices, that is placed in a cabinet or cutout box located in or flush on a wall. The assembly can only be accessed from the front and may contain switches for operation of light, heat, or power circuits.

See switchboard
parallel paths
the number of separate paths through the armature winding that exist between the brushes of a DC machine.

In a DC machine's armature, the conductors and coils are placed in their slots and connected to the commutator using either the lap winding method or the wave winding method. The number of conductors that are connected in parallel depend on the number of poles the machine has, and whether the winding connections are lap or wave. For the lap wound armature, the number of parallel paths is found by multiplying the number of poles by the number of revolutions it takes to

fill all the slots of the armature. The number of revolutions it takes to fill the slots is known as the machine's "plex" value. In a simplex wound armature, the "plex" value is 1, duplex has a "plex" value of 2, triplex has a "plex" value of 3, and so on. For the wave wound armature, the number of parallel paths is two times the "plex" value. This same concept

can also be applied to AC machinery.

parasitic capacitance
the generally undesirable and not-designed-for capacitance between two conductors in proximity of one another.

parasitic inductance
the generally undesirable and not-designed-for inductance associated with a conductor, or path of current on a conductor.

parasitic reactance
the generally undesirable and not-designed-for reactance associated with one or more conductors in a circuit.

parasitic resistance
the generally undesirable and not-designed-for resistance associated with a conductor, or path of current on a conductor.

peak let-through current
the maximum value of the available short-circuit current that is let through a current-limiting fuse.

See current limiting fuse
per-unit system
a dimensionless system for expressing each quantity in terms of a fractional part of a "base" value, often the nominal or rated value of the system. Typical electrical calculations require four base quantities (voltage, current, impedance, and apparent power), any two of which may be chosen arbitrarily. The per-unit system greatly simplifies calculations in electrical systems containing transformers with non-unity turns ratios, making the voltage differences transparent.

percent impedance
the per-unit impedance expressed as a percentage on a certain MVA and voltage base.

percent system
a variation of the per-unit system in which the ratios expressing system quantities are expressed as a percentage of the base quantity.

permanent fault
a fault that remains in existence indefinitely if no corrective actions are taken.

permanent magnet (PM)
a magnet that produces an external magnetic field by virtue of the alignment of domains inside the material and retains its magnetism after being subjected to demagnetizing fields.

permanent magnet AC motor
a generic term used to describe both permanent magnet synchronous motors and brushless DC motors.