Page 199 - An Introduction To Predictive Maintenance

Thermography 189

feeders and circuits to provide power to
devices.
Electromagnetic interference (EMI) A term that describes electrically induced
noise or transients.
Filter An electronic device that opposes the
passage of a certain frequency band while
allowing other frequencies to pass. Filters
are designed to produce four different
results: (1) a high-pass filter allows all
signals above a given frequency to pass; (2)
a low-pass filter allows only frequencies
below a given frequency to pass; (3) a
bandpass filter allows a given band of fre-
quencies to pass while attenuating all
others; and (4) a trap filter allows all fre-
quencies to pass but acts as a high-imped-
ance device to the tuned frequency of the
filter.
Flashover Arcing that is caused by the breakdown of
insulation between two conductors where a
high current flow exists, with a high poten-
tial difference between the conductors.
Fuse A device that automatically self-destructs
when the current passing through it
exceeds the rated value of the fuse. A plug-
in protector with a filament that melts or
burns out when overloaded.
Ground A general term that refers to the point
at which other portions of a circuit are
referenced when making measurements. A
power system’s grounding is that point to
which the neutral conductor, safety ground,
and building ground are connected. This
grounding electrode may be a water pipe,
driven ground rod, or the steel frame of the
building.
Harmonic A frequency that is a multiple of the fun-
damental frequency. For example, 120Hz
is the second harmonic of 60Hz, 180Hz is
the third harmonic, and so forth.
Harmonic distortion Excessive harmonic content that distorts
the normal sinusoidal waveform is har-