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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
present in an AC system, except under special cir-
(7) Do not use a megohmmeter whose terminal
cumstances. The common frequency range of har-
operating voltage exceeds that which is safe to ap-
monics is O-5 kHz with O-3 Khz being most common.
ply to the equipment under test.
If the harmonic levels are high, they may cause:
b. Operation. The following are general directions
interference to control and communication lines;
for operating a hand-driven megohmmeter. For spe-
heating of ac motors, transformers and conductors;
cific instructions, refer to the megohmmeter manu-
higher reactive power demand and hence poor
facturer's instructions. For megohmmeter connec-
power factor; misoperation of sensitive electronics;
tions when testing low voltage cables or motors,
overloading of shunt capacitors and, higher power
refer to paragraphs 6-3 and 4-5, respectively.
loss. These harmonic currents will accumulate in
(1) Place the megohmmeter on a firm and level
the neutral conductor. Therefore, it is recommended
base. Avoid large masses of iron and strong mag-
that a double ampacity neutral conductor be used.
netic fields.
a. The purpose of harmonic measurements is:
(2) If the megohmmeter has a selector switch,
set it to MEGOHMS + 1.
(1) Monitoring existing values of harmonics
and checking against recommended or admissible
(3) Check infinity by turning the hand crank in
levels.
a clockwise direction. The pointer should move
promptly to infinity. This check is made with no
(2) Testing equipment which generates har-
monics.
connections to the test terminals. If the reading is
(3) Diagnosis and trouble-shooting situations
not infinity, then use the INFINITY ADJUSTER to
where the equipment performance is unacceptable
set the pointer to infinity.
to the utility or to the user.
(4) Check zero by short-circuiting the testing
terminals. Turn the crank slowly. The pointer
(4) Observations of existing background levels
and tracking the trends in time of voltage and cur-
should move promptly to zero or off the lower end of
rent harmonics (daily, monthly, seasonal patterns).
the scale.
b. Basic equipment used for the measurement of
(5) Use well-insulated testing leads connected
nonsinusoidal voltage and currents. The techniques
to the megohmmeter terminals and with opposite
used for harmonics measurements differ from ordi-
ends separated, turn the crank. If the pointer indi-
nary power system measurement. The harmonic
cates less than infinity, there is a leak between the
measurements require more specialized instru-
leads which must be removed before proceeding
ments. Brief descriptions for three generic types of
with tests. Touch together the test ends of the leads
instruments used for harmonic measurements are
while turning the crank to make certain, by a zero
included in this section.
reading, that the leads are not open-circuited.
(1) Oscilloscope. The display of the voltage
(6) Apparatus to be tested must not be live. It
wave-form on the oscilloscope gives immediate
must be taken out of service and disconnected elec-
qualitative information on the degree and type of
trically from all other equipment (para 13-4.1).
distortion. Sometimes cases of resonances are
(7) Connect leads to apparatus to be tested. For
readily identifiable through the multiple peaks
testing to ground, connect from the LINE terminal
to a conductor of the apparatus, and from the
present in the current wave.
(2) Spectrum analyzers. These instruments dis-
EARTH terminal to the frame of a machine, the
sheath of a cable or to a good ground. For testing
play the signal as a function of frequency. A certain
between two conductors, connect test leads to the
range of frequencies is scanned and all the compo-
two conductors.
nents, harmonics and interharmonics of the ana-
lyzed signal are displayed. The display format may
(8) Turn the crank in the clockwise direction
and observe the position of the pointer over the
be a CRT or a chart recorder. For harmonic mea-
scale. It shows the value of the insulation resistance
surements, the harmonic frequencies must be iden-
under test. Take the reading while operating and at
tified by reference to the fundamental frequency. A
a fixed time, preferably 30 or 60 seconds.
wide range of analog and digital types of Spectrum
Analyzers are available in the market.
13-5. Harmonic measurements.
(3) Wave analyzers. Harmonic analyzers or
The increasing use of solid-state switching devices
wave analyzers measure the amplitude (and also
contributes to current wave forms which are
phase angle in more complex units) of a periodic
nonsinusoidal. This distorted current wave form re-
function. These instruments provide the line spec-
sults in a distorted voltage waveform. This distorted
trum of an observed signal. The cutput can be re-
wave form can be viewed as a fundamental 60 Hz
corded or can be monitored with analogue or digital
sine wave with odd multiples of 60 Hz harmonics
meters. An example of these is the Dranetz 636
wave forms. Even harmonics are usually n o t
disturbance wave analyzer. Again instruments with
13-8
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