• AC current measuring circuit in this Meter is of root-meansquare
(True-RMS) value system so this Meter can accurately measure
AC
c
urrent of non-sinusoidal wave forms including harmonics
caused by various non-linear loads. The AC crest factor of this
Meter is 3 for the frequency range of 45Hz to 1KHz.
• When measuring current, the Meter
’
s internal shunt resistors
develop a voltage across the Meter’s terminals called “burden
voltage”. Th
i
s voltage drop affect pre
c
i
sion cir
c
uits or
measurements.
Follow these steps to measure DC (or AC) Amps.
1. Set function and range switch to the de
s
i
red
D
C A (or AC A)
range. If you do not know the value of the current to be
measured, always start with the highest range and reduce the
setting as required to obtain a satisfactory reading.
2.
Plug the red test lead into the “mAµA” input terminal (if the 4A
or 10A ranges are being used, plug the red test lead into the “A”
input term
i
nal) and the b
l
ack test lead into the “COM” input
terminal of the instrument.
3. Disconnect the power from the circuit to be tested.
4. Connect the test leads in series to the circuit to be tested.
5. Reapply power to the circuit, the measured current will appear
on the display of the instrument.
6. Disconnect power to the circuit before removing the test leads
from the circuit.
4.5 THD @ 50/60 Hz
®
Mode [509THD Only]
Today, more and more ele
c
trical appliances are u
s
i
ng high
efficiency power
s
upplie
s
and adjustable
s
peed motors to conserve
energy. These appliances conserve power by drawing current in
short pulses using a solid state swit
c
h, This trend, although saving
energy, creates severe problems in power distribution.
28 29
These sol
i
d state switches create non-
l
inear load
s
on the power
distribution systems, which generate harmonics. These harmonics
are odd multiples of the fundamental power line frequen
c
y, and
may cause overheating in transformers, erratic computer operation
and premature tripping of circuit breakers.
One of the most useful measurements of harmonics is Total
Harmonic Distortion THD. This measurement requires special and
expensive instrumentation to indicate the sum of all the harmonic
frequencies present in the signal.
This Meter is designed to indicate the total rms values of the 3rd
harmonic to the infinite order harmonic as a percentage of the total
rms value including the rms value of the fundamental frequency in
the 50Hz/60Hz power lines.
In general, there are two ways to express the THD value:
THD-F = Total Harmonic Distortion as a percent of the
fundamental frequency.
THD-R = Total Harmonic Distortion as a percent of the total rms
value.
Practically speaking, either method is useful when looking at
harmonics of a h
i
gh enough value to create problems. When
measuring line voltage, a maximum THD le
v
el of 5% or less is
considered acceptable. Below the 5% level, both measurements
(THD-R and THD-F) are essentially the same. For THD levels
abo
v
e 5%, you can convert THD-R read
i
ngs to THD-F read
i
ngs
using either the following formula or conversion chart.
F (%)=
100 X R (%)
10000-R
2
(%)
Where F (%)=THD-F (%)and
R (%)=THD-R (%)
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