E N G L I S H
41
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 high
enough value to create problems.
When measuring line voltage, a maximum
THD level of 5% or less is considered
acceptable. Below the 5% level, both
measurements (TDH-R and TDH-F) are
essentially the same. For THD-R readings to
THD-F readings using either the following
formula or conversion chart.
As you can see from this table, both THD-R
(%)and THD-F (%) values are essentially
the same when measuring line voltage
where THD readings are often 5% or less.
The values differ only when THD levels exist
above about 20%.
The maximum THD levels on line voltage of
around 5% and the maximum THD levels on
line current of around 20% are common for
a power line.
THD-R (%) versus THD-F (%)
THD-R (%)
THD-F (%)
0.00
0.00
5.00
5.01
10.00
10.05
15.00
15.17
20.00
20.41
25.00
25.82
30.00
31.45
35.00
37.36
40.00
43.64
45.00
50.39
50.00
57.74
55.00
65.86
60.00
75.00
65.00
85.53
70.00
98.02
75.00
113.39
80.00
133.33
85.00
161.36
90.00
206.47
95.00
304.24
100.00
∞
CREST FACTOR versus THD-R (%)
A CREST FACTOR simply indicates whether
harmonic contents are included or not in
the power line.
However, THD-R (%) [or THD-F(%)] at
50Hz (or 60Hz) indicates what percent of
harmonic contents is included in the power
line. Both features are typically found in
much more expensive professional power
analyzing instruments.
Follow these steps to measure THD-R (%) in
the power line:
1. Set the function and range switch to the
