Application Note

3 Fluke Corporation Why true-rms matters for HVAC technicians
RMS voltage conversions for sine waves
To convert To Multiply by
Rms Average .9
Rms Peak 1.414
Average Rms 1.111
Average Peak 1.567
Peak Rms .707
Peak Average .637
Peak Peak-to-Peak 2
to the true-rms value, it means
that the tool’s internal circuit
calculates the heating value
according to the rms formula.
This method will give the correct
heating value regardless of the
current wave shape.
Average responding-rms
indicating tools don’t have true
rms circuitry. Instead, they use
a short cut method to find the
rms value. These meters cap-
ture the rectified average of an
ac waveform and multiply the
number by 1.1 to calculate the
rms value. In other words, the
value they display is not a true
value, but rather is a calculated
value based on an assumption
about the wave shape. The aver-
age responding method works
for pure sine waves but can lead
to large reading errors up to
40 percent, when a waveform
is distorted by nonlinear loads
such as adjustable speed drives
or computers. The table below
gives some examples of the
way the two different types of
meters respond to different wave
shapes.
In today’s high tech HVAC
environment the best choice is
to choose and use only true rms
test tools for the best results.
Electrical components such as
fuses, bus bars, conductors,
and thermal elements of circuit
breakers are rated in rms current
because their main limitation
has to do with heat dissipation.
If we want to check an electrical
circuit for overloading, we need
to measure the rms current and
compare the measured value to
the rated value for the compo-
nent in question.
True-rms multimeters and
other test tools respond accu-
rately to ac voltage values
regardless of whether the wave-
form is linear. If a test tool is
labeled and specified to respond
*Example and supporting data courtesy of American Technical
Publishers Inc.
RMS Calculation
V = 165 V
165
Voltage
V = ?
rms
max
+
0
A comparison of average responding and true-rms units
Multimeter type
Response to
sine wave
Response to
square wave
Response to
single phase
diode rectifier
Response to
3 D phase
diode rectifier
Average
responding
Correct 10 % high 40 % low 5 % to 30 % low
True-rms Correct Correct Correct Correct
What is true-rms?
“RMS” stands for root-mean-
square. It comes from a math-
ematical formula that calculates
the “effective” value (or heating
value) of any ac wave shape. In
electrical terms, the ac rms value
is equivalent to the dc heating
value of a particular waveform—
voltage or current. For example,
if a resistive heating element in
an electric furnace is rated at
15 kW of heat at 240 V ac rms,
then we would get the same
amount of heat if we applied
240 V of dc instead of ac.
From a measurement perspec-
tive, the rms value is equal to
.707 of the peak value of the
sine waveform.
V
rms
= V
peak
x .707
For example, say an ac volt-
age source has a positive peak
value of 165 V.*
V
rms
= V
peak
x .707
V
rms
= 165 x .707
V
rms
= 116.655 V
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Printed in U.S.A. 5/2006 2646876 A-EN-N Rev A
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