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what is the meaning of rms value
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Answer Posted / gautham
“RMS” stands for root-meansquare.
It comes from a
mathematical 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.
Electrical power system
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 component
in question.
If a current clamp is labeled
and specified to respond to the
true-rms value of current, it
means that the clamp’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.
Certain low-cost current
clamps which don’t have truerms
circuitry, use a short cut
method to find the rms value.
These meters are specified to
be “average responding-rms
indicating.” These meters
capture the rectified average of
an ac waveform and scale 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 average 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.
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