What Is True-RMS And Why Is It So Important?

What is true-rms and why is it important to us? There are two ways that meters can take readings. When they're taking an rms measurement, what we're reading is we're actually reading the rms value of a signal. What the rms relates to is if I take a 120 volt signal, ac, apply it to a resistor and then I take 120 volt DC battery, apply it to the same resistor, it'll heat that resistor to exactly the same temperature. If we use an average responding meter; what an average responding meter does is it takes the average of a signal, multiplies it by 1 point 1.11, and then displays the RMS. So, it's actually average sensing RMS display. This was fine when we had a world of nothing but linear loads. Nowadays, we have a lot of electronic loads such as drives and even simple lighting controls. So, if we take a reading on a signal that is not a clean sine wave, that has some distortion in it, the average responding meter takes the average, multiplies it by the same 1.11. But because the signal has changed, this is now the wrong number and it's giving us the wrong rms measurement. A true- rms meter actually does this conversion inside the meter and gives us the true rms reading. So, when we're working with any type of electronic load, it's important to have a true-rms meter because if we look over at the oscilloscope, here I'm showing the voltage signal in red and the current signal in blue. Even just taking this dimmer switch, you notice how the signal gets distorted. But an important aspect is as it gets distorted, the peak gets higher. So, what happens is you'll actually read a signal too low with an averaging rms versus a true-rms. This could cause you a lot of problems if you have a breaker that's popping intermittently and you measure it with an average responding meter, you're reading 13 amps on a 15 amp breaker you say, "Okay... well that's not the problem." If you take a true-rms meter, you might read 18 amps on that 15 amp breaker and then you're saying, "gee I wonder why it's not popping all the time." So in today's world of electronic loads, it's very important to have a true-rms multimeter or a true-rms clamp meter to get the correct reading on current.

Channel: Fluke Corporation

Views: 156,835

Keywords: digital multimeters, true-rms, true rms, fluke tools, fluke corporation, how to, fluke, AC, ScopeMeter, root mean square, what is true rms, what is true-rms, clamp meters, sinusoidal waveforms, true-rms meter, nonsinusoidal waveforms, non-sinusoidal waveforms, true rms vs average multimeters, true rms measurements, The concept of rms, fluke 289, fluke multimeter

Id: PHixK2d_uZU

Length: 2min 49sec (169 seconds)

Published: Mon Aug 26 2013