Treeman, Welcome to ECN. True RMS meters solve some of the inaccuracies of normal "Averaging" type meters. True RMS meters are alot more accurate with any sort of power system, but where they come into thier own, is where the AC sine wave has become distorted, for a variety of reasons, including Harmonic content in the supply. A normal averaging type meter can be up to 30% "out" (more with cheaper meters) compared to the reading you should be getting. With Non-linear loads becoming more common-place these days, a True RMS meter is becoming more a necessity, than a luxury.
Re: What circumstances require True RMS meter#53108 06/16/0508:57 PM06/16/0508:57 PM
So that brings up two questions: What causes non-linear loads, and do we throw away our old meters?
The standard AC waveform is assumed by simpler meters to be what you're measuring. When all you have are resistive loads, and simple motors, and single phase service, these simpler meters are perfectly fine.
With the advent of electronics, all this changed. Simply put, the electronics in some equipment can mess up the electricla supply to the point that your neutrals can become overloades- while a simple meter will read a far lower load than what is actually present. For this to happen, you need two things to be present: three-phase service, and a major part of a circuits' load to be run through some electronics. What sort of electronics? Fluorescent light ballasts and variable frequency controlled motors are the prime culprits. If these loads distort the power supply enough, not only are the neutrals overloaded, but transformers are also overloaded. The distorted waveform in turn can cause other electronics to misbehave, leading to other problems.
How do you tell if you have a problem? This is where you use both the "standard" meter, as well as the "true RMS" meter. Compare the readings...if they are within 12%, you don't have a problem. Over 15%, and you need to fix it.
Apart from all of the fancy, expensive stuff that is sold to supposedly correct such "power quality" issues, there are a few things you can do yourself that will greatly help. Don't share Neutrals. Don't downsize neutrals. Look for equipment -especially ballasts- that claim little "harmonic distortion." Run power for sensitive stuff through a UPS.
But- and this is often overlooked- these issues don't arise if you don't have three-phase power.
If you have lots of big motors starting up at the same time, you may have another issue- "power factor." This has the practical effect of making you pay for power you're not using- and it also upsets the power company. There are specialty companies, such as S&C Electric, that address such issues. They essentially install a bank of capacitors to off-set the reactance of the motor windings.
Either of these issues are well beyond the expertise of maintenance personnel, as well as most electricians. For most uses, the simpler meters are perfectly adequate.
Re: What circumstances require True RMS meter#53109 06/16/0511:31 PM06/16/0511:31 PM