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What circumstances require True RMS meter #53106 06/16/05 06:22 PM
Joined: Mar 2003
Posts: 6
treeman Offline OP
Junior Member
Watching and talking to the maintenance guys at our facility, I notice none of them carry/use True RMS meters/clamp meters.

The Amprobe site (tips) recommends always using a True RMS meter because you never know for sure what you are working on.

What situations require the use of a True RMS meter ie: non sinsuidol waves.

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Re: What circumstances require True RMS meter #53107 06/16/05 07:36 PM
Joined: Jul 2002
Posts: 8,329
Trumpy Offline
Welcome to ECN. [Linked Image]
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 06/16/05 08:57 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
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 06/16/05 11:31 PM
Joined: Mar 2003
Posts: 6
treeman Offline OP
Junior Member
Excellent definition of True RMS and what causes non-linear loads.

Thank you.

Re: What circumstances require True RMS meter #53110 06/17/05 07:28 AM
Joined: Jan 2003
Posts: 4,391
iwire Offline
Treeman welcome to the forum.

The Amprobe site (tips) recommends always using a True RMS meter because you never know for sure what you are working on.

That is like asking the American Dairy Council if Milk is good for you. [Linked Image]

Not the most unbiased opinion.

I agree with much of what has been said above.

However, in the course of general electrical troubleshooting I do not feel the accuracy of a true RMS meter is that important.

Yes I own some and will use them for technical work like VFDs, UPS systems or a maybe a transformer.

But when fixing an outlet that is dead it makes little difference. All we need to know is if it is live or dead. I still keep my trusty wiggys / continuity tester with me.

My true RMS DMMs are to prone to reading 'phantom' voltages when used for general troubleshooting

Bob Badger
Construction & Maintenance Electrician
Re: What circumstances require True RMS meter #53111 06/17/05 08:13 AM
Joined: Apr 2004
Posts: 810
Theelectrikid Offline
Me and my father were talking about this last night. For cable work, he needs an RMS Meter to know the precise amount of voltage on the TV cable, which comes from the power supply or amplifier.

Is there anyone on board who knows how to fly a plane?
Re: What circumstances require True RMS meter #53112 06/17/05 02:33 PM
Joined: Mar 2003
Posts: 6
treeman Offline OP
Junior Member
In my greenhouse facilities, I do have three phase circuits that feed 277v flourescent light banks. Other circuits are 1 phase for 110 outlets and 220 exhaust fan motors.

Can the three phase circuits "bleed over" to create harmonics problem in the separate 1 phase circuits?

Re: What circumstances require True RMS meter #53113 06/17/05 03:29 PM
Joined: Jul 2004
Posts: 9,513
gfretwell Online Content
The triplin harmonic that burns up neutrals is a 3p wye thing but you can still have harmonmics on single phase.

Greg Fretwell


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