Previous in Forum: Electrical Short Circuit Test in Transformer   Next in Forum: MCB. Knob Spring Grade
Close
Close
Close
12 comments
Power-User

Join Date: Jul 2011
Posts: 193
Good Answers: 3

Practical Use by Electrician of RMS Meter.

04/05/2013 6:29 AM

I know this is basic and I have tried to google to no avail.

What in actual real life do electricians use true RMS meters for? Are decisions about gauge of cable/fuses used based on use of true RMS meter? What else, if anything?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
2
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21024
Good Answers: 791
#1

Re: Practical use by electrician of RMS meter.

04/05/2013 7:13 AM

RMS meters are used for accurate power measurements. They are not used for sizing cables/fuses, which instead are sized according to electrical code rules.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply Good Answer (Score 2)
Power-User

Join Date: Jul 2011
Posts: 193
Good Answers: 3
#3
In reply to #1

Re: Practical use by electrician of RMS meter.

04/05/2013 3:25 PM

Thanks thats helpful and I do appreciate your input. To be honest the form of answer I am seeking goes like this "The most common time an electrician will utilise their meters RMS capability is when..........such and such........ and then the electrician will do..... such and such.... in response to their meter reading.

Register to Reply
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1605
Good Answers: 63
#2

Re: Practical Use by Electrician of RMS Meter.

04/05/2013 3:05 PM

Tank Circuit

You seem to be confused regarding your understanding of meters. Tell us where you received this information so you can get the right information. True RMS meters are used for more accurate measurements when harmonics are involved. Average reading meter will not give an accurate reading under these conditions. However, most electricians are not called upon to make these type readings. Consider electricians that only work on residential or small commercial projects.

http://www.escoglobal.com/resources/pdf/white-papers/True_G2.pdf

Register to Reply
Power-User

Join Date: Jul 2011
Posts: 193
Good Answers: 3
#4
In reply to #2

Re: Practical Use by Electrician of RMS Meter.

04/05/2013 3:50 PM

So are you saying that an electrician working with something like a Phase Cutting Speed Control or other industrial gear will need to use RMS meters, but electricians that work on residential property will hardly ever/never use a meters RMS ability and could just as well own an average responding meter?

Thankyou for your input.

Register to Reply
Power-User

Join Date: Sep 2011
Posts: 267
Good Answers: 7
#5
In reply to #4

Re: Practical Use by Electrician of RMS Meter.

04/05/2013 11:00 PM

I'd give that a yes.

But, back up a bit. Non TRMS meters are usually average responding, RMS reading and assume the input is a sine wave.

Estimating power from a phase angle fired SCR to a resistive load would require an TRMS meter. So would the current from a PC switchmode power supply and a florescent ballast or anytime harmonics are present.

Remember TRMS is basically the equivalent value of DC that would produce the same power.

Register to Reply Score 1 for Good Answer
Guru

Join Date: Nov 2006
Location: OH USA
Posts: 549
Good Answers: 27
#7
In reply to #5

Re: Practical Use by Electrician of RMS Meter.

04/06/2013 7:28 AM

Most RMS meters are actually peak responding, rms reading to account for variation in periodical waveshapes.

Register to Reply Score 1 for Off Topic
Power-User

Join Date: May 2010
Location: Cambridge, MA
Posts: 390
Good Answers: 82
#10
In reply to #7

Re: Practical Use by Electrician of RMS Meter.

04/06/2013 1:23 PM

Yes, "rms meters" may be and probably usually are meade with a peak responding rectifier, as you say, but "true rms meters"are another matter entirely, with a little RMS calculating machine: square the signal, average this and perform a square root on the average. Most meters use one of the high-speed AD536A or the cheaper AD636, AD637 or AD737 chips from Analog Devices. These perform the transfer function Vout = (avg Vin2)½, to an accuracy of 0.2% to 1% (max) of the reading, depending on the part you choose.

The scheme in the original Analog Devices design, shown in the AD636 drawing, was to first take the absolute value of the signal, then twice the log of the signal (two transistors in series in the feedback loop), create a current proportional to this, average the current with a capacitor, and place it in a square-rooting feedback loop. As a bonus in this circuit, you also get a logarithmic dB output.

The small 8-pin AD737 chip is probably what you find in most handheld true-rms meters. It's a low-current part (5V, 130uA), featuring a power-down mode, selling for $7 at DigiKey. It's clever circuitry performs the same functions as the schematic above, but with a different transistor layout, see Figure 1 in the datasheet.

Analog Devices has application notes explaining the use of this technology. There is some competition for the Analog Devices chips, Linear Technology offers the LTC1968, also available at DigiKey. This chip uses a delta-sigma computation method.

__________________
Thanks, Win
Register to Reply Score 1 for Good Answer
2
Power-User

Join Date: Oct 2007
Location: S. California, USA
Posts: 279
Good Answers: 12
#6

Re: Practical Use by Electrician of RMS Meter.

04/06/2013 3:24 AM

An averaging meter and a true RMS meter will read the same when measuring a sine wave. A true RMS meter is required when the ac is not sinusoidal.

Register to Reply Good Answer (Score 2)
Guru
United States - Member - Born, raised halfway 'round .....

Join Date: May 2010
Location: Metro.Manila, Philippines.
Posts: 1211
Good Answers: 25
#8

Re: Practical Use by Electrician of RMS Meter.

04/06/2013 7:48 AM

RMS or Effective power measuring meters are use for more in-depth, critical circuit analysis, as to (the actual efficiency of the system). To determine how much real power had been effectively consumed or used and therefore the equivalent rate, how much money will be coming out of your pocket!

__________________
vsar
Register to Reply
Guru
Popular Science - Cosmology - Let's keep knowledge expanding Engineering Fields - Retired Engineers / Mentors - Hobbies - HAM Radio - New Member

Join Date: Dec 2006
Location: North America, Earth
Posts: 4472
Good Answers: 105
#9

Re: Practical Use by Electrician of RMS Meter.

04/06/2013 11:58 AM

Power in places with heavy loads will have a flattened sine wave. An average responding meter will read in error in such a case where a TRMS meter will read accurately. Use of an oscilloscope will show if there is significant flattening.

__________________
“I would rather have questions that can't be answered than answers that can't be questioned.” - Richard Feynman
Register to Reply
Associate

Join Date: Jul 2012
Posts: 39
#11

Re: Practical Use by Electrician of RMS Meter.

04/08/2013 3:57 AM

Did you really do a google search?

http://en.wikipedia.org/wiki/True_RMS_converter

Register to Reply
Power-User

Join Date: Jul 2011
Posts: 193
Good Answers: 3
#12
In reply to #11

Re: Practical Use by Electrician of RMS Meter.

04/10/2013 7:53 AM

Yes I did. Whats interesting is that many people have responded to this thread by answering the question "What is an RMS meter and how does it differ from an the basic sort?" but that was not the question.

Register to Reply
Register to Reply 12 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Bluestone (1); Dodgy Solenoid (1); KeepItSimpleStupid (1); Snave (1); StandardsGuy (1); Tank Circuit (3); Tornado (1); vsar (1); wareagle (1); Winfield Hill (1)

Previous in Forum: Electrical Short Circuit Test in Transformer   Next in Forum: MCB. Knob Spring Grade

Advertisement