multimeters rms v avg. reading for res power systems

[aaatraker]

Is there any reason to use a rms meter as your daily meter for res. home work? As I understand rms meters, there needed where there is harmonic content or distorted waveforms on the system, not what you would fine in most home electrical systems in any large amounts. Someone said because of phase angle, why would that be?

thanks
kurt

 

[bob]

Re: multimeters rms v avg. reading for res power systems

As long as there is no distortion of the sin wave
the true RMS and the AVG meter will read the same.
The AVG should be sufficient in residential work.

 

[rattus]

Re: multimeters rms v avg. reading for res power systems

See no problem, and phase angle has nothing to do with the reading. If the sine wave is clean, the averaging meter or a peak detecting VM should do just fine.

 

[megloff11x]

Re: multimeters rms v avg. reading for res power systems

If the home contains a substantial load from devices with switching power supplies (i.e. many computers, some light dimmer knobs, etc.) then your sine waves may get distorted.

True RMS meters don't cost that much more in most cases. If you ever switch over to working on facility wiring, you'll want True RMS, so why not buy once and buy good?

 

[megloff11x]

Re: multimeters rms v avg. reading for res power systems

I forgot to say, the Fluke web site has a good discussion of regular vs. true RMS DMM's. Look under technical notes.

 

[iwire]

Re: multimeters rms v avg. reading for res power systems

As much as I like cool toys like DMMs I find that an 'old school' solenoid tester (Wiggy) is the best tool for troubleshooting typical line voltage power and lighting circuits.

Really, do I we need to know if the outlet has 117.55 volts or 117.56 volts?

What you want to know is it 240 or 120, is it on or is it off?

With the DMMs you end up with phantom voltage readings and worrying about theft or breakage.

My Fluke 87 stays in the truck unless I really need it.

 

[crossman]

Re: multimeters rms v avg. reading for res power systems

Agree totally with iwire.

Many years ago, I was out on a service call to a church kitchen, their mixer wouldn't work and they said the outlet was dead. I had my simpson 260, stuck the probes in the outlet, and presto, 120 volts. I told the lady that the outlet was good, the mixer must be broken. She said "no, the mixer still works if I plug it in over there."

Hmmmmm.... I go out to the truck and get a drill motor, plug it into the outlet in question, it doesn't work. But Simpson still says 120 volts.

I take the plug out... the box is filled with moisture and dead roaches, and the screw terminal connections are all gunky and green and corroded.

There was enough resistance in the connnections that under load, there was a severe voltage drop and the appliances wouldn't run, but with the Simpson's extremely high impedance, it was reading just about full voltage when not under load. Remember, in series, the largest impedance will drop the majority of the voltage.

I also worked occasionally in a place that pumped brine out of a salt dome and then evaporated the water out for making table salt. Everything in the place was wet and damp and salty. Talk about making a digital meter go nuts! (BTW, they used an ungrounded 480 delta for their equipment)

For typical troubleshooting of everyday problems, give me my Wiggy!

[ February 10, 2005, 03:47 PM: Message edited by: crossman ]