What's a "root mean squared"?

Maybe more importantly, I've see multimeters advertised as "true RMS," how do they differ from a basic meter?

A "regular" meter assumes that the waveform is a perfect sine wave. A true RMS meter can measure RMS even when the waveform is distorted.

Voltage is always a difference (AC, DC, RMS measurement or whatever). The only thing magic about the "neutral" is that it is connected to your house ground which gives a reference (this is handy because grounds are commonly everywhere which means any voltage differential from ground is a potential shock hazard).

Your 120V device will work fine as long as the two wires feeding it have a 120V differential. A 240V device similarly for 240V. The only time a neutral is needed on 240 circuits is when there is some need to also get a 120V load on the same circuit.

There are a few times where the grounded conductor (neutral) plays a safety role though this is largely been deprecated. Certain (ungrounded) two wire devices (lamps are a common one) use a polarized plug to make sure the exposed metal parts are on the grounded side of the circuit. Older dryer and range installations may be grandfathered to use the grounded conductor to ground the equipment frame as well.

Let me ask this another way, under what circumstances would a having a true RMS meter be beneficial. Or maybe more exactly, would they ever be useful to the non-pro who would just be doing work around the average home?

Thanks, Randy. I understand that part about the apparent differential in the two legs, but my question is how do they get 1/2 phase out of "sync" so to speak to create that differential? I majored in chemistry, not EE...and that was about a half-century ago.

Wait a sec...just thought that through, once I considered the differences between "Edison" electricity and Westinghouse's version...got it.... ;-)

Cheers

There's no way that your 240V is going to get out of phase as there is only ONE phase. It's just that it's voltage it comes from a transformer that is centered tapped to ground such that the legs to ground is 120V.

A true RMS voltmeter is pretty much superfluous on normal commerical power. If you are dealing with inverters or such that generate odd wave forms you might find it useful. Nice thing about true RMS is it's a function of better class meters so you may enjoy having one even if you don't need the true rms feature.

I've used an RMS meter in the lab when I was dealing with things that were not sine waves. If you're going to deal with generators or inverters that don't do true sine waves, then you might think about getting one. But otherwise there is no need to pay extra for the RMS type meter.

What electrical 'malfuction' would cause the 4th wire, ground, to prevent a unsafe situation on a 220 vac circuit? i.e. clother dryer.. Sorry for the contorted sentence composition.

On a 3-wire circuit, the frame and other metal parts of the dryer are bonded to the neutral wire. If for some reason that neutral wire became open and, say, the heating element shorts to the frame, because the neutral is open everything metal on the dryer would become energized.

On a 4-wire circuit, the metal parts of the dryer are grounded via the 4th wire, independantly of the neutral. It would take 3 faults (shorted element, open neutral and open ground) to prevent a breaker to trip while the frame is energized.