How Much DC is OK on a Power Line?

I have to take umbrage with the quote above. Unless there is a center-tap on the primary side of the power transformer (and there never is) this DC thing is of no consequence whatsoever as the power transformer will simply see that total voltage across its windings.

It may be a distorted waveform, but there won't be any DC. Keep in mind that one thing transformers get used for all the time is to block DC, and values at considerably higher levels than just a few millivolts.

However, *something* does happen, the distortion I just mentioned. Fluke Instruments published a very nice white paper about 20 years ago that explores this phenomena. It turns out that the primary (no pun intended...) cause of mechanical noise in power transformers is the 5th harmonic (300Hz in the US). The paper also gives a formula to allow one to calculate the distortion on the AC line, if you know the current drawn and the source impedance of the line transformer (perhaps the one on the power pole outside your house) winding.

The 5th harmonic can cause power transformers to become mechanically noisy, power rectifiers to radiate mechanical and electrical noise, and creates forces in hysteresis motors that can make them run backwards.

Hmm. So, with the AC line having one side at ground, there's supposed to be a DC level? The idea that a hair dryer can shift the ground potential to a DC level seem far fetched- it would require that the neutral side of the AC line be not connected to ground.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.

Well Jim, I'm certainly open to finding out I've been wrong about things. Its how we learn :)

As far as the earth connection, what I am talking about here is that the hot side of the line is measured with respect to the neutral (ground). So if there is a DC component present, the AC voltage will be shifted with respect to the neutral. *That*, since the neutral is tied to the earth connection, will take some current and will likely heat the wiring up in the process.

Now if you have as an example Eva's hair dryer, running on half power so it its only conducting when the diode conducts, I can see that that might be interpreted as DC. But, it can also be interpreted as harmonic distortion. I'm not arguing, IOW, that the phenomena does not exist, just the explanation. One could as easily argue that a 2nd harmonic in the output of an amplifier is a DC phenomena as well.

I would like to see Eva's scope readings, but the images don't load off of DIYaudio.