Mains voltage with non zero DC, how bad is it ?

[The Electrician]

I don't know about where you live but where I am, a distribution transformer will power a whole street or housing estate so it's pretty likely more than one hair dryer will be used simultaneously.

What I mean by distribution transformer is the pole mounted "pole pig":



You're probably thinking of the larger type found in substations:

[Zero999]

Lol, I've never heard the term pole pig before. Where I live there aren't any transformers on poles. There's a substation at the end of the road which powers half the village.

[sync]

What I mean by distribution transformer is the pole mounted "pole pig":

These are almost non-existent here in Germany. One of our distribution transformer supply up to a few hundreds domestic homes. It outputs 400/230V tree-phase which is directly distributed to the houses. And we are using underground cables. A typical distribution transformer looks like this.

[The Electrician]

This situation cries out for some measurements.  You guys could have many hair dryers on at the same time in the morning when people are getting ready for work.  Build the low pass filter I described and see how much DC you observe.

On my street, no more than 3 houses are served by one pole pig, so the DC from one house where a hair dryer is used on half power isn't communicated to more than 2 other houses.  You might have many hair dryers on at the same time, all on the same circuit!

[sync]

Our plugs are non-polarized. So the hairdryers cancel each other out statistically.
I made the test. I measured the DC of an outlet directly with a mulltimeter (1000V range, 100 PLC integration time). Without the hairdryer the DC level was <30mV. With the hairdryer set to half power (600W) it was ~1V on the outlet of the dryer and ~0.4V on an outlet connected to a different circuit breaker but on the same phase.

[The Electrician]

That's probably enough to cause the saturation effects (in an audio amplifier with a toroidal power transformer) reported by golden ears audiophiles. 

[johansen]

meh, tell the folks that can hear the bad mains power to increase the PSRR of their tube amps.. oh, wait, my bad. what's PSRR? 

anyhow, yes, 1 volt of dc is enough to blow up your mains.

I stick weld with a variety of nonstandard voltages and currents.
running various rods say, 7018 on ac, when they are meant for dc, sometimes you can hear the transformer saturate because the arc is pulling more dc amps than ac on various polarities. i've noticed this with other rods as well. so sometimes i switch to dc, because that rectifying action reduces the power available at the arc.

[sync]

That's probably enough to cause the saturation effects (in an audio amplifier with a toroidal power transformer) reported by golden ears audiophiles. 

Damned! How can I protect my amplifier from this evil DC? Does a blocking capacitor works? How big must it for 1.5kW? And how can I make the noisy hairdryer quieter so I can hear these bad saturation effects?

Seriously, I wounder how much DC a transformer tolerates before it goes into saturation.

[johansen]

Seriously, I wounder how much DC a transformer tolerates before it goes into saturation.

three diodes can be configured to deliver .3v drop in one direction and .6v drop in the other (use 30 amp Schottkys) for testing this yourself.
switch to silicon diodes to get a 1v dc bias, and a D cell battery to get 1.5V, etc.

[ampdoctor]

Everybody is talking about all kinds of odd possibilities but I've yet to see any mention of ghost or phantom voltage.

[CaptnYellowShirt]

Everybody is talking about all kinds of odd possibilities but I've yet to see any mention of ghost or phantom voltage.

How about *just* ghosts or phantoms?

[BravoV]

Decided to take a short video on the test, apologize for the amateur video quality. 



Few notes on the measurements :

- Hairdryer is Philips 1600 watts with 3 selections for the heat & air speed, plugged at cheap watt meter at the front for monitoring the power consumptions.
- Fluke 287 was set using DC over AC mode, while 87V was set at DC Voltage mode.
- Both dmms range were set manually at 500V for the 287 and 600V for the 87V.

So the hair dryer vs DC results :

Off -> DC = 0 Volt
Low @50 watt -> DC = 1.2 Volt 
Mid @650 watt -> DC = 0.6 Volt
High @1300 watt -> DC = 0 Volt


@The Electrician, I can't find my motor start capacitor  , probably buried somewhere in the pile of component junks, and I don't have any other high volt cap available, so no simple RC LPF measurement yet as you've suggested, sorry. 

Comments are welcome.


Attached below photos with the measurements results as above video for better details.

[BravoV]

Everybody is talking about all kinds of odd possibilities but I've yet to see any mention of ghost or phantom voltage.

Is possible that I might interpreted wrongly on my above test that the DC result is a phantom voltage ? 

[BravoV]

Do you see a potential between Neutral  and Ground? If so, how much?

Sorry, missed this question, Neutral to Ground voltage is about <= 1 Volt AC.

[johansen]

you should be able to measure some dc between neutral and ground during the load test, due to the resistance of the neutral line.

the remainder of the dc would be formed by the resistance of the hot line and the incoming supply.

[BravoV]

you should be able to measure some dc between neutral and ground during the load test, due to the resistance of the neutral line.

the remainder of the dc would be formed by the resistance of the hot line and the incoming supply.

The DC measurements between Live (L) and Neutral (N) vs Ground (G) with negative black probe was attached permanently at Ground line :

Off  -> L to G = -0.1 V  and  N to G = 0 volt
Low @50 watt -> L to G  =  0.6 V and N to G = -0.6 volt
Mid @650 watt -> L to G  =  -0.4 V and N to G = -0.2 volt
High @1300 watt -> L to G  = -0.1 V and N to G = 0 volt

What does this mean ?

[johansen]

you should be able to measure some dc between neutral and ground during the load test, due to the resistance of the neutral line.

the remainder of the dc would be formed by the resistance of the hot line and the incoming supply.

The DC measurements between Live (L) and Neutral (N) vs Ground (G) with negative black probe was attached permanently at Ground line :

Off  -> L to G = -0.1 V  and  N to G = 0 volt
Low @50 watt -> L to G  =  0.6 V and N to G = -0.6 volt
Mid @650 watt -> L to G  =  -0.4 V and N to G = -0.2 volt
High @1300 watt -> L to G  = -0.1 V and N to G = 0 volt

What does this mean ?

it means i greatly suspect the .6v reading for the low setting.

the medium setting shows that a third of the dc voltage is across the resistance of the neutral line, the other third across the hot line, and the other third appearing at the breaker box.

the 1300 watt datapoint of .1 volts dc line to ground is also suspect.

[BravoV]

johansen, thanks for reply, but honestly, I still don't get it.