A friend is on site in Mexico working on an A/V system. The power is split phase.

He measures 228.7 volts from phase to phase. That's running the projector. But he also measures 129v on one phase and 128V on the other phase (presumably to neutral!). Whenever we've measured split phase power in the US, the two phases to neutral add together to the phase to phase voltage.

Any idea what's happening here? Is it possible that the two phases are less than 180 degrees out of phase?

Thanks.

There's your problem right there. ^

But in all honesty, it sounds like the meter is malfunctioning. Does your friend have another meter to confirm with? Is he measuring at the panel with the face off?

Besides, with the neutral to phase measuring at 128 and 129 is high. Even for Mexico, as they use the same voltages as the US and Canada. Just different plug in some, but not all, areas.

[Link: adaptelec.com]

KOT

EDIT - Check that, read it as 208.7 volts not 228.7. Time to call it a day and watch some March Madness games.

Did they measure to the actual grounding stake (do they even use them?) that was driven into the ground as their reference? In DC circuits with batteries, the battery posts are the point of reference, not the cable clamp.

Was this measurement taken at the projector, or at the breaker/fuse panel? If at the breaker, I would recommend the other as an additional test because it really seems that the wiring has some resistance that shouldn't be there- possibly from bad connections at an intermediate point.

Do they have any three-way switches? Make sure they're not switching the neutral. If one of those is leaky/switching the neutral, it would be possible for AC from one phase to coincide with the other, bucking some voltage.

What voltage should they have seen, under normal conditions? If the building has metal siding with a three-way switch in a metal junction box, tell them to be careful- I got a few good jolts from my parents' garage, due to the three-way switch switching the neutral and making the box hot when the switches were in a particular position.

Ah, switching the neutral, my favorite wake-up!

He's an actual engineer, but he also knows what he's doing. I didn't want to be snarky and say what KOT said, but it was my instant answer, to be sure.

My guess is that he measured at the panel... gimme a minute

He did not answer my exact questions. I bet he measured at the projector, both phases to neutral.

His response:
I'm gonna ask the electrician to open the panel for me* tomorrow. I only traced back to the UPS.** They have transformers and backup generators and panel all over the place. Hurricane Territory

*He DOES know how to work a screwdriver, though being an engineer.
**This leaves the field wide open to issues!

Last time we worked together, the client went for a backup UPS. We ended up getting an 80 kVA unit because 40 wasn't big enough and there was a two- week delivery delay on the 60 kVA (but then it took two months for them to engineer the unit into their power plant). That system measured a draw of about 17 kVA draw when we played "The Art of Flight" at about 110 dB.... And it can play an entire movie on the UPS. Hey, client's choice, right?

Knowing how to use a screwdriver and being an engineer. Are different from using a screw driver with a chipped end (malfunction). But in this case the chipped end is a very obvious sign that something is wrong. In terms of having a malfunctioning meter, it's not so obvious unless there is another known working one.


Does anyone?

KOT

There is a netural in the plugs?
The last time I played with 220 was in Europe (when I was 12y.o.), and there were only 2 prongs plus ground.

It should be obvious but also realize that 220V world uses 50Hz vs. our 60.

Deleted - didn't notice KOT's post linking to same page.

An engineer, eh? So was the guy who wired that 3-way switch for my parents' garage.

I would shut everything off, measure both phases at the panel, close the breakers and do it again (preferably, with no major loads like 'frige, AC, etc. I would also measure from the panel's neutral to the water supply pipe and any ground rod(s).

How old is the house? If it's older, I would go around, tightening screws and looking for corrosion. What about corrosion in the panel? If the house is near the ocean,....

The chipped end shows that the engineer didn't know which end to pound on.

Makes a fine chisel, too.

I suspect you may be on the right track with the though of less than 180 degrees out of phase. Or it just may be a crummy transformer or other issue.

For most homes in the US, you have a single phase of 240 that is also tapped at the center of the pole transformer and grounded as neutral. 120 from either leg to neutral. 240 Between legs.

For commercial power you have three phases of power. You also have a center tap in the middle of all phases grounded as neutral. So 120 from any phase to neutral, but 208 between any two phases. 208 because they are 120 degrees out of phase with each other.

This web site will help:
[Link: oempanels.com]

3-phase in Mexico and other countries...
[Link: worldstandards.eu]

Single-phase in Mexico and other countries...
[Link: worldstandards.eu]

KOT

This sounds like a 120/208 volt wye style transformer. This will give you 120 volts between center tap of the transformer and each leg of the wye and then 208 volts across each leg of the wye.

Normally this is decided on by the power company in the area. The only design problem with this setup is that it takes more amps to power the same load at the lower 208 volts versus 240 volts in a delta transformer. Or your system will have lower capacity at the 208 volts.

Not likely. More likely there's a safety ground.


Like you said.


Not all of it. And most of it is not 220V but 230V, and in Saudi it's called 230V but often is as high as 240V. And 60 Hz.

Even if it were 50 Hz, that should not affect a meter reading. Even less should it affect three measurements with the same meter at the same place within minutes of one another.


I just can't see how that's possible, though!


Didn't need help with this. He has supposed 120-0-120, not three phase. And when we had to power up the 240 VAC projector for the Saudi client, we built a couple of transformers onto a hand truck to jack up 208 three phase to 240 three phase. So, been close enough to there to say been there, done that.


I know you're trying to help, but how does 128 volts and 129 volts "sound like" 208 volts?

It's not a big leap to suppose that he's telling me it's split phase but it's really three phase. It's a HUGE leap to suppose that a meter will read 230V across phases but then read 208 volts as about 130 volts! That wouldn't even make sense with an analog meter where the needle sticks and can't show full scale. You have insulted his Fluke digital meter!