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Power distribution


Jeff Sand
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If there is a better place to post this, please let me know and I will repost...

Okay, so this one (municipal) job site we have brings power up through the ground from underground vaults all over the park. This year it's been rewired into a bunch of metal enclosures (similar to LOR showtime boxes), each one with a 4-conductor 12AWG cable terminated into (2) 20A breakers with a pair of GFCI outlets. The two circuits per enclosure are (I am currently assuming, and has been in years past, but see below) different phases on a three-phase service. Here's where it gets strange. According to my Fluke meter, here's what I measure (volts AC):

Hot-Neutral on any receptical: ~120V
Hot-Hot across opposite recepticals: ~210V
Hot-ground: ~110V
Neutral-ground: 30V (this one is pretty much exact all over the park)

Now before anyone asks, no the ground pins in the recepticals are not floating. The 30V neutral-ground reads the same whether I measure to receptical ground, chassis ground, or shove the probe into the soil. Measurement is also the same regardless if circuit breaker at the box is on or off.

I know they are running three-phase service because it has given us much grief in the past. I've detected that they've done strange things with the wiring before like switching the neutral instead of switching the hot, but I have never seen this bizarre floating neutral before, and they claim to have not rewired anything underground. Although I am certain they did replace the feeder cables to the individual outlet boxes.

I guess my questions are 1) Does this situation pose any risk for the LOR controllers?, and 2) Is the LOR data network really opto-isolated like RS485 should be and do I have any risk of getting this stray voltage into the network cable?

Not really worried about the public getting zapped (our displays are set pretty far back from the sidewalks) but I'd like to not burn anything up nor re-create the cat scene from Christmas Vacation.

Would love anyone's advice.

(p.s. I realize stranded 12AWG at long distances is not correct for a 20A circuit, but we're looking past that for the time being)

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Depending on if the neutral is bonded to the ground in a different location, you may just be reading a ground loop

is there a transformer in the vault?- if not then it is quite possible to have a loop, as the neutral will be from the transformer and the grounding element could be near by.

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I should have been a little more clear with the description. What we're talking about is an outdoor park/water park complex that's about ten years old. There's a number of underground vaults spread around the area up from which these feeder cables come. The original electrical distribution in the park is at least 5 years old, and the receptical boxes and feeders were redone for us by the village over the summer. I've never seen the inside of the main panel myself. There are several hundred feet between it and some of the receptical boxes. Right now I'm leaning toward the "neutral not bonded to earth ground" theory. I wish the village was interested in investigating, but they strike me as incompitent.

I did set up some LORs this afternoon, and they are suffering no ill effects right now. When the lights are running, the neutral voltage is hovering between 26V-29V. That's with lights on (maybe) two phases so far but not on the third. The two circuits I was using today were 200' apart and I didn't feel like running cable between then to test for phase.

Should we put out foot down with the village and make them figure out what the heck is going on?

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Try connecting something like a 100 watt light bulb between neutral and ground. Then measure the voltage across the light bulb. If you still get 30 volts, then you might have a problem. But I'm betting (ok - hoping) that you won't....

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Thanks for all of the advice so far. As stated above, when under a load the potential is still measuring 26-28V, which is slightly under the original measurement of 30V. Now assuming the only transformers in question are at the main panel, if I were to shut off all of the breakers in the system and go to any receptical and measure continuity between neutral and ground on the outlet, that should tell me whether or not they're tied together, somewhere, right?

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Not load to neutral, but a 100W light bulb between neutral and ground... See if the potential still stays, or if the resistance is enough to pull the ground to ground.

If it lights the bulb, I would say you have a pretty effective indication of a problem..

Also, the standard USB-485B adapter is not isolated, as it takes the USB power, boosts it to 10V, and ships it down the line.. There is a more expensive adapter that is isolated, and still supplies accessory power.

- Kevin


Also, the 210V hot to hot is pretty good confirmation you have 208V 3 phase power...

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-klb- wrote:

Not load to neutral, but a 100W light bulb between neutral and ground... See if the potential still stays, or if the resistance is enough to pull the ground to ground.


Ah, I misread what you said above. I will try that today or tomorrow.
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Actually, it was Jim S.'s idea, but it is valid for exploring the magnitude of the issue, and it could be an interesting demonstration of the issue if it actually makes the filament glow.. If it does not pull the neutral to ground, a 24V bulb would likely make a higher impact demonstration of the issue, as it would be running above full brightness...

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