Big OOPS!

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hilltop

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I have a situation that happened recently and i am wondering if someone can tell me what actually occured electrically under these circumstances.
A shutdown was planned but was not thouroughly investigated and some 277V panels that needed to be kept energized had to have temporary power in a hurry.
The contracted electricians grabbed a 3 phase breaker from a 277/480 panel that appeared to have a nuetral. We found out later that someone had run a nuetral to the nuetral bar but it was never terminated any where in the main distribution. there was a ground, but this too only went as far as the first junction box, so technically the conduit was the only ground.
The only reason it had not been caught before hand is the only thing being supplied by this panel are transformers for other sub panels. Of course the transformers are grounded to building steel.
As a result we lost a bunch of power paks for lighting sensors. They actually were melted in the center. We lost about 15 exit lights, all led, 277V. Now 6 months later we have a bunch of ballasts that are failing. :roll:
 
So the neutral was just floating until you connected it to your single phase lighting loads. (It wasn't fixed at zero volts).

So when you connected it to your 277V lighting loads, it just floated between the 3 different phase voltages. The actual voltage depended on exactly what the phase loads were.

So the neutral voltage might have been (just for example) -100 volts. Run that and a 277V phase conductor to a light fixture, and you have a total 377 Volts across a 277 volt fixture.

Steve
 
Sounds like your lights were not originally from this panel, but got hooked up to it on your shutdown. If this is the case and the neutral is floating then the phase tied to neutral with the least resistance wins (instead of 277 it would get less 0volts maybe) . Devices on the other two phases would see a voltage greater then 277(upto 480). This is a guess based on your description. I have had some trouble with electronics in the past which expect the neutral and ground to be approx. the same potential, which it wouldn't be in your case. Your neutral to ground voltage could be as large as 277v.

I might have not understood your description, I apologize if I am off track.

-Steve
 
If the panel in question was only feeding transformers you most likely did not need a neutral. If that is the case, I don't think this has anything to do with your failures. I think we need more information.
 
Correct. The panel they tapped in to did not need a nuetral but somewhere along the line someone had put a wire under the nuetral lug in this panel (which shouldn't have been there as well) and just left it hang back at the distribution source. Also the ground in this panel was only bonded to a junction box in the home run. It was not taken back to the ground bar at the distribution panel.
So i'm trying to figure out what exactly happened when they tied in to this mess to feed the 277V lighting panel that needed to be kept hot.
The 277V lighting panel was fed with 3 ungrounded phases and basically a floating nuetral with the Panel ground only tied to the home run conduit.
 
The absence of the neutral definetly caused your problems, I stress to my guys if they install a straight three phase panel that does not require a neutral, Remove the neutral bar and clearly mark the panel with the available voltage. Since someone apparently pulled a neutral, but did not terminate it at the source, some blame can be put on the original installers, but some blame could also be placed on the temporary power installer not verifing correct voltages to the load.
 
don_resqcapt19 said:
If the panel in question was only feeding transformers you most likely did not need a neutral. If that is the case, I don't think this has anything to do with your failures. I think we need more information.
Click to expand...

I agree. As I understand it, transformers were the only loads coming out of that panel, and the transformers were bonded to the building steel (point of reference for ground), then the transformers should create their own neutral. I believe your problem is elsewhere. Maybe the transformer's bond to building steel has high resistance for some reason. Or maybe somehow during the shutdown, in the process of keeping the 277v panel hot, someone backfed 277v on the neutral for that circuit and fired everything and is saying nothing. Just a thought
 
It sounds like you're lucky that equipment was the only casualties. It could have been people.
 
I can imagine how this happened. Someone saw the white wire and then measured L-L and L-G voltages only. I can not believe the number of times I have heard from electricians who did not include L-N voltage in their measurements.
 
jim dungar said:
I can imagine how this happened. Someone saw the white wire and then measured L-L and L-G voltages only. I can not believe the number of times I have heard from electricians who did not include L-N voltage in their measurements.
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I agree but still have a hard time believing it though. Even a first period apprentice knows this....

People too often get in a hurry and take shortcuts.:roll:
 
What it sounds like is you applied 480 across your loads in series because of an open neutral, NOT a floating neutral. Floating neutral in field terms refers to a ungrounded neutral.



Per IEEE

Floating Neutral: One whose voltage to ground is free to vary when circuit conditions change.
 
hilltop said:
Correct. The panel they tapped in to did not need a nuetral but somewhere along the line someone had put a wire under the nuetral lug in this panel (which shouldn't have been there as well) and just left it hang back at the distribution source. Also the ground in this panel was only bonded to a junction box in the home run. It was not taken back to the ground bar at the distribution panel.
So i'm trying to figure out what exactly happened when they tied in to this mess to feed the 277V lighting panel that needed to be kept hot.
The 277V lighting panel was fed with 3 ungrounded phases and basically a floating nuetral with the Panel ground only tied to the home run conduit.
Click to expand...
So the original load on this panel was line to line (the transformers) and line to neutral loads were added. If that is the case then you have the classic open neutral case with a large series/parallel circuits with some loads getting high voltage and others getting low voltage. Failures of equipmetn is to be expected in this case.
As far as the contractor making a connection to this panel without checking, even if he checked he would have read the correct voltages with only line to line loads connected. There was a path back to the grounded conductor.
 
brian john said:
What it sounds like is you applied 480 across your loads in series because of an open neutral, NOT a floating neutral. Floating neutral in field terms refers to a ungrounded neutral.



Per IEEE

Floating Neutral: One whose voltage to ground is free to vary when circuit conditions change.
Click to expand...

That's exactly what they had after connecting the Line-Neutral loads. The line to neutral loads connected the neutral, but not to ground. Thus, the neutral voltage was free to vary with the changing circuit conditions.
 
steve66 said:
That's exactly what they had after connecting the Line-Neutral loads. The line to neutral loads connected the neutral, but not to ground. Thus, the neutral voltage was free to vary with the changing circuit conditions.
Click to expand...
The lack of a neutral to ground connection would not cuase this type of problem as that does not change the line to neutral votlage. It appears to me that the issue is that there was not a good path between XO at the transformer and the neutral bus in the panel where the loads were connected.
 
I'd say you learned your lesson about not planning electrical shutdowns and wanting your electrician to try and do things fast in a emergency situation.

The first post was hard to follow so I'm not going to speculate what went wrong. Your electrician acted in good faith by trying to remedy a emergency situation. Perhaps he made a mistake? Better planning and no emergency hook up to whatever panel is available will keep this from happening to you again.
 
don_resqcapt19 said:
The lack of a neutral to ground connection would not cuase this type of problem as that does not change the line to neutral votlage. It appears to me that the issue is that there was not a good path between XO at the transformer and the neutral bus in the panel where the loads were connected.
Click to expand...

It doesn't seem like we are talking about the same scenario. From the original post, it sounds like the neutral bar in the panel was initially open. (The one wire that was connected to the neutral bar wasn't connected to anything else.) So we start with an open neutral.

Then the electricians connected the neutral bar to some single phase 277V loads. Now we have a floating neutral. The voltage at that neutral changes as the ratio's of the loads on the different phases change.
 
steve66 said:
It doesn't seem like we are talking about the same scenario. From the original post, it sounds like the neutral bar in the panel was initially open. (The one wire that was connected to the neutral bar wasn't connected to anything else.) So we start with an open neutral.

Then the electricians connected the neutral bar to some single phase 277V loads. Now we have a floating neutral. The voltage at that neutral changes as the ratio's of the loads on the different phases change.
Click to expand...

There has been many apost on this subject, as I see it, "floating neutral" (really a technical misnoamer) means not bonded to ground, thus not being common to the grounding electrodes and such. voltages between phases and neutral are constant, but voltages to ground are not. With an open neutral, loads are series and parrallel, causing loads with neutrals to be in series, with some loads having higher voltages, and the opposite loads having lower voltages, but the total being 480 volts in this case.
 
For those of you that forgot the OP it said this was clearly an open neutral situation. My emphasis.

hilltop said:
We found out later that someone had run a nuetral to the nuetral bar but it was never terminated any where in the main distribution.
Click to expand...
 
The way that I understand it is that there are two different scenarios, which are a floating neutral and and open neutral.

With a floating neutral all loads have a neutral wire connecting the load back to the neutral point of the transformer secondary, however the netural point is not bonded to ground. In this case, depending on the load balance, the neutral voltage with reference to ground is free to float to any voltage determinded by the current impbalance. (Ia + Ib + Ic = In). It is possible in this case to then have a neutral reference with 100V like someone had mentioned, and therefore have an higher than rated voltage across L-N loads. Bonding this neutral point to ground ensures that the neutral point has a 0V reference and all L-N loads are correct voltge.

With an open neutral, there is no neutral return wire from the load to back to the neutral point on the transformer. Therefore no current will flow in the neutral but will instead flow in series through any loads connected L-N. These loads in series will then create voltge dividers which will have different voltages dropped across the loads depending on the impedences of the loads. This situation can lead to higher than rated L-N voltage being across a given load.

For the particular situation mentioned in the OP I agree that this is a case of an Open Nuetral. The OP mentioned that the neutral path back to the source was broken.
 
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