Strange 3 Phase Delta Reading

[fifty60]

I have a 240V 3PH outlet that I am plugging in a 240V Delta to 480V Delta transformer. When I check the voltage to ground at the outlet, I get 240V between L1-G, L2-G, and L3-G. This all appears fine. But when I check the phase to phase voltage, I get 240V betwee L1-L3, 240V between L2 and L3, but less than 1 volt between L1 and L2.

My immediate instincts was that it could be a corner grounded delta system, but it doesn't add up. I still should be reading 240V between phases on a corner grounded delta system, right? Why would I be seeing 240V L-G, and 240V Ph-Ph except on L1 and L2 where I only see less than 1 V?

 

[jaggedben]

If you're seeing 240V between L1-G, L2-G, and L3-G on a 'delta', that doesn't 'appear fine' to me in the first place. It shouldn't be true on either a corner grounded or ungrounded system, not to mention high-leg. I'm not sure how you get that on a delta at all.

My first thought was that ground was somehow swapped with L1 or L2 (perhaps in your mind, if not physically). But then one of your hot to ground readings should still be zero, so that doesn't add up either.

 

[Jraef]

You don't have 3 phase...
Someone fed your 3 phase panel with single phase and just tied L2 to two of the bus bars...

I've seen it, twice! Someone likely making do with what they had available on the day it had to be installed. If nothing in the facility actually needed 3 phase power, it has worked fine the entire time, so long as nobody tried to hook up a 240V load between L2 and L3, which is how I found it both times.

 

[JFletcher]

They make plug-in 240/480 xfmrs?

You are correct in that a corner grounded delta would have 240V L-L x3, the corner would have 0V to ground. You have a different problem. Could be as jraef mentioned. Could be the outlet is miswired or mis-breakered and has two of one line and one of a second. That would be a fairly simple fix in the panel, assuming that the panel is 3ph, not 1ph.

 

[jaggedben]

You don't have 3 phase...
Someone fed your 3 phase panel with single phase and just tied L2 to two of the bus bars...

I've seen it, twice! Someone likely making do with what they had available on the day it had to be installed. If nothing in the facility actually needed 3 phase power, it has worked fine the entire time, so long as nobody tried to hook up a 240V load between L2 and L3, which is how I found it both times.

How do you know they didn't tie L1 to two of the busbars?

When you found it twice, was it 240 to ground though? Or 120?

 

[Jraef]

How do you know they didn't tie L1 to two of the busbars?

When you found it twice, was it 240 to ground though? Or 120?

Both times it was L2 jumpered to both bus bars, I attributed it to people reading left to right... L1 is starting on the left, L2 is next, then since there is no L3, you jump over from L2.

You're right though, it was 120V to ground on "all 3 legs" on mine, not 240V. That does make this scenario not fit then...

That really only leaves JFletcher's hypothesis then, a mis-wired outlet; they ran L2 to it twice. It would have to be a 240V 3 phase 3 wire system as well, no center tapped leg.

 

[jumper]

You don't have 3 phase...
Someone fed your 3 phase panel with single phase and just tied L2 to two of the bus bars...

I've seen it, twice! Someone likely making do with what they had available on the day it had to be installed. If nothing in the facility actually needed 3 phase power, it has worked fine the entire time, so long as nobody tried to hook up a 240V load between L2 and L3, which is how I found it both times.

Nice job.

I saw this thread earlier and was “ I got no clue”.

 

[GoldDigger]

Both times it was L2 jumpered to both bus bars, I attributed it to people reading left to right... L1 is starting on the left, L2 is next, then since there is no L3, you jump over from L2.

You're right though, it was 120V to ground on "all 3 legs" on mine, not 240V. That does make this scenario not fit then...

That really only leaves JFletcher's hypothesis then, a mis-wired outlet; they ran L2 to it twice. It would have to be a 240V 3 phase 3 wire system as well, no center tapped leg.

One simple way to run the same L twice is not to realize that both poles of a tandem/duplex breaker are fed from the same bus stab. Putting different colored wires on them does not help.

 

[JFletcher]

One simple way to run the same L twice is not to realize that both poles of a tandem/duplex breaker are fed from the same bus stab. Putting different colored wires on them does not help.

That's still working from the 3ph panel wired with 1ph theory. anything is possible, tho I want to believe this receptacle is cobbled together from 2 breakers rather than someone wasted a 3ph panel with a 1ph feed.


For all we know there could be a jumper on the receptacle rather than an idiotic and catastrophic panel feed failure.

 

[fifty60]

I believe the issue is something similar to what Jraef initially suggested with one phase spread to two seperate phases....still looking into this. For the life of my however, I am not able to figure out what the voltages to ground should be on an ungrounded 3 phase Delta 240V system should be.

I know that between the phases should be 240V, and there is not neutral, but what should the voltage be in an ungrounded delta, phase to ground?

 

[electrofelon]

With a high impedence meter, you would get about 138 volts phase to ground on an ungrounded 240V delta, but this could vary, potentially significantly due to different capacitive values and leakage current across the conductors and coils.

 

[jim dungar]

I know that between the phases should be 240V, and there is not neutral, but what should the voltage be in an ungrounded delta, phase to ground?

The voltage can be almost any amount. The value will depend on the current balance between phases and the amount of 'coupling capacitance' between each line conductor and ground.
In a perfect world everything would be balanced so Vlg = Vll/1.73.

 

[kwired]

I believe the issue is something similar to what Jraef initially suggested with one phase spread to two seperate phases....still looking into this. For the life of my however, I am not able to figure out what the voltages to ground should be on an ungrounded 3 phase Delta 240V system should be.

I know that between the phases should be 240V, and there is not neutral, but what should the voltage be in an ungrounded delta, phase to ground?

With no ground reference the voltage to ground can be anything depending on conditions at the time of measurement.

I do agree that what Jraef initially suggested may not be exactly what is going on but is probably something close to that scenario.


The two lines that have little/no voltage between them are connected together somewhere or even through a load, which could happen if you had open circuit in a phase conductor somewhere between your measurement point and the source.

Blown fuse in one line could give that reading.