grisprince
Member
- Location
- la, ca
three phase 240 volt delta, ungrounded, no neutral. Voltage to ground phase 1 is 120 volt, phase 2 is 120 volt, phase 3 is 240 volt, all voltages between phases is 240 volts: whats up?
3 phase voltage
- Thread starter grisprince
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If it's truly an ungrounded 3 phase delta system, voltages to ground are somewhat meaningless.. Might be "capacitance" type voltages which will change depending on various items including the type meter being used to make the measurement.
ptonsparky
Tom
- Occupation
- EC - retired
It sounds more like a Hi Leg Delta where one of the transformers has a center tap ground. Typically the hi leg would be about 208 to ground.
A fault on an ungrounded system would give some odd voltages to ground but yours sound too even. Better educated will chime in. Welcome to the Forum.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Sounds like a high-leg to me, too, and either a typo or other error on the high-leg to ground reading.
If you are looking at a feeder or branch circuit that does not have a neutral then you might think it's 'ungrounded', but if the system has a grounded neutral back at the source then it is still a grounded system.
Welcome to the forum.
If you are looking at a feeder or branch circuit that does not have a neutral then you might think it's 'ungrounded', but if the system has a grounded neutral back at the source then it is still a grounded system.
Welcome to the forum.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190608-1918 EDT
grisprince:
There is clearly an error in your first post. Your voltage from the third phase to ground is not 240, but closer to 208. Others have pointed this out.
I would have to do something very strange in a circuit to get the voltages you report.
If you have a 3 phase system and get 240 between each of the pairs AB, BC, and CA you confirm that you have 240 delta or wye 3 phase source.
That you read exactly 120 between two lines and earth implies that there is a center tapped winding with its center tap connected to earth between those two lines. If you draw the phasor diagram for these conditions you will find that the voltage from the third line to the center tap or earth is 208 V.
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grisprince:
There is clearly an error in your first post. Your voltage from the third phase to ground is not 240, but closer to 208. Others have pointed this out.
I would have to do something very strange in a circuit to get the voltages you report.
If you have a 3 phase system and get 240 between each of the pairs AB, BC, and CA you confirm that you have 240 delta or wye 3 phase source.
That you read exactly 120 between two lines and earth implies that there is a center tapped winding with its center tap connected to earth between those two lines. If you draw the phasor diagram for these conditions you will find that the voltage from the third line to the center tap or earth is 208 V.
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grisprince
Member
- Location
- la, ca
it is an electronic meter the voltage to ground on one leg is 240/250 volt it is a true floating sys which pasadena no longer lets installiation thanks for the reply
It can not be an ungrounded system with the voltages you posted if the meter is correct.
Please give us the meter manufacturer and model number.
Roger
Are there true ungrounded deltas..?
I was directed several years ago by an engineer to install a system in an existing building for several machine tools.
Why I ask is several years before that it was mentioned to me that you always want to ground one leg because if you don't, a leg may ground in the field and you would never know and you would be creating potential to ground.
Thank you
I was directed several years ago by an engineer to install a system in an existing building for several machine tools.
Why I ask is several years before that it was mentioned to me that you always want to ground one leg because if you don't, a leg may ground in the field and you would never know and you would be creating potential to ground.
Thank you
Yes but they are dinosaurs, if you do use one you will have to supply a GF detector, see 250.21 (B)
Roger
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190608-224 EDT
There are probably many 1,000,000 sq-ft automotive factories that run on ungrounded delta systems. For a very good reason. A single short to ground does not shutdown a $500,000 per hour factory.
Ground fault detection is used to indicate a fault. Troubleshooting can occur while production continues, or delayed to a later time.
I doubt that this has changed since I had close contact with these factories.
Continuous production is an absolute necessity.
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There are probably many 1,000,000 sq-ft automotive factories that run on ungrounded delta systems. For a very good reason. A single short to ground does not shutdown a $500,000 per hour factory.
Ground fault detection is used to indicate a fault. Troubleshooting can occur while production continues, or delayed to a later time.
I doubt that this has changed since I had close contact with these factories.
Continuous production is an absolute necessity.
.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I agree. Decades ago I worked at a steel mill with a 480V ungrounded delta system. On day one of orientation I was told to cease using phase-to-ground measurements for anything because they are meaningless on an ungrounded delta system. “Officially” the voltage reference to ground from any phase is zero, but in practice the readings can be anywhere from 0 to the full line voltage depending on total circuit leakage capacitance. In this case there could simply be machines using single phase 240V connected to those legs that have components in them with enough capacitance to ground that it reads 120V out of pure coincidence.
So bottom line; either you have a high leg delta system and are reading the one phase to ground voltage incorrectly (which you confirmed is not the case) or you have a truly ungrounded delta system in which case you should just ignore any phase to ground readings as meaningless.
The OP says it is ungrounded.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190609-1036 EDT
Besoeker3:
The first post states
His wild leg to earth measurement makes no sense, and implies a measurement error.
Depending on the system stiffness it may be possible to place a 120 load on a leg that reads 120 to earth between that leg and earth. If no major change in this voltage reading, then there is a center tapped winding with its center tap earthed.
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Besoeker3:
The first post states
I read this to say that since he sees no neutral that the assumption is made that it is ungrounded. He did not provide any solid proof that it was ungrounded. While his voltage measurements to earth clearly imply a secondary winding with a center tap that is earthed.
His wild leg to earth measurement makes no sense, and implies a measurement error.
Depending on the system stiffness it may be possible to place a 120 load on a leg that reads 120 to earth between that leg and earth. If no major change in this voltage reading, then there is a center tapped winding with its center tap earthed.
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
I worked in a building on and off for years that had a true ungrounded 600v service. There was a ground fault in one of the phases, probably been like that a long time. Later I found it when I was upgrading the feeder serving the elevator.
Edit: after the fixing the fault, despite this thing being super old, the phase to ground voltage returned to a nearly perfect 347 to ground.
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I agree. It may not have been intentionally grounded by original installer, but at some point there had become a ground at midpoint of one side. Someone may have improperly tried to connect some 120 volt loads and created this condition.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
190609-1542 EDT
grisprince:
We need to hear back from you with a redo of your measurements, and some new measurements.
On the meter, even though I don't think it is the problem. What does an electronic meter mean? I would say a Simpson 260 is not electronic, while a Fluke 27 would be. Whether a meter uses an average reading method, a bridge rectifier followed by averaging, and probably calibrate in RMS on a sine wave, or a so called true RMS meter should make no difference.
Redo the previous measurements the same way as originally done. You should not get 240 from the wild leg to earth. Next take a 1500 W space heater, after the coil heats it is about a 10 A load at 120 V, and connect it between one of the 120 V lines and earth. Does the 120 from that line to earth still remain at about 120? If so, then for sure you have a grounded center tapped winding.
Assume you did not have a center tapped winding across the two lines you read as 120 to ground. If you had an ungrounded delta and someone put two 100 W bulbs in series across that 240 side and grounded the mid point of the bulbs you would get the 120 V to ground measurements even though the delta was not grounded. But you still are not going to get 240 V from the wild leg to ground.
Give us some more feedback.
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grisprince:
We need to hear back from you with a redo of your measurements, and some new measurements.
On the meter, even though I don't think it is the problem. What does an electronic meter mean? I would say a Simpson 260 is not electronic, while a Fluke 27 would be. Whether a meter uses an average reading method, a bridge rectifier followed by averaging, and probably calibrate in RMS on a sine wave, or a so called true RMS meter should make no difference.
Redo the previous measurements the same way as originally done. You should not get 240 from the wild leg to earth. Next take a 1500 W space heater, after the coil heats it is about a 10 A load at 120 V, and connect it between one of the 120 V lines and earth. Does the 120 from that line to earth still remain at about 120? If so, then for sure you have a grounded center tapped winding.
Assume you did not have a center tapped winding across the two lines you read as 120 to ground. If you had an ungrounded delta and someone put two 100 W bulbs in series across that 240 side and grounded the mid point of the bulbs you would get the 120 V to ground measurements even though the delta was not grounded. But you still are not going to get 240 V from the wild leg to ground.
Give us some more feedback.
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