Working with an older 480V 3-wire system which I would assume to be a delta. The MCC had voltage meters for each phase (assumed L-G). They were strange so I got the meter out. When I read the voltages, I measured each phase to ground. With the following readings: 306V, 281V and 294V. Had this been a Y system, I would consider the readings strange. Is this typical for a delta? L-L voltages were right on.
480V delta strange voltages
- Thread starter mjmike
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Are you sure it's not a Y system where they often just brought out the 3 lines and EGC only? As such it may be misunderstood as being a delta because of not having a neutral. With a delta it is often corner grounded but could also be an ungrounded delta which I doubt that it is either of those.
With a Y sysem the transformer's X0 is most likely grounded which is where the EGC is taken from. With the strange voltage readings that you are getting I would varify that XO is solidly grounded and that the EGC is connected to the XO in my opinion.
kwired
Electron manager
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- NE Nebraska
Sounds like you have an ungrounded system to me, or if it is supposed to be grounded it has lost, never had the bonding jumper installed and is essentially an ungrounded system.
Add - if this is a wye system you lost the neutral.
iceworm
Curmudgeon still using printed IEEE Color Books
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If it is ungrounded delta, there should be grounding indicators - maybe just three lights.
Recommending looking for a neutral grounding resistor. The system could be High Resistance Grounded. It should have grounding indicators as well.
ice
ATSman
ATSman
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Yes, I agree with your logical assumptions. And as the other post noted, if the system is designed to be either ungrounded or resistance/ zigzag grounded then it should have ground detection lights to aid in troubleshooting the fault.
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Back in the 60's, there would have been no requirement for indicators lights.
Get out your binoculars and see how the indivual transformers are interconnected.
On an ungrounded system you can read L-G voltages, with a high resistance meter, due to coupling capacitance. The more balanced your loading, the closer to 277V you will see.
Phil Corso
Senior Member
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- Boca Raton, Fl, USA
MJMike,
Q1) Do the 3 voltage-to-ground measurements vary at different times of the day?
Q2) What are the Line-to-Line measurements?
Regards, Phil Corso
Q1) Do the 3 voltage-to-ground measurements vary at different times of the day?
Q2) What are the Line-to-Line measurements?
Regards, Phil Corso
iceworm
Curmudgeon still using printed IEEE Color Books
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If I had to guess, I'd say:
But that would just be a guess baised on ....:roll:
ice
iceworm
Curmudgeon still using printed IEEE Color Books
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Hummm .... mid 60s?
If it was originally corner grounded, check the motor starter boxes. A lot of these were only two overloads. If that is all you see in the controllers, go look for a broken corner grounding conductor.
ice
ptonsparky
Tom
- Occupation
- EC - retired
Two overloads were common for 3 phase systems years ago and have no relevance as to wye or corner / ungrounded delta.
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
Hummm....
I recall my dad talking about two overlaods being the norm during the 1940s. This, of course, would be the one with "no relevance as to wye or corner / ungrounded delta". no, I'm not old enough to have seen those.
I recall seeing only two overloads in Corner Grounded 480D (only) during the 1960s. I am old enough to have seen and worked that.
However, if you are dead certain, and have researched the applicable age group standards and codes, I'll believe you over my recollection from 50+ years ago.
ice
Phil Corso
Senior Member
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- Boca Raton, Fl, USA
Iceworm...
I was there in that era and can confirm your Dad's observation was correct! That is, until someone proved that under certain load conditions protection would be compromised.
Regards, Phil
I was there in that era and can confirm your Dad's observation was correct! That is, until someone proved that under certain load conditions protection would be compromised.
Regards, Phil
kwired
Electron manager
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- NE Nebraska
I am not that old to have been installing them, but have run into plenty of older still operating equipment that did use only two overload elements and pretty sure it happened on all three phase systems whether grounded phase or not. I know of some instances where those starters are still in operation with only two overloads.
Motor overload protection will not care what is grounded and what is not grounded, it is supposed to monitor motor current for overloading. Short circuit and ground fault protection is where it is more important to not have an interrupt able component in the circuit unless it interrupts all conductors of the circuit.
I think the concept back then, and is still somewhat valid concept, is that if a three phase motor is in good condition and input voltage is balanced the current will be balanced. Only having one overload unit would monitor that but problems arise when imbalance of any kind gets thrown into the picture, and it often happens. Two overload units however will offer fairly good overall protection, because if one phase goes down (fully or partially) it will result in a rise in current in at least one of the other conductors. I think two overloads probably provided sufficient protection in most general duty applications, but at some point they decided we need to monitor all three lines to get better accuracy and better protection. My problem with either situation is I see so many applications where motors are not protected at proper level - that they may as well just have short circuit and ground fault protection only - as they are ultimately going to fail from overload anyway
Even when overload is properly sized, often you may see people resetting them as they continue to run the machine - maybe even every few minutes while the motor continues to get hotter and hotter. If the device is tripping, there is a reason, and most of the time it is not because the trip device is the defect, though that is what owners/managers/operators wish to believe.
Motor overload protection will not care what is grounded and what is not grounded, it is supposed to monitor motor current for overloading. Short circuit and ground fault protection is where it is more important to not have an interrupt able component in the circuit unless it interrupts all conductors of the circuit.
I think the concept back then, and is still somewhat valid concept, is that if a three phase motor is in good condition and input voltage is balanced the current will be balanced. Only having one overload unit would monitor that but problems arise when imbalance of any kind gets thrown into the picture, and it often happens. Two overload units however will offer fairly good overall protection, because if one phase goes down (fully or partially) it will result in a rise in current in at least one of the other conductors. I think two overloads probably provided sufficient protection in most general duty applications, but at some point they decided we need to monitor all three lines to get better accuracy and better protection. My problem with either situation is I see so many applications where motors are not protected at proper level - that they may as well just have short circuit and ground fault protection only - as they are ultimately going to fail from overload anyway
Even when overload is properly sized, often you may see people resetting them as they continue to run the machine - maybe even every few minutes while the motor continues to get hotter and hotter. If the device is tripping, there is a reason, and most of the time it is not because the trip device is the defect, though that is what owners/managers/operators wish to believe.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
If the phase-to-phase voltages are equal and the phase-to-ground voltages are different then the neutral point is not in the center and that means it is not grounded. The voltage to ground is measured through system capacity-cables, transformers etc.as jim dungar said. Attached is one of angle possibilities.
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
Okay. I suspect one would have to be in their 90s to have been around common installs. I'm only in my 60s.
What I recall (fuzzy at best), is the 1960's issue of installing only two overloads had to do with the corner ground was considered a neutral.
This is where all jump in and tell me I'm screwed up. Could be. As I said, " if you are dead certain, and have researched the applicable age group standards and codes, I'll believe you over my recollection from 50+ years ago."
ice
ATSman
ATSman
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- San Francisco Bay Area
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- Electrical Engineer/ Electrical Testing & Controls
If the phase-to-phase voltages are equal and the phase-to-ground voltages are different then the neutral point is not in the center and that means it is not grounded. The voltage to ground is measured through system capacity-cables, transformers etc.as jim dungar said. Attached is one of angle possibilities.
View attachment 11532
Julius,
Thanks for the refresher course in vectors. Couple of points:
In the diagram shouldn't the CG be CN since there is no gnd?
Also, if you use a ruler and measure AN and compare it to CN, CN is longer but the numbers show different (306/294) Why is that?
Can you show the math or formula used to arrive at these numbers?
Tks
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Whatever vectors and PFs you use in your calculation, a single series overload contact will leave the motor single phased. There has to be a second contact for that.
And the NEC does not require opening the grounded conductor, even though current in that conductor may be monitored .
And the NEC does not require opening the grounded conductor, even though current in that conductor may be monitored .
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