480 volt 3 phase testing at panel

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hardhat

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I was testing a 480 volt 3 phase panel to industrial metal working shop.
I was reading 480 volt to ground on 2 legs and less than 30 volt ac on the third leg.
I have worked in numerous locations industrial and have not seen. seems to be a grounded leg??
I called electric company whom came out and found nothing wrong so I look stupid on this.
Any one can explain the different configurations I have yet to run into and feel like a dumbass again.
Thanks, Rick
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Did you really get 30 volts or 30 millivolts?

If this is corner grounded delta 30 volts on the grounded leg suggests a poor bonding jumper.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
hardhat said:
I was testing a 480 volt 3 phase panel to industrial metal working shop.
I was reading 480 volt to ground on 2 legs and less than 30 volt ac on the third leg.
I have worked in numerous locations industrial and have not seen. seems to be a grounded leg??
I called electric company whom came out and found nothing wrong so I look stupid on this.
Any one can explain the different configurations I have yet to run into and feel like a dumbass again.
Thanks, Rick
Click to expand...



Welcome Rick, but calm down, this is a family forum:grin:

I agree with Bob, can you clarify more?
 
H

hardhat

Member
power measurement

power measurement

Sorry all,
Yes , it may have been 30 mv. since I have new Fluke auto adjust not used to.
One panel read near 0 v on third leg and I checked several spots through out plant even from different power in.
I must assume I am wrong thinking an open leg since elec. co. said no problem after they were there 2 hrs.
Any info will be greatly appreciated and sorry.
I hate looking STUPID but if the shoe fits.
Thanks , Rick
 
R

rcwilson

Senior Member
Location
Redmond, WA
Rick- It sounds like you have a corner grounded delta service. To visualize this, imagine the voltage triangle with all three sides = 480V and the phase connections at the three corners. Now connect one corner of the triangle to ground. All phase-to-phase voltages are still = 480V. Two phase-ground voltages = 480V and one phase-ground voltage = 0 Volts.

That matches the readings you got, if we assume that the 30V reading was a meter reading anomoly.

There is no "neutral" on the system to get 277V phase-to-neutral voltages.

It is also possible that you have an ungrounded delta system with a ground fault on one phase- an unintentionally grounded corner grounded delta.
 
H

hardhat

Member
480 panel voltage check

480 panel voltage check

Hello and thank you,
You have described the situation perfectly.
SO,
If faced with this again- how is one to know/test voltages in these and all possible wiring configs. etc
Have a good eve,
Rick
 
H

hardhat

Member
Hello and thank you,
No tape nor color coding on wires in.
Very old plant I am told.
How would one be assured their testing is accurate on such as this wire configuration.
Most commonly I would assume burnt fuse on power co. side as it was the last time I was there many years ago.
Rick
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
hardhat said:
Hello and thank you,
You have described the situation perfectly.
SO,
If faced with this again- how is one to know/test voltages in these and all possible wiring configs. etc
Have a good eve,
Rick
Click to expand...
Make all of the possible voltage measurements

L1-L2
L2-L3
L3-L1
L1-N
L2-N
L3-N
L1-G
L2-G
L3-G
N-G

Simple analysis.
Line-Line: all should be close to equal
Line-Neutral: all should be close to equal (if not system may be a high-leg)
Line to Ground: all should be close to equal (if not one leg may be grounded on purpose or because of a fault)
Neutral-Ground: should be close to 0V (actual number depends on amount of neutral current)
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
jim dungar said:
Make all of the possible voltage measurements

L1-L2
L2-L3
L3-L1
L1-N
L2-N
L3-N
L1-G
L2-G
L3-G
N-G

Simple analysis.
Line-Line: all should be close to equal
Line-Neutral: all should be close to equal (if not system may be a high-leg)
Line to Ground: all should be close to equal (if not one leg may be grounded on purpose or because of a fault)
Neutral-Ground: should be close to 0V (actual number depends on amount of neutral current)
Click to expand...

Jim,
If he has a corner grounded delta, he may not have a neutral. ;)
delta_cornergrnd.gif


hardhat,
Look at the charts on this guy's page, it's a nice summary of most of the configurations you might find out there. I don't know if anyone is allowing new corner grounded delta systems to be installed, but there are a lot still out there. Utilities don't want to run around installing new transformers if they don't have to. The NEC requires that the grounded conductors be marked, but if your installation is old enough that may not have been the case at that time. I used to have a building in Seattle with a corner grounded delta system installed in 1910. No wire identification whatsoever. Whenever we had problems with the service, I had to educate the utility techs about what CGD was and how we could have 3 phase 480V when there were only 2 wires coming off the pole!
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I had a POCO field engineer who had to ask around his department before he OKed a smaller high-leg conductor.
 
chris kennedy

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
don_resqcapt19 said:
Chris,
Are you suggesting that someone would have followed the code and used white for the grounded conductor of the corner grounded system? That is something I have never seen.
Click to expand...

I used gray on the last one I did. Reverse wired 480 delta primary 208y/120 secondary. I get your point, the old systems here do not reflect 200.6 requirements. Wonder which cycle 200.6 came to be?
 
H

hardhat

Member
480 panel testing

480 panel testing

chris kennedy said:
I used gray on the last one I did. Reverse wired 480 delta primary 208y/120 secondary. I get your point, the old systems here do not reflect 200.6 requirements. Wonder which cycle 200.6 came to be?
Click to expand...

Thank you all who replied.
I appreciate your help.
Yes-no color coding on the wire in. I believe you are correct with the corner grounded delta. I have worked in industry for years and never faced this somehow even though I was aware of this type of service many years ago but forgot.Yes -No neutral in panel. Thanks, Rick
 
mbeatty

mbeatty

Senior Member
Location
Illinois
rcwilson said:
Rick- It sounds like you have a corner grounded delta service. To visualize this, imagine the voltage triangle with all three sides = 480V and the phase connections at the three corners. Now connect one corner of the triangle to ground. All phase-to-phase voltages are still = 480V. Two phase-ground voltages = 480V and one phase-ground voltage = 0 Volts.

That matches the readings you got, if we assume that the 30V reading was a meter reading anomoly.

There is no "neutral" on the system to get 277V phase-to-neutral voltages.

It is also possible that you have an ungrounded delta system with a ground fault on one phase- an unintentionally grounded corner grounded delta.
Click to expand...


I agree. This definitely looks like the case.
Mark
 
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