We have and old facility with a 1948 ITE 200KVA 4160V Primary Delta to a 480V 3 phase secondary. When measuring phase to ground on each phase of the secondary side we have one phase that measures zero, the other two measure 480V. Phase to phase they all measure 480V. I'm inquiring if anyone knows if this could be a safety concern or why this might of have been done this way?
1 Phase of our 3 phase system measures zero volts to ground?
- Thread starter Pcore
- Start date
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- Location
- Connecticut
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- Engineer
You have a corner grounded delta. I can't think of any particular safety concerns with that system, but maybe someone else will have more insight.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
As David mentions, you may have a 'corner grounded delta', where one of the three phases is intentionally connected to ground to provide a solid ground reference. This is done for much the same reasons that a neutral (if available) gets grounded.
Another possibility is that you have what is supposed to be an _ungrounded_ delta system, and one phase is _unintentionally_ grounded by a fault. Ungrounded delta systems are used to permit systems to continue operating in the event of a single ground fault. However when this fault occurs, it is important to locate and repair it.
-Jon
Another possibility is that you have what is supposed to be an _ungrounded_ delta system, and one phase is _unintentionally_ grounded by a fault. Ungrounded delta systems are used to permit systems to continue operating in the event of a single ground fault. However when this fault occurs, it is important to locate and repair it.
-Jon
big john
Senior Member
- Location
- Portland, ME
The only suggestions I have:
Be sure it's deliberately corner grounded and not an ungrounded system with a single phase fault.
If it is a deliberate corner ground, be sure it was installed properly: Real common to open feeder disconnects and find fuses in the grounded phase.
Be sure it's deliberately corner grounded and not an ungrounded system with a single phase fault.
If it is a deliberate corner ground, be sure it was installed properly: Real common to open feeder disconnects and find fuses in the grounded phase.
JeffBabineaux
Member
- Location
- Minden, LA
It's not unheard of. At a chemical plant where I worked in Southern Arkansas, they are still using corner grounded delta systems to feed water pumps. Schneider Electric has put together some relevant information that you might be interested in.
http://static.schneider-electric.us...n/Molded Case Circuit Breakers/2700DB0202.pdf
See pages 2-3 for relevant codes.
A few notes:
If you lose a phase, your other phase voltage doesn't change.
You're reading 480V across the two high legs because they are perfectly out of phase with one another (when one AC is high, the other is 0).
Don't fuse the grounded leg, and make sure that it's clearly marked all the way down (there was some confusion in AR on this one, led to an electrician almost losing a hand when he went to troubleshoot the pump)
2-pole breakers should be marked 1?-3?
http://static.schneider-electric.us...n/Molded Case Circuit Breakers/2700DB0202.pdf
See pages 2-3 for relevant codes.
A few notes:
If you lose a phase, your other phase voltage doesn't change.
You're reading 480V across the two high legs because they are perfectly out of phase with one another (when one AC is high, the other is 0).
Don't fuse the grounded leg, and make sure that it's clearly marked all the way down (there was some confusion in AR on this one, led to an electrician almost losing a hand when he went to troubleshoot the pump)
2-pole breakers should be marked 1?-3?
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
As it says in the link Geff posted. Corner grounded delta doesn?t really have a place in the modern world.
You need to check at the transformer or panel for a link of some kind. If there isn?t one you have a ground fault that needs to be found as a matter of urgency. Can you isolate the load and test at the panel? It would take the guesswork out of the problem.
You need to check at the transformer or panel for a link of some kind. If there isn?t one you have a ground fault that needs to be found as a matter of urgency. Can you isolate the load and test at the panel? It would take the guesswork out of the problem.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
As mentioned, Delta power systems were used, mostly in the past, as a form of fault tolerance, meaning the facility could withstand the FIRST ground fault and keep going. The NEC technically still allows for this, but the system must be EITHER: 1) Grounded, meaning "corner grounded delta"; or 2) Monitored, meaning a Ground Fault Monitoring system.
If it is a corner grounded delta, the grounded phase must be* grounded AT the transformer terminal, meaning you will be able to SEE the ground conductor in the termination box. If you don't see one, then it was an UNGROUNDED delta, and you have a ground fault somewhere else. if it is an UNgrounded delta system, you are supposed to have a ground monitoring system installed.
That said, Tony's comment is more true than ever now. Delta power systems are a bad thing for power electronics, like VFDs and UPS systems, requiring special attention and/or isolation of them.
*Actually, I guess not, but it USUALLY is...
If it is a corner grounded delta, the grounded phase must be* grounded AT the transformer terminal, meaning you will be able to SEE the ground conductor in the termination box. If you don't see one, then it was an UNGROUNDED delta, and you have a ground fault somewhere else. if it is an UNgrounded delta system, you are supposed to have a ground monitoring system installed.
That said, Tony's comment is more true than ever now. Delta power systems are a bad thing for power electronics, like VFDs and UPS systems, requiring special attention and/or isolation of them.
*Actually, I guess not, but it USUALLY is...
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meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
As some know who have seen my posts before regarding corner grounded 480 delta, I'm not a big fan. Reason being that if you have either of the ungrounded phases with a ground fault, you have a phase-to-phase fault. Much fault current. Also, if you must work it hot for testing or troubleshooting, contact with an ungrounded phase while touching ground is a 480V shock. I usually get all kinds of flak but it's just my opinion working around ungrounded 480 delta for much of my career that they are "safer". Current code requires a ground detection system on ungrounded systems as stated above, but many older systems don't have one. Best bet is 277/480 grounded Wye. Ditto on the other opinions, though...be sure it's intended to be grounded. If not, you have a safety concern.
- Location
- Massachusetts
meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
How can you get a shock when you are wearing the required PPE?
Uh Oh...caught me in the act.....I suspect I'm the only one who has ever had to troubleshoot a 480V control while in near darkness, standing in water with half a dozen water guys breathing down my neck in the middle of the night on a callout and I can't fix it because I'm in a 40 cal arc flash suit and can't even hang onto my meter leads, let alone see anything. Not to mention that I'm a geezer and started learning the trade in the Navy on a submarine in the 1970's. PPE? What's that? I just learned to let go before it got to my elbow. No excuses, though. WEAR YOUR PPE!
SceneryDriver
Senior Member
- Location
- NJ
- Occupation
- Electrical and Automation Designer
Even ungrounded delta systems will most likely give you a phase to ground shock from capacitively coupled current. I wouldn't bet me life on a floating delta making things "safer" if I might be touching things.
SceneryDriver
I have not seen a grounded delta since I started some thirty years ago. It was mentioned to me the importance of grounding one leg. I think it also had lights that monitored the switch gear.
Question. You should not fuse the grounded leg ?? If you used fuses you would only use two? Or if a breaker you would use a two pole for the three phase load?
Have me thinking too. Wired a machine shop a few years ago. The engineer designed it with an ungrounded delta. I mentioned to the guy I was working for and the engineer about what if a legg grounded. I was directed to install it ungrounded. Cable tray. Rubber cord. Hope all is well ...
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
JR, how would you do earth/ground fault monitoring?
I worked on LF induction furnaces where the LV supply wasn?t earthed/grounded due to the cables and coils being water cooled. We had enough trouble with electrolysis without adding a path to earth/ground.
The earth detection was by DC injection through two high impedance inductors, one for each leg of the LV supply. I?m retired and the furnaces are now in Spain so it?s of no great concern but I?ve always thought there was a better way to do it.
Sorry I should have said the furnaces were single phase. 2.6MVA @660V = 4000A. Not something to be around when it went wrong.
meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
True....my opinion is just that...I just think I'd rather have incidental ground contact with my body or a conductive tool or wire from ungrounded rather than solid phase to phase. Obviously, neither is fun. But we have had several unintentional grounds on one phase of equipment due to water, rodent damage, trees or other nasties...all of which allowed the equipment to continue running with an ungrounded delta until we could safely shut it down. That was the thinking when they wired pump stations ungrounded delta. Long before I started working there. Most have now been changed to 277/480 grounded Wye. Power outages can still cause problems, such as water hammer when the pump stops, but arc damage from a 277V to ground fault prior to breaker tripping or fuse blowing is usually much less than 480V on a corner ground. As I said above, I was trained as a newbie on a nuclear sub, where NOTHING was grounded, AC or DC. If a depth charge shakes thing up, a single ground would not affect operation at all. Maybe it's still in the back of my mind. Again...only my opinion.
JeffBabineaux
Member
- Location
- Minden, LA
My understanding is that fusing the grounding leg risks cutting off the path to ground, so yes. Two fuses instead of three. That's the way it was wired up there. I'm the new guy on the block, so I won't pretend to honestly believe that means it was done right. I'm just the guy that filled out the incident report when we had an arc flash incident. As I said before, though, we determined that the problem was in improper wiring, because the ground leg got mixed up with one of the high legs, so the high leg wasn't fused. With all the problems that it caused, I doubt it benefited us enough to cover the eventual loss.
kwired
Electron manager
- Location
- NE Nebraska
People get confused with the grounded phase. You must remember you can ground any point of the system- we normally are required to ground the neutral point of systems that have a neutral point. This system doesn't have a neutral point so we can ground any other point and basically have same system no matter which point we ground.
We call the conductor that is grounded the grounded conductor whether it is a neutral conductor or not - it is still the grounded conductor.
We don't put fuses in the grounded conductor - because it is grounded, do you agree that it is not desired to have a fuse in a neutral conductor of a 120/240 single phase system? This grounded phase is still the grounded conductor just like the neutral is in the 120/240 single phase system. It will have the main or system bonding jumper connected to it, the equipment grounding conductor must still be a separate conductor beyond the service or first disconnect of a separately derived system. We can have switches and circuit breakers that simultaneously open all circuit conductors installed in the grounded conductor, just not fuses. Same is true for the neutral of 120/240 single phase applications - you just don't see it all that often.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Basically, what the NEC requires is that the only time that you can interrupt the grounded conductor is through a device that opens all ungrounded wires at the same time. Like a three or four pole common trip breaker, or the equivalent using signalling fuses that trip the other wires too.
What they are trying to avoid is having line voltage appear on what was the grounded conductor because a single line fuse opened. That would, among other things, be a hazard to the electrician expecting the grounded conductor to be grounded!
What they are trying to avoid is having line voltage appear on what was the grounded conductor because a single line fuse opened. That would, among other things, be a hazard to the electrician expecting the grounded conductor to be grounded!
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