Corner Ground Delta

K

kjroller

Senior Member
Location
Dawson Mn
Occupation
Master electrician
If the NEC code is supposed to be about life and safety why haven't they outlawed the corner ground delta system?
 
Why? These systems work. Do you have a particular example of such a system being more dangerous than any other grounded system of the same voltage?
 
All grounded systems are tapping or connecting to a winding.
 
Is an ungrounded delta worse than a corner grounded one, I don’t think so. That’s why corner grounded came about. Something to limit the voltage spikes from an arcing fault, gives it a fault return path to trip the overcurrent protection. And stop the arcing fault.
 
I guess what im trying to say is lets your corner delta loses its bond to the earth wouldnt this essentially give you a ungrounded conducter without someone being aware of thr potiential couldnt it hurt someone or am I thinking wrong
 
I guess on a wye or delta what is the centertaps voltage if not bonded to ground I suppose i was under the impression that the centertap carrys 0 potiential but maybe im wrong
 
kjroller said:
I guess what im trying to say is lets your corner delta loses its bond to the earth wouldnt this essentially give you a ungrounded conducter without someone being aware of thr potiential couldnt it hurt someone or am I thinking wrong
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Why would you be touching a conductor without checking? There is no now nor will there ever be a reason to rely on wire color to determine the function and or voltage of that conductor.
 
kjroller said:
I wouldnt but ive seen dumber things happen is all im saying
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How is this any different from when you lose the service neutral on a 120/240 volt system? The voltage on the neutral and on anything that is connected to the electrical bonding system rises to a voltage between 0 and 240.
 
don_resqcapt19 said:
How is this any different from when you lose the service neutral on a 120/240 volt system? The voltage on the neutral and on anything that is connected to the electrical bonding system rises to a voltage between 0 and 240.
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It may not be any different im trying to learn so thats why i asked the question I get how a open neutral works when I use that train of thought maybe it is no different
 
I don't like the corner grounded delta but we use it all the time to test equipment since it's the only way I've found to give us power in the lab that matches the ship's power setup.

Corner Grounded Delta Question.

We have a test setup in our lab that simulates shipboard, corner grounded delta power. We are required to power some of the distribution racks this way because the Eaton power distribution units require the main and secondary inputs to be 120 degrees apart. If we try to simply power each input...
forums.mikeholt.com
 
kjroller said:
It may not be any different im trying to learn so thats why i asked the question I get how a open neutral works when I use that train of thought maybe it is no different
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kjroller said:
I guess on a wye or delta what is the centertaps voltage if not bonded to ground I suppose i was under the impression that the centertap carrys 0 potiential but maybe im wrong
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When you make an electrical system, such as with a transformer, There is nothing "special" about any of the system connections. One conductor is not "hotter" than any others, and none are "not hot". There is an electrical joke (kinda not even a joke) that the first time you energize a corner grounded delta you just set up, you hesitate and turn away when you throw the switch because it just seems off to connect that "hot" conductor right to ground. However there is nothing different about that and connecting the center tap of a 120/240 system transformer to ground.
 
kjroller said:
Cause if for some reason the ground is ever disconnected someone could get seriously hurt.
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This statement is just as true with other grounded systems.

The bigger hazard with corner ground systems is people that don't understand things well enough and end up bonding the grounded conductor beyond the main/system bonding jumper. That allows parallel current of the grounded conductor to flow on otherwise non current carrying paths, which is exactly the same thing that happens if you do the same with the neutral of 120/240 systems.
 
kjroller said:
I guess on a wye or delta what is the centertaps voltage if not bonded to ground I suppose i was under the impression that the centertap carrys 0 potiential but maybe im wrong
Click to expand...
Potential is relative to a certain point. We ground systems to make earth that reference. Outside of code telling us that a neutral conductor is the one that usually will be required to be the one that is grounded theory wise you could ground any point of any system as there is no earth reference until you ground something. Take a 208/120 wye system and ground one of the outer points but not the neutral. Many have it in their mind that would result in possibly high level of fault current. But it does not. voltage between every point of the wye remains the same, you just have a ground reference on one of the outer points instead of on the neutral. Ground a second point of the system (whether intentional or not) and you will have fault current.

With a three wire delta you only have three possible choices of what to ground. Any one of them still makes it a corner grounded delta. If you ground the mid point of one side of the delta you get the high leg delta. You still have about 87% of line to line voltage to ground on that one leg. The bigger advantage of high leg delta over corner ground delta on 240 volt systems is you can operate 120 volt loads off said system (within limits of that portion of the supply) without further transformation being needed.
 
kwired said:
Potential is relative to a certain point. We ground systems to make earth that reference. Outside of code telling us that a neutral conductor is the one that usually will be required to be the one that is grounded theory wise you could ground any point of any system as there is no earth reference until you ground something. Take a 208/120 wye system and ground one of the outer points but not the neutral. Many have it in their mind that would result in possibly high level of fault current. But it does not. voltage between every point of the wye remains the same, you just have a ground reference on one of the outer points instead of on the neutral. Ground a second point of the system (whether intentional or not) and you will have fault current.

With a three wire delta you only have three possible choices of what to ground. Any one of them still makes it a corner grounded delta. If you ground the mid point of one side of the delta you get the high leg delta. You still have about 87% of line to line voltage to ground on that one leg. The bigger advantage of high leg delta over corner ground delta on 240 volt systems is you can operate 120 volt loads off said system (within limits of that portion of the supply) without further transformation being needed.
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Edit time expired. We ground systems to make earth the "zero" reference. But any point of the system can be made the zero reference. Code just happens to require it be the neutral conductor in nearly all cases if the system has a neutral.
 
don_resqcapt19 said:
How is this any different from when you lose the service neutral on a 120/240 volt system? The voltage on the neutral and on anything that is connected to the electrical bonding system rises to a voltage between 0 and 240.
Click to expand...
Lost neutral when you have line to neutral loads and the variance of voltage to the connected 120 volt loads is different situation than losing the ground reference itself though. Remove bonding jumper and you still have 120/240. Voltage measured to earth may vary as loading conditions vary though, and capacitance is a factor in what voltage readings you will see to ground. Grounding the system gives your measured voltages to earth more stability as the system has a fixed reference.
 
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