Corner Ground Delta

In my mind, there is one major difference between a grounded neutral conductor and a grounded phase conductor on a large system:

In general, when the system is maximally loaded, the current on the grounded neutral will be at a minimum. In general, when the system is maximally loaded, the current on a grounded phase conductor will be at a maximum.

In both cases the potential of the unbroken grounded conductor is near ground potential. And in both cases opening the grounded conductor will cause grief unless you've also opened the hot conductors. In general I think a grounded phase conductor is more alike than different from a grounded neutral, but they do have different flavors in terms of expected operating current.
 
There is considerable current flowing on the blue highlighted conductor here when the 3 phase secondary has a balanced 3 phase load. The blue conductor is supplying the 3rd phase. So is it a neutral, or a phase? And if it is a phase, where does it change back to the MGN?

Screenshot 2023-07-21 195144.jpg
 
electrofelon said:
Where is this from? It sounds like they don't really know what they're talking about, it makes no sense. That right there is exactly the problem, people think a corner grounded grounded conductor is different for some reason. The fact is there is absolutely nothing whatsoever different about a corner grounded grounded conductor than a wye or split phase grounded conductor.
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Interesting, show me a normal three phase wye power disconnect (with overcurrent device ) where the neutral ( grounded conductor )is switched with the ungrounded conductors. Show me a split phase ungrounded conductor (neutral) that in a normal situation is switched with the ungrounded conductors overcurrent device.
 
IMHO that blue wire remains a MGN/neutral.

But it is only neutral relative to all 3 phase conductors; it _isn't_ neutral relative to phase 1 and phase 2 alone; that is why it is carrying full current.

-Jonathan
 
Joethemechanic said:
There is considerable current flowing on the blue highlighted conductor here when the 3 phase secondary has a balanced 3 phase load. The blue conductor is supplying the 3rd phase. So is it a neutral, or a phase? And if it is a phase, where does it change back to the MGN?
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It's the primary neutral; same as with 1ph 120/208v.
 
Rjryan said:
Interesting, show me a normal three phase wye power disconnect (with overcurrent device ) where the neutral ( grounded conductor )is switched with the ungrounded conductors. Show me a split phase ungrounded conductor (neutral) that in a normal situation is switched with the ungrounded conductors overcurrent device.
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You can switch the grounded conductor, even the neutral in the wye system or the neutral of a split phase system, you just can't put a fuse in the grounded conductor. Switching must be simultaneous with the ungrounded conductors and common trip circuit breakers will fall into this simultaneous switching as well. What you don't want is for a fuse or non common trip breaker end up opening the grounded conductor without also opening all the ungrounded conductors.

Separately derived standby power sources get the grounded conductor switched in the transfer switch at any voltage or system type.

Many GFCI's open the neutral (grounded) conductor as well when they trip.

Class 1 division 1 (maybe division 2 as well I'd have to double check) also require opening the grounded conductor if the circuit has one this so you don't get incidental spark and ignite the hazardous environment should the grounded conductor have some voltage, as in neutral to earth voltage, and it touches some other grounded object while working on the circuit
 
roger said:
The same as any grounded two wire circuit. The NEC's attempt two define the word "neutral" falls short.
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IMO comparing three phase three wire system to single phase two wire system is one the best ways to help explain both corner grounded and ungrounded delta systems. Two wire is simpler for learning purposes. You have a two wire source. There is no reference to ground until you make one. There is no neutral either. A neutral would have to be equal to at least to other points and you only have two points.

So all you can have with a two wire system is either ungrounded, or ground a conductor (either one) and now you have a grounded and an ungrounded conductor. Current on both will be equal as with two wire system what goes out on one lead has to come back on the other. EGC is derived from the grounded conductor at the system bonding jumper location but only is intended to carry current upon a fault from an ungrounded conductor, which in two wire system you only have one ungrounded to choose from.

Three phase three wire most of what I just said applies except you now have three conductors from the source, but none are neutral conductors as they are not midpoint of any winding. You can ground any of them should you want a grounded system, but only one of them can be grounded. EGC is still derived at the system bonding jumper location. The grounded phase conductor still carries current of any load connected to it just like the grounded conductor of a two wire system does.

Many tend to call the grounded conductor of a two wire system the neutral but technically is wrong, it is simply grounded conductor.

NEC messed with definitions when they wanted to include the grounded mid point of one side of high leg delta systems. It is "neutral" to that one side but is not neutral to the entire delta system, until NEC tried playing with definitions anyway.
Rjryan said:
  • Grounding method: In a corner-grounded delta system, one of the phase conductors is intentionally grounded. This grounded phase essentially serves a similar function to a neutral in that it provides a path to ground. However, it is fundamentally a phase conductor, not a dedicated neutral conductor derived from a center tap or wye connection.
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I have run into this twice where an electrical contractor failed to ground the center tap on a 480-208/120 transformer. Both times were buildings that the company I worked for occupied.

The first building my company leased was fed with a feeder from another building. When we leased it the EC hired by the building owner installed a 480/208 transformer and a new service and the feeder was disconnected.

The first think that happened it wouldn't pass inspection because the POCO wanted a switch in front of the meter "cold sequence". After he fixed that our automation dept was installing some building controls and the tech came to me and said he was getting weird voltages. I found the transformer not grounded. The owner called the electrician back and he said he wouldn't ground it because it would 'blow up". But we insisted and he did it and it was fine.

The second one was a different Co I worked for put an addition on their building and the EC they hired (been in business for 90 years) put in a 480 volt panel a 480-208 transformer and a 208/120 panel.

Sometime after that we bought a used overhead crane and installed in in our shop and found some strange voltages. Found the transformer ungrounded. The electrician that worked for me said "if we ground it it will blow up" we grounded it and it was fine.
 
We always call this blue conductor a "neutral" in a subpanel like this. But is it ever really a neutral? No matter how you balance your loads in this subpanel, there will always be current on the "neutral". So it is never really a neutral, although it is a grounded conductor. And there are actually 3 phases in the subpanel, although they are at unusual angles and not all the same voltage.

three-phase-four-wire-y-connection.jpg

Don't believe me? Hook a 3 phase motor to both hots and that grounded conductor and watch it start. I had a service call one time for motor overloads tripping all the time. And I found that a maintenance guy hooked a 3 phase saw up to a 120/208 "Single Phase" subpanel. It only tripped out when they tried to take a heavy cut
 
winnie said:
In my mind, there is one major difference between a grounded neutral conductor and a grounded phase conductor on a large system:

In general, when the system is maximally loaded, the current on the grounded neutral will be at a minimum. In general, when the system is maximally loaded, the current on a grounded phase conductor will be at a maximum.

In both cases the potential of the unbroken grounded conductor is near ground potential. And in both cases opening the grounded conductor will cause grief unless you've also opened the hot conductors. In general I think a grounded phase conductor is more alike than different from a grounded neutral, but they do have different flavors in terms of expected operating current.
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Yes I could see the grounded conductor to ground potential being higher on a corner grounded system since the current could be much higher. Probably doubtful it would get high enough to be a shock hazard however.
 
There is one other minor difference between a corner grounded system and a split phase or wye system (but not a high leg Delta), and that is the symmetry of the grounding point. I have found that in ungrounded systems, the line to ground voltage usually floats pretty consistently at the geometric center. Even in an ungrounded Delta your phase to ground voltages will all be consistent, assuming there are no high or low impedance faults. You are sort of "fighting" that with a corner grounded system, but really there are no practical negative implications to this as the capacitive coupling voltage doesn't really mean much.
 
Rjryan said:
Interesting, show me a normal three phase wye power disconnect (with overcurrent device ) where the neutral ( grounded conductor )is switched with the ungrounded conductors. Show me a split phase ungrounded conductor (neutral) that in a normal situation is switched with the ungrounded conductors overcurrent device.
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Done all the time for gas pumps. It's required by code.
 
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