Corner Ground Delta

[kwired]

Same here, I worked on very old mill building with a corner grounded 480.
The only thing fed out of the main switch-gear was the three elevators and two other new transformers (208 & 480 y), I always asked why it was originally used as everything else in the building was fed from the 480y277 or 208y120 transformer nobody ever had an answer. Anyone know the history on why you'd need one?

Before we had VFD's and other electronic items that are basically intended to operate on wye systems with equal volts to ground on each conductor it maybe made more sense to some degree. 277 volt lighting might been about the only thing that would cause a definite need for a system with that voltage. Otherwise all your motors or other higher power loads were 480 volts and everything else was separately derived from 120/240 single phase or 208/120 three phase system.

Now one the more common places to see a corner ground on something rather new would be if it is separately derived on site from a back fed delta-wye transformer where you have no neutral on the delta side of the transformer. Most the time such setups are feeding specific load and not so much a general distribution system at a facility.

 

[Joethemechanic]

Is there any real reason why you have to use 3 pole discos or contactors on a corner ground system? What is the need to open a grounded conductor?

 

[electrofelon]

Is there any real reason why you have to use 3 pole discos or contactors on a corner ground system? What is the need to open a grounded conductor?

There is no reason to. I think it just goes back to many people thinking it's only "kinda" a grounded conductor and not being able to let go that it's a hot phase conductor. If you had a panel board for a corner grounded three phase system it would look pretty much exactly like a split phase panel.

 

[jim dungar]

Is there any real reason why you have to use 3 pole discos or contactors on a corner ground system? What is the need to open a grounded conductor?

Three pole contractors and non-fusible disconnects are Listed and labeled for 3-phase motors. While 2-pole devices could be used their labels usually only show 1-phase motors. Also running overload protection, like thermal units, needs to be supplied for all phases

Some equipment, like Motor Control Centers, only come in 3-pole versions.

The installer just uses what was on hand or in stock.

 

[Rjryan]

NEC 2023 240.22 Grounded Conductor. No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following conditions is met: (1) The overcurrent device opens all conductors of the circuit, and is designed so that no pole can be operated independently.
While it is the grounded conductor, it is still the third power phase of your circuit.

 

[Joethemechanic]

NEC 2023 240.22 Grounded Conductor. No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following conditions is met: (1) The overcurrent device opens all conductors of the circuit, and is designed so that no pole can be operated independently.
While it is the grounded conductor, it is still the third power phase of your circuit.

While it is the 3rd phase, being that it is grounded there is no need for an overcurrent device on it. In fact an overcurrent device on the corner grounded conductor is kind of dangerous if it doesn't simultaneously open the other two phases.

In fact an open delta primary on a 120/240 high leg service is using the MGN as the 3rd phase, and there is no way I'd put an overcurrent device in a MGN

 

[kwired]

Three pole contractors and non-fusible disconnects are Listed and labeled for 3-phase motors. While 2-pole devices could be used their labels usually only show 1-phase motors. Also running overload protection, like thermal units, needs to be supplied for all phases

Some equipment, like Motor Control Centers, only come in 3-pole versions.

The installer just uses what was on hand or in stock.

240 volt is easier to find two pole disconnects. 600 volts, might be a catalog number for several, but suppliers won't likely be stocking them.

Straight 240 volt rated breakers are more difficult to find in stock and cost as much or even more then a three pole breaker which is rated straight 240.

Similar with 277/480 rated breakers- certain product lines won't have straight 480 or 600 volt rated breakers and would require you to use a different product line. With Square D products you basically have to use I line panels on a 480 corner ground system and an appropriate rated breaker, but they have them in this series, and it would be available in two pole breakers if that is desired. With I line you get three phase assembly bus even if you only intend to use two of them.

 

[Rjryan]

While it is the 3rd phase, being that it is grounded there is no need for an overcurrent device on it. In fact an overcurrent device on the corner grounded conductor is kind of dangerous if it doesn't simultaneously open the other two phases.

In fact an open delta primary on a 120/240 high leg service is using the MGN as the 3rd phase, and there is no way I'd put an overcurrent device in a MGN

NEC 240.22 Exception allows you to at the fused disconnect to switch the grounded connector in effect simultaneously with the ungrounded connectors. Remember that the grounded leg is your third leg of power and is not fused. Remember this the Exception (1) to NEC 240.22.

 

[Joethemechanic]

I'm not seeing the need to open the grounded conductor at all. Here is a grounded conductor used as a 3rd leg. No overcurrent, no means of opening it. I know that it's NESC, but same principals apply.



The grounded conductor is at the same potential as the dirt you are standing on just like the neutral for your 120/240 single phase service, or even like the neutral for your 480/277 service, in fact all of them are connected through the MGN, and through the earth, and who knows all what else, building steel. plumbing, whatever. Your home's neutral is at the same potential as the factory with the corner ground, and the other plant 2 blocks away or across town with the 480/277 because they are all connected (keep in mind that I am ignoring differences in potential due to conductor impedance)

When you touch the neutral/ground bar in your home, you are touching the factory down the street's corner grounded phase because they are connected by a piece of wire

 

[pipe_bender]

Now one the more common places to see a corner ground on something rather new would be if it is separately derived on site from a back fed delta-wye transformer where you have no neutral on the delta side of the transformer. Most the time such setups are feeding specific load and not so much a general distribution system at a facility.

Thanks that makes sense so by "back fed" you mean someone running old transformer backwards perhaps for a small work shop where they don't have 480 service but have a 480v piece of equipment?
I wonder what the arc flash / ppe requirements are for 480 to ground vs 277 if any?

 

[Joethemechanic]

I wonder what the arc flash / ppe requirements are for 480 to ground vs 277 if any?

I'm pretty sure you need to calculate the worst case incident energy to know exactly. But having been bit a few times by 208 from a high leg, and 277 from 480 WYEs I can tell you I wouldn't want to be part of a 480 to ground circuit.

 

[Rjryan]

I'm not seeing the need to open the grounded conductor at all. Here is a grounded conductor used as a 3rd leg. No overcurrent, no means of opening it. I know that it's NESC, but same principals apply.

View attachment 2579025

The grounded conductor is at the same potential as the dirt you are standing on just like the neutral for your 120/240 single phase service, or even like the neutral for your 480/277 service, in fact all of them are connected through the MGN, and through the earth, and who knows all what else, building steel. plumbing, whatever. Your home's neutral is at the same potential as the factory with the corner ground, and the other plant 2 blocks away or across town with the 480/277 because they are all connected (keep in mind that I am ignoring differences in potential due to conductor impedance)

When you touch the neutral/ground bar in your home, you are touching the factory down the street's corner grounded phase because they are connected by a piece of wire

 

[Rjryan]

This the best I can find for switching the grounded conductor in a grounded delta system is: “ The core principle is that if you’re going to disconnect power to a circuit, you want to ensure that all the conductors carrying current, both phase conductors and the grounded conductor, are interrupted. -Ensuring Complete Isolation.” The best I could find from the internet. Could not find an exact NEC code article dealing with this, but it makes sense from a safety standpoint. If someone has an NEC reference please come forward with the exact code.

 

[Rjryan]

This the best I can find for switching the grounded conductor in a grounded delta system is: “ The core principle is that if you’re going to disconnect power to a circuit, you want to ensure that all the conductors carrying current, both phase conductors and the grounded conductor, are interrupted. -Ensuring Complete Isolation.” The best I could find from the internet. Could not find an exact NEC code article dealing with this, but it makes sense from a safety standpoint. If someone has an NEC reference please come forward with the exact code.

"If the grounded conductor is contacted by a "better" ground, and there are other faults or conditions that cause voltage differences to exist, a person touching both the "better" grounded object and another conductive surface in the area could be shocked."

 

[kwired]

Thanks that makes sense so by "back fed" you mean someone running old transformer backwards perhaps for a small work shop where they don't have 480 service but have a 480v piece of equipment?
I wonder what the arc flash / ppe requirements are for 480 to ground vs 277 if any?

Arc flash protecteion - like everything else you need to start with possible incident energy and that starts with source impedance, then conductor impedance between the fault location and the source factor in, and then response time of the overcurrent protection factors in as well.

Just because you have 480 volts to ground doesn't automatically mean some 120 volt to ground circuit somewhere else won't have higher incident energy during a ground fault event. Maybe your 480 volt system is a limited capacity system and has much higher source impedance than say the 208/120 system it is derived from.

PPE regarding protection from shock- pretty much the same for everything from ~50 to 1000 volts AFAIK.

 

[jim dungar]

I wonder what the arc flash / ppe requirements are for 480 to ground vs 277 if any?

Arc Flash calculations are based on 3-phase Line-Line-Line bolted faults.
The presence or absence of a neutral does not come into the equation.

 

[electrofelon]

This the best I can find for switching the grounded conductor in a grounded delta system is: “ The core principle is that if you’re going to disconnect power to a circuit, you want to ensure that all the conductors carrying current, both phase conductors and the grounded conductor, are interrupted. -Ensuring Complete Isolation.” The best I could find from the internet. Could not find an exact NEC code article dealing with this, but it makes sense from a safety standpoint. If someone has an NEC reference please come forward with the exact code.

So why don't we disconnect the grounded conductor in split phase and wye systems if it's such a good idea?

 

[Rjryan]

• 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.

 

[roger]

• 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.

The same as any grounded two wire circuit. The NEC's attempt two define the word "neutral" falls short.

 

[electrofelon]

• 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.

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.