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