Transformer xo bonding

[kwired]

Also guessing this was installed before that rule came about.

Still needs one of the 480 volt leads grounded, not sure if there is an EGC on the supply side, but needs one.

 

[romex jockey]

This has been a very informative thread to me fellas.

Please bear with me as i need to ask the 'stupid Q' because my xformer theory is weak

What happens , in this scenario, if we mess with XO?

Keep in mind i would have instinctively gone for XO....

~RJ~

 

[kwired]

This has been a very informative thread to me fellas.

Please bear with me as i need to ask the 'stupid Q' because my xformer theory is weak

What happens , in this scenario, if we mess with XO?

Keep in mind i would have instinctively gone for XO....

~RJ~

It works fine when POCO builds a wye-delta bank out of three single phase transformers, it is when you have a common core for all three coils that you have problems. Connecting to neutral results in high circulating currents within the transformer, especially if the "secondary" has unbalance load on it, and it don't have to be much imbalance at all from my understanding.

I don't fully recall all the details of why but that is what I recall. There have been several threads on this topic in the past.

 

[romex jockey]

Thx, I think i get the idea Kwired

2nd dopey Q

So we take a secondary leg of the delta and make it to the bottom mess of grounding conductors

It's really not and SDS, no noodle.....or an MBJ for that matter......should it have all the normal GEC's we're used to installing?

~RJ~

 

[jumper]

What happens , in this scenario, if we mess with XO?



~RJ~

Bad stuff.

 

[JFletcher]

Oooh! On a scale of good to bad, that ^ would be "bad". :lol:

OP said there were no loads utilizing a neutral in his OP.

work space violation - maybe, maybe not, depends on how strict you read into it. If the outer enclosure is no more then 6 inches deeper then the inner enclosure, I think it is even harder to say there is a violation.

True however he called it as "277/480" - there is no 277V anywhere on a 480V delta secondary.

If one were to attempt to use the neutral from the 4 wire 208V primary as the neutral point for the 480V secondary to run 277V loads, bypassing the transformer, what bad things would happen? Inevitable transformer fire as pictured above? It is the center point on the 208V side - would the phase shifts in the transformer 'cancel out' and still leave it the center point on the 480V side, would any current imbalance on the secondary result in a shift in voltage from phase to neutral, thereby giving 0-480V?

Backing up a bit, if the primary neutral was not used, and you had a proper transformer setup with a corner grounded 480V delta secondary, what readings would you get measuring secondary phases to that primary neutral?

 

[jumper]

Thx, I think i get the idea Kwired

2nd dopey Q

So we take a secondary leg of the delta and make it to the bottom mess of grounding conductors

It's really not and SDS, no noodle.....or an MBJ for that matter......should it have all the normal GEC's we're used to installing?

~RJ~

Use this pic to start.

Imagine a box around the tranny, it is the case.

The grounded phase conductor, the GEC, and the case are all tied together. Prolly a grounding bus attached to case.

 

[ActionDave]

Thx, I think i get the idea Kwired

2nd dopey Q

So we take a secondary leg of the delta and make it to the bottom mess of grounding conductors

It's really not and SDS, no noodle.....or an MBJ for that matter......should it have all the normal GEC's we're used to installing?

~RJ~

A corner ground delta is nothing more than the end of a transformer winding tied to earth instead of the center. After that it acts and is treated like any other grounded neutral. One could tap say, three quarters of the winding and then you'd have 360V and 120V to ground on each leg respectively but that would not be very useful.

 

[kwired]

Thx, I think i get the idea Kwired

2nd dopey Q

So we take a secondary leg of the delta and make it to the bottom mess of grounding conductors

It's really not and SDS, no noodle.....or an MBJ for that matter......should it have all the normal GEC's we're used to installing?

~RJ~

It is a "grounded conductor". It follows all the same rules as grounded conductors that are a "neutral".

Bond it at service or first disconnect and separate into grounded (current carrying) conductor and equipment grounding conductor (non normally current carrying) beyond that main/system bonding jumper. Because it is a "grounded conductor" it need to be identified with gray or white.

 

[romex jockey]

Oooh! On a scale of good to bad, that ^ would be "bad". :lol:

9.9 baaaaAAaaaad .....

Backing up a bit, if the primary neutral was not used, and you had a proper transformer setup with a corner grounded 480V delta secondary, what readings would you get measuring secondary phases to that primary neutral?

hmmm, well wouldn't there be a primary GEC somewhere on the premisis?


~RJ~

 

[GoldDigger]

It works fine when POCO builds a wye-delta bank out of three single phase transformers, it is when you have a common core for all three coils that you have problems. Connecting to neutral results in high circulating currents within the transformer, especially if the "secondary" has unbalance load on it, and it don't have to be much imbalance at all from my understanding.

I don't fully recall all the details of why but that is what I recall. There have been several threads on this topic in the past.

It is not so much unbalanced load on the secondary that is a problem, but unbalanced voltages on the wye primary for any upstream reason or unbalanced local load.
The delta connection imposes a condition on the sum of the delta voltage phasors (must sum to zero) which the primary wye voltages may not satisfy. If that happens enough current flows in the primary neutral and the delta windings to make the equation work by dropping voltages across the windings.

 

[kwired]

hmmm, well wouldn't there be a primary GEC somewhere on the premisis?


~RJ~

Yes, is still same 120 volts from each phase to GEC just as it would be if the transformer primary were delta connected.

Until you bond something on the secondary there is no solid reference to ground on the secondary conductors.

 

[nickelec]

Very informative thread so far .

Sent from my SM-G950U using Tapatalk

 

[LarryFine]

If one were to attempt to use the neutral from the 4 wire 208V primary as the neutral point for the 480V secondary to run 277V loads, bypassing the transformer, what bad things would happen?

There would be no benefit, as no current can flow between any of the secondary lines and the primary grounded conductor, because they're isolated from each other.

 

[kwired]

There would be no benefit, as no current can flow between any of the secondary lines and the primary grounded conductor, because they're isolated from each other.

If you connected 277 volt loads to it it would create a neutral point, if they are equal loads it will be a fairly stable 277 volts. Isn't that how ground fault indicators are connected on ungrounded systems, no fault all three lights are equal brightness as they all see ~277 volts.

Add: It won't be a low impedance path for fault clearing though

 

[romex jockey]

There would be no benefit, as no current can flow between any of the secondary lines and the primary grounded conductor, because they're isolated from each other.

Use this pic to start.

Imagine a box around the tranny, it is the case.

The grounded phase conductor, the GEC, and the case are all tied together. Prolly a grounding bus attached to case.

Ok, i'm 'imagining' those little grounding symbols going to GEC's water lines, building steel

I can imagine the guy who installed the incoming service did something similar

Maybe I've a tad too much imagination?

Where do we want this to 'fault' to?

~RJ~

 

[Sahib]

I don't fully recall all the detais

Grounding neutral of star delta transformer is not always harmful. In fact grounding star delta transformer with neutral grounded is sometimes used to supply phase to ground connected loads in ungrounded systems.

 

[kwired]

Ok, i'm 'imagining' those little grounding symbols going to GEC's water lines, building steel

I can imagine the guy who installed the incoming service did something similar

Maybe I've a tad too much imagination?

Where do we want this to 'fault' to?

~RJ~

You have bonded one point of the supply just like you do with a system that has a "neutral".

Reality is you could ground a "phase" conductor of a 4 wire wye system and you still have same scheme when it comes to grounding and bonding, you just made that ground reference with a different point on the system. NEC just happens to require us to ground the neutral conductor on those systems.

Grounding/bonding just puts all non current carrying metal components at same potential as earth as well as the point of the system that was bonded. Create some system with 10 points of potential and you still just ground one point of the system if it is to be a grounded system. You still separate "grounded conductors" from "equipment grounding conductors" beyond the main/system bonding jumper to avoid stray currents from flowing in the EGC and non current carrying components.

The question isn't what do we want this to fault to, it is where do we want ground fault current to return to?

 

[romex jockey]

The question isn't what do we want this to fault to, it is where do we want ground fault current to return to?

oh man....

the delta secondary?

~RJ~

 

[jumper]

oh man....

the delta secondary?

~RJ~

Assuming fault is in motor, we want fault to return towards secondary source and the OLs in controller to heat up and open.

If it faulted from case to grounded conductor and OL does not open, the fault would eventually cause enough current flow to open primary OCPD. At this point motor is prolly toast.