# Corner Grounding Help



## Janjak (Feb 2, 2016)

We hook up a 480-208 transformer backwards. We take 208V power in building and bring up to 480V. We left off X0 but boss said to run a piece of #4 Copper wire from H3 to ground lug. I picture a big huge boom with sparks when you touch a hot to a ground. Can someone help me understand this concept. I know it is to give a reference to ground if there is a fault but still picture big boom.

Janjak


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## micromind (Aug 11, 2007)

No boom, it'll work just fine. 

The reason is because there is no electrical connection between the 480 side and the 208 side of the transformer. 

Leaving X0 unconnected is a good idea as well. If it is left connected, eventually the transformer would burn up regardless of load.


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## AllWIRES (Apr 10, 2014)

micromind said:


> Leaving X0 unconnected is a good idea as well. If it is left connected, eventually the transformer would burn up regardless of load.


I am interested to know more. Please go on.


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## Bad Electrician (May 20, 2014)

AllWIRES said:


> I am interested to know more. Please go on.


When you connect a delta wye in "reverse" wye primary, delta secondary you never connect the Neutral or ground the XO

Good reference

http://www.automationdirect.com/static/manuals/hpsimperator/backfeedingtransformers.pdf


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## AK_sparky (Aug 13, 2013)

Bad Electrician said:


> When you connect a delta wye in "reverse" wye primary, delta secondary you never connect the Neutral or ground the XO


But why? I understand there is no reason to need to do it, but why does connecting the center of the wye to neutral cause issues?


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## RePhase277 (Feb 5, 2008)

AK_sparky said:


> But why? I understand there is no reason to need to do it, but why does connecting the center of the wye to neutral cause issues?


Seems like connecting the neutral would in effect make three 120 volt transformers, instead of three 120 volt transformers in series across 208 volts. The saturation current in each coil would be higher because the full 120 volts is impressed across each coil... but I'm just guessing.

Besides that, every time I have seen it done, it wasn't just the neutral connected, but the case bond too. In that case you now have current flowing over the EGC too.


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## Bad Electrician (May 20, 2014)

AK_sparky said:


> But why? I understand there is no reason to need to do it, but why does connecting the center of the wye to neutral cause issues?


I looked on google for the answer to your question but nothing detailed. I do know in the cases we have inspected where the neutral was connected the insulation was melted off the conductor heading towards a fault.


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## micromind (Aug 11, 2007)

In theory, the fact that the neutral is connected or not should make no difference but in actual reality, it does. 

I'm not sure, and I realize this makes no sense, but I suspect it has to do with basic waveforms of a Y connected system.

If you draw out the sine waves of a 3Ø Y system, you have 3 voltage peaks in a row followed by 3 more peaks and so on. Constant evenly spaced peaks. If you erase one of the phases, you have 2 peaks and a gap followed by 2 more peaks and a gap and so on. 

If you look at the waveform of 2 legs of a Y, on a scope, you see a sine wave, not 2 peaks and a gap. 

There are things about electricity that we simply do not understand yet......... 

It would be interesting to see which transformer would burn up if the supply was smaller than the load and the neutrals were connected.


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## don_resqcapt19 (Jul 18, 2010)

It is my understanding that when you connect XO on this type of installation, any voltage imbalance on supply side causes current flow on XO as the transformer will try to balance the supply voltage.
Jefferson Transformer says:


> CAUTION:
> When connecting a Delta-Wye transformer in reverse the primary should be connected as a Delta (ignoring the X0 terminal). Connecting the X0 on the primary may cause unsafe conditions in fault situations.


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## telsa (May 22, 2015)

The cleanest explanation is by thinking of energy flow.

The juice pumped into the primary Wye coils sends energy into the ground as current AND sends energy into the magnetic circuit.

If the secondary circuit is open, then the magnetic circuit reaches its maximum and chokes off most of the current that head into the neutral connection.

Since there are three (3) circuits trying to use the same 'drain' or grounded neutral one can see where trouble comes.

&&&&

Once the secondary is loaded -- pulling energy out of the magnetic circuit -- the choking power is reduced -- and the current passing down and through the neutral rises past critical levels. ( And the over all current passing through the coils takes off.)

This condition is most of the way towards having three hots feeding a single neutral conductor but without the back-emf seen in a three-phase wye connected motor.

Just because it's grounded does not mean that it's not carrying current. 

!


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## AK_sparky (Aug 13, 2013)

micromind said:


> In theory, the fact that the neutral is connected or not should make no difference but in actual reality, it does.
> supply was smaller than the load and the neutrals were connected.


I'm sure there is theory to back up why it matters; just nobody here knows what it is!



micromind said:


> If you draw out the sine waves of a 3Ø Y system, you have 3 voltage peaks in a row followed by 3 more peaks and so on. Constant evenly spaced peaks. If you erase one of the phases, you have 2 peaks and a gap followed by 2 more peaks and a gap and so on.
> 
> If you look at the waveform of 2 legs of a Y, on a scope, you see a sine wave, not 2 peaks and a gap.


That is not true. You are hooking up the 'scope wrong for what you are trying to achieve. If you want to see the waveforms of 2 phases it is most certainly possible and easy.


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## AK_sparky (Aug 13, 2013)

Here is some heavy reading that I think explains it. I haven't read through it, but it looks like connecting the neutral on a wye primary causes issues because of harmonics.

I might read through this in more detail another time.

http://apps.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic


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## telsa (May 22, 2015)

To elaborate -- the electric currents and the magnetic currents are out of phase.

Note that when a delta-to-wye dry-type transformer is creating 208Y120 from 480 or 600 3-phase voltage -- that the neutral currents are 30 degrees out of phase.

This same lagging delay occurs when the energy flow goes the other way.

That's a whole lot of electrons jiggering up and down.

See the foot notes in Uglies.

And note the geometry between Delta and Wye.

Each interior angle of the delta is 60 degrees.

The Wye -- when overlaid -- splits that 60 degrees in half.

Yes, THAT'S where the 30 degrees of lag comes from.

NO HARMONICS are involved. Its the phase shift -- as inductive elements always LAG the current. Triplens and all the rest are not involved.

Get it ?


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## telsa (May 22, 2015)

The delta and wye are actually Tensor diagrams -- from mathematics.

They are related to Vector mathematics.

Yes, yes, they beat this to death in EE college.


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## RePhase277 (Feb 5, 2008)

AK_sparky said:


> I'm sure there is theory to back up why it matters; just nobody here knows what it is!
> 
> 
> 
> That is not true. You are hooking up the 'scope wrong for what you are trying to achieve. If you want to see the waveforms of 2 phases it is most certainly possible and easy.


He means between any two phases referenced against each other. If you put your scope ground on A phase and probe B phase, you will only see one sine wave.

Now, if you put the scope ground to the neutral, and channel 1 to A phase and channel 2 to B phase, yes you will see two waveforms.


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## Southeast Power (Jan 18, 2009)

micromind said:


> No boom, it'll work just fine.
> 
> The reason is because there is no electrical connection between the 480 side and the 208 side of the transformer.
> 
> Leaving X0 unconnected is a good idea as well. If it is left connected, eventually the transformer would burn up regardless of load.


The line and load windings are not physically connected and bringing XO across to the other transformer and connecting it to the same reference point as XO, doesn't seem like a healthy choice. Somewhat like a condom with a hole in it, you have breached a barrier.


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## chicken steve (Mar 22, 2011)

10-4 JRannis

But if H3 is grounded i'd wager is a point of ref & current flow through the building structure occurs back to poco source IF the load is not balanced 3ph


~CS~


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## AK_sparky (Aug 13, 2013)

InPhase277 said:


> He means between any two phases referenced against each other. If you put your scope ground on A phase and probe B phase, you will only see one sine wave.


Which is what you should expect to see.


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## AK_sparky (Aug 13, 2013)

telsa said:


> NO HARMONICS are involved. Its the phase shift -- as inductive elements always LAG the current. Triplens and all the rest are not involved.
> 
> Get it ?


The reference material from General Electric would suggest otherwise.


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## AK_sparky (Aug 13, 2013)

Some more reading that might provide some answers.

http://electrical-engineering-portal.com/transformer-connection-star-star


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## RePhase277 (Feb 5, 2008)

jrannis said:


> The line and load windings are not physically connected and bringing XO across to the other transformer and connecting it to the same reference point as XO, doesn't seem like a healthy choice. Somewhat like a condom with a hole in it, you have breached a barrier.


A corner grounded Delta isn't bringing X0 over to the other side. X0 is left unconnected on the primary wye. The secondary delta is electrically isolated from the primary. You take one of the delta phases, often B, and ground it. You color any wires connected to it white and in almost every way, treat it like a neutral.


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## wendon (Sep 27, 2010)

The only issue I've seen with these back fed transformers is the inrush current.


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## chicken steve (Mar 22, 2011)

InPhase277 said:


> A corner grounded Delta isn't bringing X0 over to the other side. X0 is left unconnected on the primary wye. The secondary delta is electrically isolated from the primary. You take one of the delta phases, often B, and ground it. You color any wires connected to it white and in almost every way, treat it like a neutral.


What happens in a fault situation...

~CS~


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## RePhase277 (Feb 5, 2008)

chicken steve said:


> What happens in a fault situation...
> 
> ~CS~


Same exact thing that happens in a grounded wye system: current from the faulted phase flows along an equipment grounding conductor to the system bonding jumper and into the grounded phase, completing the circuit and tripping the OCPD.


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