Grounding one leg of 120/240V transformer

[PhaseShift]

What would happen if instead of grounding the neutral of a 120/240V transformer, we removed this ground and grounded one of the hot legs?

I believe we would still have 120V L-N on each leg?

I guess we would be able to have ground fault current for a L-G fault on the ungrounded hot leg, but for a L-G fault on the grounded leg there would be no current? Is there any downside to this?

 

[jim dungar]

What would happen if instead of grounding the neutral of a 120/240V transformer, we removed this ground and grounded one of the hot legs?

I believe we would still have 120V L-N on each leg?

I guess we would be able to have ground fault current for a L-G fault on the ungrounded hot leg, but for a L-G fault on the grounded leg there would be no current? Is there any downside to this?

You would have one leg of 120V L-G and the other of 240V L-G.

 

[gar]

100120-2202 EST

If neutral is floated off of ground, then you still have 120-N-120.

Otherwise if neutral, the center tap, is still connected to ground, then you have a dead short across one previous hot to the neutral. Lots of sparks and heat.

.

 

[infinity]

I agree with Jim. Is this a hypothetical question?

 

[ptonsparky]

The neutral is the grounded conductor

You would have 240v L1 to G and 120v L2 to G.

L2 is now the center tap of the xfmr (X2&X3)

The transformer doesn't care which tap is grounded.

 

[iwire]

The neutral is the grounded conductor

I disagree, the neutral conductor would remain the midpoint.

 

[mivey]

I disagree, the neutral conductor would remain the midpoint.

Correct. Jim & Gar are both correct. The OP is correct.

 

[mivey]

mivey is correct.

 

[John Paul]

You would have one leg of 120V L-G and the other of 240V L-G.

And 120V L-L ?

 

[iwire]

mivey is correct.

Doubt it.

 

[mivey]

And 120V L-L ?

240 L-L

Summary (neutral is not tied to ground):
L1-G = 120
L2-G = 240
L2-L1 = 240
L1-N = 120
L2-N = 120
N-G = 120

 

[mivey]

Doubt it.

Oh ye of little faith. Extend your vision and read my post #11. :grin:

 

[mivey]

Corrected

240 L-L

Summary (neutral is not tied to ground):
L1-G = 0
L2-G = 240
L2-L1 = 240
L1-N = 120
L2-N = 120
N-G = 120

Ooops. Jim was wrong. And so was mivey.

add:

Oh ye of little faith. Extend your vision and read my post #11. :grin:

Don't listen to this idiot. :grin:

 

[PhaseShift]

You would have one leg of 120V L-G and the other of 240V L-G.

This is a hypothetical question.

If we assume that L1 is the ungrounded leg, the neutral is not grounded, and L2 is the grounded leg then I see that we will have L1-G be 240V but why would L2-G be 120V? Wouldn't it be 0V since it would be at the same potential?

 

[ptonsparky]

And 120V L-L ?

yes, if you consider that L1 and L2 are not grounded.

X1 = L1
X4 = G
X2 & X3 = L2 (and yes the neutral because it is the center point of xfmr)

L1-G = 240V
L2-G = 120V
L1-L2 = 120V

 

[jim dungar]

Jim was wrong.

Not really.
If you want to to take the midpoint connection and call it a neutral you are describing a different system.

I just did not consider the system as having a neutral. In my description there are two Line conductors and one Grounded conductor. Which is why I did not list a L-L voltage.

This may not be a hypothetical installation. There really are multi-voltage transformers used in some industrial control panels. The typical voltage for these are 480-120/24 (i.e. Square D #9070TxxxD15).

 

[PhaseShift]

So when we remove the ground from the center tap and move it to what was the L2 (far right) leg then we essentially have moved the neutral and the center tap now becomes L2 and the far right leg the neutral. Any down side to doing this?

 

[jim dungar]

So when we remove the ground from the center tap and move it to what was the L2 (far right) leg then we essentially have moved the neutral and the center tap now becomes L2 and the far right leg the neutral. Any down side to doing this?

The neutral point does not change. All you are doing is changing the grounded conductor.

 

[iwire]

we essentially have moved the neutral and the center tap now becomes

I do not think it is possible to move the neutral, the neutral is the conductor that carries the imbalance from the others. Changing which one conductor is grounded does not change this.

See 2008 NEC Article 100 "Neutral Point"

 

[PhaseShift]

The neutral point does not change. All you are doing is changing the grounded conductor.

So why then are we saying that L2-G is 120V if L2 is the grounded conductor? Wouldn't they be at the same potential and therefore be 0V?