Transformer reverse connected...

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http://magazine.iaei.org/magazine/02_a/johnston.htm
 
Chevy, in Stickboys illustration ( and since the discussion is an SDS) there should be an EGC from the disconect back to the frame of the transformer.

Or, if the bonding was done at the transformer there should be an EGC from that point to the disconnect enclosure.

If there was a garauntee that there would never be a conductive path between the transformer and the disconnect the bonding could take place at both locations, see 250.30(A)(1) EX 2

Roger
 
jim dungar said:
Particularly, beware of using circuit breakers or distribution panels on grounded-phase systems. UL has specific listings for any equipment (including fusible switches) used on these systems.
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How expensive would a 3 phase 25 HP rated disconnect on a corner grounded delta system be? Would a normal 3 phase 600v 25 HP rated disconnect be sufficient??
 
chevyx92 said:
Have I got this all correct????
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Yes, with one additional requirement: the groundeed conductor should be disconnected along with the others, as in Stick's drawing.

chevyx92 said:
Would a normal 3 phase 600v 25 HP rated disconnect be sufficient??
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Yes.
 
LarryFine said:
Yes, with one additional requirement: the groundeed conductor should be disconnected along with the others, as in Stick's drawing.
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Larry I do not believe that is a requirement, just a design decision.

Sometimes you can use single phase panels for corner grounded delta.
 
LarryFine said:
Yes, with one additional requirement: the groundeed conductor should be disconnected along with the others, as in Stick's drawing.
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Not according 240.22. If a breaker were used I would agree that you could disconnect the grounded conductor because all phases would open on a fault. But if you use a fused disconnect then that won't meet the requirements of 240.22. Or am I missing something here?
 
chevyx92 said:
Ok I think I have a real good picture of how you're suppose to corner ground a delta. 3 phase plus EGC in with no neutral, 3 phase plus EGC out no neutral. XO will not have any connections to it. Take and ground "B" phase of delta side to frame of Xfmr and building steel and EGC. "B" phase will be taped white and have no fused protection per code. And the disconnect has to be rated for a corner grounded system. Have I got this all correct???? Also are there any links of diagrams or pics or official documentation as to the way we are hooking this up?? Thanks.
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You do not necessarily have to ground "B" phase you can ground "A or C". I grounded "B" phase because I could come straight up to the double lug with the EGC from the shell lug on the transformer.
 
chevyx92 said:
Not according 240.22. If a breaker were used I would agree that you could disconnect the grounded conductor because all phases would open on a fault. But if you use a fused disconnect then that won't meet the requirements of 240.22. Or am I missing something here?
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You can't place a fuse in the grounded phase's fuse-holder, but the switch can open all conductors.
 
LarryFine said:
You can't place a fuse in the grounded phase's fuse-holder, but the switch can open all conductors.
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Thats what I was saying you can't fuse the grounded phase unless all phases open on a fault. A breaker will open all phases includung the grounded conductor if you do it this way. If you use a fused disconnect and a fault happens it doesnt necsesarily open all the phases.
 
chevyx92 said:
post #37
Let me ask this. By grounding the "B" phase that phase will have zero volts correct? If thats correct how would a 3 phase motor work on this corner grounded system? You would have phases A,C and the B is the grounded one with zero volts. Wouldn't the 3 phase motor be single phasing on this setup???
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Single phasing is when you lose a phase so that you have no current flow on that phase.

LarryFine said:
post #39
Yes, B phase would have zero volts relative to earth, but the voltages among the three conductors remains the same.
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Think of it this way:
If you had a 3 wire HR in a single phase panel. . If the black hot carried 20a of current and the red hot carried 20a of current, the neutral would carry 0a, right ? . It?s easy to see it as a neutral when there?s no current flow.

Now if the black hot carried 20a and the red hot carried 5a, the neutral would carry 15a, right ? . So it?s a grounded neutral with current flow. . Voltage to ground is zero and yet there is current flow back to the transformer.

The corner grounded phase of a corner grounded delta has zero voltage to ground and yet there is current flow back to the transformer on that grounded conductor. . The fact that it has no voltage to ground and yet still carries current isn?t a contradiction.

Any one point in the system can be grounded without affecting current flow. . Current isn?t searching for ground, it?s searching for a return path back to the transformer. . So if there?s only one point connected to ground then there?s no path thru the ground back to the transformer. . With single point grounding, there?s no current flow to ground there?s only stabilizing of voltage for the system. . The grounded point [whatever point that is] stabilizes at zero v to ground.

David
 
chevyx92 said:
Thats what I was saying you can't fuse the grounded phase unless all phases open on a fault. A breaker will open all phases includung the grounded conductor if you do it this way.
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That is a bit off, you can use a fused disconnect with a 'dummy fuse' in the grounded phase position.
 
Stickboy,
I realize that the picture from the IAEI magazine that you posted on post #41 is an installation allowed by 250.30(A)(1)x2 but I think electrode connections at multiple points is always a bad idea. . And I think doing it on a corner grounded delta is worse than on a center tapped single phase or on a wye. . You get full current flow on the corner tapped phase and it's connected to an electrode at both meter and main disconnect.

I know that 250.30(A)(1)x2 says, "For the purposes of this exception, connection through earth shall not be considered as providing a parallel path" but that doesn't mean that there isn't current flow thru the ground and it also doesn't mean that multiple point earth connection is a good idea.

David
 
dnem said:
but that doesn't mean that there isn't current flow thru the ground and it also doesn't mean that multiple point earth connection is a good idea.

David
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David what makes it a bad idea?

What is the connection to earth going to acomplish in the first place.
 
I believe that David is talking about _multiple_ connections between the grounded conductor and earth, forming a parallel path for current flow.

You will always have current flowing on such a parallel path, and since for services of the same KVA and voltage, you would expect higher current flow on the phase conductor than on the neutral, one would expect higher 'parallel path' current flow in a corner grounded system.

-Jon
 
winnie said:
I believe that David is talking about _multiple_ connections between the grounded conductor and earth, forming a parallel path for current flow.
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Jon, I understand that, why is that bad?

Lets consider for a moment how much current we are talking about, I bet it will be very low.

Take a steel framed building for instance, every column is another ground connection to earth.

It would be vitally imposable for an NEC compliant SDS to not have multiple connections to earth
 
iwire said:
Jon, I understand that, why is that bad?

Lets consider for a moment how much current we are talking about, I bet it will be very low.

Take a steel framed building for instance, every column is another ground connection to earth.

It would be vitally imposable for an NEC compliant SDS to not have multiple connections to earth
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If each building column is connected to the service disconnect there would only be one point of connection between the system and the earth. . Whether the electrode conductors were brought back to the service independently or were "daisy-chained" and had one wire going back, it's still just one single connection point between service and earth.

Why would you connect one building column to the meterbase and another building column to the service disconnect ? . That creates a electrode-earth-electrode path that's parallel to the service neutral. . Adding currents to the ground is never a good idea even if there isn't a body of water close by.

David
 
I would have to agree with dnem,
Just like any SDS you would only bond the system neutral at a single point. Either at the transformer or first disconnect, not both.
 
k-Rail

k-Rail

Actually there is no need to bond one of the phases to ground. I personally think that having 480v. potential on 2 of the legs is a lot more dangerous than having the delta system ungrounded. With the system ungrounded you won't have potential to ground and it's a lot safer.
250-20 only requires the 4 wire delta to be grounded
 
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