Xfmr Reverse Connected

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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Bjenks said:
I am not understanding your response... You would have the A phase and the ground conductors now carring the current in parallel.

Exactly. If a phase leg is intentionally bonded to ground, and also faults to ground at another location, there will be no significant change in phase current, and thus the breaker won't trip.

But this is exactly the same situation as a neutral to ground fault in a wye system with a grounded neutral. You have a _grounded_ conductor, which faults to bonded metal at an inappropriate location. There is no significant fault current flow, and thus breakers will not trip.

In either case, there is 'objectionable' current on bonded metal (EGC, building steel, etc.), and in either case ground fault detection may be able to discern this problem.

Because neutral conductors generally carry less current than phase conductors, I agree that the danger of a grounded conductor to ground fault is greater with a corner grounded delta...but have no way of quantifying 'greater'.

-Jon
 

iwire

Moderator
Staff member
Location
Massachusetts
Bjenks said:
I am not understanding your response...

Actually I think you did. :)

If you corner ground the A phase, then the A phase and ground are at the same potential.

Agreed.....they are both the same potential as all bonded objects and the earth in the vicinity of the grounding electrodes


So if you have a fault on the B phase the breaker would trip,

Again we agree. :)


but if you have a fault on the A phase I wouldn't see how it would trip the breaker.

It would not.

The NEC does not even require this grounded phase to have Overcurrent protection.

(Just like the neutral of a Wye system)

You would have the A phase and the ground conductors now carring the current in parallel.

Yes, just like what would happen if the neutral of a Wye system had a ground fault.
 

Bjenks

Senior Member
Location
East Coast of FL
OK, thanks. So I am still thinking that the best way to engineer this solution to to put some type of CT/ground fault detector in so that a warning of a ground fault on the grounded phase (Forget that it is a cost issue and think about safety) right? As an engineer, I don't feel comfortable not putting some type of alarm or trip mechanism in place for a ground fault on grounded phase. Yes, now I understand that it is no difference than a N-G fault, but in my mind I am imagining more current and a larger chance of that type of fault happening. Again, maybe it just doesn't make any difference as everything is on the same ground plane. Also, do you know what code I would use to NOT put OCPD on the grounded phase?
 

iwire

Moderator
Staff member
Location
Massachusetts
Bjenks said:
So I am still thinking that the best way to engineer this solution to to put some type of CT/ground fault detector in so that a warning of a ground fault on the grounded phase (Forget that it is a cost issue and think about safety) right?

I am not sure it is such a big issue or I would think the NEC would require this ground fault detection for grounded conductors.

Yes, now I understand that it is no difference than a N-G fault, but in my mind I am imagining more current

I agree it is likely that a phase conductor will be carrying more current than a neutral conductor.

and a larger chance of that type of fault happening.

What type of fault do you envision?

Also, do you know what code I would use to NOT put OCPD on the grounded phase?

240.22 prohibits over current protection on a grounded conductor unless.

1) The device will open all conductors of the circuit simultaneously and can not be operated independently

2) Or as required for Motor overloads in 430.
 

Minuteman

Senior Member
Delta to Delta

Delta to Delta

Okay, just read (actually skimmed through) all the previous posts. I have a different scenario.

Plant has 240 volt Delta, new machine is 480 Delta. IF I can find a delta/delta "step up" transformer, than would I be okay to "step up" the voltage for this machine?
 
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vrizz@comcast

New member
Transformers

Transformers

i UNDERSTAND THAT TO PRODUCE A 480VOLT 3 PHASE SOURCE VIA A STEP DOWN TRANS FORMER, WE WIRE IT WITH THE 240 3PHASE AS PRIMARY, NO NEUTRAL AND CORNER GROUND ONE OF THE SECONDARY LEGS . IS THAT CORRECT? OF COURSE GROUNDING IT TO THE STRUCTURE AS WELL.
 
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