have not seen this before....can anyone explain

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iamdent

Member
The job I am currently on has a primary feed from a "substation" to a 600 amp disc feeding a 300KVA delta/wye transformer. The line drawing shows a "slug" fuse in the B phase of the disconnect and the B phase of the primary tap (delta side)on the transformer grounded? I do not understand this and it puzzles me as to how it works this way. The maintenance folks at this plant say they do it all the time????

signed
confused
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

This appears to be a B phase ground secondary transmission line supplying the primary.

Check the number of high voltage lines to the transformer bank.

How many high voltage bushings are on the transformers, or is the transformer the dry type?
 

templdl

Senior Member
Location
Wisconsin
Re: have not seen this before....can anyone explain

It appears as though the feed is from a grounded 'B' phase. If the OCP is provide with fuses then only the 'A' and 'C' phases are fuse with a "slug" fuse in the 'B' phase.
However, you implied that the 'B' phase of the primary of the 300KVA delta/wye transformer was grounded. Am I to understand that the 'B' phase conductor is grounded at the supply and also the 300KVA delta/wye transformer (the B phase of the primary tap (delta side)on the transformer grounded)? If so you may have a code violation as the "grounded" conductor has been grounded at more than on location.
If being fed with an ungrounded delta all 3 phases should be fused and can find no logic as to why the " B phase of the primary tap (delta side)on the transformer grounded " as you post implies.
 

iamdent

Member
Re: have not seen this before....can anyone explain

Yes sir you read the posting correctly, the B phase conductor is grounded at the 300 kva transformer, and yes it seems to be a code violation in my opnion also. This is a pretty prominent orginazation we are doing this install for though and they say they do this in all of thier many locations. I am going to do a little more research when I get back to the job on Tues. morning maybe more there than I'm seeing. I will try to keep posting here with comments.
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

This is a perfectly good installation. Review the principals of a B phase grounded 480 volt system.
 

russ

Senior Member
Location
Burbank IL
Re: have not seen this before....can anyone explain

I understand that the B phase can be grounded. On the 480 volt service it would make the potential voltage, Phase to ground 480 volts.
What I don't understand is the location of the ground and bond. Why is it at the end of the feeder, in the transformer that is being fed. Should it not be grounded and bonded in the Six hundred amp disconnect? Just asking!

Russ

[ August 31, 2003, 11:30 AM: Message edited by: russ ]
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

The bond from the secondary X-O to the B phase of the primary must be done at the transformer to prevent high voltage from appearing on the premises wiring in the event of a winding to winding fault.

This confriguration is definitely not a separately derived system. The active B phase of the other system is solidly connected to the neutral of the user secondary.

This is another case to substantiate my definition of a separately derived system.
 

russ

Senior Member
Location
Burbank IL
Re: have not seen this before....can anyone explain

Bennie:

Would it be less safe of a installation on the secondary side, if the transformer were fed from a ungrounded 3phase delta. In that case bonding to the primary anywhere is not done.
Just trying to sort this out,in my own head.

Russ

[ August 31, 2003, 02:45 PM: Message edited by: russ ]
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

A system with an ungrounded supply to a delta primary, will not have any containment for a through fault. The primary voltage will appear on the secondary ground/neutral from a winding feed through or an insulator flash over.

This is the reason for the MGN system extending to all the Ground/neutrals in the premises system.

This excludes a separately derived system from being supplied by a primary grounded transformer.
 

iamdent

Member
Re: have not seen this before....can anyone explain

Bennie, I think I understand most of that but what in your opinion makes this "not" a seperately derived system??
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

The grounded B phase, of the primary, is connected to the ground/neutral of the secondary.

I am trying to learn how to post drawings.
 

russ

Senior Member
Location
Burbank IL
Re: have not seen this before....can anyone explain

Is the B phase from the utility sub station, grounded before it gets to the plants six hundred amp disconect.
I not sure you can ground an ungrounded utility service. :)

Russ
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

The B phase will be grounded at the source and at least four times per mile until it reaches the user end. Charlie 59 can check this out, I may not be accurate.

The utility calls the grounded phase, the multigrounded neutral conductor. This again is a reason to leave the definition of neutral alone.

The active conductor of transmission lines is often the MGN. An open wye is supplied by two phase conductors and the MGN for the third phase.
 

iamdent

Member
Re: have not seen this before....can anyone explain

bennie, check out the posting titled "seperaly derived source grounding" there is a drawing posted there (which I will try to copy here) which acurately depicts what we are talking about here. If you ground H2 of the transformer at the transformer.
 

iamdent

Member
Re: have not seen this before....can anyone explain


This would be correct if you drew a green line from H2 to the ground or building steel in our case.
 

bennie

Esteemed Member
Re: have not seen this before....can anyone explain

Correct, the B phase will become a part of the MGN system. This is definitely not a separately derived system.
 

Ed MacLaren

Senior Member
Re: have not seen this before....can anyone explain

Bennie, the wiring system and loads connected to the transformer secondary (in the sketch above) are not supplied at the utility's service voltage. They are supplied with a voltage produced by a separate source, the transformer.

If you don't like the term "Separately Derived System", what do you suggest such a system be called?

Ed
 
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