Lost leg on Delta Transformer?

[Joethemechanic]

I worry with all the DIY hackwork, there may be additional ground bonds in the building and/or equipment

250 amp total at 240V is 60 KVA I wouldn't worry too much about a 30 KVA trans on that service, not normally, but with all the hackwork, everything could be loaded on one winding.

Or maybe one winding is defective, and this place is operating on an open delta, then the trans, or bank of trans has to be derated to 57.7% capacity.

You are going to end up having to check everything, starting with the transformer windings.

 

[Joethemechanic]

Nobody who knows what they are doing, even in the slightest way buys single phase equipment for a place with a 3 phase service. For the most part, 3 phase equipment is cheaper to buy. And on the used market most times 3 phase equipment can be had for next to nothing. The 3 phase scares all the DIY guys so it limits the market

 

[Psychlo]

Be careful about using terms incorrectly.
Stop believing what an uninformed layperson tells you, trust your training and the experts on this forum.

A grounded conductor is not always a neutral.
Corner grounded 3-phase systems have been used for almost 100yrs. Typically the only special requirement is that 2-pole breakers can not be 'slash rated' (in the Square D world you cannot use a QO220, instead you need a QO220H).

Corner grounded systems are not necessarily more unsafe than any other system just because they are 240V L-G, after all a center-tapped 240/120V system has one leg that is 208V L-G.

I guess I was referring to the following link (which was posted earlier in this thread), and others like it: http://www.google.com/url?sa=t&rct=j...zVaRQWqgJFVpbQ

But I'm beginning to think that the best way to go, is to let a sleeping dog lie and just add another panel off of the corner ground transformer. I can test phases for balance. If I need to upsize the trans, I could just get the same style, and corner ground it as well?

 

[Joethemechanic]

I guess I was referring to the following link (which was posted earlier in this thread), and others like it: http://www.google.com/url?sa=t&rct=j...zVaRQWqgJFVpbQ

But I'm beginning to think that the best way to go, is to let a sleeping dog lie and just add another panel off of the corner ground transformer. I can test phases for balance. If I need to upsize the trans, I could just get the same style, and corner ground it as well?

Well I would get one capable of being hooked up as a high leg 120/240 high leg delta just to leave future options open, BUT I would, for now, connect it as a a corner ground so it fits into the same electrical "footprint"

 

[kwired]

It would take a set of double lugs to come off of the trans again. But I may need to upsize the tranny anyway. It's a 30kVA. I'll have two panels coming off of it - 150amp and 100amp. I don't know the exact connected load, yet.

In the articles I've been reading, I'm finding that most people are recommending to steer clear of corner ground setups. If I have to change out the tranny anyway, I thought it might be easier (and safer) to switch it over to a more standard scenario. Since the existing loads are all 1-phase, 240v, it wouldn't require rewiring of the branch circuits, right? Or is there a possibility that they tied B-phase to ground at the equipment also?

As you may have noticed, I'm no transformer wiz. Even so, this whole system seems counter-intuitive. Identifying B-phase as a neutral, even though it is a phase conductor? Seems like we're in the Twilight Zone! But that's just my inexperience talking.

Jim commented on this already but I will add my $.02.

You are not identifying the B phase as a neutral, you are identifying it as a grounded conductor, there is a difference. Whenever there is a "neutral" it is usually grounded, so on those systems they are the same conductor.

As far as changing it to 4 wire delta system, you will need a fourth service conductor, if you don't have breakers on the B phase now you will need overcurrent devices on it after the change. Although there is concern about B phase being bonded someplace besides the service equipment, that is improper installation anyway. Just like other systems the equipment grounding conductor and system grounded conductors are to be separate past the service equipment. If there is a bond past the service and you change this to a 4 wire delta system you will find out quickly which circuits have a ground connection on B phase.

 

[Psychlo]

I worry with all the DIY hackwork, there may be additional ground bonds in the building and/or equipment

250 amp total at 240V is 60 KVA I wouldn't worry too much about a 30 KVA trans on that service, not normally, but with all the hackwork, everything could be loaded on one winding.

Or maybe one winding is defective, and this place is operating on an open delta, then the trans, or bank of trans has to be derated to 57.7% capacity.

You are going to end up having to check everything, starting with the transformer windings.

Agreed.

But if one winding was defective, wouldn't the voltage readings be different? What would the readings be if it was an open delta?

After hearing all of you guys, it sounds like the transformer probably is working fine. I'll chat with the owner tomorrow and see why he thinks he lost a leg ten years ago.

 

[jim dungar]

I worry with all the DIY hackwork, there may be additional ground bonds in the building and/or equipment

There has been no mention form the OP about hack work at this site. From the discussion so far, a layperson incorrectly described a grounded systems as having an open phase. Nothing else seems to be amiss.

250 amp total at 240V is 60 KVA I wouldn't worry too much about a 30 KVA trans on that service, not normally, but with all the hackwork, everything could be loaded on one winding.

This is does not appear to be a service. I believe it has been described as a customer owned 30kVA transformer.

 

[Joethemechanic]

Open Delta

 

[Joethemechanic]

Another view

 

[Joethemechanic]

This is an older fab shop that has had a lot of mickey-mouse repairs and add-ons over the years.

Jim,

This says DIY hackwork to me

 

[jim dungar]

I guess I was referring to the following link (which was posted earlier in this thread), and others like it: http://www.google.com/url?sa=t&rct=j...zVaRQWqgJFVpbQ

I happen to think that document is very good.
Is there a difference between it and what I have been saying?

All neutral 'are' grounded conductors, but not all grounded conducotrs are neutrals.
I know that a large majority of people feel that in the US a 240V electical system is 'always' 120V to ground, but that is just not the case. This forum probably discusses 240V 3-wire delta systems at least 5 or 6 times per year.

But, as far as adding a new transformer, if the existing distribution system is not very large (i.e. not expensive to upgrade) and you are sure that you have no improper (i.e. downstream) 'ground' connections, then I would recommend any new transformer be at least a 240/120 4-wire connection.

 

[Joethemechanic]

I just worry that if he touches the owner's DIY hacked up stuff, he is kinda now responsible for it, and I'll bet not only is it a mess, I'll bet the owner's perception is "It's been working for years, there is nothing wrong with it"

That's a hard battle to win sometimes

 

[Joethemechanic]

BTW, my original post of this diagram was incorrect. A primary connection was missing

 

[Psychlo]

This is does not appear to be a service. I believe it has been described as a customer owned 30kVA transformer.

That's correct. The transformer is fed off of the MDP.

 

[Psychlo]

I happen to think that document is very good.
Is there a difference between it and what I have been saying?

I just meant in regard to the preference of a 120/240v vs. 240v corner ground. This Schneider link makes it clear that they think corner grounds are obsolete and quite inferior. Other material I have read confirms that opinion. So they started to sway me that way. But you fellas have brought me back around to thinking it is best to leave the system as is, if possible.

If I have to upgrade the trans, then perhaps I'll consider changing it all over. There are really only six or seven loads in the panel - easy enough to check.

 

[Joethemechanic]

BTW, my original post of this diagram was incorrect. A primary connection was missing


View attachment 6274

Oh god, I messed this one up too, Way to tired when I drew the connections. I'll repost later

 

[jimbo123]

I don't know much about this type of transformer and would like to ask a question if i may.
What if a service is in place with a 480v corner ground and a generator is needed for what ever reason . How do you spec out the correct generator for a corner grounded system ? Will generator service know the differrence?

 

[Joethemechanic]

I cheated and photocopied the pages instead of drawing them.

This gives you a little more information too.

 

[Joethemechanic]

ok if it don't work this time,,,,,,,,,,,,,,,,,,,,

 

[masterinbama]

You also need to make absolutely sure that the B phase was purposely grounded. Had a bad experience with an open Delta 480volt service that had a phase going to ground only when a certain piece of equipment was turned on. Got bit pretty good by the leg that I had tested before working on it.