Closed Delta 120/240V 3-phase service

In the case of a commercial building served by 3 phase - closed delta 240/120 Vservice, what is the best way to detect a ground fault on a three phase motor circuit on phase C were it to come into contact with the ground?
Whereas the service transformer on phase AB has the only centered tapped (neutral) windings 120V phase-to-neutral and 240V phase-to-phase and phase C (not connected to the phase AB transformer) is the wild phase.
It seems to me that (without additional protective equipment), if phase C accidently touches the ground there would be a voltage imbalance, but the overcurrent protection would not necessarily trip off.
Am I wrong by assuming this?
Also, what, in general, are the pros and cons of a 120/240V 3 phase closed delta connection?
Is this a good choice for a small commercial building with a 3 story hydraulic elevator and mostly 120V. appliance and lighting loads?
Beachcomber

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I don't know what you mean by the first question. In answer to your second; In general terms you want the 240 system when your predominate load is small to medium hp 3-phase motors. If you are going to supply primarily lighting and receptacle loads, then a 208 4-wire system is an obvious advantage.
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On Sun, 10 Feb 2008 06:10:33 GMT, snipped-for-privacy@notreal.none (Beachcomber) wrote:
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The 3d leg is 208v above ground and it would trip the O/C device if it faults.

| In the case of a commercial building served by 3 phase - closed delta | 240/120 Vservice, what is the best way to detect a ground fault on a | three phase motor circuit on phase C were it to come into contact with | the ground?
A GFI device? All 4 wires (A,B,C,N) would run through the CT together.
| Whereas the service transformer on phase AB has the only centered | tapped (neutral) windings 120V phase-to-neutral and 240V | phase-to-phase and phase C (not connected to the phase AB transformer) | is the wild phase.
It is 208 volts relative to ground.
| It seems to me that (without additional protective equipment), if | phase C accidently touches the ground there would be a voltage | imbalance, but the overcurrent protection would not necessarily trip | off. | | Am I wrong by assuming this?
That depends on how solid the ground path is. If the impedance is high to get back to the source, maybe not enough current will flow. That could easily be the case if it contacts earth. It could even be the case if it contacts a metal part of the building. But if the impedance is low enough, you get high current.
Still, if there is any risk of a ground fault, and especially one that could be contacted by a person as part of the ground path, then ground fault protection would be a good thing to have. A GFI device?
| Also, what, in general, are the pros and cons of a 120/240V 3 phase | closed delta connection?
If there is a loss of one phase in this system it can lead to trouble. Delta can have back-energizing issues as well as lopsided loading if a phase is lost. And 240D/120 would be quickly out of balance if the 120 volt loads predominate. Alternatively, a Scott-T version of that could help in limited 3-phase loading environments.
| Is this a good choice for a small commercial building with a 3 story | hydraulic elevator and mostly 120V. appliance and lighting loads?
208Y/120. Configure the elevator system for 208 volts. It is the more common electrical system in the USA, now. An elevator manufacturer not being able to do 208 volts is limit their market severely.

Beachcomber wrote:
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The protective device will trip if phase C is grounded, just as for the other phases. The delta system will not allow full utilization of the available kVA, since one phase can not be used for the single-phase loads. With mostly 120V lighting & appliances, I would go 208/120 4-wire wye, so you get the benefit of all three phases for the 120V loads. Run the elevators & A/C on 208 3-phase.
Benjamin D Miller, PE www.bmillerengineering.com

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I have only seen center tapped "closed delta" a few times, notably in the alley next to the Hog's Breath Salloon in Key West (look at the pole behind the stage) and I bet from the looks of it they added the 3d transformer as the load increased. Usually it is "open delta" with 2 transformers as a cheap way to get some 3 phase to a customer who has mostly single phase loads. The open delta was actually pretty popular in the light industrial areas of East Naples (Florida).

On Tue, 12 Feb 2008 11:53:59 -0500 snipped-for-privacy@aol.com wrote:
| wrote: | |>Beachcomber wrote: |>> In the case of a commercial building served by 3 phase - closed delta |>> 240/120 Vservice, what is the best way to detect a ground fault on a |>> three phase motor circuit on phase C were it to come into contact with |>> the ground? |>> |>> Whereas the service transformer on phase AB has the only centered |>> tapped (neutral) windings 120V phase-to-neutral and 240V |>> phase-to-phase and phase C (not connected to the phase AB transformer) |>> is the wild phase. |>> |>> It seems to me that (without additional protective equipment), if |>> phase C accidently touches the ground there would be a voltage |>> imbalance, but the overcurrent protection would not necessarily trip |>> off. |>> |>> Am I wrong by assuming this? |>> |>> Also, what, in general, are the pros and cons of a 120/240V 3 phase |>> closed delta connection? |>> |>> Is this a good choice for a small commercial building with a 3 story |>> hydraulic elevator and mostly 120V. appliance and lighting loads? |>> |>> Beachcomber |> |> |>The protective device will trip if phase C is grounded, just as for the |>other phases. The delta system will not allow full utilization of the |>available kVA, since one phase can not be used for the single-phase loads. |>With mostly 120V lighting & appliances, I would go 208/120 4-wire wye, so |>you get the benefit of all three phases for the 120V loads. Run the |>elevators & A/C on 208 3-phase. |> |> |>Benjamin D Miller, PE |>www.bmillerengineering.com |> | | I have only seen center tapped "closed delta" a few times, notably in | the alley next to the Hog's Breath Salloon in Key West (look at the | pole behind the stage) and I bet from the looks of it they added the | 3d transformer as the load increased. Usually it is "open delta" with | 2 transformers as a cheap way to get some 3 phase to a customer who | has mostly single phase loads. | The open delta was actually pretty popular in the light industrial | areas of East Naples (Florida).
I've seen a few setups with one big fat transformer and one small one. Usually, the big one has 2 bushings and the small one has 1 bushing. On the secondary, the big one has 3 wired lugs and the small one has only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where I have seen these is in business (not industrial) areas of small towns in West Virginia and Ohio, usually servicing stores or restaurants. Some of those smaller pole pigs were the smallest pole pigs I've seen. My guess is the smallest ones were no more than 3 or 5 kVA.

On 12 Feb 2008 18:07:12 GMT, snipped-for-privacy@ipal.net wrote:
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The two transfomer delta around here is just 2 of the expected 3 in true delta. They connect A to C and leave B floating off of the AB transformer that is center tapped. As long as you have a well balanced 3 phase load it works fine. We did get in trouble in the computer biz since we had some single phase loads derived from the incoming 3 phase inside the machine. I have had to juggle machines around or "roll the phases" to balance the phase loads.

On Tue, 12 Feb 2008 14:37:13 -0500 snipped-for-privacy@aol.com wrote: | On 12 Feb 2008 18:07:12 GMT, snipped-for-privacy@ipal.net wrote: | |>| I have only seen center tapped "closed delta" a few times, notably in |>| the alley next to the Hog's Breath Salloon in Key West (look at the |>| pole behind the stage) and I bet from the looks of it they added the |>| 3d transformer as the load increased. Usually it is "open delta" with |>| 2 transformers as a cheap way to get some 3 phase to a customer who |>| has mostly single phase loads. |>| The open delta was actually pretty popular in the light industrial |>| areas of East Naples (Florida). |> |>I've seen a few setups with one big fat transformer and one small one. |>Usually, the big one has 2 bushings and the small one has 1 bushing. |>On the secondary, the big one has 3 wired lugs and the small one has |>only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where |>I have seen these is in business (not industrial) areas of small towns |>in West Virginia and Ohio, usually servicing stores or restaurants. |>Some of those smaller pole pigs were the smallest pole pigs I've seen. |>My guess is the smallest ones were no more than 3 or 5 kVA. | | The two transfomer delta around here is just 2 of the expected 3 in | true delta. They connect A to C and leave B floating off of the AB | transformer that is center tapped. As long as you have a well balanced | 3 phase load it works fine. We did get in trouble in the computer biz | since we had some single phase loads derived from the incoming 3 phase | inside the machine. I have had to juggle machines around or "roll the | phases" to balance the phase loads.
What I wonder is how 240 volt L-L loads would do with either the closed or open 240D/120 systems, when connected between the high-leg and either pole of the 120/240 side. Consider the simple 240 volt water heater, assuming it treats both wires as hot, which it must in USA single phase systems. Also consider a computer PSU that has two-pole switching and can handle a 230 volt German Schuko that has no polarity to it (e.g. either wire it gets from being plugged in could be the hot). I do remember reading some power utility requirements that nothing but three phase loads is allowed to connect to the high-leg. But what is the effect of doing so?

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I'm wondering if the simplicity of a single phase 240/120 V service for a small elevator building is in some way superior to the slightly more complex 3 phase- 208/120 V. wye service.
Can a small hydraulic passenger elevator serving a maximum of 3 flloor run OK on a 240 V. single phase circuit? If wired the other way, this would probably be the only 3 phase load.
My concern with the 3 phase is that if the utility loses a phase, a 208V 3 phase motor will stop rotating and burn up (without special protection). I've seen this happen in a condo building. Also, because of the partially powered delta-wye at the transformer and some of the 120 V lines might go to something like 68 V or so.
I know that there are probably protective devices that can be installed to protect this. I wonder though, if it might just be better to stick with a 120/240 V. single phase service.
Any thoughts?
Beachcomber

|>What I wonder is how 240 volt L-L loads would do with either the closed or |>open 240D/120 systems, when connected between the high-leg and either pole |>of the 120/240 side. Consider the simple 240 volt water heater, assuming |>it treats both wires as hot, which it must in USA single phase systems. |>Also consider a computer PSU that has two-pole switching and can handle |>a 230 volt German Schuko that has no polarity to it (e.g. either wire it |>gets from being plugged in could be the hot). I do remember reading some |>power utility requirements that nothing but three phase loads is allowed |>to connect to the high-leg. But what is the effect of doing so? |> | | I'm wondering if the simplicity of a single phase 240/120 V service | for a small elevator building is in some way superior to the slightly | more complex 3 phase- 208/120 V. wye service.
It would be simpler. For a small building it can be OK. For a larger building, the utility would want more balance in the phase loading.
One option is to use a three phase transformer where all three secondary windings are 240 volt center tapped to 120/240 volts. What you get are SIX separate hot phase wires, and a neutral. I'll label the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three phase systems from this, connecting to A,C,E or B,D,F. They would be 208Y/120. And you can have 3 single phase systems from this, connecting to A,D and B,E and C,F. Divide the tenants that get single phase three equal ways and this way they get genuine 120/240. This is actually a bit more complex than the common systems (for example you would have 3 separate distributions for the single phase). You probably can't meter the whole thing at once. But I'd prefer such a system, myself.
If for some reason you really need 240 volts on the threee phase loads, that either forces the 240D/120 system, or just 240D, or a 240Y/139 system just for three phase.
Ultimately, I'd prefer to have 3 120/240 volt systems plus 480Y/277 for the three phase loads.

snipped-for-privacy@ipal.net wrote:
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I am obviously missing something. How do you connect the center taps of three windings together, and then connect them in a wye, and not blow up the transformer? And how do you get 240/120 & 208Y/120 from the same windings?

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wrote:
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He is essentially describing a 6 phase secondary. Each 120 to neutral and separated by 60 degrees but you can get 240 between the legs that are 180 degrees apart. as well as 208 between the legs that are 120 degrees apart. So you can have 2 -120/208V Ysystems with common neutral as well as 3 120/240V single phase circuits as well as some 120V D with one terminal tied to neutral.
There is no real advantage but the chance of problems with mixed up wiring will be increased.
The question then arises- why bother?

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| wrote: |>> One option is to use a three phase transformer where all three |>> secondary windings are 240 volt center tapped to 120/240 volts. What |>> you get are SIX separate hot phase wires, and a neutral. I'll label |>> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three |>> phase systems from this, connecting to A,C,E or B,D,F. They would be |>> 208Y/120. And you can have 3 single phase systems from this, |>> connecting to A,D and B,E and C,F. Divide the tenants that get single |>> phase three equal ways and this way they get genuine 120/240. This |>> is actually a bit more complex than the common systems (for example |>> you would have 3 separate distributions for the single phase). You |>> probably can't meter the whole thing at once. |>> But I'd prefer such a system, myself. |>> |> |> I am obviously missing something. How do you connect the center taps of |> three windings together, and then connect them in a wye, and not blow up |> the transformer? And how do you get 240/120 & 208Y/120 from the same |> windings? |> |> |> |> -- |> Benjamin D Miller, PE |> www.bmillerengineering.com | | He is essentially describing a 6 phase secondary. Each 120 to neutral and | separated by 60 degrees but you can get 240 between the legs that are 180 | degrees apart. as well as 208 between the legs that are 120 degrees apart. | So you can have 2 -120/208V Ysystems with common neutral as well as 3 | 120/240V single phase circuits as well as some 120V D with one terminal | tied to neutral. | | There is no real advantage but the chance of problems with mixed up wiring | will be increased.
The advantage as I see it is the combination of keeping loading balanced (for a large building) and having genuine 240 volts. I would divide up such a building into 3 segments and feed only one phase to each. So the mixed up wiring risk would only be from the transformer out to each of the 3 single phase main panels. I've never actually seen a transformer like that on the market. If it came down to it, I'd just do it with 3 separate single phase transformers. I am rather adamant about getting genuine 120/240 to any and all residential units. If the utility says they need to have the phases balanced more that a single phase drop would allow (in a big building, I would expect that), I'll suggest the "6 phase" answer, either externally or internally (via 480 perhaps). That is, if I ever get into the landlord business (not too likely).

| snipped-for-privacy@ipal.net wrote: |> One option is to use a three phase transformer where all three |> secondary windings are 240 volt center tapped to 120/240 volts. What |> you get are SIX separate hot phase wires, and a neutral. I'll label |> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three |> phase systems from this, connecting to A,C,E or B,D,F. They would be |> 208Y/120. And you can have 3 single phase systems from this, |> connecting to A,D and B,E and C,F. Divide the tenants that get single |> phase three equal ways and this way they get genuine 120/240. This |> is actually a bit more complex than the common systems (for example |> you would have 3 separate distributions for the single phase). You |> probably can't meter the whole thing at once. |> But I'd prefer such a system, myself. |> | | I am obviously missing something. How do you connect the center taps of | three windings together, and then connect them in a wye, and not blow up the | transformer? And how do you get 240/120 & 208Y/120 from the same windings?
Maybe an ASCII art picture will help:
B C \ / \ / A----N----D / \ / \ F E
A-N and N-D are both wound on the first core. B-N and N-E are both wound on the second core. C-N and N-F are both wound on the third core. A third of the 120/240 loads would be served from A-N-D. A third of the 120/240 loads would be served from B-N-E. A third of the 120/240 loads would be served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E and N. Half of the 208Y/120 loads would be served from B,D,F and N.
You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A, but you would not want to.
You could also ignore the center tap and rewire it for 416Y/240.
If the windings can be split and wired in parallel, you could rewire it for 208Y/120 with double the amperage.
A transformer with dual secondary 120 volt windings on each of the three cores would be quite flexible, being able to be configured for any of these three systems (but you would have 12 wires coming off the cores on the secondary side).

On 13 Feb 2008 05:46:10 GMT, snipped-for-privacy@ipal.net wrote:
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I agree with Mr Kelly
They use delta vee to save money, why would they do this 6 pole thing?

On Wed, 13 Feb 2008 01:05:22 -0500 snipped-for-privacy@aol.com wrote: | On 13 Feb 2008 05:46:10 GMT, snipped-for-privacy@ipal.net wrote: |
|>|> One option is to use a three phase transformer where all three |>|> secondary windings are 240 volt center tapped to 120/240 volts. What |>|> you get are SIX separate hot phase wires, and a neutral. I'll label |>|> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three |>|> phase systems from this, connecting to A,C,E or B,D,F. They would be |>|> 208Y/120. And you can have 3 single phase systems from this, |>|> connecting to A,D and B,E and C,F. Divide the tenants that get single |>|> phase three equal ways and this way they get genuine 120/240. This |>|> is actually a bit more complex than the common systems (for example |>|> you would have 3 separate distributions for the single phase). You |>|> probably can't meter the whole thing at once. |>|> But I'd prefer such a system, myself. |>|> |>| |>| I am obviously missing something. How do you connect the center taps of |>| three windings together, and then connect them in a wye, and not blow up the |>| transformer? And how do you get 240/120 & 208Y/120 from the same windings? |> |>Maybe an ASCII art picture will help: |> |> B C |> \ / |> \ / |>A----N----D |> / \ |> / \ |> F E |> |>A-N and N-D are both wound on the first core. B-N and N-E are both wound |>on the second core. C-N and N-F are both wound on the third core. A third |>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |>loads would be served from B-N-E. A third of the 120/240 loads would be |>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |>You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A, |>but you would not want to. |> |>You could also ignore the center tap and rewire it for 416Y/240. |> |>If the windings can be split and wired in parallel, you could rewire it |>for 208Y/120 with double the amperage. |> |>A transformer with dual secondary 120 volt windings on each of the three |>cores would be quite flexible, being able to be configured for any of |>these three systems (but you would have 12 wires coming off the cores on |>the secondary side). | | I agree with Mr Kelly | | They use delta vee to save money, why would they do this 6 pole thing?
To serve a lot of 120/240 volt single phase loads AND keep phases balanced? It might depend on how large a building is involved. Suppose it is a 10 floor building with 9 floors that have 12 residential apartments, each. Would you in the role of a utility engineer be satisified running single phase service into that building? I would think not. Then would delta with only one side having all the 120/240 loads really be any better? What I would ultimately propose for such a project would be to bring in 480Y/277 to the building, run that to the elevators, and put single phase transformers on floors 3 (serving 2-4), 6 (serving 5-7) and 9 (serving 8-10), fed from diverse 480 volt connections (480 to 120/240 volt single phase dry transformers are pretty common). Various outside HID lights could also be fed from the 277 volts.
The risk of mixing the wiring with 6 phases is probably realistic. But isn't that what color marking of conductors is for? Besides, the only places to get that mixed up is between a common single transformer and the place where the phases split into 3 sets of single phase. If this concept had been chosen a long time ago and been an accepted standard, I'm sure we would have a color code standard, now, for each phase angle.
If anyone does make a three phase transformer that could be wired for either 208Y/120 or 416Y/240, my bet is someone with the know-how could wire it for the scheme I suggested (although it might not have enough outgoing terminal lugs, or enough conduit space, to do it).

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wrote:
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The "6 phase " scheme wouldn't cost any more in terms of the transformers and could be done using a bank of 3 single phase pole pigs. However, it would appear that there would be additional costs involved with no net gain as well as a more complex system. Where possible one should avoid adding complexity (KISS). This doesn't mean that it can't be done but may mean that it shouldn't be done. There has been some playing around in the past on the basis that 3 phase is good so maybe 6 or 12 phase is better- tain't so except in some special cases. Where 6 phase has been used is in some urban distribution where there is a savings in terms of space necessary at a given line to neutral voltage because the line to line voltage between adjacent phases is then the same as to neutral. Whether this is still being done is something that I don't know. It also has been used for rectifier supplies in order to reduce harmonics and ripple.
Your scheme of 480Y and single phase transformers is far better.

| The "6 phase " scheme wouldn't cost any more in terms of the transformers | and could be done using a bank of 3 single phase pole pigs. However, it | would appear that there would be additional costs involved with no net gain | as well as a more complex system. Where possible one should avoid adding | complexity (KISS). This doesn't mean that it can't be done but may mean | that it shouldn't be done.
So what would you have instead, keeping to the requirement of genuine 240 volts at the single phase circuits?
| There has been some playing around in the past on the basis that 3 phase is | good so maybe 6 or 12 phase is better- tain't so except in some special | cases. | Where 6 phase has been used is in some urban distribution where there is a | savings in terms of space necessary at a given line to neutral voltage | because the line to line voltage between adjacent phases is then the same as | to neutral. Whether this is still being done is something that I don't know. | It also has been used for rectifier supplies in order to reduce harmonics | and ripple.
I've wondered if any of those 6-wire transmission lines might be phased this way, even if the loads end up being split at the far end.
| Your scheme of 480Y and single phase transformers is far better.
So that's the way you would go? What if the utility says they don't want to have the loss of an extra set of transformers with MV -> 480 -> 120/240 and insist on no more than one transformer per MV -> meter path?

snipped-for-privacy@ipal.net wrote:
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The utility cannot insist on anything if the service you order is tariffed by the state public utilities commission they must provide it on demand. The tariff may allow them to surcharge you if the service does not draw enough current to justify the additional expense. If the type of service you want is not tariffed then the utility must apply for a tariff for that new class of service and obtain it prior to connecting the service. If they don't want to provide it then you would be faced with the insurmountable cost of applying for the tariff against their opposition.
Dairy farmers in several states have obtained tariffs for ungrounded delta service to supply customer owned transformers in order to keep utility neutral currents off of the farm and away from their livestock. The utilities initially fought the tariffs very hard for reasons that were not clear to me. Once the problem with dairy cows and stray Multi Grounded Neutral (MGN) currents became well known some utilities began offering special transformers that do not permit the MGN currents to flow on the premise service wiring. I've yet to learn how they accomplish that but if it were my livelihood I'd want the ungrounded Delta supply that was tariffed in spite of the utilities opposition.