wierd voltages

HIREDHAND

Member
So I'm in a disconnect today and check voltage to ground (no neutral in system)
The voltages were..
A phase to ground 500 V
B phase to ground 0 V
C phase to ground 500 V
Phase to phase was all 500 V.

I work for a municipality, so the first thing I do is call the inspector over and tell him I think we need to get a lineman out here. He checks voltage and says "its a wild leg" then walks off. I assume he was talking about a center tappdd delta system, but it doesn't make any sense to me because its not a 240 V system and it doesn't have a high leg, it has a dead leg and supper high voltages on the others.

Anyone know what's goin on? I'm lost here and we have about 1 million dollars in AC equipment running off this system. Seems to me something is bad wrong, but all my experience is with Wye systems.
 

iwire

Moderator
Staff member
Location
Massachusetts
That appears to be corner grounded delta 480 running a bit high.

There is no neutral so B phase is bonded to ground.

It is the gronded conductor and should be white



OR, it is ungrounded delta with a ground fault.
 

jcassity

Senior Member
Location
24941
i need to educate myself on stuff like this,, i periodically run into strange readings and when i see no N leg, i know its gotta be a delta drop.

20miles down the road from the last 3phase source, the power co made 208 3ph out of two 240v transformers.

as soon as i walked outside and saw one larger and one smaller transformer at the customer aerial drop, i knew i better measure twice and cut once on my site survey for our new gear.
 

HIREDHAND

Member
That appears to be corner grounded delta 480 running a bit high.

There is no neutral so B phase is bonded to ground.

It is the gronded conductor and should be white



OR, it is ungrounded delta with a ground fault.

That makes a lot of sense. Thanks for the education. In commercial work I usually only see Wye systems. It seems I need to read up on deltas.

Thanks again
 

HIREDHAND

Member
i need to educate myself on stuff like this,, i periodically run into strange readings and when i see no N leg, i know its gotta be a delta drop.

20miles down the road from the last 3phase source, the power co made 208 3ph out of two 240v transformers.

as soon as i walked outside and saw one larger and one smaller transformer at the customer aerial drop, i knew i better measure twice and cut once on my site survey for our new gear.

Sounds like an "open delta" system.

Never worked on one but I hear if the load isn't balaced the voltages go crazy.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Sounds like an "open delta" system.

Never worked on one but I hear if the load isn't balaced the voltages go crazy.
Actually, the combination of a large transformer and a small transformer is a strong indication that it is a "high leg" system. The primary of the large pot goes between two line phases and the secondary is center-tapped 240, which provides 120-0-120 (120/240 3 wire) to a 120/240 panel. Often for lighting and miscellaneous loads. Call these wires L1, L2 and N (the center tap). Now you add the smaller pot, whose primary is across a different pair of phases and the secondary is from one end of the large pot secondary. This gives you L3 for use with three phase loads. But the L3 to N voltage will now be 208V. Hence the "high leg", and L3 cannot be used for line to neutral loads. The second pot is smaller because there is a smaller amount of 3-phase loads and all of the single phase loads are on the large pot, along with its share of the three-phase loads.

If the two pots were similar in size, it is more likely to be a simple open delta with no line to neutral loads because there is no "neutral" wire at all.
 

HIREDHAND

Member
Actually, the combination of a large transformer and a small transformer is a strong indication that it is a "high leg" system. The primary of the large pot goes between two line phases and the secondary is center-tapped 240, which provides 120-0-120 (120/240 3 wire) to a 120/240 panel. Often for lighting and miscellaneous loads. Call these wires L1, L2 and N (the center tap). Now you add the smaller pot, whose primary is across a different pair of phases and the secondary is from one end of the large pot secondary. This gives you L3 for use with three phase loads. But the L3 to N voltage will now be 208V. Hence the "high leg", and L3 cannot be used for line to neutral loads. The second pot is smaller because there is a smaller amount of 3-phase loads and all of the single phase loads are on the large pot, along with its share of the three-phase loads.

If the two pots were similar in size, it is more likely to be a simple open delta with no line to neutral loads because there is no "neutral" wire at all.
So what I gather from that is there are two different open delta systems, one with a high leg and one without a nuetral at all and also I guess that means there would be two different high leg systems. One is the grounded open delta mentioned and the other would be the center tapped delta. Correct?

I'm trying to figure this stuff out, but it seems there's about 100 delta systems. Lol. Its a lot to wrap my head around
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So what I gather from that is there are two different open delta systems, one with a high leg and one without a nuetral at all and also I guess that means there would be two different high leg systems. One is the grounded open delta mentioned and the other would be the center tapped delta. Correct?

I'm trying to figure this stuff out, but it seems there's about 100 delta systems. Lol. Its a lot to wrap my head around
In the case of the grounded open (or closed) delta, there would be no high leg. There would just be two phase conductors the same voltage away from the grounded conductor, just with a different phasing. The center tapped winding with the center tap grounded and called the neutral is the only one which can properly be called high leg (or wild leg, or...) since just one of the phase lines will be different from the rest.

And either of those scenarios (grounded or high leg) could just as well be closed delta as open delta. Open delta provides a more economical way of getting three phase service as long as the balanced three phase power required is low enough. But there will be bigger problems with voltage regulation under changes of load than for a closed delta.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
So what I gather from that is there are two different open delta systems, one with a high leg and one without a nuetral at all and also I guess that means there would be two different high leg systems. One is the grounded open delta mentioned and the other would be the center tapped delta.
Delta configuration- can be open, using 2 transformers, or closed using three transformers.

Wild leg/ High leg/ Center-tapped delta, or a lot of other names - a delta that has one of its winding center tapped to create a neutral point between 2 of its 3 line conductors. The laws of physics dictate that the high voltage L-N nominally will be 1.732 x the low L-N voltage (e.g. 208V versus 120V)

Corner grounded/ Grounded B phase, or other names - a delta where one of the Lines/phase is intentionally connected to ground. These systems are one of the reasons the NEC uses the longer phrase 'grounded conductor' instead of just saying 'neutral'.

Ungrounded - a system with not intentional connection to ground.

A qualified electrician understands that the electrical systems in the US involve more than just wye connections, and that L-N voltages are not always 120V or 277V. However, it seems to be the rare individual that has worked on more than just a few system types.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
So I'm in a disconnect today and check voltage to ground (no neutral in system)
The voltages were..
A phase to ground 500 V
B phase to ground 0 V
C phase to ground 500 V
Phase to phase was all 500 V.

I work for a municipality, so the first thing I do is call the inspector over and tell him I think we need to get a lineman out here. He checks voltage and says "its a wild leg" then walks off. I assume he was talking about a center tappdd delta system, but it doesn't make any sense to me because its not a 240 V system and it doesn't have a high leg, it has a dead leg and supper high voltages on the others.

Anyone know what's goin on? I'm lost here and we have about 1 million dollars in AC equipment running off this system. Seems to me something is bad wrong, but all my experience is with Wye systems.
Sounds like an "open delta" system.

Never worked on one but I hear if the load isn't balaced the voltages go crazy.
It is not a "High Leg", your inspector is using the term incorrectly. It may be an open delta, but not necessarily, it wouldn't make a difference if it was open or closed in what is being seen here.

It is however a delta system, that much we know and like iwire said, it's either a 'corner grounded delta', or it is an UNGROUNDED delta where one phase has gone to ground. That's why if you have an ungrounded delta system, you must also have some form of ground fault indication, otherwise you can't tell.

That also could be an open delta or a closed delta as well, like I said, you would not know from what was reported here. Simplest way to tell is to count the transformers on the pole. 2 transformers, open delta; 3 transformers (or one large transformer), closed delta.

But if it was a "high leg" delta, either open OR closed, then you would not read 0V from one phase to ground. You cannot have a grounded delta system that is ALSO a high leg system.
 

tek9

Member *
Location
Australia
Definetly corner ground delta,saw this in a factory wired by the American Army in WW2,from memory the wires were orange,yellow,white and green connected to the generator,A12 lead reconnectable job,using a gearbox that could be run using incoming 3 phase 50 Hz or straight from the diesel engine,a wild leg is a center tapped delta,wild meaning 120 v a and c to earth 208 b phase,240 v between phases.
 

HIREDHAND

Member
It is not a "High Leg", your inspector is using the term incorrectly. It may be an open delta, but not necessarily, it wouldn't make a difference if it was open or closed in what is being seen here.

It is however a delta system, that much we know and like iwire said, it's either a 'corner grounded delta', or it is an UNGROUNDED delta where one phase has gone to ground. That's why if you have an ungrounded delta system, you must also have some form of ground fault indication, otherwise you can't tell.

That also could be an open delta or a closed delta as well, like I said, you would not know from what was reported here. Simplest way to tell is to count the transformers on the pole. 2 transformers, open delta; 3 transformers (or one large transformer), closed delta.

But if it was a "high leg" delta, either open OR closed, then you would not read 0V from one phase to ground. You cannot have a grounded delta system that is ALSO a high leg system.

Sorry for themisunderstanding. There's actually two different topics there. One fella was talking about a delta with two Xformers. It has nothing to do with the building I'm talking about (it has three)

So there's only one delta configuration (center tapped) that will give you a high leg then? Seems I'm getting mixed responces
 

HIREDHAND

Member
Thanks again for the info guys! I don't really have any industrial experience, only commercial. That's why I'm only familiar with wye.

One more question, is it possible to get 277 volts with delta? Or is it a system only used for motor loads?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Thanks again for the info guys! I don't really have any industrial experience, only commercial. That's why I'm only familiar with wye.

One more question, is it possible to get 277 volts with delta? Or is it a system only used for motor loads?
You cannot get a useable 277V from a delta connection.
Although, depending on 'capacitive coupling', it is possible to get a measurement of 277V.

Buildings with delta systems often use step-down transformers to get their 480Y/277V, if it is needed for equipment like lighting.
 

kwired

Electron manager
Location
NE Nebraska
So what I gather from that is there are two different open delta systems, one with a high leg and one without a nuetral at all and also I guess that means there would be two different high leg systems. One is the grounded open delta mentioned and the other would be the center tapped delta. Correct?

I'm trying to figure this stuff out, but it seems there's about 100 delta systems. Lol. Its a lot to wrap my head around
I think it has been answered already, but open delta primarily means the primary side only has two coils instead of three. The derived secondary can still be ungrounded, corner grounded, or mid point of one phase grounded, and is not limited to 480 or 240 volts though those are the most commonly seen.

Thanks again for the info guys! I don't really have any industrial experience, only commercial. That's why I'm only familiar with wye.

One more question, is it possible to get 277 volts with delta? Or is it a system only used for motor loads?
Yes you could get 277 volts, but unless all you were feeding was 277 volt loads it wouldn't have much other purpose, as you would have to build the transformer bank from 277 volt output coils and would have 277 volts phase to phase, if you grounded a midpoint of one of them then the line to ground voltage would be half 138.5 volts and it would have a wild leg that was 240 volts to ground(138.5 x 1.732).
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
So there's only one delta configuration (center tapped) that will give you a high leg then? Seems I'm getting mixed responces
Yes and No. "High Leg" is actually a misnomer. All three legs are the same with respect to each other. The term "high leg" refers only to the fact that if you have a grounded center tapped leg, and you measure line to ground, you will see a higher voltage reference to ground on the line that is NOT connected to the winding that has the center tap. That one line, usually B phase, is floating with regard to reference to ground, so your measurement will be anywhere between Line voltage and Line divided by the sq. rt of 3, but never the same as the Line to the grounded center tap. So Yes there is only one configuration that ends up as a "high leg" because it must be a grounded center tap transformer. But No, because you can have a center tapped leg on either an open delta OR a closed delta.

The only issues that are mutually exclusive are a grounded center tapped leg AND corner grounded delta.
 
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Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Screwed up but ran out of edit time. The high leg on a 240V delta usually shows as 208V because it is the square root of the sum of the line voltage sq. minus the line to N voltage sq. so 2402 - 1202, then take the sq rt of that, =208V
 
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