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Tool Talk Discussion Forum

Basic AC Power Systems

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John T

04-22-2013 05:37:10




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Okay, there still seems to be some "corn"fusion out there about typical single phase and three phase AC power systems so heres a summary of typical installations:

1) TYPICAL HOUSEHOLD Thats 120/240 Volt Single Phase Three Wire Its a single transformer thats tapped in its center. Its has 120 volt from EITHER outside leg L1 or L2 to Neutral but 240 Line to Line. Out at the transformer PLUS at the service entrance, the Neutral is tied to Mother Earth, a GroundING Electrode.

2) A TYPICAL COMMERCIAL SERVICE that uses mostly 120 volt but has some 208 3 phase HVAC requirement.

Thats a 208 Y 120 Volt Three Phase FOUR Wire It has three legs of single phase 120 volt ANY leg to Neutral is 120 but its 208 line to line. The 208 three phase can be used to run smaller HVAC loads. Its a Grounded system

3) A TYPICAL OLDER INDUSTRIAL PLANT with lots of heavy three phase or single phase 240 or 480 equipment and a small amount of 120

May be a 240 or 480 Volt Three Phase Three Wire DELTA. It can serve 240 or 480 single or three phase equipment. Generally it floats its NOT a grounded system!!!!!!! In later years we used them less and less as we preferred a grounded system

If the utility didnt give us a 120/240 then we used dry transformers to get it.

4) AN INDUSTRIAL PLANT with lots of 240 single or three phase and a small amount of 120 That could be the 120/240 volt Three Phase FOUR Wire center tapped DELTA.

It has three legs of 240 single phase and 240 volt three phase and can supply 120. It has two legs of 120 to Neutral (via the center tapped transformer) but the red/high leg is 208 to Neutral (NOT used)

There are OTHER systems including open deltas etc but in all my years we didnt get into them. I forgot, sometime we used a 480 Y 277 Volt Three Phase Four Wire for heavy 40 3 phase HVAC and we used the 277 for lights. Of course, dry transformers for 120 as needed. Our Primary was 12470 Y 7200 Three Phase Four Wire. NOTE how as above, 12470 = 7200 x square root of 3.

Nuff said, got it?? Any questions??

John T toooooo long retired Electrical Engineer this stuff is gettin more stale alll the time lol SO AS ALWAYS NO FREAKIN WARRANTY

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GarryinNC

04-29-2013 21:02:47




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
John T, It is all making more sense the more that I read and think about it. This has always been a subject of interest for me, and I have always wished for 3 phase power in our shop, but the biggest motor we have is a true 5hp single phase air compressor, so it would never be practical.

Thanks again to all for taking time to reply.

Garry



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GarryinNC

04-25-2013 18:53:52




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
If one leg of a floating 3 phase system went to ground,would there be a circuit breaker tripping somewhere? Or not, since there was no reference to ground in the system previously? Did I even say that right?

It is still confusing in the high leg system that the voltage between any two legs is 240, and high leg to neutral is still 208, but other legs to neutral are less. I know JMOR explained the math, but that does not 'soak in' very well sometimes either!

Also, and I know this has probably been explained a thousand times on here, in 240 single phase: are the two hot legs 180 degrees out of phase with each other?

Also, John T, JMOR, Dusty, Dr. Sportster, and all others, I believe that there are many of us who read on here 99 percent of the time and may not, if ever, post much. We very much appreciate all of the time and effort expended to make knowledge on ANY subject available. The arguments are a good read also!

Garry

Garry
This post was edited by GarryinNC at 20:48:25 04/25/13.

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John T

04-26-2013 05:59:32




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 Re: Basic AC Power Systems in reply to GarryinNC, 04-25-2013 18:53:52  
QUESTION

If one leg of a floating 3 phase system went to ground,would there be a circuit breaker tripping somewhere? Or not, since there was no reference to ground in the system previously? Did I even say that right?

ANSWER

NO TRIPPING of a circuit breaker somewhere. Theres no voltage with respect to ground on any leg....Theres no circuit breaker between any leg and ground (there can be detectors)

QUESTION

It is still confusing in the high leg system that the voltage between any two legs is 240, and high leg to neutral is still 208, but other legs to neutral are less. I know JMOR explained the math, but that does not 'soak in' very well sometimes either!

ANSWER

Okay there are three transformers connected in DELTA (picture an equilatteral triangle) and the voltage across each of the legs (L1 to L2,,L2 to L3,,L3 to L1) is 240 VAC. In ONE of the transformers (L1 to L3) lets tap in the dead center and run a wire out there for NEUTRAL and lets run that also to Mother earth......HMMMMMMMMM think about it, since that one L1 L3 transformer has a mid point tap HECK its only 120 VAC (1/2 of the full 240) from L1 or L3 to Neutral. Buttttttttt if you go way up there to L2, the Red/High Leg top of triangle, its 120 x Square Root of Three or 208 volts to Neutral. Thats the infamous 120/240 Volt Three Phase FOUR Wire Red/High Leg Delta System.

QUESTION

Also, and I know this has probably been explained a thousand times on here, in 240 single phase: are the two hot legs 180 degrees out of phase with each other?

ANSWER YES In the household 120/240 Volt Single Phase Three Wire System theres only ONE transformer and its 240 Line to Line. However with a tap in the center mid point (Neutral) its only 1/2 or 120 VAC either Line to Neutral yes 180 out of phase from the other

GREAT QUESTIONS, has this and all else I posted making any sense and does it help???? Dont expect to understand what takes libraries to explain and education and years of experience. I cant put that in a few sentences here

John T BSEE, JD Longgggggg retired Electrical Engineer

John T

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GarryinNC

04-24-2013 19:11:58




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
Thanks for the answers,always interesting. Does three phase power date all the way back to the Tesla/Westinghouse days or did it develop later? I have stayed up late some nights reading the history of power generation/distribution on the web but can't recall about 3 phase. I know Tesla developed the AC 'Polyphase' system,among other things.

I believe that rural electrification was one of the greatest things that happened to country people. To country women for sure. I am told that around here(NW North Carolina) a lot of the dairy farms had 3 phase high leg power. There were over 100 dairy farms in my county in the 1950s, down to about 6 today.

Garry
This post was edited by GarryinNC at 19:17:26 04/24/13.

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John T

04-25-2013 06:47:45




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 Re: Basic AC Power Systems in reply to GarryinNC, 04-24-2013 19:11:58  
Youre really gettin into this stuff, neat...

I have no idea when 3 phase was first used commercially. I do know when it comes to motors its so much more efficient and self starting etc etc. At our plant any motor over 5 HP we tried to use 3 phase for lighter cheaper more efficient use.

I thought about joining the "Tesla Society" Back in the days he and Westinghouse and Edison really got into it I understand. I dont think things worked out that well for Tesla but he was a genius in his time. At Niagra Falls thay have a huge statue and a lot of history about him.

John T Tooo long retired EE but as I look back, ya know, "its been a good ride"

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Dusty MI

04-25-2013 09:42:05




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 Re: Basic AC Power Systems in reply to John T, 04-25-2013 06:47:45  
A 3 phase motor can be reversed instantly, that's so fast that you can't see it happen or see that it's turning the other way.

In most cases the voltage to ground is half of what the nominal is, that is if the voltage is 480 between the legs, it's 240 to ground. In a home 120/240, it's 120 to ground. It's surprising the Mr. home owner will try to work on something that's 120 but will not touch 240, even though he is more likely to get bit on one leg to ground, 120 volts.

Along that line the isolated/floating 3 phase that we were talking about, most often 480, if one leg does go to ground, then it's 480 from each of the other legs to ground.

Where as a 480 Y is 277 to ground.

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GarryinNC

04-24-2013 09:27:59




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
Thanks John, that makes sense, it just is hard to think of an AC system without a ground when grounded systems are what we are most familiar with. What was the advantage to this type of floating distribution system?
I really enjoy reading these discussions, they always make me think and hopefully some of the knowledge soaks in, if only a little at a time!

Garry



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John T

04-24-2013 10:49:18




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 Re: Basic AC Power Systems in reply to GarryinNC, 04-24-2013 09:27:59  
Gary, AGAIN DARN GOOD QUESTIONS

You ask "What was the advantage to this type of floating distribution system?"

Okay, these were sort of before my time, (I always speecified a Grounded system in all my years) but I have been "told" one advanatge in the early days of manufacturing was they didnt want a plant or production line to shut down if a ground fault occurred so with a "floating" non grounded system the machines and assembly line kept in production with a single ground fault. Also in a pure floating system theres "theoretically" zero voltage with respect to mother earth and zero voltage with respect to the machines or metal or conduit etc. in the vicinity. The Voltage is ONLY with respect to line to line NOT any line to earth or other metal or machines.. HOWEVER due to mutual inductive coupling and capacitance the farther away from the source and more wire runs etc the more voltage there can be developed albeit low energy.

NOW NO WARRANTY ON THIS maybe some old timer electricians and plant managers can expand on this but the above is what I was told and also is supported by the theory at least behind a floating system. In a floating system they can use detectors to sense if a leg is shorted that way they can fix it before a second leg shorts also YIKES Maybe some plant men and sparkies can comment on the advantages of the floating Delta also BECAUSE I NEVER USED THEM.....
and just arent familair with all their advantages. My Deltas had one transformer center tapped which I grounded and when I did design a 3 wire delta I still grounded one corner.

John T

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Dusty MI

04-24-2013 11:35:55




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 Re: Basic AC Power Systems in reply to John T, 04-24-2013 10:49:18  
John, you got half of it. They didn't want to shut production.
There are/were indicator lights on the electric buss. So if one leg did go to ground the lights would indicate there was a short, and it could be repaired when production was not running.

Dusty



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John T

04-24-2013 13:11:26




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 Re: Basic AC Power Systems in reply to Dusty MI, 04-24-2013 11:35:55  
Thanks Dusty my sparky friend (the guy with the AC Hat at Flywheelers), yep I mentioned the warning indication but it was so good to hear a confirmation of what I had been told as I lack expreience on those floaters......Even though theres such low energy from capacitance or inductance induced voltage way down the lines, I bet a guy could feel a tingle if he got across one line and say a machine or other conductive object way out in the plant somewhere???

Thanks for your help

John T

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GarryinNC

04-23-2013 16:45:06




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
When you talk about some of these ungrounded systems, you use the terms float and floating. I have heard that before and cannot wrap my mind around what it means. John T or others can you explain it to a neophyte?

Thanks, Garry



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John T

04-24-2013 04:25:00




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 Re: Basic AC Power Systems in reply to GarryinNC, 04-23-2013 16:45:06  
Can do, A Float or Floating AC power distribution method system simply means ITS NOT GROUNDED. By the way GREAT QUESTION

For example, in your household 120/240 Volt Single Phase Three Wire system that uses just one transformer (perhaps say 7200 or 9600 etc VAC down to 240 VAC) the center of that transformer is tapped, serves as the Neutral, and that point is tied to a #4 Bare Coper Wire that leads down the pole to MOTHER EARTH GROUND. Its a Grounded NOT floating system. Likewise at the main service entrance the Neutral is again tied to a Grounding Electrode via the GroundING Electrode Conductor # 4 bare copper wire.

LIKEWISE In a typical say 208 Y 120 Volt Three Phase Four Wire commercial system, the common tie point where each of the three transformers are all joined together, ALSO SERVES AS NEUTRAL AND IS ALSO TIED TO MOTHER EARTH GROUND (perhaps via a driven into the earth rod or rods or perhaps other suitable Grounding Electrodes), A GROUNDED (NOT Floating) System.

HOWEVER and as used a lot in the thirties through 60's, if three transformers are connected in a Delta configuration (usually 240 or 480 line to line) and theres no connection to earth ground (or any Grounding Electrodes), THEN THAT SYSTEM IS FLOATING. NOTE Im talkin a 240 or 480 Three Phase THREEEEEEEEEEEE Wire here, not 3 phase FOUR wire where (Red/High Leg system) one transformer is center tapped and that point is tied to a grounding electerode.

So in theory if you ignored capacitive and inductive coupling you can be standing barefooted on the ground and touch any lead and not receive a shock since there shouldnt be any voltage with respect to mother earth ground in a floating system. Similar in a plant if one leg got shorted out to a machine or building steel or conduit etc the motor keeps running THATS A FLOATING SYSTEM

That help?? that what you were looking for??

Again GREAT QUESTIONS YALL Hope I helped and didnt confuse by use of my technical jargon, but thats sort of inbred after Purdue and 40 + years lol

John T

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Dusty MI

04-24-2013 06:53:24




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 Re: Basic AC Power Systems in reply to John T, 04-24-2013 04:25:00  
Quoting Removed, click Modern View to see

I got nailed once on one of those systems once where one leg had unintentionally gone to ground. That hurt worse than 277.

Dusty



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John T

04-24-2013 08:40:34




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 Re: Basic AC Power Systems in reply to Dusty MI, 04-24-2013 06:53:24  
Yep, after one leg gets unintentionally grounded (machine or conduit or steel etc) on a Floating Delta, then there exists Line to Line Voltage from the 2 other Legs to conduit or machinery frame or building steel etc OUCH THAT CAN KILL YOU DEAD

John T



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cannonball

04-23-2013 11:23:31




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
John what do you know about 762 voltage....where they ground a phase ..



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John T

04-23-2013 13:08:06




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 Re: Basic AC Power Systems in reply to cannonball, 04-23-2013 11:23:31  
Sorry, I dont know beans about it, as Dirty Harry said in the movie "A mans got to know his limitations" lol maybe if someone else does it will refresh my memory and then something may come to me, but I'm drawing a blank for now at least. I have specified a 480 Y 277 Three Phase Four Wire but that was my limit......

John T



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George Marsh

04-23-2013 05:53:04




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
JohnT,
Doesn't a transformer with a center tap produce two outputs that are 180 degrees out of phase with eachohter, making the electricity coming to your house 2 phase?

I know we call it single phase.

George



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John T

04-23-2013 07:14:04




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 Re: Basic AC Power Systems in reply to George Marsh, 04-23-2013 05:53:04  
Good question George, yep that single phase transformer at your house is tapped in the center and the two legs of 120 are 180 opposite (that way only 120 not the full 240) but theres only one transformer and splitting it in the middle doesnt make it two phase. Sorry not a great explanation but we need pictures lol. I think JMOR posted a picture the other day of two phase. A picture is worth a thoudsand words so maybe he can re post it or find it down there below. I'm not smart enough to post a lot of pictures here and that would better answer your question which is a good one.

John T

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George Marsh

04-23-2013 10:49:23




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 Re: Basic AC Power Systems in reply to John T, 04-23-2013 07:14:04  
JohnT,
I have used a dual trace scope and looked at 3 phase waves that are 120 degrees out of phase and two waves from a transformer with center tap that are 180 degrees out.

Doesn't matter how you cut it, when waves are 120 out of phase, you want to call it 3 phase. When 2 waves are 180 out of phase, you want to call it single phase. Do you see the irony?

However, there are motors working on single phase that use a run capacitor on the start windings and you call them poly-phase motors.
Guess you don't want to call a poly-phase motor a 2 phase motor. Poly sounds better!

No need to explain, I understand phase angles, how 3 phase generators are made.

BTW, most alternators are actually 3 phase, some heavy duty are 4 phase.

George

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DH in Carolina

04-22-2013 15:55:14




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
What about a zig/zag transformer system to get a neutral from a ungrounded three phase system. I have only did one of these years ago in a plant using three single phase transformers. DH



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David G

04-22-2013 17:10:22




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 Re: Basic AC Power Systems in reply to DH in Carolina, 04-22-2013 15:55:14  
That would probably be a delta wye transformer with the center tap of the wye grounded.



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John T

04-22-2013 17:10:16




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 Re: Basic AC Power Systems in reply to DH in Carolina, 04-22-2013 15:55:14  
In all the Industry I worked in we didnt use what you mentioned or any open delta etc. However I did design the first Corner Grounded Delta ever used at our large Navy complex and the electricians got all excited tellin me one phase was dead since it measured 0 volts to which I responed YES IT BETTER because its dead grounded in that one corner !!!!

Fun chattin with ya

John T



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george md

04-22-2013 21:24:37




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 17:10:16  
John,

I ran into one of those systems in a small
commercial garage in Aberdeen md. 3 wires in what looked like a single phase breaker box,
2 wires on the normally hot posts ,the third
on the neutral . measured 240 across any two. Thought this is odd, followed the service cable (2wire with wraped neutral) outside, it's connected to a triplex (2 insulated and a bare neutral), followed it several pole spans down the street to a pole with 3 small transformers. That was the first I had seen that ,and I don't remember that set up from college.

george

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John T

04-23-2013 05:33:36




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 Re: Basic AC Power Systems in reply to george md, 04-22-2013 21:24:37  
In college we didnt get into corner grounded Deltas, thats something I discovered years later and based on my preference for a grounded versus a floating system.

Fun chat

John T



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David G

04-22-2013 17:11:16




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 17:10:16  
There was a lot of ungrounded delta put in industrial plants in the 50s. Most of it has been replaced.



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John T

04-23-2013 05:36:24




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 Re: Basic AC Power Systems in reply to David G, 04-22-2013 17:11:16  
TRUE even in the 30's through the 60's many industrical plants used non grounded straight Delta but that has went out of favor for the same reasons I didnt like it either, I just preferred grounded systems and still do.

Fun chat

John T



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David G

04-23-2013 06:03:09




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 Re: Basic AC Power Systems in reply to John T, 04-23-2013 05:36:24  
I think it was about the time distribution changed from 2400 delta to 12477 or 4160 Wye. There is still a little 2400 around here in the smaller communities.



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WyoDave

04-22-2013 15:54:18




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
And if you have a farm with 8 irrigation services, and 3 farmyards, you get to use all 4 of those setups.

David



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dr sportster

04-22-2013 08:48:35




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
High leg /red leg . When you see orange tape on the {most likely panel] B phase and no brown and yellow tape indicating 480 service. Now for the music --- Step back and think before you install those single pole breakers -step back and think before you install those single pole breakers. To any tune you want -sing this song. Do a little dance. Individual spaces need not be marked by code . Use the meter while you dance and sing and not make 120 volt stuff smoke at 208.

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Dr. Walt

04-22-2013 08:03:07




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
Two questions; first, does the Neutral work the same as a Ground? and second, can a Neutral be carrying Current? .



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John T

04-22-2013 11:18:24




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 Re: Basic AC Power Systems in reply to Dr. Walt, 04-22-2013 08:03:07  
NOOOOOOO The Neutral (A GrounDED Conductor) DOES NOT I REPEAT NOTTTTTTTTTT WORK SAME AS THE EQUIPMENT GroundING CONDUCTOR.

Neutral (A GrounDED Conductor) is for the normal return current.

Equipment GroundING Conducotr is for fault current ONLY NOTTTTTTT normal return current. Its there to provide a dedicated low resistance current path for fault current ONLY

John T



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David G

04-22-2013 08:33:43




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 Re: Basic AC Power Systems in reply to Dr. Walt, 04-22-2013 08:03:07  
The neutral should carry current, the ground is only for fault protection. They should be bonded together at one location.



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Dr. Walt

04-22-2013 09:49:55




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 Re: Basic AC Power Systems in reply to David G, 04-22-2013 08:33:43  
David G:

If the Neutral IS carrying Current, & as you say, the Neutral and the Ground should be bonded at one location - then wouldn't that create a DEAD SHORT ?



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John T

04-22-2013 11:26:53




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 Re: Basic AC Power Systems in reply to Dr. Walt, 04-22-2013 09:49:55  
The Neutral (A GrounDED Conductor) carries return current back from the load to the Panels Neutral Buss. The Equipment GroundING Conductor DOES NOT carry any current back from the load, ONLY fault current if a fault exists.

At the main service entrance and only there the Neutral Buss in bonded to the Equipment Ground Buss but still the equipment ground does NOT carry any current ONLY carries current if theres a fault.

PS for the poster and all here: Its IMPOSSIBLE to explain in a paragraph here what can take books to fill and years of engineering practice and experience to fully comprehend SO DONT FEEL A BIT BAD IF YOU DONT UNDERSTAND IT

John T BSEE, JD

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David G

04-22-2013 11:14:43




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 Re: Basic AC Power Systems in reply to Dr. Walt, 04-22-2013 09:49:55  
The neutral is not considered potential carrying, it should only have voltage on it caused by resistance and current referenced to ground.



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David G

04-22-2013 10:13:22




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 Re: Basic AC Power Systems in reply to Dr. Walt, 04-22-2013 09:49:55  
No, because current must have a return path. The current in the neutral is after the load.



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David G

04-22-2013 07:13:12




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
John,

The 480V Y is probably the most common in industrial facilities now, with 277 single phase.

they will put in a 277-120 transformer for 120VAC power.



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greg k

04-22-2013 07:13:06




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 Re: Basic AC Power Systems in reply to John T, 04-22-2013 05:37:10  
Can you explain 2 phase power? I have seen it addressed in the code book, but don't know what it is. Then there is the 120/208 "high leg". Basically a 3 phase delta, it has 2 legs of 120v to neutral and one leg of 208 to neutral, but 208v between any two legs. Good lesson John, and tomorrow someone will ask about grounding :)



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John T

04-22-2013 11:20:41




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 Re: Basic AC Power Systems in reply to greg k, 04-22-2013 07:13:06  
Thanks, heck they already did lol see above

John T



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JMOR

04-22-2013 07:40:01




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 Re: Basic AC Power Systems in reply to greg k, 04-22-2013 07:13:06  
Quoting Removed, click Modern View to see

[URL=http://s192.photobucket.com/user/JMOR_photo/media/Other/two-ph_pwr.jpg.html][/URL]



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