embarassing maybe, but I don't get this. In a single phase 3-wire system for instance, I've always read that the center point of the secondary winding has had this neutral wire physically spliced/connected to the winding, and then going to a ground connection point.
But why isn't that a short circuit to ground? the same as it would be 50 feet away from one of those phase wires if they were connected to ground?
if this is too complicated, never mind

Good Q, Cindy...

It is my understanding that any one phase (or point between) may be grounded... But only one. The ground reference helps to define what 0 volts is, and also provides a (rather high impedance) return path.

Please correct me if I have erred, I wish to understand this a little better too.

If I understand your question, you are right. The neutral wire (which comes off the center tap of the secondary transformer at the pole) is shorted to ground. Or more accurately, forced to stay at ground voltage or zero volts.

If the world was wired without grounding neutrals everything would still work fine...but the neutral in my house would be at a different voltage than it would be in yours or for the guy next door.

A 220/110V transformer provides 100V between each hot leg and neutral but that can be done by providing three wires at -110V, 0V, and 110V or by providing 4,300V, 4,510V, and 4,620V. The difference between the wires is still 110V and equipment would operate properly if you don't mind your plumbing, building framing, electrical boxes all being at 4000V above ground. Hurts to turn on a light while standing outside.

Hope this helps.

last time i asked someone this question i got the same dumb look on my face when i listened to the answer. this may be one of those questions like who came first, the dinosaur or the egg?

Mxguy, you said it is a short to ground. why don't it go boom?

The answer is the same as that for a corner grounded delta, where one of the phases is intentionally grounded. As long as it is the sole connection to ground, there is no short circuit, because ther is no current path. That is, until a second (unintentional) ground connection (fault) occurs on another phase.

whether single or 3-ph, like the 3 windings on a trxfmr or generator stator, they say that all phase conductors are connected together at a common point in a wye system, or looped together in a delta system, and my understanding of phase conductors is that they are bent on finding the easiest path to ground. [my next question was going to be how can these phase conductors connect without another boom? but i won't go there now]
I'm reading that the common conductor attached to that midpoint or corner is "grounded to earth" so why isn't that a "current path" as Redsy says? its line to ground, right? why does it need to be a different phase to ground before its a short? why does calling it "intentional" make this midpoint or corner grounding less of a current path than if grounding those phase conductors 2 feet away or 10 feet, etc.
if you look at an impedance grounded neutral system i think they put a resistor between the trxfmr and the GE, so if they are putting a resistor there, then what are they resisting? i know, dumb questions probably, but i'm missing something fundamental obviously

Scott had a number of X-former pix in the Tech ref area @ one time, I found this,,
https://www.electrical-contractor.net/ubb/Forum15/HTML/000044.html

The theory of a 'center tap' is viewable here. Myself, i think it helps to think of electricity simplistically as a circle
( all hardcore theory aside)

It's just that one point in that circle should be at 0 volts....

Cindy, you apparently understand it better than I do... You point out a lot of interesting things...

I have a similar Q concerning what would happen if a non-grounded delta conductor were to "fault" to a grounded guy wire of a wye system... like on a shared pole, 19.9KV Delta on top and 4160/7200 Wye on the bottom...



In this photo, there are fiberglass insulators to prevent the scenario I've created here for discussion...

Go here for the other thread about this photo where the insulators were "brought to light"...
https://www.electrical-contractor.net/ubb/Forum5/HTML/000095.html

[This message has been edited by sparky66wv (edited 12-23-2001).]

Hoo Boy!,now were gettin' technical! It's funny how one 'afraid to ask' Q can lead to us all partaking in the Tarte de corneillefor X-mas din-din....

& by the way, Merry Xmas to all !!!

Steve
aka sparky

Here's an experiment that might make sense. Of course just do it in your head.

Take two AA batteries and put one stacked against the other just like they would be in a flashlight. Pretend this is your tiny little secondary winding on the pole. How the electric gets into these batteries doesn't really matter and the fact that it's 1.5VDC instead of 110VAC really doesn't either.

So take the point where these two batteries touch together and ground it...for no reason at all. With your meter, you measure 1.5V between on end of this battery stack and ground and the same between the other end and your ground, right?

Now from one end of your stack to the other end is 3V, right? Now you've got two hot legs and a neutral.

What if instead of grounding the center point, you left it ungrounded, or even attached it to one end of some other battery somewhere? You still get 1.5V on each leg to ground and 3V across both batteries. But...the voltage of the center point is not necessarily at the same voltage as ground anymore. That's okay, there's nothing wrong with that. you still get the voltage you had before.

The problem is that in our houses, we want our neutrals to all be at the same voltage. To do this they all have to be tied to some reference point, and we chose to use the ground.

But even the earth's ground voltage changes all the time. When there's a lightening storm, it vary's all over the place, light hitting the earth causes a negative charge to form as do all sorts of radiation. But we don't notice because we all change our ground voltage along with the earth. As long as the whole neighborhood is on the same ground, no body gets hurt.

When you walk across a carpet and get a shock touching the doorknob that happens because you and the doorknob had a different ground voltage. Had you both been attached to a driven rod, that wouldn't have happened.

If your house and garage aren't attached to the same ground, the results of moving from one to the other would be worse than that static shock. So everything gets grounded for no reason other than to keep us all working at the same ground voltage...whatever that is today.

I know this is long but I can't come up with a short way to say it. Hopefully somebody more elequent than me will come along.

Maintguy,
I liked your battery example until you said that if you didn't ground the center point of the batteries, you would still get 1.5 volts to ground. I don't believe that to be the case.
Cindy,
There is no current path because the secondary is isolated from the primary. Where would the electricity flow to when you ground a midpoint of a secondary if no other line is also connected to ground? Nowhere, because there is no way for it to get back to the secondary windings, until a second line is grounded through an unintentional ground fault.
The resistance grounded systems don't conduct to ground either, unless there is a second, unintentional ground fault. The resistor then limits the current flow to ground to minimize equipment damage, but allows enough current to flow to operate protective devices.
Correct, anyone?



[This message has been edited by Redsy (edited 12-23-2001).]

>>>Where would the electricity flow to when you ground a midpoint of a secondary if no other line is also connected to ground?<<<

i've pictured the secondary [like the primary] as a lot of freed negative electons searching for the positive ground, and don't see why that path isn't provided by that center tap off the negatively charged secondary to the positively charged ground

we still can't connect a wire from an ungrounded conductor to the ground without flashes and sparks. thats what i'm working on.

i get this, 7200v is generated, comes to the pole on one phase wire into the top of the txfr, across the txfr secondary its stepped down to 240v, a neutral is center-tapped on the secondary phase windings, 2 lugs on the side of the txfr are the ends of those windings 180 degrees out of phase and will measure 120v from each one to the neutral tap that is a third lug on the txfr cannister that has a connection to the pole-to-pole ground wire and to a pole-to-ground grounding wire. but there's a point up there inside that can where the neutral wire comes off the secondary windings and is connected to the neutral lug that is connected to ground, WHY DOESN'T THAT SHORT WIRE CONNECTION BLOW UP?

i've read Timmy Transformer and it didn't help http://www.waukeshaelectric.com/ref_library/pdf/general_lit/Timmy_Transformer_Book.pdf

Ok, I'll jump into the fray. I liked maintenanceguy's battery example..that's the first example that popped up in my head when I read Cindy's question. Redsy's comments were right on too. Instead of the 'center-tap' secondary situation..let's simplify things and just think of a single winding for the secondary, with the "neutral" gounded (technically its not a neutral, just the "grounded conductor", like the code describes it). Now, the secondary current will only go out and back from that one winding..and it will take the path of least resistance. The current has no reason to go to ground as long as the wires are intact and/or the ground doesn't link both ends of the secondary. If someone were to connect a ground connection at the load end of the circuit, but still at the "neutral" side of the load, then the current would divide a little, with most of the current going through the "neutral" conductor, since it has the least resistance,and the rest going through ground..the earth and the "neutral would be in parallel. If the "hot" side of the circuit accidentally connected to ground, then we'd have the ground fault situation that causes things to blow up and melt things..unless the breaker or fuse beats it to the punch.
Ok, so, since we're talking ac current, it would seem that the selection of which end is the "neutral" vs which end is the "hot" would be somehow dangerous. What would happen if you picked the "wrong end"..and connected the "hot" to ground? It just wouldn't matter. The secondary acts like its own little alternator..pushing and pulling ..and if there is an external loop of conductor and resistance..current will flow. It wouldn't inject electrons into, or pull electrons out of, the earth unless the opposite end of the winding was also connected to earth.. or the "neutral" was paralleled with the earth.
That is the basic stuff..we haven't touched on statics and capacitive coupling and charging currents of the primary and the derived system..all that is stuff that MAY have some bearing on Cindy's question..but I can't tell at this point.
I'm glad Cindy had the guts to ask this question..I suspect that there are lots of lurkers who are curious about the same thing.

"I liked your battery example until you said that if you didn't ground the center point of the batteries, you would still get 1.5 volts to ground. I don't believe that to be the case."

You are right. I should have said to neutral, not to ground.

If you take a car battery and ground one terminal why doesn't it spark and send current to ground? Same reason.

Too many have been taught that electricity is always trying to get to "ground". That just isn't true. Electricity is only trying to get back to its source. In a grounded system, ground becomes a path back to the source.
Don(resqcapt19)

Tarte de corneille? tarte is french for a pie, corneille? is that french for crow?

anyway, thanks to all for the help. i think i have altered my view of the electrical circle or loop. before don's last input, i was just about to say, maybe electrons are not necessarily always looking for the earth.

that seems to be the answer to my misunderstanding [i hope]. normally the loop is from the secondary txfr lugs thru the service, loads, and back on the grounded conductor to the txfr neutral lug [without a load, then that path would be a short, but with loads it works, same as a wire plugged into an outlet] or if faulted it returns via egc's, and gec > ge > earth > ground wire up the pole > pole mounted txfr neutral lug > back to the source at the center tapped point. and most significant for my clarification is that the electrons aren't as interested in getting from the center point tap to the ground as they are in taking the route above to return to the source. i'm kinda thinkin outloud here, so probably isn't all together, but i can sorta accept the secondary tap to ground not being a short with this understanding. so don't tell me i'm out to lunch tonight, at least wait till tomorrow.

Hi Cindy,

Glad you have chosen to ask this question here. The answer[s] are not going to be really easy to explain, even harder to comprehend, so don't get discouraged if the consepts take too much time to grasp.

I'll try and cover this subject without going too far overboard with tech.

The first thing to do is to drop all ideas of Electricity wanting to go to ground. It really doesn't want to go to ground, it is at times "forced" to ground, or is "attracted" to ground by what's called Capacitance [AKA Admittance, Permitance, Electrostatic Fields, Capacitive Charging and Capacitive Coupling]. These items are involved, but at this point let's forget them all together.

Now, let's use one of my Transformer Schematics as a reference model for the first part of the discussion. We will use the simple 1 phase 2 wire Isolation Transformer shown in this Schematic:

1 phase 2 wire series connected transformer

Let's set the systems voltage ratings at 1000 VAC for the primary, and 100 VAC for the Secondary.

For the first part here, assume that we have removed the ground bond [connection to ground] on the "Line A" of the Secondary.

We can connect any load to the Secondary which requires 100 VAC to drive current through it. For this one, we connect a fixed Resistor of 100 Ohms to the Secondary. The Transformer drives 1 amp through the load and the overall result is 100 watts true power dissipated.

Notice that this system works just fine being ungrounded. Keep this in mind here, because this is the 1st key item.

Now if I take one of the Secondary Lines [either A or B] and physically connect it to the earth - AKA Grounding It - nothing happens at all to the system, and the load will continue to dissipate 100 watts of true power. No high current levels will flow from any part of the Transformer's circuitry to the earth - hence there are no sparks flying or smoke being let out.

With the same Line connected to the earth ground [let's say it's Line A as shown in the schematic], if we take a tap off of Line B and also connect it to the earth, there will now be a flow of current from the Transformer through the earth, then back to the Transformer. Here's the 2nd key issue to remember - flowing from the Transformer [or power source], through the earth [a closed circuit], then back to the Transformer [or power source].

To invision these situations as a high fault current example [AKA Sparks Flying / Smoke Let Out], let's take that Line A and connect it to both earth ground and to the metallic enclosure of our main service panel.

If we take a wire and physically connect Line A and Line B together without something in series to limit current, we have the classic fault situation [AKA Short Circuit - the breaker tripping, spark throwing situation].
Sparks fly and breakers trip due to the high level of amperes flowing during this fault condition.

Now, if we have Line A bonded to earth and the enclosure, the enclosure is almost as if it was Line A it's self [not really, but at this point it can be thought of this way and be very helpful].
Take a tapped wire from Line B and connect it directly to the metallic enclosure. The results are the same as shown for the Line A to Line B fault shown above - sparks flying / breakers tripping / smoke being let out.
In this case, the fault is known as a Ground Fault. It's called this because the referenced parts of the fault[s] are between Transformer, an Ungrounded Conductor, and a Grounded piece of equipment [the panel's enclosure] which is physically bonded [connected] to both the Grounded Conductor and a "Reference Ground" [typically earth].
During this Ground fault, there's only fractional current flowing through the earth ground - and only if the Transformer ALSO has the Grounded Conductor bonded to Earth at the Secondary.

If the Ground Fault's level was 10,000 AIC [10,000 Amps], the level of current flowing from the Transformer, through the circuit conductors and the faulted point, then back to the Transformer will be 9,999 Amps. The last 1 amp will flow from the Transformer, through the Ungrounded Conductor, through the faulted point to the grounded enclosure, to the earth, then through the earth back to the Transformer.
This would indicate the Impedance of the earth [the dirt it's self] between the Transformer's ground bond and the main service's ground bond is 100 Ohms Z [using the figure of Secondary voltage being 100 VAC]
This would be a value which is not too uncommon.

Now for some terminologies :

In the Ground Fault situations, the current flows from source [Transformer] through the Ungrounded Conductor ["Hot"], then back to the source [Transformer] through the Grounded Conductor [known by the slang term "Neutral"].
In a Line to Line fault [Short Circuit], the current flows from the source and back to the source through the Ungrounded Conductors.
A Corner Grounded Delta can have a Ground Fault or a Line to Line fault occur between the Grounded circuit conductor [AKA "Grounded Phase"] and one of the 2 Ungrounded Conductors.
A "Bolted Fault" in which systems are max rated would be a Line to Line to Line short circuit on a 3 phase system, or Line to Line short circuit on 1 phase systems.

One last example of a Short Circuit would be a direct short from an Ungrounded Conductor to the Grounded Conductor, without the fault current flowing through grounded equipment [equipment grounding conductors, conduits, enclosures, etc.]

Keep in mind that the Ground Fault is one which the fault current flows between the source [Transformer, for example] from an Ungrounded Conductor into bonded metallic equipment / conductors, back to the point where the grounded conductor is bonded to the metallic equipment and the grounding electrode, then flows back to the source on the Grounded Conductor.

OK, now back to the Transformer model

If we remove all the connections to earth ground from the selected conductor [line A] and also remove the bond to the metallic enclosure of the main service panel, we could easilly connect a wire tapped from Line B to the enclosure [or even earth ground] and not have any sparks flying what so ever!!!

There's no LOW IMPEDANCE PATH for current to flow, so the highest level of current we could expect to see flowing here would be in the milliamps range [0.01 to 0.00001 amps].
The reasons for this are due to the Capacitive Coupling effects I mentioned at the beginning of this long winded mini series

If you ever run across 3 phase 3 wire ungrounded Deltas, you will be able to test this first hand.
Take your wiggy and measure the voltage to ground. The wiggy will attempt to show a voltage level, but will rapidly decrease to a tiny hum and very faintly glowing neon lamp - even though the system might be a 480 VAC Delta [the wiggy will not even show a level of 120 VAC]. When you disconnect the wiggy, it will "pulse" or "surge", so the solenoid will be heard as it slams down [like it does when you perform any voltage tests with the wiggy], and the neon lamp will brighten up.

Try this test with a High Input Impedance DVM, and you will get strange voltage readings. Levels of 300 to 900 VAC to ground on an Ungrounded 480 VAC 3 phase 3 wire Delta are not uncommon!

These all suggest that we have a Capacitor, in which we are first charging it [the rapid drop in levels as the wiggy is first connected], then is discharged [as observed by the surges as the wiggy is disconnected].
The high potentials observed with the High Impedance DVM reflects a similar Capacitive situation.

You could take any line conductor in this system and let it hit the enclosures of metallic equipment - but not have the resulting sparks flying / breakers tripping / smoke let out situations.
You would be creating a corner grounded Delta.
Don't go ahead and try this at work!!! It's just an example of a common system. If the system is Grounded and you try this, there will be a high fault level.

OK, now to rap things up, let's apply all this baloney to the 3 wire center tapped 1 phase Transformer.
Check out this schematic:

1 phase center tapped secondary

Apply the same ideas here as we used in the examples above, and this should unmask the whole thing!!!

The same can be said of when using "Split Coil" Transformers as shown in this schematic:

1 phase split coil schematic

The reason we have opted to ground the common / center tapped conductor [common conductor on 4 wire Wyes, Center Tapped Conductor on 1 phase 3 wire and 3 phase 4 wire Deltas] is that this will offer the lowest system voltage to ground across a person.
Any system line conductor can be ground bonded, but one AND ONLY ONE can be ground bonded per system - otherwise...well you know the results!!!...sparks / smoke / trips, etc.

Well, let's cut the message off here and see if this has been helpful enough to you.

Let me know if there's still doubts.

Scott SET

P.S. edits reflect major typos!!!

SET



[This message has been edited by Scott35 (edited 12-24-2001).]

Hi Cindy,

I only just picked up on this topic, so there's not much left that's not already been said. Just my 2 cents though.....

As the others have said, forget the idea that current ALWAYS tries to get to the earth/ground. Just try to think of the earth as another conductor which you can connect to by driving a rod into the soil. (O.K., so it's a very big conductor about 8000 miles across! )

If you took a length of wire, you could connect one end of it to any point in a circuit, and nothing would happen. It's only when the other end is also connected that current can flow.

With a circuit connected to ground at only one point, no current can flow to the ground from that circuit. It's only when some other part of the circuit is grounded (intentionally or by a fault) that current flows through the earth.

In practice, that common "conductor" called earth or ground is shared by many other systems, but that doesn't matter, because the systems are linked only at that one point. (e.g. If you drive two cars together so that their bumpers are touching, both cars' electrical systems still function perfectly well, because there is only one point of contact between the two.)

We have an example of an ungrounded circuit in many British homes: It is used for an electric shaver outlet in a bathroom.

The standard (in Britain) 240V supply has one wire grounded, as in the States, so if anyone were to touch the hot wire while holding onto, say, a faucet, he would get a shock. Yes the current is going to ground through his body, but only because the ground (and pipework) happens to be a convenient path back to the neutral of the utility company's transformer.

To reduce the shock risk, the shaver outlet in a bathroom here is fed by an isolation transformer. The primary connects to the standard 240V supply with one side grounded.
The secondary is also 240V, BUT it has no ground connection to it. It's possible therefore, to accidentally touch either side of the outlet and a faucet at the same time without getting lightly frazzled, because the path through the body is then the ONLY connection to ground on that circuit. (You might feel a very slight tingle, due to the capacitance effect that Scott mentioned.)

In fact, you could take a length of wire, ground one end to the faucet, and shove the other end in either side of the shaver receptacle, and nothing would happen: No big sparks, no bang, no blown fuse. Again, the circuit has only one point grounded.

The fact that the shaver supply is ungrounded means that the only way to get zapped by it is to touch BOTH sides simultaneously -- Much less likely to happen than contacting one side while grounded.

I hope I haven't confused you any more. Merry Christmas!

so the negatively charged electrons are not attracted to the earth until they are given a path, both to get to the earth, and return from the earth, i.e. the earth is only a conductor [ruins the picture I used to have of the earth as a mass of positive protons].

and re: a portable generator, it doesn't have a connection to ground, so if I were to split a cord apart and hold the ungrounded conductor to a ground rod, would nothing happen? no current would flow because there is no return path to the source, generator? the reference would probably be the generator frame?

Yes, that's pretty much the size of it. If we go into STATIC discharges (e.g. lightning strikes) then it gets a bit more complicated, but as far as normal power distribution systems are concerned no current can flow to ground so long as there is only one point of the system connected to ground.

And yes, if you have a generator which has no ground connection you could take a wire and ground either either conductor and nothing would happen -- No sparks, no flash, no current to ground (except perhaps a very tiny leakage current due to the aforementioned capacitance effect).

cindy
so much has been said about the topic, that there isn't much left to say. but i'd like to pitch in the way i see it.

to understand the concept of grounding, first of all you have to understand the concept of the star point, or more common neutral. why isn't there a short circuit when all 3 phases are shorted together. the answer is that in a balanced three phase system (same amount of current flowing through all three phases, equal voltages, 120 degrees apart) then at one given moment the summ of the three voltages will be zero, thus if the three wires are shorted, a zero potential point is created which is called the neutral. So no current flows out of a zero potential point (neutral).

but there will be curent flowing from one phase to another.

i don't want to make this too long, if u have understood what i've tried to explain, tell me so and i will try to explain the rest of it to u.

happy new year

This is a bit off topic, but think of what happens to maintenance guy's example if you use standard 120 single phase and a scope instead of a volt meter.

I've never tried it, but I assume that If you use the neutral/ground as 0 and the look at the two hots, you will see two sine waves seperated by a phase shift of 180 degrees, so you have two phase current (but not real two phase).

But if you use one of the hots as 0, and look at the neutral and the other hot, you will see 2 sine waves in phase 1 with twice the amplitude of the other, so you only have single phase current.

I guess it just depends on your point of view.

rbiro,

I would imagine that if you scope'd out a 1 phase 3 wire system and used a "common" probe on the center tapped grounded neutral conductor - with probes also on the 2 ungrounded conductors, the results would be sine waves of equal intensity and equal time, rather than any time offset.

Seems likely that the wave displayed for "Line A - Center Tap" circuit [let's call this one "Ckt. 1"], the wave would begin at the zero line off of the ungrounded conductor Line "A".

The wave displayed for "Center Tap - Line B" circuit [let's name this one "Ckt. 2"] would begin at the zero line on the center tapped common netutral, and in-time with the wave for Ckt. 1 [no time offset or other type of lag/lead - except those of Reactance per wave].

I have never scope'd the described connections, so if anyone has conflicting information, please chime in!!!

Would like to perform this test ASAP. If I can get hold of a scope for a day or so, definitely will run this analysis!

Scott SET.

BTW, has this topic's original question[s] been covered to everyone's liking, or are there still some pending Q's???
If there are, feel free to toss them in!
As said many times before - "There's no Stupid Questions in this forum, just occasional Silly answers".
No one needs to feel intimidated if they are unsure of ANY topic in which this forum may help with. Throw it in and let everyone chime in.

As far as the tech stuff goes, don't feel bad if it doesn't make 100% sense right away!
Electrophysics is by far one of the most in-depth and complex theories in the Scientific Community, but with a little time and patients, ANYONE interested can grasp the principles.

S.E.T.

If you connect a dual-beam 'scope with its common to the neutral, channel 1 to one hot leg and channel 2 to the other hot leg, then you will see two sinewaves of equal amplitude but 180 deg. out of phase (i.e. the zero crossings will coincide but the positive peak of one will coincide with the negative peak of the other).

A similar center-tapped arrangement is common for obtaining DC supplies in much electronic equipment, because full-wave rectification can then be achieved with just two diodes.

Connecting the scope common to one hot leg with channel 1 on the neutral and channel 2 on the other hot changes the reference point. In this case you would see two sinewaves in phase, but ch. 2 with twice the amplitude of ch. 1.

Notes if anyone tries these experiments:
1. Many 'scopes have the chassis and common point grounded via the cord. Obviously, connecting the common of such a unit to a hot leg will result in a flash and a tripped breaker. Use an isolated scope or a separate isolation xfmr, and remember that the case of the scope could end up at 120V to ground, so take appropriate precautions.

2. Modern dual-beam scopes usually have different trigger modes for the horizontal timebase. To get a true comparison of the phase of the two inputs, make sure the triggering is set to one channel only, otherwise it will synchronize to each sinewave individually and you won't get a true phase comparison.