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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?