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).]