Why does the center tap in a transformer not short when bonded to ground?

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Gary Alzaga

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Hi everyone,
Basic question here. Residential pole mounted transformers and many other transformers that produce a neutral have that neutral "center taped" from a current carrying coil in the transformer and bonded to ground. My question is why doesn't the current in the coil short threw that neutral to ground?


I understand a neutral in a transformer before bonded to ground has equal potential with reference to the ungrounded/hot conductors and once it is bonded to ground it creates a conductor that has zero potential to its surrounding environment. This is all done in the name of safety in case an ungrounded/hot conductor comes in contact with ground or anything grounded it will trip an over-currnet protection device.


Now Ive read if just one current carrying conductor is connected to ground their is originally no potential difference so nothing happens. That makes sense but theoretically their is not just one conductor connected to ground. At every transformer there is a neutral connected to ground. Is their too much resistance between the different neutrals to constitute a short or is it cause all the neutrals are in phase? Or am I totally off? Thanks in advance for the help. It is much appreciated.


Gary
 
First: Throw away the idea that electricity is trying to seek 'ground'. This concept is false. Rather electricity is always trying to find a way back to its source.

The transformer secondary coil is an isolated source of electricity. The power for the transformer secondary is supplied by a changing magnetic field, and the power for this changing magnetic field is provided by the current flowing in the primary coil. So power is linked from primary to secondary, but electric current is not.

This means that electric current flowing out of one terminal on a transformer secondary is going to try to find its way back to some other terminal on that transformer secondary. No where else, just _that_ transformer secondary.

You can take any _single_ terminal of the transformer secondary and intentionally connect it to ground, and essentially no current will flow through this ground connection. (Ignore capacitive coupling for now...) This means that you can 'corner ground' a delta secondary, or ground either leg of a single phase secondary, or ground the 'high leg' (not allowed by code but physically possible), etc. Most commonly a 'neutral' terminal will be grounded, but this is not required by the physics of the transformer.

Whichever transformer terminal that you connect to ground will be at ground potential. The connection itself forces that terminal to be at ground potential. All of the other terminals will keep their same relative voltage to the 'grounded' terminal, and will now have that same relative voltage to ground.

You _cannot_ ground two terminals of a transformer, even if one ground is at the pole and the other at a structure. The current flow through ordinary soil with ordinary ground rods will not be enough to trip a breaker, but will be pretty substantial. You'll cook a volume of soil and a few worms, and create a shock hazard.

-Jon
 
winnie said:
First: Throw away the idea that electricity is trying to seek 'ground'. This concept is false. Rather electricity is always trying to find a way back to its source.

The transformer secondary coil is an isolated source of electricity. The power for the transformer secondary is supplied by a changing magnetic field, and the power for this changing magnetic field is provided by the current flowing in the primary coil. So power is linked from primary to secondary, but electric current is not.

This means that electric current flowing out of one terminal on a transformer secondary is going to try to find its way back to some other terminal on that transformer secondary. No where else, just _that_ transformer secondary.

You can take any _single_ terminal of the transformer secondary and intentionally connect it to ground, and essentially no current will flow through this ground connection. (Ignore capacitive coupling for now...) This means that you can 'corner ground' a delta secondary, or ground either leg of a single phase secondary, or ground the 'high leg' (not allowed by code but physically possible), etc. Most commonly a 'neutral' terminal will be grounded, but this is not required by the physics of the transformer.

Whichever transformer terminal that you connect to ground will be at ground potential. The connection itself forces that terminal to be at ground potential. All of the other terminals will keep their same relative voltage to the 'grounded' terminal, and will now have that same relative voltage to ground.

You _cannot_ ground two terminals of a transformer, even if one ground is at the pole and the other at a structure. The current flow through ordinary soil with ordinary ground rods will not be enough to trip a breaker, but will be pretty substantial. You'll cook a volume of soil and a few worms, and create a shock hazard.

-Jon
Click to expand...
electricity is trying to find its way back to the source. I always wondered does that mean its trying to get back to a generator somewhere but never really considered the secondary it derived from to be a source. I know the secondary has no physical connection to the primary and the voltage in it is there by mutual induction. Still the idea that the current is trying to get back to the secondary where it originated and no where else doesn't seem completely logical. When you think of the concept of difference in potential wouldn't it make sense that if you ground the lets say for simplistic purposes the "a" phase of a secondary on one transformer and the "b" phase of the secondary on another transformer a few houses down, both transformers from the same utility, their would be a difference in potential between the two phases?
 
The transformer is just a winding. If you call one end of the winding phase A then the other end is phase B. You could just as easily call them left and right or top and bottom.

Ground the left side of the first utility transformer and the right side of the second and the center of the third. Each transformer primary is fed from the same utility source, but each secondary is its own source, its own winding.
 
Even more drastic of a situation. If it is true electricity is trying to get back to its source and no where else. what would happen if you took an "a" phase from the secondary of one transformer and connected it directly "b" phase of the secondary of another transformer. If the, "only back to its source", thing is true then nothing would happen. So theres no physical connection to the primary, isn't there still a push and pull effect on the electrons in a/c voltage, regardless of its source? I know these scenarios sound crazy. Im using them cause it makes my question easier to convey.
 
Gary Alzaga said:
Even more drastic of a situation. If it is true electricity is trying to get back to its source and no where else. what would happen if you took an "a" phase from the secondary of one transformer and connected it directly "b" phase of the secondary of another transformer. If the, "only back to its source", thing is true then nothing would happen. So theres no physical connection to the primary, isn't there still a push and pull effect on the electrons in a/c voltage, regardless of its source? I know these scenarios sound crazy. Im using them cause it makes my question easier to convey.
Click to expand...
Not crazy, you are on your way towards a three phase power supply.
 
Gary Alzaga said:
Even more drastic of a situation. If it is true electricity is trying to get back to its source and no where else. what would happen if you took an "a" phase from the secondary of one transformer and connected it directly "b" phase of the secondary of another transformer. If the, "only back to its source", thing is true then nothing would happen. So theres no physical connection to the primary, isn't there still a push and pull effect on the electrons in a/c voltage, regardless of its source? I know these scenarios sound crazy. Im using them cause it makes my question easier to convey.
Click to expand...


Not, not a crazy situation at all, and as ActionDave mentioned various transformer secondaries can be connected together to make 3 phase supplies.

Let's ignore 3 phase for a moment, and consider just standard single phase transformers such as power a home.

These transformers have a high voltage primary and a 240V center tapped secondary. Normally the center tap gets grounded, and you have your standard 120/240 residential service.

If, instead, you grounded the 'B' leg of the transformer, then the center point (the 'neutral') would become a 'hot' at 120V relative to ground, and the 'A' leg would be a 'hot' at 240V relative to ground.

Now, take _another_ transformer and connect its 'A' leg directly to the grounded 'B' leg of the first transformer. The center point of this transformer would become a 'hot' at 120V relative to ground, and the 'B' leg of this second transformer would be a 'hot' at 240V relative to ground.

Assuming that 'A' and 'B' indicate the 'phasing' of the transformer, then the voltage from the 'hot A' leg of the first transformer to the 'hot B' leg of the second transformer will be 480V.

The current from each transformer secondary is trying to get back to that secondary. When you connect the transformers together, then one of the possible paths is through _both_ transformers, and the voltages can add up.

-Jon
 
Gary Alzaga said:
Even more drastic of a situation. If it is true electricity is trying to get back to its source and no where else. what would happen if you took an "a" phase from the secondary of one transformer and connected it directly "b" phase of the secondary of another transformer. If the, "only back to its source", thing is true then nothing would happen. So theres no physical connection to the primary, isn't there still a push and pull effect on the electrons in a/c voltage, regardless of its source? I know these scenarios sound crazy. Im using them cause it makes my question easier to convey.
Click to expand...

And we actually do this when we use buck/boost transformers, we are inserting the secondary output of one directly into the circuit of another. Hooked one way it will subtract the voltage, the other way it will add to it.
 
Thanks guys for the thorough explanations. I was having a hard time wrapping my head around the, ONLY back to its source concept. It makes sense to me now.

Just in case anyone reading this thread is still having a hard time understanding the logic, here are some additional ways to think about it that some one told me.



"To have a circuit, all charge must find a path back to its source, wether that source be a battery in orbit around pluto or a transformer on the pole in your back yard."

" With a battery electrons pushed out the negative end must get back to the positive end, else the battery would run out of electrons almost immediately.

NOW- your transformer winding is not a lot different from that battery.
its motive force is electromagnetic not chemical
and direction of charge reverses direction 120 times per second

but every electron that comes out one end of that transformer winding
must get back to the other end of same winding"
 
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Gary Alzaga said:
...

but every electron that comes out one end of that transformer winding
must get back to the other end of same winding"
Click to expand...

More precisely...

"for every electron that comes out one end of that transformer winding, a corresponding electron
must get back to the other end of same winding"

With respect to circuits though, it amounts to the same thing. There must be a continuous path (or paths) from the positive pole of the source to the negative pole.
 
Gary Alzaga said:
"To have a circuit, all charge must find a path back to its source, wether that source be a battery in orbit around pluto or a transformer on the pole in your back yard."
Click to expand...
Be sure you stick with the path and not stretch that to mean the charge has to complete the path.

Gary Alzaga said:
" With a battery electrons pushed out the negative end must get back to the positive end, else the battery would run out of electrons almost immediately.
Click to expand...
A battery does not "supply" electrons nor does it run out of electrons. It is a charge pump and can run out of energy.

Gary Alzaga said:
but every electron that comes out one end of that transformer winding must get back to the other end of same winding"
Click to expand...
It needs a path but does not have to complete the journey. The moving charges (current) and separation of charges (voltage) just creates a medium (electromagnetic field) that we use to exchange energy. Sort of like the water is the medium in the pool that we use to swim in or send waves from one end to the other: the water does not have to make the journey.
 
Gary, I use a silly little game to teach my apprentices how to troubleshoot and it is applicable here. It is not accurate in terms of electrical theory, but makes understanding a little easier in my experience. For all intents and purposes it works. Until you get to the capacitive and inductive transfer of electricity:

Always pretend you are "one little electron" You always start at the source and your goal is to end up at the same location. Ultimately that is it. You very astutely asked about the term "source" and it is a variable depending on what you are trying to accomplish. For troubleshooting a branch circuit, for example, we don't need to worry about the feeder, meter, Utility drops or utility transformer. so we can designate the panel as the source, which would mean we can either get back to it on the neutral, ground or other hot leg. In you case, since you are questioning the secondary of the transformer, that is a good source. Your follow up question about the "actual source" would be applicable if you were (as the one little electron) trying to run through the Utility companies generator or substation.

So here is an example of how to use this simple tool:

Here is a diagram of a simple 120/240 single phase grounded transformer.

circuit 1.jpg
You can start at any point but let's start at X2 and head towards X1. We can run along no problem. We have no choice but to go out along the red wire, which we know will end up feeding through breakers fuses etc. to a load. For this case let's say it is an incandescent light. So we run along a wire and get to a switch. Where we wait, until someone turns it on, then we take off again. We run to the lamp, an enter it. We run through the filament, heating it up, then we exit through the shell to the neutral. We run along the white wire back to the service panel, through the neutral bus, here we actually have a few choices. We can continue on the white wire back to X2, or we can take the side road along the green wire, through the earth and then up the bare wire, to the neutral and ultimately back to X2. Or we can run back out another white wire backwards through a different light bulb only one on the other phase, through the light bulb back to the panel and out the other hot to X4, and ultimately back to X2. Or we can run along the neutral straight to X4. This is the only place we can end up. We must take the "path of least resistance" in quotations because this is another buzz phrase you will always hear. In a proper installation, that path is first along the other hot, split with the neutral for unbalanced loads (this is where the one electron thing breaks down a little), but ultimately the ground path is NEVER the easiest path back to X2 unless there is a problem.
Try this, but remember you can clone yourself and send a lesser version in two directions. A light bulb or load represents a resistance. The term is electrical but also a noun that means just that, an opposing force. So, for example, if I create a ground anywhere along the neutral wire, current won't flow, until or unless there is a problem CLOSER to the source, from the ground. If I do the same on the hot wire, the ground may or may not offer less resistance that the light bulb, but either way, I am likely to clone myself and go both ways, partially lighting the lamp and partially along ground until or unless I am powerful enough to trip the circuit breaker.


I actually have fun doing this exercise because it is sort of like a logic puzzle. When you start getting in to complex schematics etc. the rules don't change.

PS you can think of capacitance and inductance as like passing a baton in a race.
 
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