Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can a transformer be grounded and any point in the winding?

  1. Feb 12, 2019 at 5:42 PM #1
    Conceptual question here. Not really positive if it should be asked here or in general physics...

    In U.S. grid power supply could the secondary winding in a step-down transformer providing split phase power be grounded at any point in the winding?

    Instead of calling the center tap neutral, if I use L1, L2, and L3 from 'top to bottom' where L2 is the center tap and L3 is grounded: Voltage between lines wouldn't change, but now voltage from L1 to ground would be the full 240 V, and voltage between L2 (center-tap) and ground would be 120 (so L3 is now the grounded conductor).

    The implications of this, if true, make me reel a bit, but would be enlightening.
  2. jcsd
  3. Feb 12, 2019 at 6:53 PM #2
    Yes, but with caution.
    Single ground tap do not change anything, but complications may happen if primary and secondary grounds are connected.
    Many low-voltage transformers with split secondary winding have L1 grounding as manufacturer tested option.
    For higher voltage at output (24V or such) you will likely have reduced transformer lifespan due to over-stressed insulation on secondary winding.
    Last edited: Feb 12, 2019 at 7:28 PM
  4. Feb 12, 2019 at 8:23 PM #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    That is exactly correct.

    Think of a humble two cell flashlight with a metal case
    where the negative end of the bottom battery is connected to the case by that coil spring in the end cap..
    If the flashlight is placed either on the ground or on a grounded metal surface,
    voltage at the junction of the two cells is 1.5 volts to ground
    and voltage at top of the upper cell , where it touches the bulb, is 3 volts to ground.

    Just as with your transformer.

    If instead you set the flashlight on an insulating blanket
    voltage to ground is indeterminate , unknown, undefined and could well be very high due to static electricity charges.

    That's why we ground power systems.
    Electrical codes ensure that grounding is done in accordance with standards that keep us safe,
    and what you described would violate code,
    but conceptually your thinking is straight.

    If you want to read a great little book on grounding try IEEE142 "The Green Book"

    old jim
    Last edited: Feb 12, 2019 at 8:49 PM
  5. Feb 12, 2019 at 9:22 PM #4
    Great thanks.
    This is purely for conceptual understanding of grounding (and voltage for that matter).

    I assume it wouldn't be incorrect, then, to say that the voltage of one of the 'hot' lines from a transformer is not a voltage to 'earth' in any inherent way (when no part of the winding is connected to earth).

    I'll have to look more into floating systems as I feel this is part of that thought. The concept of a voltage being indeterminate/unknown when un-grounded is a bit confusing. If an electrical system could truly function without any over voltages and other things I'm probably not aware of (in other words, was simplified and idealized) would an un-grounded system simply have no voltage to actual 'earth'?

    Thanks for the book suggestion, I very well may check that out.
  6. Feb 12, 2019 at 9:44 PM #5
    Just to be clear: I realize there are major safety reasons, namely clearing faults, associated with the 'grounding system' (EGC's). Not trying to ignore that aspect; I'm more using the concept of 'earth' and ground to help understand the dynamics of the system.
  7. Feb 12, 2019 at 10:07 PM #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    Yes, it's emotionally difficult to accept "you cannot predict" as an answer. That's our vanity getting in the way of our science. We humans like to be in control.

    It could have substantial voltage to earth, or it could have zero. You can't predict..
    Remember the definition of a capacitor- two conductors separated by a distance
    The conductors of your electrical system form one plate of a capacitor
    Earth is the other plate
    just they're not nice rectangles like the ones in our textbooks.
    We call that "Distributed Capacitance" and it is one property of any electrical system.
    You can estimate it from the length and size of the wires comprising the system.

    Any opposite charges present on the plates will establish voltage between them

    and that's part of the concept you asked about.

    your system could operate just fine with substantial voltage to earth
    and electrical systems in aircraft in fact do
    i was once in a passenger flight that flew through a lightning bolt, big streams of brilliant white light shot out behind the wing like fire from every static discharge wick and from the navigation light on the wingtip.
    The interior lights didn't even blink.
    (image credit to www.b737.org.uk/wingtips.htm )

    In electrical systems the trouble comes when voltage gets high enough to pierce insulation.
    That has happened in industrial environments

    and that's why any electrical system should be "grounded" by an impedance not more than that of its distributed capacitance---
    -- it gives a path to discharge the system's distributed capacitance---
    You'll read about that in "The Green Book"

    it's good that you ask these questions. Too many people think there's some 'magic' about ground, that electric current has some affinity for it. Glad to see you investigating the concept.
    Remember - the humble flashlight works just fine on the moon.

    old jim
    Last edited: Feb 12, 2019 at 10:22 PM
  8. Feb 13, 2019 at 8:00 AM #7


    Staff: Mentor

    See if this helps.

    In the pictures below, I show a simple circuit with 6 variations, moving the ground symbol around. In case 5, there is no ground; the whole circuit is floating. In the table of results, I show "potentials" each at a single point, and voltages each s measured between two points. The potentials are expressed relative to ground, but they could be expressed relative to any other reference.

    In these examples, you can consider the ground symbol as either an imaginary reference point, or an actual physical connection to Earth.

    The whole point is to show that the potentials and the zero reference point, are arbitrary and make no difference when calculating the voltage and current. Therefore, potential relative to infinity is a useful concept in physics teaching, when we get to actual circuits like a transformer, it is useless and we typically don't discuss it at all. There are exceptions, but those are advanced cases, not basic cases. We always try to understand the basic rule before the advanced exceptions.

    Unfortunately, we sometimes get lazy and sloppy in our speech. Once the ground point in a circuit is decided and we start talking about voltages, we presume that the second wire of the voltmeter is attached to ground and start talking about voltage "VA" rather than "VA with respect to ground." In other words, using the language of potentials when discussing voltages. It is wrong but it happens every day.

    Does that help or does it confuse more?

    6 cases.jpg 6 cases.png
  9. Feb 13, 2019 at 9:18 AM #8
    Thank you both.

    I do think that helps confirm and enhance my understanding of the reference / ground and its arbitrary nature.

    I think where my understanding breaks down a bit is in case 6 above: where the 1000V is 'coming from.' (or similarly, the ???(unknown) voltage in case 5)

    One question that might clarify for me (or help lead me in the right direction): If there is voltage between a floating system and an outside reference point, specifically the earth (such as displayed in case 6) does this mean that current would flow between those points if connected? (This would be like stepping out of a car on a dry day and getting a shock, yes?)
    If yes, what is the source of this voltage? Is it induced atmospherically? Said differently, how does a floating circuit reach a differing potential with actual earth (Does the actual circuit voltage supply create any of it, or is it all external to that?).

    Perhaps distributed capacitance answers some of this, and so I will look more into that!
  10. Feb 13, 2019 at 9:19 AM #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    A most excellent point, and subtlety that confuses plenty of people.

    A voltmeter has TWO wires not just one.
    That's because voltage is "potential difference" and it takes two of anything to have a difference between them.

    So voltage is always between two points - the one where our voltmeter's black lead is hooked and the one where its red lead is hooked.
    We usually choose to place the black lead on the spot in the circuit where currents from the various sections collect for return to the power supply.
    On better schematics you'll see it called "Circuit Common" or "Power Supply Return(PSR).

    The word "Ground" is so frequently misapplied , using it to describe the concept of 'circuit common', that it's become part of everyday speech.
    A yellow bulldozer isn't a "Cat" unless it was made by Caterpillar Corporation
    Circuit Common isn't "Ground" unless a wire connects it to Mother Earth. But everybody calls it that sometimes..

    Think of your car shop manual - what they call "Ground" is really "Chassis" and is insulated from "Ground" by the tires. It's where currents from various places collect for return to the battery.

    Just something to keep in the back of your mind . It'll keep you from building on a false concept.

    Thanks @anorlunda
    Last edited: Feb 13, 2019 at 2:36 PM
  11. Feb 13, 2019 at 9:41 AM #10


    Staff: Mentor

    That's an inprecise question. But if you mean is there a current in the 1000V battery shown in case 6, the answer is "No."

    Use Kirchoff's current law; the sum of all currents at a node is zero. Then consider the node where the 1000V battery connects to the circuit. 1 amp come in from the right. 1 amp goes out to the left. Therefore, the current through the third branch (the one with the 1000V battery) much be zero, regardless of where the 3rd leg goes or what it connects to.

    It is a frequent misconception that current just disappears into the ground.
  12. Feb 13, 2019 at 9:56 AM #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    Ahh, you posted while i was typing this unnecessary tome.

    You've felt that shock. So the answer is yes.

    And you've seen static electricity experiments

    friction can drag electrons from one material to the other, charging their distributed capacitances.

    distributed capacitance is small, usually in picofarads so the stored energy isn't much
    let us say 100 picofarads(approximate human body capacitance) is charged to 10,000 volts

    Q = CV = 100X10^-12 (Farads) X 10+4 Volts = 1 microcoulomb , not much charge ..
    If an amp flowed through your finger it'd discharge in a microsecond .

    Energy = ½CV2 = ½ 10-12 X (104 ) ^2 = ½ milljoule, not much at all. (check my arithmetic?)

    You've felt the effect when separating styrofoam coffee cups or removing the dry cleaner's bag from a sweater.

    Imagine those electrons being dragged from one surface to the other. When the surfaces are in contact there's no electric field between them so no work is involved in that transfer.

    Now as you separate the surfaces, the opposite charges on the two surfaces are attracted to one another by coulombic force Q1Q2/r2.
    As you increase the separation you do work FXD against that force, and it shows up as energy in an electric field occupying the volume between the surfaces.
    That energy manifests itself as voltage between the surfaces,
    and that's how static electricity comes about.

    200 years ago it was all a mysterious parlor game - how far we've come , eh ?

    old jim
    Last edited: Feb 13, 2019 at 10:42 PM
  13. Feb 13, 2019 at 8:47 PM #12
    Thanks fellas, I appreciate the time you've taken to help guide me in the right direction.
    I'm sure I could easily just keep coming up with questions, but you've given me a good idea of which questions to ask and where to look.

    Here's one last thought:
    The concept that current doesn't just 'disappear' into the earth is in essence what started my head scratching over the current flows witnessed in the shock from a car to ground and by the purported stray voltages that are 'dissipated' or 'bled off' into the earth in a grounded electrical system.

    I see in case 6 from above that there is no complete circuit between the 1,000 volt battery and the ground. Standing on the ground and touching the negative terminal wouldn't present any potential; yet if I were to touch the positive side of that battery (the rest of the circuit), I would be the return for the 1,000 volts into ground, and thereby back to source of the battery.

    Perhaps trying to think in circuits is not always possible or practical, but I can't help but want to think of the circuit that is being completed when these stray voltages find current return paths (and where the voltages came from). If charge wants to return to source, then it would seem that these charges have come from the earth. (or is it more accurate to say the earth is one side of the capacitor (edit: just reread above and see you said this exact thing Jim)... But perhaps the actual source (generator) of voltage is variable: from the likes of tires spinning on the road in the case of the car, to atmospheric turbulence in the case of a large electrical grid?)

    If we can approximate this concept with a circuit diagram would something like the following be at all correct?
    circuit case 7.jpg
    Where capacitor plate c represents the entire circuit (so not actually having a node of attachment)
    and capacitor plate e represents earth (so again, no point attachment)
    and the unknown voltages we've been referring to would be across this capacitor, which is why current could flow into earth if the system were grounded by a conductive path (i.e. the shock received when stepping out of the car onto the ground and touching the car).

    There's probably some imprecise notions and notations here... But hopefully its the right track.
    I did just read in The Green Book: "Consequently, the so-called ungrounded system is in reality a capacitance grounded system, by virtue of the distributed capacitance from the system conductors to ground."
    I also re-read above Jim and see you pretty much already spelled this out, but I guess I had to work through it for it to sink in.
    Last edited: Feb 13, 2019 at 9:05 PM
  14. Feb 13, 2019 at 10:23 PM #13


    Staff: Mentor

  15. Feb 13, 2019 at 10:36 PM #14

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    YES ! And BRAVO !

    A year from now you will be unable to remember when it wasn't intuitive.
    But we have to train our mind to envision things the way Mother Nature built them.
    She gave us magnificent imagination and that's a double edged sword - we can believe lots of things that "just ain't so".

    Perhaps we're imprinted in youth by lightning - it usually goes to ground so we think that's what electricity does.
    But the charge that was carried aloft probably rode up from the ground on water molecules (they're quite polar ) in the evaporation and upflow that drives thunderstorms.
    So returning to ground in lightning strokes IS completing the circuit , albeit after a time delay.
    I tell my students "That doesn't violate Kirchoff - he will allow an occasional Rain Check "

    I really shouldn't be so colloquial in an academic forum.,
    But i believe we should use our everyday sensory experiences to solidify our basic physics.
    and that's why i use so many boring anecdotes.

    To see you reading The Green Book and putting together examples that are spot on i find rewarding to say the least.
    Makes an old guy feel a little bit useful.

    Go man, GO !
    old jim
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?