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

What happens if "hot" wire touches Earth ground?

  1. Feb 28, 2015 #1
    Greetings !New to the forums.
    Here expanding my concept of what AC Electricity is
    My question (correct me if wrong)..
    If AC power alternates between +,- at a rate of 60Hertz/second and negative is used as -earth- at the AC generator

    If the hot wire touches the ground would happen? (I think Nothing)

    Cause by the principle of my understanding if I touch a live "Hot" ac wire and I am standing still to the ground current from the "hot" wire will travel through me to reach the potencial difference cause there is no voltage in ground .

    But since the current is actually passing through my body I will get electrocuted.

    Question is if this happens would the voltage go back to neutral?
     
  2. jcsd
  3. Mar 1, 2015 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I think you need to get straight in your mind the basic concepts of

    energy
    work
    power

    charge
    potential
    potential difference


    current

    and learn the units of each

    before you become totally confused.
     
  4. Mar 1, 2015 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Current will return to the source from which it came.
    If enough of it it travels through you to get there, you will indeed be electrocuted.
    If earth also provides a path back to that source, current might go through earth too.
     
  5. Mar 1, 2015 #4

    Nugatory

    User Avatar

    Staff: Mentor

    That is not right. If the hot wire touches the ground, there will be a somewhat exciting spark (I have some melted screwdrivers to prove it) and a rush of current from the hot wire to the ground. If a circuit breaker or other protection device does not open, things will explode, melt, catch on fire. If you happen to be part of that path to ground, you will receive a dangerous, perhaps lethal, electrical shock.

    What you may be missing is that the earth at the AC generator is not negative, it is zero. In a North American 60Hz 120 system, the hot wire is alternating between negative -170 and +170 (120 volts is the RMS average - google for "root mean square electrical power" for more), so there is a voltage difference from ground, and hence current flow in one direction or the other throughout the cycle.
     
  6. Mar 1, 2015 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    a compilation of electrical explosions


     
    Last edited: Mar 1, 2015
  7. Mar 1, 2015 #6
    Thanks very much for the replies !
     
  8. Mar 1, 2015 #7

    berkeman

    User Avatar

    Staff: Mentor

    Yikes! Did you see the Fire Engine that pulled up to the burning substation around 2:44 in the video? They thought better of it and drove right on by. Not a thing in the world they could do until the power was turned off... :wideeyed:
     
  9. Mar 1, 2015 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Indeed you don't put water on an electrical fire . The stream will conduct current right back to the fire hose and whoever's holding it.

    Country wisdom : never pee on an electric fence.
     
  10. Mar 2, 2015 #9

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Last edited: Jul 13, 2015
  11. Mar 2, 2015 #10

    DrClaude

    User Avatar

    Staff: Mentor

    City wisdom: never pee on the third rail :wink:
     
  12. Mar 2, 2015 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Don - you taught me that nuance of term "Step Voltage".
    DrClaude - I married a NYC girl. She confirms your advice .

    Thanks, guys !

    old jim
     
  13. Mar 2, 2015 #12


    Hehe.
     
  14. Mar 2, 2015 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    We tried an insulator wash system that used the principle of breaking the stream into droplets separated by enough distance to prevent conduction..
    It worked okay the first couple of times...... wish i had a video of that 240KV flashover

    Thanks for the confirmation !
     
  15. Mar 2, 2015 #14
    I've heard of power line insulators washing systems where they use water spray under high pressures. Water is of low conductivity or even tap water.
    Sounds insane but...
     
  16. Mar 2, 2015 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Very pure water is a good insulator. Some generator windings are water cooled.

    I've heard of success washing insulators on transmission lines. Our trial on a transformer didn't work out well.
     
  17. Mar 2, 2015 #16
    System was the same used for washing insulators on transmission lines? Maybe it was used in a wrong way...
     
  18. Mar 2, 2015 #17

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Maybe. It was one of the earliest offerings. I see it's still done a lot of places.
    Washing%20insulators%20affect%20ADSS%20cables.png

    On a high line like that there's nothing immediately below the insulator, so runoff falls and separates.

    I'd be doggone nervous around this:
    image26.jpg
    Our failure was on a transformer with vertical insulators like these.
     
  19. Mar 3, 2015 #18

    rollingstein

    User Avatar
    Gold Member

    Why is it not possible to design the analog of an automated circuit breaker? A short like that, doesn't it have some signature? I suppose you cannot just detect the high current because there might be legitimate reasons for a high current too?

    Some of those shorts seem to last for ever. It'd be nice to have an automated system trip the feeders. Too many false alarms?
     
  20. Jul 13, 2015 #19

    dbc

    User Avatar

    k..... now im confused....

    taking what Nugatory said into account.... A/C transformers primary coil are a direct path from hot ( >4000V ) to ground/neutral (unless you are considering the reactance of the inductor coil).... so why dont A/C transformers in substations and on poles blow up when they are hooked up since there is no resistance? Is the coil's inductor's reactance actually acting as the resistance of the circuit thus preventing current from flowing at a substantial rate? And what happens when the secondary side is disconnected? Does the primary current continue flowing?
     
  21. Jul 13, 2015 #20

    rollingstein

    User Avatar
    Gold Member

    @dbc

    How so? On a delta-star AC transformer (assume you mean step down) the neutral grounding is on the secondary not primary side.
     
  22. Jul 14, 2015 #21
    (We'll talk about single phase for ease of communication) If the secondary side is disconnected so there is an open, current will cease to flow in the secondary and current in the primary will depend on the equivalent values of the primary circuit - coil resistance and impedance, along with magnetization current and hysteresis losses of the core material. When the secondary is connected to a load, the load will draw some amount of current (whatever is needed to drive whatever device is connected or there're multiple references to ground). This current draw will have an effect on the voltage and current phasors of the primary side.

    In three phase transformers this is complicated slightly depending upon how the respective sides are connected. For instance: if you have a Delta-Wye transformer and you have a fault on the Wye side that goes to ground, it will manifest itself as a 2 phase fault on the Delta high side. There are also zero sequence networks thrown into the mix, it's these currents that make it important to ground a system - or have some form of stability insurance. If you had a large power transformer that didn't have some type of grounding, you could have massive circulating currents in the core and other parts where current was not designed to flow (this could also happen if it is improperly grounded): The ground is used to stabilize the system (though you can have ungrounded systems so long as the relays monitoring the presence of ground faults are set to operate on a very slight disturbances, or so I'm told). There are some large transformers that have three and four windings to eliminate as much as possible these zero sequence currents (along with other reasons).

    Hope this helps...I'm still not sure I completely understand your question.
     
    Last edited: Jul 14, 2015
  23. Jul 14, 2015 #22
    On large Power Transformers, there is also a grounding (Wye) tertiary winding usually included on the Delta side of the transformer that has a 1:1 ratio and is connected to ground.
     
  24. Jul 14, 2015 #23
    There is, and it is, itself, and entire profession. :wink:
     
  25. Jul 15, 2015 #24

    psparky

    User Avatar
    Gold Member

    The neutral of the secondary is grounded just like the neutral is grounded in a house panel. The only thing going thru here is perhaps a small trickle current, a perhaps a ground fault or things of that nature.

    When the secondary of transformer gets disconnected, the primary sees an open circuit and NO power flows.
    Why, because the neutral of the transformer has a potential of zero volts....and the earth ground has the potential of zero volts. Zero minus zero is.......Zero. Therefore, according to V=IR, Zero current flows.

    Hypothetically speaking, read the voltage in your house receptacles with a meter. Hot to neutral or hot to ground you get 120 volts. Now measure the voltage from neutral to ground. You will get zero volts. Same thing for 3 phase panel, neutral to ground is zero volts.....same thing for said transformer above.

    To answer the question about what happens when hot wire touches ground.....again, we'll use our friendly V=IR. The current will simply be the supply voltage divided by the resistance of the ground back to the neutral of the panel, or the neutral of the secondary of its transformer. Or a combination of both would be even more realistic. Both of these neutrals are obviously tied to ground.
     
    Last edited: Jul 15, 2015
  26. Aug 6, 2015 #25

    dbc

    User Avatar

    K. I guess I must be misunderstanding something. This is how I've always visualized how high voltage (power line) transformers (7 kV, 36 kV, 500 kV, etc...) are hooked up. Am I wrong?


    physicsImage1.jpg
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook