What happens if "hot" wire touches Earth ground?

[dmtyper]

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?

 

[jim hardy]

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

energy
work
power

charge
potential
potential differencecurrent

and learn the units of each

before you become totally confused.

 

[jim hardy]

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.

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.

 

[Nugatory]

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)

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.

 

[jim hardy]

a compilation of electrical explosions

 

[dmtyper]

Thanks very much for the replies !

 

[berkeman]

a compilation of electrical explosions

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...

 

[jim hardy]

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...

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.

 

[dlgoff]

They thought better of it and drove right on by.

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.

That and ...

http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-step-and-touch-potential.php

 

[DrClaude]

Country wisdom : never pee on an electric fence.

City wisdom: never pee on the third rail

 

[jim hardy]

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

Thanks, guys !

old jim

 

[zoki85]

City wisdom: never pee on the third rail

Hehe.

 

[jim hardy]

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 !

 

[zoki85]

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 !

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...

 

[jim hardy]

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.

 

[zoki85]

I've heard of success washing insulators on transmission lines. Our trial on a transformer didn't work out well.

System was the same used for washing insulators on transmission lines? Maybe it was used in a wrong way...

 

[jim hardy]

System was the same used for washing insulators on transmission lines? Maybe it was used in a wrong way...

Maybe. It was one of the earliest offerings. I see it's still done a lot of places.

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:

Our failure was on a transformer with vertical insulators like these.

 

[rollingstein]

Not a thing in the world they could do until the power was turned off...

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?

 

[dbc]

k... now I am 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 don't 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?

 

[rollingstein]

@dbc

How so? On a delta-star AC transformer (assume you mean step down) the neutral grounding is on the secondary not primary side.

 

[EverGreen1231]

k... now I am 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 don't 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?

(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.

 

[EverGreen1231]

@dbc

How so? On a delta-star AC transformer (assume you mean step down) the neutral grounding is on the secondary not primary side.

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.

 

[EverGreen1231]

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?

There is, and it is, itself, and entire profession.

 

[psparky]

k... now I am 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 don't 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?

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.

 

[dbc]

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?

 

[davenn]

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?

you missed the connection from the other side of the generator ... as shown, the circuit is incomplete

 

[dbc]

you missed the connection from the other side of the generator ... as shown, the circuit is incomplete

transmission circuits don't have a return (neutral) wire. so it must go to ground.

also... neutral wires on distribution circuits are connected to ground at poles so it never makes it back to the substation.

in either case, the circuit never makes it back to the source.

nevertheless, is this acknowlegement of the fact that there is a constant flow of electricity since the circuit is complete regardless of whether the secondary is hooked up?

 

[davenn]

transmission circuits don't have a return (neutral) wire. so it must go to ground.

also... neutral wires on distribution circuits are connected to ground at poles so it never makes it back to the substation.

dbc, I know how its supposed to be connected !, just making sure you did/do

...in either case, the circuit never makes it back to the source.

that's incorrect, of course it does. The Earth return IS part of the circuit

it pays to show everything, else misunderstandings ariseDave

 

[davenn]

nevertheless, is this acknowlegement of the fact that there is a constant flow of electricity since the circuit is complete regardless of whether the secondary is hooked up?

a constant flow would be indicative of DC. You pointed out in your diag. text that it's AC and is oscillating, correct.
keep in mind that the electrons in the wire in the coil of the generator never get to your house and likewise the ones
in the cable in your house, never get to the generator at the power station. They are just osc. back and foreward in situ

Yes, there is still current flowing in the primary, even when the secondary is open circuit ( no load connected)

some years ago in this thread ...
https://www.physicsforums.com/threads/transformer-secondary-coil-question.187441/

Claude made a couple of excellent posts ...

There are 4 laws at work simultaneously. All of them must apply, namely, Ohm's law, Ampere's law, Faraday's law, and the conservation of energy law.

A transformer cannot output more power than inputted, due to CEL (conservation of energy law). Right off the bat, it should be obvious that the secondary power, which is Isec*Vsec must equal the primary power, Ipri*Vpri, minus losses.

Faraday's law, FL, relates the magnetic flux density, B, to the voltage, V, and the frequency, f. Ampere's law, AL, relates the magnetic field intensity, H, to the current, I. Also, B and H are interrelated through the permeability mu, which is analogous to Ohm's law (for magnetics). Ohm's law, OL, relate the current and voltage at the secondary with the load resistance.

Most transformers are driven at their primary by an independent power source which is generally a "constant voltage" source. Current transformers can be discussed later. The voltage source magnitude, V, frequency, f, and primary turns number, Np, and the core area, A, determine the magnetic flux density, B, per FL. This magnetic flux almost completely links, or couples into the secondary winding. Since the core area is the same, as well as the frequency, only the number of turns differs, Ns. Just as FL describes the relation between Vp, f, A, and Np, it holds equally for Vs, f, A, and Ns. Again, f and A don't change, so that the ratio of volts to turns cannot change. Hence Vp/Np = Vs/Ns, since B, f, & A remain constant.

When current is drawn by loading the secondary, a magnetomotive force, mmf, occurs, which tends to counter the existing core flux, which tends to reduce the voltage. But, by definition, the primary power source is a constant voltage type, which will supply whatever current needed to maintain a fixed voltage value. The primary current increases to a value needed to maintain the core flux. AL describes the relation. For a given secondary current, Is, and Ns, a magnetic field intensity, H, is given by AL. In the primary an equal and opposite H, or mmf if you will, must exist. Since H is almost equal in the primary and secondary, Np*Ip = Ns*Is.

Since the volt per turn RATIO must be the same on each side, the winding with higher turns has a higher voltage, or emf. Since the amp-turns PRODUCT must be the same on both sides, the winding with the higher turns has the lower current, or mmf.

It can't be any other way, as the CEL would be breached. Does this explanation make it clear?

and ...

Because the primary has a constant voltage source, CVS, connected across its terminals. Remember the definition of a CVS. It will output any current neded to sustain a fixed voltage value. When the secondary is open, and a CVS is connected to the primary, a small current flows, called exciting current. The B-H curve of the core material, and the CVS value of voltage determine the magnetizing portion, Imag, of the exciting current. Hystersis and eddy current losses determine the loss portion of the exciting current. FL describes the relation between V, f, N, A, and B. For a transformer whose core has area A, and primary turns Np, the flux density B will be determined by the voltage and frequency of the CVS. AL describes the relation between magnetizing current Imag, primary turns Np, magnetic path length including gap(s) lc, and magnetic field intensity H.

This B value links the secondary almost entirely. For a given B, f, A, and Ns, Vs is determined. Just as a fixed voltage determines the flux density value, a fixed flux density determines the induced voltage value. FL is bi-directional. Likewise with AL.

When a load resistance is connected across the secondary, Is is induced in accordance with OL, Is=Vs/Rs. This current Is is accompanied by an mmf which counters the original emf, known as Lenz' law, or LL. This mmf would tend to reduce the flux density B, and the field intensity H. But remember that the primary flux (& secondary since the coupling is very good) is determined by the value of the CVS. A CVS forces a specific voltage value, and consequently a specific B & H value. When the primary voltage Vp is countered by the mmf of the secondary load, the CVS outputs a larger current whose mmf opposes the secondary mmf. In other words, just as Is and its mmf tend to reduce the flux and emf, Ip does just the opposite and restores them. Otherwise power would diminish greatly as soon as a load is connected.

Does this make things clear? BR.

Claude

I wonder if those help ?Dave

 

[dbc]

this confirms my suspicions. thank you.

most people never realize this until the see both the abstract and physical versions of electrical construction, and it is never actually mentioned point blank in physics texts, but AC primary sources are actually connected directly to ground and is constantly flowing.

coming from a background of an electrician to lineman to potential engineer... this at first seemed ludicrous. all my life I've been dealing with power on a source > breaker > ground relationship. the fact that something operates on a source > ground basis and doesn't cause an explosion somewhere was hard for me to get my hands on.

taking all this into consideration now... I'm curious now how many amps are flowing in the transmission lines when there is no load connected at the substation (secondary is open circuit). what's actually going on at the power station? do the generates go into some idle state?