# doubt in domestic current

Discussion in 'Homework Help' started by bhuvanesh, Mar 4, 2015.

1. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
i have two question

1)voltage coming from our home wall socket is sine wave or sinusoidal wave.my confusion is we use vrms to get it average since average of sin wave is zero that same if there is phase shift it may become cos wave so there is chance for we can call it as sinusoidal wave isnt it?

2) For current to get through ,the ground is need.so if i touch wall socket current with slipper.does current pass through me or if i hang the power wire that going on post(space) without my leg touching ground ,does the current pass through?

Jul 18, 2013
10,813
2,491
Sine wave (sinusoidal in shape).
You do not need a ground to pass any current, the earth ground is there as a safety measure and should not be used as a circuit conductor.
If you have a grounded system, if coming in contact with a live conductor and earth ground you will receive an electrical shock.
Max.

bhuvanesh likes this.
3. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
RMS and Average are NOT the same, so we don't use Vrms to get an average.

cosine and sine are the same except for a 90° phase shift. An absolute phase only has meaning if you have an absolute time reference. Since you don't have that for the voltage at your home wall socket, the distinction between sine and cosine is without meaning.

In general, a ground is NOT needed. You need a return path from the line (hot) wire back to the source (generally a transformer nearby). That's the other wire, usually called the neutral. That may or may not be grounded. In either case, if you just touch the system at one point and otherwise remain electrically isolated from everything else, then no appreciable current will flow through you.

Last edited by a moderator: Mar 8, 2015
bhuvanesh likes this.
4. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
so if i float in air by touching phase alone .do current pass through?

Jul 18, 2013
10,813
2,491
No, but send a pic of you doing it!
Max.

bhuvanesh likes this.
6. ### sheldons Well-Known Member

Oct 26, 2011
616
101
A short video of you in contact between phase and earth or neutral would be good (single phase 240v ac supply) but phase to phase (on a 3 phase 415v ac supply) would be much better to watch......

bhuvanesh likes this.
7. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
Remote high voltage power lines are often inspected by someone being deposited onto the line by a helicopter. The helicopter first makes contact with the line using a probe to place the helicopter at the same potential as the power line. There is a small transient surge of current when this happens, mostly due to the static charge on the helicopter owing to the rotor blades "rubbing" the air. Then the person climbs out onto the line (which are generally bundled conductors so they straddle the two conductors) and then they scoot along the line inspecting it. When it's time to get picked up, the process is repeated.

bhuvanesh likes this.

Jul 18, 2013
10,813
2,491
NEC on Grounding.
Max.

• ###### NEC-grounding-MREC2010.pdf
File size:
2.6 MB
Views:
18
bhuvanesh likes this.
9. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,130
766
Ever see birds sat on a power cable in the sky, wondered why they don't get electrocuted?

bhuvanesh likes this.
10. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
because its feet have high resistance,right?

11. ### MrChips Moderator

Oct 2, 2009
12,622
3,451
That is not the correct answer.

bhuvanesh likes this.
12. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
What would the current path be in order for the bird to get electrocuted? Where would the current enter the bird's body and where would it exit? What would it take to develop the kind of voltage that would be needed between the entry and exit points?

bhuvanesh likes this.
13. ### darrough Member

Jan 18, 2015
86
19
bhuvanesh, I have read some of your questions and it seems like you are really having a hard time. Could I ask what book you are working from? Could I ask what level of math you have completed? I suspect that you are either missing a lot due to poor english or the book requires more math than you have. I think you would make more progress if you got a book in your native language or one where the math is less demanding.

bhuvanesh likes this.
14. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
yes i am having hard time .yes i am missing lot because of poor english.i can understand english but sometimes the people reply with how jack sparrow speak in pirates of carrebean movie ,that time i feel like i am missing the knowledge of respective members reply.Anyhow my question may be silly,dump,whatever may be but i dont quit,and i am thinking ,yet it is not too late to learn all things,Thanks for bearing with me

current need closed path
1)if i hold the electric wire as same as bird does ,without any connection with land(like i am isolated in space and hands alone touching the wire).In this case there is only entry path and no exit path so no current pass through my body,right?
2)if the same bird place one leg on electric wire and other leg on nearest building.sure it is electrocuted since building is grounded isn't it?

15. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
You are getting it. Birds and other critters, such as squirrels, sometimes get electrocuted when they step onto or off of a wire onto the tower arm.

The point I was trying to steer you toward was that the bird is generally standing on the wire with two points of contact -- the left leg and the right leg. Similarly, if you are hanging from the wire using both hands, then you have two points of contact -- the left hand and the right hand. So you do have both an entry point and an exit point. Yet you don't get electrocuted. Why not?

This is related to what is known as the "step potential", which is literally the potential difference between one foot and the other when you take a step on electrified ground (such as ground that has high currents flowing in it due to a lightening strike).

bhuvanesh likes this.
16. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
1) No exit path i have only entry path.you question('So you do have both an entry point and an exit point') seems to me like connecting two ends of wire to positive terminal of battery alone.
i try to state myself why current not passing even though we hold wire with two hands ,correct me if i am wrong or give me if you have better answer to steer my mind to right way
The current does not pass because random moment of electron is not current ,the movement, of electron in particular direction is current .if i hold my two hands electron move randomly but dont where to end up, so no current ,as soon as i hit the ground it will get directional guidance to ground so electrons flow towards the ground through my body and cause current
what i have been talking above is Touch potential
Now Step potential
step potential is something that current flow in ground for around ten meters,i have seen guidance for people who stuck in such place not to have normal walk(not long step walk),have jump with two legs of get out of the current zone.why do they say if we place one foot far away from other cause electric current through body and that not occur when we place our two legs close and move.

17. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
You're getting there.

Let's say that you have a resistor that has some current flowing in it and you grab one side of the resistor with one hand on the other side of the resistor with the other hand.

Now let's say that the resistor is 10Ω and that there is 100A of current flowing in it. What voltage are you placing your hands across?

Let's instead say that the resistor is 0.01Ω and that there is 1000A flowing in it. What voltage are you placing your hands across?

Any normal conductor can be modeled as a whole bunch of resistors in series. When you hang from a power line you are putting your hands across a small valued resistor with a fairly large current in it. When you are standing on ground that has just had a lightening strike, your feet are across a fairly large resistor with a huge amount of current flowing through it.

bhuvanesh likes this.
18. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
I have two things on my mind say which is right sire

1) while holding electric wire current no current passes through us because we are isolated In space
2) compared to wire our body have high resistance so current taking least resistance path .so passing only through wire

Which among the two exactly make sense

19. ### WBahn Moderator

Mar 31, 2012
18,064
4,904
Both are more or less true, with the "less true" being largely in the fine print.

If you are touching any electrical source at more than one point then there is the possibility for current to flow through your body by entering one point and leaving another. The degree to which this happens depends on how much voltage there is between the various points of contact and the impedance of the electrical paths between those points (i.e., Ohm's Law).

The "current takes the path of least resistance" is fine as a simple guide, but don't take it as an absolute. If I have two parallel paths and one has 1000Ω of resistance and the other has 1001Ω of resistance, it is completely wrong to conclude that all of the current is going to go through the 1000Ω resistor because it has the "least resistance". There will be current flowing in all parallel paths, but the greatest share of the current will be in the path with the smallest resistance and the smallest share will be in the path with the greatest resistance. That is true for the example above and the 1000Ω resistor will have more current than the 1001Ω resistor, but the difference between the currents will be very slight.

This is important to keep in mind because if you are in contact with a conductor carrying 1000A, and considering that you need to keep the current flowing through you to under 10mA to be reasonably safe, it is far from sufficient just to have the resistance of the wire between where you are grabbing to be less than the resistance through your body. It will have to be 100,000 times smaller. If it's only 10,000 times smaller, then you have a serious safety risk.

When I was working at NIST we had our high current bus bars mounted on the wall and they carried up to 4000A to our samples. But you could (and I did on several occasions) walk up to them and put your hands right across them. The only thing it would do was inject noise into the measurement (which is why I would do it as a means of verifying that we were still connected to the sample which was inaccessible down in a superconducting magnet submersed in liquid helium).

bhuvanesh likes this.
20. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
No or less current because my body resistance is higher than that, May i know what is the resistance of body,approximately

my body resistance is low compared to ground so current passes through me considerably