Understanding Earth/Shock

[Gigsy]

Hi I am studying to become an electrician.

There are a few things I do not fully understand regarding shocks and earth and I cannot find the answers to some of my questions.

Can anyone help me understand the following points regarding a standard TN system.

1) If you are in a loft or a 2nd floor, how can you get a shock from touching a live conductor, does the building structure ground you? if so surly the shock cannot be that bad, surly the resistance of the building should be really high. How does the electricity find a path to earth? The building is wood and brick. Wood and brick have a high resistance. This makes no sense to me.



2) If I stood on a rubber mat (or a different high resistance insulator) can I then touch a live conductor, without getting a shock?



3) Even when you are just standing on the ground, how can there be enough current passing through your body to harm you if you touch a live conductor. I mean The resistance of the earth is reasonably high, the current would have to go through your body then through the earth all the way back to the transformer. Surly not much current would flow. If I touched a light bulb to the line conductor and attached a conductor to the negative of the light bulb, then touched the other end of the conductor to earth, would the bulb light up?

 

[Sparkie30]

Gigsy surely you dont want to experience an electric shock because it is very Bad.

Good luck with your studies pal

 

[Richard Burns]

1) if you touch a live conductor then any chance earthing that may exist in contact with you e.g. an earthed metal pipe could present a path to earth and generate enough current to kill. Mainly the resistance path to earth is reasonably high and you only get a minor shock, say less than a couple of mA, not nice but not deadly.
Even wet surfaces in contact with an earthed part may be enough of a path for the shock to hurt.

2) if you are totally insulated from earth (and neutral) then touching the line conductor would not cause a shock. I would never test this in practice as it could be very dangerous if there were an unanticipated path to earth.
Your body would rise to the line potential but there would be no current flow.

3) If you are in contact with ground even though the resistance may be in the thousands of ohms range there will still be enough current flowing to potentially cause death, the effect is made worse by skin contact with the ground and water on the body and / or ground as this reduces the resistance of the earth path.
Even a current of 50mA can kill and this (at a 230V supply) would be generated with a resistance of 4600Ω, well with in the range of earth contact resistance.
A light bulb would require much more current to cause the filament to glow and so (unless there is a low resistance path to earth) it would not light up, an LED lamp might experience enough current to glow. Again do not try connecting a line conductor to earth for safety's sake.

 

[Gigsy]

well wikipedia says resistance of the soil is between 10 - 1000 ohms per meter, so lets say 500 ohms, smack in the middle.

Say the transformer is 100 meters away. Ohms law current = volts / resistance,
so 500 ohms x 100 meters = 50,000 ohms.

I = 230 / 50,000 = 0.0046A
0.0046 amps is not much

i just dont get it
you could not even light up an LED with 0.0046A, LED's need 0.020 amps to light.

There is something i am not getting

 

[Gigsy]

If i am on the 2nd floor or in a loft, how can I get much of a shock? The resistance path is through my body, through the building structure then through the earth to the transformer. That is a massive resistance

 

[Gigsy]

Is the part I am not getting is that the current flow uses metal underground pipes to get back to the transformer?

 

[Pat H]

If i am on the 2nd floor or in a loft, how can I get much of a shock? The resistance path is through my body, through the building structure then through the earth to the transformer. That is a massive resistance

How can you be sure no part of the loft is earthed already?
No pipework there? no wiring that is already earthed?
No aerial feed for the TV?
Lots of potential earths around and any damp can help lower the resistance.

Also even if completely isolated touching a high voltage will have some current flow as the potential of your body rises to that of the live conductor. And again a small current when you then touch ground as that potential is released. very small currents but still a current will flow. Think of the human body as a large capacitor.

 

[telectrix]

basically, a 10mA current through you is usually safe.so, yourbody resistance is around 1k ohms, so if the resistance from your body to earth is >22 kohms, you'd survive.

I = V/R

I =230/23000 = 0.01 = 10mA.

higher than that.it's betfred.

bear in mind thebird on a wire. the wire may be 33 kV. the bird is also at 33kV. he sitting pretty, thinking about his next shag. if said bird was earthed , he'd be toast. so bye bye shag.

 

[Gigsy]

you say the resistance between your body and earth. But the current still has to get back to the transformer. Why have you not factored in the resistance of the earth to the transformer?

 

[Gigsy]

How can you be sure no part of the loft is earthed already?
No pipework there? no wiring that is already earthed?
No aerial feed for the TV?
Lots of potential earths around and any damp can help lower the resistance.

Also even if completely isolated touching a high voltage will have some current flow as the potential of your body rises to that of the live conductor. And again a small current when you then touch ground as that potential is released. very small currents but still a current will flow. Think of the human body as a large capacitor.

So it is the various different paths back to the transformer that I am not factoring in?

 

[snowhead]

well wikipedia says resistance of the soil is between 10 - 1000 ohms per meter, so lets say 500 ohms, smack in the middle.

Say the transformer is 100 meters away. Ohms law current = volts / resistance,
so 500 ohms x 100 meters = 50,000 ohms.

There is something i am not getting

The current won't flow through the ground all the way back to the TX it will always find the path of least resistance.

The current may flow through the Ground first but then when it finds a better earth eg consumers earth rod, underground metal pipework that's bonded inside a house or one of the thousands of suppliers earths not located at the transformer it'll flow more easily.

If what you are thinking was true there wouldn't be as many deaths due to electrocution as there is.

 

[Deleted member 26818]

One of the problems is, that you don't need a high current to kill.
6mA across the heart is enough to stop it.
If you are stood outside on some damp grass and were to pick up the wet severed end of a live cable, the current would not be enough to trip the breaker or fuse, but it would be enough to kill after a short period of time.

 

[Richard Burns]

The resistance back to the transformer through the earth can cover a very large volume and so is not just based on the one square meter of soil you are considering. The pinch point is the contact area with earth, thereafter the effective resistance is very low because of the large volume through which the current can pass and any conductive paths within the earth.

If you consider an earth rod may provide a resistance of 100 ohms from 1.2m rod the sphere of influence of that rod is very large.

 

[Gigsy]

The current won't flow through the ground all the way back to the TX it will always find the path of least resistance.

The current may flow through the Ground first but then when it finds a better earth eg consumers earth rod, underground metal pipework that's bonded inside a house or one of the thousands of suppliers earths not located at the transformer it'll flow more easily.

If what you are thinking was true there wouldn't be as many deaths due to electrocution as there is.

Thank you.

I do understand that people do get killed by electricity, I just didn't understand why.

So is it because we our systems that causes the lethal situation?

 

[Richard Burns]

Originally it was expected that electricity would not flow through the earth as they were all insulated from earth, but then in practice it was found that there were always partial accidental earth paths present to cause electric shock. Therefore the design of electrical systems was modified to provide very good earthing and a means of disconnecting the fault quickly to avoid these problems, this has developed to the system we have today.