Understanding Earth/Shock

[Gigsy]

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.

I just do not understand


I kind of understand, I think, The larger the CSA of a conductor the less the resistance.


So are you saying that I have to treat the entire earth as its CSA?


If so then it all makes slightly more sense to me.

 

[FatAlan]

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.

Is that the sea bird Shag?

 

[D Skelton]

1 & 3 have been answered for you Gigsy, but in answer to number 2; in theory no, but in practice, expect a nice belt as that live conductor treats your body as a rather hefty capacitor.

One example of this you can see happening in real life. Head on to YouTube and watch videos of helicopter linesmen in the US mounting high voltage power cables wearing faraday cages. They still have to attach a discharge protector before they mount the cable from the helicopter despite the fact that there is no potential difference (atmospheric discharge is also at play here as well as capacitance).

This question puzzled me very early on in my career when I was stood on a fibreglass ladder wearing trainers with insulated soles and still got a belt from a live conductor (it was the only lesson I ever needed to take safe isolation seriously!).

Theory is all very well, but you need to understand that the theory you get taught at college and read in textbooks is about as basic as it comes. In practice, it goes out the window, because things are much more complex than initially thought.

As for your guesswork regarding the resistance of earth per meter, you haven't factored in to the equation that at any point you're more often than not mere feet away from a vast network of underground metallic structures all interconnected and all ultimately leading back to the point of origin.

When you get a shock in the loft and all you're standing on is a wooden beam? Well also attached to that wooden beam a few inches from your feet remember is bonded metal pipework, or the earthed metallic conduit of a switch drop.

To summarise:

Make sure the bloody circuit is dead!

 

[Darkwood]

There is a big flaw in many calculations here, to realise these results the resistance would have to be maintained throughout the shock but in reality once the electricity has breached the bodies resistance you end up with a crash of resistive values of the skin, couple this with the fact that if the current flow travels through the heart then even 1mA can trigger fibrillation and kill you.

Using maths to work out a survival chance is very crude considering the variables and the concept that electricity flow will break down the resistance of the body as it flows, so with all respect to standard mains voltage, you have to realise that regardless of whether your in a loft sat on the wooden rafter or stood on the actual earth it does not mean you will survive one and not the other, the many variables including duration, voltage, current path, your health, humidity etc etc all play a vital role.