Confused about simple electrical safety concept (pics)

[Blaze]

I think the answer is :

I humbly disagree with all the answers given above and infact also with the question slightly. In my opinion the following is the answer :

In picture 1 there are two parallel paths to ground after the resistance element :

1: Through the wire which is very low resistance path to ground
2: Through the man which is comparitively a very high resistance path to ground

So, when given two paths, the maximum current flows through the lest resistance path according to Ohm's Law which states that V=I*R, hence I is inversely proportional to R for a constant V.

Hence the maximum current flows through the wire and a very small part of the current flows through the man.Hence in picture 1, very little current flows through the man.So NO SHOCK!

Now in picture 2, Only and if only we assume that the resistance connected in the circuit is as high as or much higher than human body resistance, a considerable part of current flows through the man giving him SHOCK.

Now coming to the Bird, I agree with the previous answer which says there must be a potential difference for current flow or atleast a very small arcing distance.Since no current flows there is no SHOCK for the birds.

Here the underlying fact is that air has a very high resistance and the bird is connected between two points of same potential in the wire.So NO closed circuit, NO potential difference and NO shock.

Hope I was clear in presenting my thoughts......Please give your feedback or correct me if Im wrong.

By the way I just joined the Forum.Jus wanted to say HI to all my friends out there.

 

[Someone Electro]

I seen an video of guy trying to make an suicide by toching the electrcal lines and he climed all the way to the top and wen he tohed the high voltage lines (it arced is the proper thing to say) kabom...adn big firebal formed and he falen down.amazinly he SURVIED!!! :shock:

I aslo seen an bear climb up an electic lines pole and he tryed to lay on the insulator and it stared to shake him around and he jumped of it and he almost falen down.The firemen saved him in the end.

 

[bonxer]

luckuly it switced fast enugh so it only fryed a fuse in the domophone

I was wondering what a domophone was. A quick google image search turned up the following. I totally want one.

 

[andrew2022]

I seen an video of guy trying to make an suicide by toching the electrcal lines and he climed all the way to the top and wen he tohed the high voltage lines (it arced is the proper thing to say) kabom...adn big firebal formed and he falen down.amazinly he SURVIED!!! :shock:

i can think of less painful was to kill myself.

Now with RCDs (residual current device), if you are not grounded they will not trip.

Whilst we're on the topic of RCDs (GFCI's for the yanks), can anybody explain to me why bridging the neutral (line and load) terminals on an RCD, causes it to NOT trip, with or without an earth fault.

I have done it a number of times, always works, but no-one can explain it to me!

shorting the neutral on the in and out should trip the RCD, if there is a load on the output. if it doesnt, there could be a a fault with the RCD. same with live. it should trip, because the resistance will be lower accross where you shorted it, so some current will flow that way, so it detects an imbalance and trip. shorting neutral (load) to earth should also trip an RCD (as ive found out the hard way many times). obviously, touchin live will trip it since there is a potential and current can flow. touchin live and neutral on the output side will NOT trip an RCD because it sees it as a load, not a fault. thats what the MCB/fuse is for

 

[samcheetah]

I humbly disagree with all the answers given above and infact also with the question slightly. In my opinion the following is the answer :

In picture 1 there are two parallel paths to ground after the resistance element :

1: Through the wire which is very low resistance path to ground
2: Through the man which is comparitively a very high resistance path to ground

So, when given two paths, the maximum current flows through the lest resistance path according to ohm's Law which states that V=I*R, hence I is inversely proportional to R for a constant V.

look you have said yourself that

I agree with the previous answer which says there must be a potential difference for current flow or atleast a very small arcing distance

so there will be no flow of current in the first pic because there is no potential difference.

ill explain this to you in simple words. a man named "current" has to choose between two paths to get to his home. one path has the greater resistance and the other has a lower resistance. but if the man named "current" himself is absent there is no question of what path he will choose. the reason he isnt there is because a guy named Ohm has said that for the current to flow there should be a potential difference.

so its simple. no potential difference, no flow of charges, no current, no chosing between two paths and eventually no shock effect

i hope that helps

 

[Blaze]

Absolutly not! I strongly disagree.

There IS a closed circuit and also a potential difference in both the parallel paths i.e. the wire and the man.I will tell you how

The ground is always a node of zero potential. The point in the circuit where the man is touching indeed has a potential of V(source) - (I*R) according to Kirchoff's Voltage law which certainly has some value. Here the (I*R) is the voltage dissipated in the resistor and V(source) is the source or battery voltage. The current I definetly flows in the circuit since there is a potential diffrence of [ V(source) - (I*R)] - 0 (Point of contact to ground). It is just that it has to choose between the two parallel paths.

 

[Phasor]

shorting the neutral on the in and out should trip the RCD, if there is a load on the output. if it doesnt, there could be a a fault with the RCD. same with live. it should trip, because the resistance will be lower accross where you shorted it, so some current will flow that way, so it detects an imbalance and trip.

Yes, this is why we can't understand it!! It should trip, because there is an alternative path for the neutral return current, but it doesn't. We do it regularly, and it works EVERY time.

 

[samcheetah]

Absolutly not! I strongly disagree.

There IS a closed circuit and also a potential difference in both the parallel paths i.e. the wire and the man.I will tell you how

The ground is always a node of zero potential. The point in the circuit where the man is touching indeed has a potential of V(source) - (I*R) according to Kirchoff's Voltage law which certainly has some value. Here the (I*R) is the voltage dissipated in the resistor and V(source) is the source or battery voltage. The current I definetly flows in the circuit since there is a potential diffrence of [ V(source) - (I*R)] - 0 (Point of contact to ground). It is just that it has to choose between the two parallel paths.

look ill try to explain it to you. in fig 1 the hand of the man is touching the ground node right???? and his foot is also touching the ground node right????? now the difference of potential between his hand and his foot is 0-0 = 0 volts

so there is no potential difference. and hence no flow of current.

and one thing more your kirchoff's voltage calculations are wrong.

i would recommend that you get a good book on electric circuit theory

 

[andrew2022]

shorting the neutral on the in and out should trip the RCD, if there is a load on the output. if it doesnt, there could be a a fault with the RCD. same with live. it should trip, because the resistance will be lower accross where you shorted it, so some current will flow that way, so it detects an imbalance and trip.

Yes, this is why we can't understand it!! It should trip, because there is an alternative path for the neutral return current, but it doesn't. We do it regularly, and it works EVERY time.

you should get a proper RCD tester to make sure it is functioning properly. does the test button make it trip? could you get a hold of another RCD to swap them and try that? you say you do it regularly and it works. what do you mean by works?

also, can i ask why your bypassing the neutral on the RCD?

 

[spuffock]

Said before, in the RCD is a current transformer. It is a ring of iron tape. It has 3 windings, live supply to live load, 1turn, neutral supply to neutral load, 1 turn, and sense, a couple dozen turns. A current through a load flows through both 1 turn windings and the fields cancel. Nothing in the sense winding. A current to ground only flows in the live winding, and is not cancelled. Current is sensed, trip goes. If you ground the neutral load side there will be current due to the voltage drop in the neutral line, this current is only in the neutral winding,and the trip goes. If you short the neutral supply and load together, you put a shorted turn on the transformer. The current induced in the neutral winding will oppose the fault current, and the trip sensitivity will be reduced a LOT. The same applies if you short live supply and load together.

 

[nos_slived]

Back in the 1960s, Life magazine ran a two-panel photograph. The first panel showed a long row of little birds on an electric wire, all huddled together because of the cold weather, little feathers fluffed out. The next picture showed the end bird that reached over and found a nearby ground, sending the entire line-up to birdie heaven as the entire strip, to a bird, ended up hanging upside-down on the wire, little feet clenched in death.

The birds have no problem as long as there's not a complete circuit. But add that path and ......

Dean

I saw that happen to a bird a few years ago. It was on a lamp post(ground) and moved it's leg to the wire to get on. All you could see after that was a crow go flying through the air.