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Greetings.
I have a question, this has been puzzling me for a while.
If a put an earth rod in the ground and use it as my main earthing method and measure the resistance I find it to be 160 Ohms.
Ok, Assume there are no RCD's and a link is made between the phase/line and earth at the distribution board, a fault current would flow of 230/160 = 1.44Amps.
This would not blow any circuit breakers and effectively this fault current would flow to earth indefinitely.
Now assume the building is equipotentially bonded and so all metalwork is bonded together. Disregard parallel paths, assume we have plastic pipes for water and gas.
What sort of voltage would I get if I touched the metalwork in the house assuming my feet were wet and placed on the ground?
Is it a simple or complex mathematical equation?
The reason I am asking is because my friend lives in Asia and has an earth rod with no RCD protection at all and I am trying to work out the worse case scenario.
Any thoughts would be appreciated.
Thanks.
 
Thanks.
How about within the equipotential zone?
I mean if all the metalwork within his property is connected together through bonding etc, and a fault occurs then all the metalwork will become live.
The only thing stopping a current from flowing through his body would be the resistance of say the floor he is standing on and his shoes, bear in mind sometimes in hot climates people don't wear shoes.
Now considering this guy lives on a farm in a third world country the floor could be made of anything, he might not even have a floor in some of the buildings so is it fair to say that even within the equipotential zone there is a strong possibilty of him receiving a 230V shock if an earth fault develops?
Is there anything wrong with my logic?
Thanks again.
 
Now considering this guy lives on a farm in a third world country the floor could be made of anything, he might not even have a floor in some of the buildings so is it fair to say that even within the equipotential zone there is a strong possibilty of him receiving a 230V shock if an earth fault develops?
Is there anything wrong with my logic?
Thanks again.

If the floor is the general mass of earth,has a equipotetial zone been created?

is there is a strong possibilty of him receiving a 230V

Indeed
 
The calculation is simple ohms law, inside an equipotential zone created by a full bonding regime the Touch voltage Ut = Fault current If x R2 Resistance of the CPC from the point of fault to the MET. The result though is academic because, as you have alluded to in the question, in a TT system if the required fast disconnection times of the current regulations are to be achieved it is almost impossible to do this without the use of RCDs due to the variable low earth fault current due to soil conditions and other external factors. However, because the earth fault current is low (less that 10A) and the R2 (less that 2 Ohms) the Touch voltage is below 20V in most practical situations. This why in earlier versions of the wiring regulations RCDs were not required if the installation was FULLY earth bonded and the maximum Touch Voltage could not rise above 50V.
 
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However, because the earth fault current is low (less that 10A) and the R2 (less that 2 Ohms) the Touch voltage is below 20V in most practical situations.
I'm getting confused now.
R2 is the resistance of the earth cable back to the MET which is an earth spike. (I think I am right in saying this)
In my original post I was using the figure of 160 Ohms as the earth fault loop impedance back to the transformer earth (presuming the transformer has an earth, this is Thailand)
So when working out the touch voltage we don't use the earth fault loop impedance, we use the resistance of the cable that leads back to the MET.
Is this correct, is this what you are saying? Because this is where I am getting confused.
Also to create a fully equipotential zone within this guys property would not be practical, he would haveto lay new insulated floors and everything. They don't have the cash.
If anyone could recommend some links for further reading I would greatly appreciate it, this touch voltage using earth spikes is something I really need to study more about.
Thanks.
 
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Creating a full equipotential zone is extremely difficult to achieve and maintain, even in our country and is the reason that the regulations were changed and why we now use RCD protection almost exclusively in TT installations. In all other circumstances you don't have an equipotential zone and therefore the touch voltage will approach the supply voltage and calculation or measurement becomes irrelevant.
 
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In an equipotential zone its as mike said Ut = If R2

Now without bonding it would be Ut= If(Ra + Rb + R2)

So, RA = 160 ohms Rb(transformer electrode) 21 ohms R2 = 1ohm

Voltage 230v, so if = 230/183 = 1.26 amps

So no bonding ut = 1.26x(160 +21+1) = 230v, the same as outside the equipotential zone.

Bonding connected = 1.26 X 1= 1.26v.

So in essence at the fault(exposed conductive part) we have 230v
at the met we have dropped a small amount of voltage across the cpc so the potential difference between the exposed and extraneous conductive parts connected to the met are relativity low.
 
A human body has a variable resistance depending on a number of factors but a figure of 1000Ω is often quoted and this rises depending on footwear etc. If metalwork becomes live then I = V/R
=230?/(160Ω + 1000Ω) = approx 20mA

20mA current could induce a hold on response.
 
my calc. is based on the 160ohms ( rod ) and the 1000 ohms ( body ) being in parallel. this would result in a current through the rod of 1.43A, and a current through the body of 0.24A, ( using ohms law and kirchoff's ). if you can find fault with that, please do so. this is just my take on it, might be wrong.
 
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If you can find fault with that, please do so.
This would result in a current through the rod of 1.43ohms, and a current through the body of 0.24ohms,
Current is not in Ohms but I understand where you are coming from.
It is a much better way of looking at it, calculating the current through the body as opposed to the volt drop.
The more rods we put in the less current will travel through the body although the volt drop will still be the same.
So as I understand it so far it is very likely if a person were to step outside the equipotential zone they would be liable to a shock of 230V, if we were to put in multiple rods and get the resistance to earth right down the chance of a fatal current travelling through the body would be greatly reduced.
Any thoughts?
 
OMF i had ohms on the brain, should have said amps. will edit before somebody else sees it,
 
Current is not in Ohms but I understand where you are coming from.
It is a much better way of looking at it, calculating the current through the body as opposed to the volt drop.
The more rods we put in the less current will travel through the body although the volt drop will still be the same.
So as I understand it so far it is very likely if a person were to step outside the equipotential zone they would be liable to a shock of 230V, if we were to put in multiple rods and get the resistance to earth right down the chance of a fatal current travelling through the body would be greatly reduced.
Any thoughts?
DOH, said ohms, not amps, insufficient blood alcohol levels. reducing the resistance of the earth fault path by adding extra rods looks , in theory , to help. do you agree though with mty theory of parallel paths?
 
my calc. is based on the 160ohms ( rod ) and the 1000 ohms ( body ) being in parallel. this would result in a current through the rod of 1.43A, and a current through the body of 0.24A, ( using ohms law and kirchoff's ). if you can find fault with that, please do so. this is just my take on it, might be wrong.

First of all you need to decide the path of shock current, hand to foot hand to hand etc(different resistances for different paths). Then you need to know what voltages are involved, then we can calculate what current will flow.

Then using the heart factor for various shock paths we can see if the current that will flow will cause ventricular fibrillation.
 
my calc. is based on the 160ohms ( rod ) and the 1000 ohms ( body ) being in parallel. this would result in a current through the rod of 1.43A, and a current through the body of 0.24A, ( using ohms law and kirchoff's ). if you can find fault with that, please do so. this is just my take on it, might be wrong.

Actually yeah you are right that somebody getting a shock would be in parallel, because the earth path is already there! I humbly do doth my cap to you sir. I got 230mA ish but what is 10 mA between friends ;)
 
Well I was going to assume a voltage of 230V and a shock path from hand to foot.
I would put the human body as 500 Ohms just to be on the safe side.
I mean lets assume a fatal current is 30ma, judging by the regulations in this country.
So as long as I can get a figure below that at a voltage of 230V with a resistance of 500 Ohms I will assume it will be safe.
Well. a lot safer than it is at the moment.
Any thoughts?
 
too many variables to calculate. need to do a real test with a couple of horses. rig 'em up with a clamp meter and get the barby ready.
 

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