earthing concept (i know this aint the place ... but)

[rahulkothari]

first i would like to say sorry for posting this kind of doubt in this forum but this was coz i couldnt find any useful "electronics forum" after searching from www.google.com (http://www.google.com)

doubt:- y doesnt current pass from our body when an malfunctioning electronic object is touched ? although the current passes from earthing wire , is should also pass from our body given teh laws of electronics?

ex- if u connect a 1ohm and 2ohm resistance in parallel, then current passes thru both the resistors, the same should happen with our body and earthing wire?

hey... wait a minute .. the earthing wire has zero or resistance less than our body and current flows thru the least resistance path. is it so, that if in the above ex if one of the resistors is of 0ohm(or 0.1ohm , anythin lesser than 2ohm) and other of 2ohm, then current shouldnt flow from even the 2ohm resistance. if this is so, then i have my ans, but if it aint .....
..... thx in advance.

------------------
An eye for an eye makes the whole world blind.
-Mahatma Gandhi

[bassman]

Hello Rahulkothari,
Yes current passes thru the path of leased resistance.
Yes current can pass thru two objects of different resistance.

The question in this is, "How much current is being passed?"
The path of leased resistance will only accomidate a certain amount of volume (amps/watts), and can only perform with the correct pressure (volts).
If you override the volume, it passes excess on to the next path.
If you override the pressure, it blows the circuit.

This is why all NON-Double insulated, high voltage devices today have 3 wires. 1)HOT-power coming in. 2)Neutral-power going out. 3)Ground-excess power.

Hope that helps http://www.PCGuide.com/ubb/cool.gif

------------------
A real Christian is one who can give his pet parrot to the town gossip.
Frank's Place (http://dreamwater.net/tech/frankscomp/)

[yawningdog]

If you override the volume, it passes excess on to the next path.


I'm afraid this isn't accurate.

The total resistance of two resistors in parallel is somewhat less than the smallest resistor. When total resistance is calculated (or measured) then the applied voltage determines the total current. Current passes through all paths whether or not one path is saturated. (A term normally reserved for semi-conductors, I know.)The smaller of the two resistors will conduct the most current.

The resistance of your body varies from thousands of ohms to as low as 30 ohms depending on conditions. If memory serves, 1 amp is considered lethal.

------------------
Linux- Because you wouldn't buy a car with the hood welded shut, why should your O.S. be different?

[rahulkothari]

i will give u a real practical example:-

my keybrd has a metal-back. when i touched the metal back , i experienced shock and this happened while my power supply (socket of comp) wasnt
earthed.

then i called an electrician, he earthed the supply (he said the earthing wire(of the socket) had lost contact...) and now i dont experience shock.

now would anyone tell me:-
when the comp wasn't earthed, i was the only "resistance" and hence current flowed thru me and i xperienced a shock.
when the comp was earthed, there were two resistances in parallel, myself and the earthing wire

if...
R1 (my resistance) = 50 ohms
R2 (resistance of ideal earthing wire) = 0 ohms
V (voltage) = 250 v
then...
I1 (current across myself) = V/R1 = 250/50 = 5 amps
I2 (current across earthing wire) = V/R2 = 250/0 = "infinite" amps

(so going by the above equations, i should have been dead http://www.PCGuide.com/ubb/mad.gif )
the question is why didnt current flow thru my body after earthing, although the above equations prove that 5 amps current passed thru my body.
plz clr my doubt.(i.e. my electronics funda)

http://www.PCGuide.com/ubb/eek.gif http://www.PCGuide.com/ubb/eek.gif http://www.PCGuide.com/ubb/eek.gif DEAD MAN POSTING http://www.PCGuide.com/ubb/eek.gif http://www.PCGuide.com/ubb/eek.gif http://www.PCGuide.com/ubb/eek.gif

------------------
An eye for an eye makes the whole world blind.
-Mahatma Gandhi

[This message has been edited by rahulkothari (edited 03-20-2002).]

[bassman]

For starters Rahulkothari, your keyboard does not carry 250V. The power coming into the comp gets broke down to verious levels of low voltage by the power supply.

Electrical theory can be easily be explained by thinking of water flow. It is not always 100% accurate though (as Yawningdog pointed out http://www.PCGuide.com/ubb/wink.gif )
According to his explination, I believe you are thinking your resistance is much lower then it really is.


The resistance of your body varies from thousands of ohms to as low as 30 ohms depending on conditions. If memory serves, 1 amp is considered lethal.



30 ohms would be "Naked, in the bathtub, with a blowdryer" http://www.PCGuide.com/ubb/eek.gif

Glad you got it fixed and are still around to tell us about it http://www.PCGuide.com/ubb/cool.gif



------------------
A real Christian is one who can give his pet parrot to the town gossip.
Frank's Place (http://dreamwater.net/tech/frankscomp/)

[rahulkothari]

thx 4 all the help...so far. i think i need to rephrase my question in simple words:-

if a earthing wire (i.e. ideally 0 resistance wire) and a human body(or any conducting material with some resistance) are connected in parallel in a malfunctioning device, then y doesn't current flow thru the human body ?

------------------
An eye for an eye makes the whole world blind.
-Mahatma Gandhi

[bassman]

IT DOES, just not at a rate that you can conciously feel.

------------------
A real Christian is one who can give his pet parrot to the town gossip.
Frank's Place (http://dreamwater.net/tech/frankscomp/)

[Reid]

Originally posted by rahulkothari:
if...
R1 (my resistance) = 50 ohms
R2 (resistance of ideal earthing wire) = 0 ohms
V (voltage) = 250 v
then...
I1 (current across myself) = V/R1 = 250/50 = 5 amps
I2 (current across earthing wire) = V/R2 = 250/0 = "infinite" amps

(so going by the above equations, i should have been dead http://www.PCGuide.com/ubb/mad.gif )
the question is why didnt current flow thru my body after earthing, although the above equations prove that 5 amps current passed thru my body.



You don't get current "across" an object; the current flows through an object and the voltage is across the object. If the earthing (ground) path was perfect (zero ohms), then there would be no voltage across it, so you would not receive a shock. Voltage = current x resistance.

If the resistance in the defect path is very low, then the circuit breaker or fuse should open, but many faults will have enough resistance so that the protective device's rating is not exceeded.


[This message has been edited by Reid (edited 03-20-2002).]

[Paul Komski]

Unless there was some serious mis-wiring or malfunction I don't believe you could have got a shock from the metal back of a keyboard that originated from the computer or its power-supply. That voltage is unlikely to be more than 12-24V DC. More likely it was a static discharge. If your keyboard is made of conductive material and is now "earthed" then it can't build up static anymore.

If the keyboard became "live" then installing an earthing wire would give you much (though not total) protection against such mains current (by flowing preferentially down the earthing wire) but if it was actually doing this - either a fuse would blow or something would have burnt-out in your computer by now. The main protection coming from the fact that this burnout would occur. Without an earthing wire there would be enough current to damage you (once you became the earthing material) without necessarily burning anything out in the component you were touching (because generally speaking you have high resistance); in other words, you would become the fuse that would blow!!!

The lethal dose of current is complicated by whether it is AC or DC and for how long the current is applied. The lethal amount (unless you are totally fried) is determined by the current flowing through the muscle of the heart. A few milliamps of AC flowing through the heart can certainly be lethal, with the heart being made to fibrillate; the longer this persists the worse. Since the body is not a uniform conductor the percentage of the current passing through the heart also depends on where the entry and exit points on the body are and whether they are on or under the skin.

That's my two cents. Happy frying everyone. http://www.PCGuide.com/ubb/biggrin.gif http://www.PCGuide.com/ubb/biggrin.gif

------------------
Take nice care of yourselves - Paul
"There are old pilots and bold pilots but there are no old bold pilots."


[This message has been edited by Paul Komski (edited 03-20-2002).]

[hiredgoonz]

I remember the last time I was grounded working on stuff...checking for spark on a friend's Bronco...

Simple enough, pull a spark plug wire, stick a screwdriver in there and look for a spark...

Well, make sure you have a well insulated screwdriver and that the asphalt isn't wet...weird feeling, whole body locked up....not pleasent...

But from what I've heard, your body's resistance decreases slightly every time you get a big shock...unfortunately, I still can't do the Uncle Fester light bulb trick http://www.PCGuide.com/ubb/wink.gif

------------------
When all else fails, read the instructions.

Microsoft Knowledge Base (http://support.microsoft.com/default.aspx?scid=fh;rid;kbinfo)

Drivers (http://www.driverguide.com)

Google (http://www.google.com)

[integral]

If memory serves me, the US Navy taught us that 20mA through the heart could be fatal. The lesson always keep one hand in your back pocket when working on live circiuts. (I was an electronics Tech)

If your keyboard was poorly grounded, it may could been "floating" at a higher then ground potential. Thus when you touched it you became the path to ground and were shocked. Now that that the keyboard is properly grounded, the case is held at ground potential so you cannot recieve a shock. Stray capacitance and other circuit elements can cause the common lines of a circuit to float to a DC potenital if not properly grounded. Grounding simply maintains a constant potential on the common lines preventing them from creeping up.

[rahulkothari]

thx guys. thx REID for v=i*r. i was thinking the same but the other way round. i was thinking this way, i was calculating the current :-
v=i*r . therefore, i=v/r = 250/0 = "infinite" amps and "infinite" amps is as good as open-circuit, right ?

but wat actually happens is, current is already leaking (i.e. faulty app),and when it flows thru the earthing wire, voltage drops:-

v= i*r= 100000000*0 = 0 volts (assuming huge amt of current is flowing)

... and so as the voltage becomes 0,the appliance attains 0 (ground) potential and so the Potential Difference between "earth" and "app" becomes 0 and hence eventually no current flows from the "appliance" to the "gnd"....... or is it "all the current flows" from app-gnd and no current passes thru humanbody-gnd ?

i.e. at this pt of time if v touch a tester (screw drvier with a light on the back) to the earthing wire, will it glow ?
if the ans is NO, then i think my concept is clear.
if yes, then ...



------------------
An eye for an eye makes the whole world blind.
-Mahatma Gandhi

[yawningdog]

The answer to you most recent question is no, unless the other side of your tester is secured to some potential.

I don't believe you're dealing with a "voltage" such as we in the electronics racket think of it, rather than an electrostatic potential. Electrons have built up on your keyboard back, probably as a result of imperfections in your monitor's design. (Lots of spare electrons float around in there.) When you touched it, you became the ground path. You weren't harmed because your resistance was probably a several thousand ohms. After your machine was grounded, no electrostatic charge could build up because stray electrons were immediately grounded.

It is possible to have very high voltage which is safe because of the lack of current it may draw from its design. A transformer can step up 5 volts to 2000, but the current capacity drops dramatically in such a case.

By the way, anything divided by zero is not infinite, but undefined.

------------------
Linux- Because you wouldn't buy a car with the hood welded shut, why should your O.S. be different?

[Paul Komski]

Suggestion: concentrate more on the concepts than on the mathematics. The water analogy can be very useful in this regard. Imagine a tank that can hold water (this is the equivalent of your keyboard casing). Normallly the tank should be empty (no electric potential with respect to earth).

If the casing became live with mains electricity then this is equivalent to the tank being filled with water from a pump; a high pressure pump would be like a high voltage ac/dc supply; and a high-flow pump would be like high amps. The tank would fill to the same height of water that equalled the pressure of the pump. Without an earth this potential energy would just stay there.

Attaching a good earthing wire would be like having a very wide bore drain pipe from which the water could drain from the tank very rapidly and as long as the pump was still pumping (no wires of fuses blown) this flow would continue. In this situation if you were to touch the keyboard it would be like you becoming a second very small bore (high resistance) drain pipe through which water could also flow and so you could experience a shock equivalent to the height of water in the tank at the time. The better the earthing wire (the greater the size of the drain pipe) then the lower the height of the water in the tank, since it could drain very rapidly.

With electrostatic build up the electrons (ie the water) are not pumped but carried in very small buckets into a tank that is tiny in area but extremely tall; it has a very small capacitance but the water in it can be at extreme pressure. If this funny-looking tank has a drain pipe (an earth) then it takes very little to keep it empty. If you become the drain pipe (by earthing it) you will experience a one-off shock; it will hurt because of the high-pressure but is unlikely to harm you because the quantity and flow of the water are miniscule and over in an instant.

If you can get your head round this analogy then you can perhaps work-out, meaningfully, when a phase-tester will and wont glow and consider other possibilities and combinations. It is the flow of water that is dangerous of itself; but there can be no flow without a head of pressure. http://www.PCGuide.com/ubb/wink.gif http://www.PCGuide.com/ubb/tongue.gif


------------------
Take nice care of yourselves - Paul
"There are old pilots and bold pilots but there are no old bold pilots."

[YODA74]

Read this and you will get a good concept of electronics and OHMS Law
http://chemistry.uttyler.edu/~inst/electronics.PDF

------------------
Death has come to our windows.

-- Jeremiah 9:21
(undoubtedly a Biblical reference to a Microsoft product!)

YODA74@windows-sucks.com