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Joined: Nov 2000
Posts: 2,148
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DS,
This is a series/parallel circuit and the current at this point in the circuit is limited to 6 amps for a cold lamp and 1 amp for a hot one, by the resistance of the lamp. In your example we would have 6 amps going into a parallel circuit with resistances of 0.3 for the #14 and 0.015 for your metallic short. In this case 5% of the 6 amps or .3 amps would flow in the #14 and 95% or 5.7 amps would flow in your metallic short. The same thing happened in my example except that 99.97% of the current flows through the #14 and 0.03% flows through our 1000 ohm human.
Don(resqcapt19)
Don(resqcapt19)


Don(resqcapt19)
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Bill request was "Please include work so we can follow calculations."

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In the original post the cold filament resistance was given at 20 ohms.
I=E/R so I=120/20 = 6 amps
We now have 6 amps flowing in the grounded conductor that has a resistance of approximately 0.3 ohms base on my assumption of 100 feet of wire from the lamp to the panel. If we add the human in parallel at this point with his given resistance of 1000 ohms we can calculate the current flow in the human and in the #14. We can use I=E/R if we know the voltage drop on the #14 from the screw shell to the panel. We find the voltage drop using E=IR
E =6(0.3) = 1.8 volts.
The 6 is the current flow with a cold filament and the 0.3 is the resistance of 100' of #14. This drop will change slightly with the human in parallel. The total resistance with the human in the circuit becomes 1/Rt = 1/R1 + 1/R2
1/Rt= 1/0.3 + 1/1000 = 3.3333 + 0.001 = 3.3343
Rt = 1/3.3343 = 0.29991 ohms
This gives us a new voltage drop of E =6(.29991)= 1.799 volts.
Now the current in the human is I= 1.799/1000 = 0.001799 amps.
The current in the #14 is I= 1.799/.3 = 5.998.
Note there is some rounding in these numbers and the current does not total 6 but is only off by 0.0002 amps.
If we replace the human in this circuit with DS's 0.015 ohm metallic short we have to repeat the calculations. 1/Rt = 1/0.3 + 1/0.015 = 70
Rt = 1/70 = 0.01428 ohms.
This does not change the 6 amps that we are working with because this current is limited by the lamp resistance. We have a new voltage drop of E = 6(0.01428) = 0.08568 volts.
We can now look at the respective currents in the #14 and the short.
For the #14 we have I = 0.08568/0.3 = 0.2856 amps
For the short we have I = 0.08568/0.015 = 5.712 amps.
Don(resqcapt19)


Don(resqcapt19)
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>his does not change the 6 amps that we are working with because this current is limited by the lamp resistance.

I used the same assumption in my calculations just because the difference is not significant. But technically we should take the reciprocal of the sum of the reciprocals of the two parallel resistances then add in the lamp resistance and the first half of the conductor resistance to figure the total resistance, and figure the current from that. Anyway, you used the parallel resistance formula to get Rt, and that is what matters.


I don't mind rounding at all. In fact I think we both were using too many significant digits.


Now about Bill question on human resistance, I have not seen a published number even quote as low as 1000 ohms.

I have measured myself on a 50 V battery operated ohmmeter with the ohms X 100000 range. I don't recall the readings. But they were quite variable, like 5000 to 30000 or something like that.

I suspect that the resistance from head to toe is different from that from thumb to index finger tip. Cuts or wet wounds definitely have lower resistance when contacted.

Smear a little crazy glue over your finger tip rubbing it rapidly like ointment until it dries and you can literally stick your finger in a light socket. I wonder if Scott knew about that trick for invisible insulation. [Linked Image]

Joined: Nov 2000
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DS,
You are correct that the whole circuit should be looked at as a unit as there will be some small changes in the numbers, but these changes are so small as to be negligible. In my first post I did not resolve for Rt and had 0.0018 amps in the human conductor, when I did use the Rt formula, I had 0.001799 amps in the human.
A quick way to do this with somewhat less accuracy is just to divide the low resistance by the high resistance and use this a percentage multiplier of the total current. In this case we would have 0.3/1000 = 0.0003 = 0.03%. This would put 0.03% of the 6 amps through the human and 99.97% of the current through the #14.
I found a number of sites that list the human skin resistance as being between 1000 ohms (for wet skin) to 500,000 plus ohms for dry skin. The two contact points on the body also have an effect on the resistance. Internal resistance is often listed as 200 to 500 ohms. Open cuts and shocks become very very dangerous.
Don(resqcapt19)

[This message has been edited by resqcapt19 (edited 08-03-2001).]


Don(resqcapt19)
A
Anonymous
Unregistered
>these changes are so small as to be negligible.
I agree.

>A quick way to do this with somewhat less accuracy is just to divide the low resistance by the high resistance and use this a percentage multiplier of the total current.
Actually, divide by the sum of the two resistances. Pick resistances of 1 ohm for both paths and it is obvious why.

>Open cuts and shocks become very very dangerous.
Be sure to put crazy glue over them.


[This message has been edited by Dspark (edited 08-03-2001).]

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Dspark,

Didn't know about the Crazy Glue on the fingertip insulation thing!!!

Thanks for the info!!!

If I had known of this, I would have done it while attempting to troubleshoot/fix a camera's flash circuitry. After about the 3rd blast from the tank circuit, I gave up [before throwing the camera [Linked Image]]
Needed to charge the tank circuit for this one since probing was completely impossible - plus it was a mechanical deal that went haywire.

Once again, Thanks!!!

Scott SET


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Sep 2001
Posts: 3
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Junior Member
would any one like to chat about electricity, transformers, motors,
nec requirements, successful electrical contracting traits,habbits,marketing etc....
I am a 15 year electrician and would like to learn more..I take my California State contracting exam on 9/13/01......... my icq and email should be visable to users....

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Welcome to the forum!
I sure hope you aren't going to reply to every thread. Just once is enough.

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Posts: 5,392
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welcome scespark !

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