Got a question.

120v system supplying an outside Light Fixture. It is on. Assuming worst case you are standing in a puddle with your Ground rod or by your pool's bonding points (which are connected back to panel) If you touch the screwshell (grounded) how much current will flow through you back to source?
(This gives us a series-parallel circuit)

• 120v
• Lets use a bulb resistance (hot) of 120 ohms
• body resistance of 1000 ohms
  (can it go lower? I don't know)

What would it be when the light is first turned on and that resistance is say 20 ohms?

Please include work so We can follow calculations.


Bill

[This message has been edited by Bill Addiss (edited 05-27-2001).]

Bill,
Assuming that the grounded conductor to the fixture is intact, there will be almost no current through the body. The only voltage to ground from this point on the circuit will be the voltage drop on the grounded conductor.
Assuming the fixture is fed with 100' of #14, the voltage drop will be about 1.8 (1/2 of the voltage drop for the circuit as we have only sent the current through 1/2 of the wire at this point) volts for the cold lamp and 0.3 volts for the hot lamp. I=E/R so the current through the 1000 ohm person would be 0.0018 amps for the cold lamp and 0.0003 amps for the hot filiment. A GFCI is designed to trip at 0.004 to 0.006 amps. This current flow is well below the GRCI trip point.
Don(resqcapt19)

[This message has been edited by resqcapt19 (edited 05-28-2001).]

Bill,

Great Qs!!!

One of the major factors here is what the load current is past the lamp's screwshell. This will give the complete voltage drop [total current in circuit] at that point.

This Q kinda' looks like it's the spawn of the Mike Holt Forum thread about standing on the ground rod and grabbing hot wires - which turned into a Hot File [>15 posts].

You have a valid thought on the Inrush current the Incandescent Lamp will have - it might have an effect on the results. That inrush, for the most part, is looking at maybe a R of 17 OHMs for a cold-fillament on a 120 VAC 60 Watt A17 lamp. That would last maybe 1/30 second [2 cycles], upto 1/8 second [8 cycles], then taper down to a running value [hot-fillament] within 2 seconds [120 cycles].

Don has given better numbers than I can, so I'll not complicate the thread with jibberish.

Scott SET

Don & Scott,

Thanks for the quick reply. Does anybody know how low the resistance of the body can go? If it wouldn't be too much trouble could someone help out with the math? (show work)?? It would be a good refresher.

Bill

Bill,

Have you seen this EE training series by Integrated Publishing on the web: http://www.tpub.com/neets/index.htm Here is a page for series-parallel circuits showing the math http://www.tpub.com/neets/book1/chapter3/1-30.htm I have been learning a lot there; but I still have a lot to read.

Siemans also has a series of web pages on basic electricity http://www.sea.siemens.com/training/step2000/courses/electricity/index.asp with info on series-parallel circuits.

Phil H

edited to remove long url to siemens page on series parallel circuits. It caused message width to increase, and some people might have to scroll to read.

[This message has been edited by Phil H (edited 05-28-2001).]

Phil,

Thanks, good Link!

Scott,

Yes it's a spawn of that thread. I've conceded the point that voltages and current will be low under normal conditions or even these 'test' conditions mentioned.

I would like to do some permutations sometime to see what scenarios could be possible aside from a broken neutral. I figure that there could be 20 to 30 connections in the circuit who's integrity would have to be good to match our test Question. I'm wondering what variations could be expected in the field and still be a working circuit, and how those numbers affect the outcome here.

Bill

How about the farmer who had a light on a pole between the house and the barn. The farmer wanted to switch the light on/off from the house or the pole but there were only two overhead wires existing from house to pole. If the farmer added the three way switches and made the circuit work without changing the wiring between house and pole, what would this do to your calcs?

If the voltage drop is 1.8 for the return path, then the resistance is 0.015 ohm (1.8 V / 120 V). For a nominal 120 W bulb the current flow is ~1 A (~120 VA / ~120 V). (Don's number may have been figured on a larger current).

The parallel resistance for a standing human (nominal 1000 ohms) plus the copper is 1/((1/0.015)+(1/1000)), which is 0.0149998 ohm.

The current that flows is inversely proportional to the resistance of each parallel path.

The human to the copper ratio is 1000 ohms / 0.015 ohms = 66667, meaning that 66667 times as much current flows through the copper as through the human. 1 A / 66667 = 0.000015 A through the human.

I have done this in rather round numbers.
More exact calculations would require the resistance of both halves of the circuit with one including the bulb, the resistance of the human pathway back to the circuit source, and the exact voltage at the circuit source.

The problem I with Don's calculation using a 1.8 V drop is that it does not invoke the parallel resistance formula. If I provide a parallel path with a resistance of 0.015 ohms, his formula yields an amperage of 120... and you know it's not a 14500 W bulb.

DS,
I don't understand what you are telling me here.
Don

Okay, Don, forgive me for copying your words above and changing a few...

Assume the fixture is fed with wire having a voltage drop of 1.8 volts for the cold lamp and 0.3 volts for the hot lamp. I=E/R so the current through the 0.015 ohm metallic path would be 120 amps for the cold lamp and 20 amps for the hot filament.


What I did was instead the human I substituted a metallic path with a 0.015 ohm resistance. I kept your math the same.

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)

Bill request was "Please include work so we can follow calculations."

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)

>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.

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).]

>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).]

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 ]
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

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....

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

welcome scespark !

Hey! I missed that before!

Dspark, you use CyA (cyanoacrylate HCl -"Super Glue") to seal cuts? So do I!

I'm told CyA was developed during 'Nam for medics as a field suture...

Go here for the latest uses... Gotta love that Brand name!

Go here for Dave Barry's response to same...

I use Pacer Technologies "Zap" of various viscosities and cure times both for work and for my airplanes and for toughening up sore finger tips while playing the git-fiddle...

[This message has been edited by sparky66wv (edited 09-05-2001).]

I've used something like that in the Bowling Alley. I think they called it "Liquid Skin"


Bill

>I'm told CyA was developed during 'Nam for medics as a field suture...
I heard pretty much the same except that it was used to repair shrapnel injuries internally, e.g., guts, that were too shredded or too painstaking difficult to suture conventionally.

I heard the story about gluing chicken butts shut. The part apart dying it red didn't make sense.

Perhaps I should market it under the name Anti-Shock. Put the liquid kind (not the gel) on any finger and rub it out with the same finger of the opposite hand just like lotion until it is thin and dry. (And I stress that you do just one pair of fingers at a time.) When you are done, the coating will be quite flexible and does not interfere with most work (sense of touch is somewhat impaired). But so far as I can tell, shock proof, at least until damaged.
I never tried anything above 120 V, but at that voltage, it seems as good as a fingernail for keeping out those pesky amperes. As Scott35 suggested, a camera flash circuit might be a safe test for high voltage.

It is important to get a thin layer of glue so that it is flexible. If it is thick, it will crack and leak.

Of course it is better to apply in a well ventilated area. The fumes are tremendous.

How did it smell, Bill? I doubt that cyanoacrylate is approved for use on skin - at least it is contrary to the manufacturers' instructions.

CyA is regular ol' super-glue or crazy-glue, depending on brand. I don't know what liquid skin is... Does it look like super glue?

The various viscosities of Zap and other brands are for hobby use... Watch the thinner varieties, they cure so quickly they'll burn your skin! Accelerators (catalyst?) make it even quicker if "instant" isn't quick enough, cures so fast it smokes!... (Good for when one is holding a dimension critical part in place, like the incidence on a tailplane...) "removers" are also available, but expensive, and they also make "odorless" varieties of glues, albeit more expensive.

I'm a "Superglue" expert!

[This message has been edited by sparky66wv (edited 09-05-2001).]

I don't remember what it smelled like but it could be peeled off like PVC cement when it dries on your skin. They used to sell it in the vending machines with the talcum powder and rosin (resin?) bags.

Bill

I should mention that I don't use it for the purpose of a PPE, but literally as a suture for the inevitable cuts that I get... I've cut to the bone before, getting in a hurry, not paying attention, new blade... or those dern canister lights... they'll getcha too...

Without CyA, good ol' Scotch Super 33 (couple of wraps) will stop the bleedin'!

>it could be peeled off like PVC cement when it dries on your skin.
That could be many things. CyA doesn't peel. And if you put it on that thick, it would crack or flake. But it would be hard as stone.

>I don't use it for the purpose of a PPE, but literally as a suture for the inevitable cuts that I get...
Same for me. But that's how I discovered that it is PPE, sort of.

The 1000 ohms resistance from hand to hand goes completely out the window if contact is made with a wound. Therefore, for your own safety, you should have all wounds sealed just in case. I know of nothing that seals a wound like super glue.

Most clean slices heal in three days with no itching or soreness or burning or infection or all those awful things, and that is about two weeks faster than the next competitor.

For wire cuts and encounters with box corners and such that leave a jagged edge or remove too much skin to glue it back together, I use waterproof adhesive tape. It keeps the wound clean and moist.