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Old 04-24-2011, 06:59 PM   #141
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Quote:
Originally Posted by Electric_Light View Post
It's not that complicated.


If you touch that red wire, as far as shock is concerned, its the same as touching the hot. If it "shorts" to the ground, the current is limited by load rather than the power source. If it's on a GFCI it will trip. If it's not on GFCI, the bulb lights up and remains lit until fault is cleared.

A fully lit 100W light bulb is around 145 ohms, or around 10 ohms when its off.

Even if you assume your body is 1kOhm, if you touch that wire within the red circle say on a bulb end of a lamp cord that got damaged.

You've basically got

1000/1010 of 120v allocated to your body, so for practical purpose, you can safely assume the same amount of current will flow through you as if you touched the hot directly.

I suppose neutral return path shock can be worse from the surprise factor since you go touch it with confidence that it is definitely not energized only to find it is.
You're talking about a series circuit, I'm talking about a parallel circuit.
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Old 04-24-2011, 07:03 PM   #142
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Quote:
Originally Posted by Big John View Post
Nah, I bet English teachers and editors would as well. I did the math four pages ago!

-John
You're taking it the wrong way. I meant the math with all the variables plugged in. I didn't mean to sound ungrateful, it was wishful thinking that's all. Like I said I'm being lazy.
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Old 04-24-2011, 07:19 PM   #143
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Quote:
Originally Posted by brian john View Post
And that link I provided has the math.

1 (touch open neutral) +2 (Be grounded or in the circuit) =3, (Shock or DEATH, or as we call it electrocution)
Yeah, and I appreciate that. You're the one who got me thinking about this in the first place. You made a post a while back in which you mentioned that a difference in potential will exist between neutral and ground on a properly installed circuit doue to the voltage drop inn the circuit. The other day when I was talking to a maintenance guy we got to talking about shock. I was telling him that you can recieve a shock by touching an "unopened" neutral and ground. He said impossible because there is no potential between them. I found myself explaining to him that a difference in potential does exist for the very reasons you stated. Then I realised that I couldn't fully explain it because those weren't my words, I was just repeating what I had read. I had to go home and search to find out where I got that and it was your post.

I have plenty of books to turn to, to find out how to apply ohm's law, but no real world example like what I described. For some reason I had this thought in my head that the electronic ballasts, along with the voltage being 277v, set up a unique circumstance that would create a bigger shock than a resistive load at say 120v.

You guys have steered me back in the right direction and I appreciate that.
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Old 04-24-2011, 08:44 PM   #144
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Quote:
Originally Posted by CFL View Post
...I meant the math with all the variables plugged in....
I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity Length Current) Circular Mils
VD = (12.9 90 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.

A person is standing barefoot on the concrete floor in a basement. The grounding electrode conductor goes into that floor. The resistance of the person from hand to foot is 1,500 ohms and we'll call the ground resistance 50 ohms. Total resistance of the parallel path back to the panel is 1550 ohms.

The person touches the receptacle box
I = V R
I = 3.39 1550
I = 0.0022 or 2.2 milliamps flowing through the person

The threshold of sensation is between 0.5 and 1 milliamp, so someone would definitely feel that. And if they were sweaty, or like one instance I saw where a woman's hands were wet, the body resistance would go down so the current flow would go up.

-John
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Old 04-24-2011, 08:49 PM   #145
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Quote:
Originally Posted by Big John View Post
I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity Length Current) Circular Mils
VD = (12.9 90 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.

A person is standing barefoot on the concrete floor in a basement. The grounding electrode conductor goes into that floor. The resistance of the person from hand to foot is 1,500 ohms and we'll call the ground resistance 50 ohms. Total resistance of the parallel path back to the panel is 1550 ohms.

The person touches the receptacle box
I = V R
I = 3.39 1550
I = 0.0022 or 2.2 milliamps flowing through the person

The threshold of sensation is between 0.5 and 1 milliamp, so someone would definitely feel that. And if they were sweaty, or like one instance I saw where a woman's hands were wet, the body resistance would go down so the current flow would go up.

-John
Where were her hands?
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Old 04-24-2011, 10:29 PM   #146
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Quote:
Originally Posted by Big John View Post
I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity Length Current) Circular Mils
VD = (12.9 90 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.

A person is standing barefoot on the concrete floor in a basement. The grounding electrode conductor goes into that floor. The resistance of the person from hand to foot is 1,500 ohms and we'll call the ground resistance 50 ohms. Total resistance of the parallel path back to the panel is 1550 ohms.

The person touches the receptacle box
I = V R
I = 3.39 1550
I = 0.0022 or 2.2 milliamps flowing through the person

The threshold of sensation is between 0.5 and 1 milliamp, so someone would definitely feel that. And if they were sweaty, or like one instance I saw where a woman's hands were wet, the body resistance would go down so the current flow would go up.

-John
That's what I'm talking about!
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Old 04-24-2011, 10:45 PM   #147
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Quote:
Originally Posted by Big John View Post
I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity Length Current) Circular Mils
VD = (12.9 90 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.
The circuit length is 180' because it has two wires.
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Old 04-25-2011, 06:55 AM   #148
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New on the job - I showed up to a large retail remodel. My first task was replacing lighting control panels. The LCP was next to it's associated breaker panel. After turning off the panels main breaker, I disconnected all the hots, then the neutrals. I noticed arcing on the neutrals. Took out my meter and read varying voltages (neutral to ground), 0v up to 250v. I notified the foreman on the job. At that point they informed me an employee had started the same task and claimed to have been shocked (injured/out on disability). They didn't believe him because they had not measured anything over 30v at the panel. They figured he was being a big p***y. Therefore, they didn't even bother to warn me. I found the same situation at other locations in the store.
I figure the troffers that ran the length of the store wich were fed with different systems/panels had neutrals crossed.

In this case, Ohm's law applied ;

(Impedance of employee/ (impedance of employee + impedance of connected load) X ~277v) / impedance of employee = current flow through employee.
It's preferred to have this value less than 100mA
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Old 04-25-2011, 07:38 AM   #149
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Quote:
Originally Posted by noname View Post
.

It's preferred to have this value less than 100mA
Only two ways to get that. GFCI or do as you did WORK SAFE. Because crossed neutrals happen all the time, especially after the 3rd remodel.
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Old 04-01-2017, 02:11 PM   #150
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[Q UOTE=Big John;434662]
Quote:
Originally Posted by CFL View Post
...I meant the math with all the variables plugged in....
I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity ? Length ? Current) Circular Mils
VD = (12.9 ? 90 ? 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.

A person is standing barefoot on the concrete floor in a basement. The grounding electrode conductor goes into that floor. The resistance of the person from hand to foot is 1,500 ohms and we'll call the ground resistance 50 ohms. Total resistance of the parallel path back to the panel is 1550 ohms.

The person touches the receptacle box
I = V R
I = 3.39 1550
I = 0.0022 or 2.2 milliamps flowing through the person

The threshold of sensation is between 0.5 and 1 milliamp, so someone would definitely feel that. And if they were sweaty, or like one instance I saw where a woman's hands were wet, the body resistance would go down so the current flow would go up.

-John[/QUOTE]

Thanks for the post, very informative but i have one question on your math John. Please help clear my confusion. When calculating the current on the neutral why are you calculating just the voltage drop and not subtracting the original voltage. ex. 120v -3.39vd

Why isnt the formula this:
I = V R
I = (V-VD) or (116.61 1550)
I = 0.075

Thanks for your help.
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Old 04-01-2017, 02:56 PM   #151
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Quote:
Originally Posted by MonkeySeeMonkeyDo View Post
[Q UOTE=Big John;434662]I'll give it a try:

There's a receptacle neutral tied to the equipment ground at the end of a 90' run of 14/2. There's an air conditioner plugged in drawing 12 amps.

Voltage Drop = (Resistivity ? Length ? Current) Circular Mils
VD = (12.9 ? 90 ? 12) 4110
VD = (13932) 4110
VD = 3.39 volts between the neutral bond at the receptacle box and the panel where the neutral bar is grounded.

A person is standing barefoot on the concrete floor in a basement. The grounding electrode conductor goes into that floor. The resistance of the person from hand to foot is 1,500 ohms and we'll call the ground resistance 50 ohms. Total resistance of the parallel path back to the panel is 1550 ohms.

The person touches the receptacle box
I = V R
I = 3.39 1550
I = 0.0022 or 2.2 milliamps flowing through the person

The threshold of sensation is between 0.5 and 1 milliamp, so someone would definitely feel that. And if they were sweaty, or like one instance I saw where a woman's hands were wet, the body resistance would go down so the current flow would go up.

-John
Thanks for the post, very informative but i have one question on your math John. Please help clear my confusion. When calculating the current on the neutral why are you calculating just the voltage drop and not subtracting the original voltage. ex. 120v -3.39vd

Why isnt the formula this:
I = V R
I = (V-VD) or (116.61 1550)
I = 0.075

Thanks for your help.[/QUOTE]

The reason is that the V in V=IR is the voltage difference between the two ends of the resistor.
The body part touching the neutral is at 3.39V and the other body part is at ground. The voltage across the body is 3.39V
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Old 04-01-2017, 03:06 PM   #152
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But the circuit for example is a 120v circuit. The length of the run is just the voltage drop so shouldnt 116 v still be going throuhh the neutral?
http://m.ecmweb.com/code-basics/char...tral-conductor
"When sizing the load for a 2-wire circuit, the grounded neutral conductor carries the same amount of current as the ungrounded phase conductor. This type of installation has no unbalanced load; therefore, the neutral conductor carries full current."
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Old 04-01-2017, 07:49 PM   #153
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This one almost went 6 years from the last posts.... How does one find the dinosaurs of this forum and decide "I'll reply to this one"?? lol...
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Old 04-01-2017, 07:59 PM   #154
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LoL right. I saw that but i was left hanging at the end and i noticed John was online. Just glad i haven't been waiting 6 years for the sequal to start up. LoL. I hope someone could clear the explation up in lameman terms. Because i just dont get how th voltage on the neutral would change from 120v to 3v.... Ive been hit with 277 plenty of times and shook it off. Then got hit one time and got cooked so i figured i caught a neutral with all the return current on it. So i live off the theory dont get caught on the return path.
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Old 04-02-2017, 12:46 PM   #155
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For the folks who remember CRTs in the old TVs they were supplied 20KV, with very little current.
I've been shocked many times working on them. Sure beats a cup of coffee to wake up!

So that's why GFCIs work on current
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