Current through body as a result of touching a neutral

[philly]

I had a discussion recently about what would happen if someone were to touch a neutral conductor (grounded neutral) that had current flowing through it.

I was thinking that because there is no voltage potential on the neutral in relationship to ground, there would be no new ground currnent introduced through the body. However because there is current flowing through the neutral your body would set up a current divider for this current with the neutral wire connected back to the source, and a small amount of current would go through your body. The amount of current would be dictated strictly by the amount of neutral current determined by upstream loads, and by the impedance ratio between your body and the neutral path returning back to the source.

Am I looking at this correctly?

 

[SEO]

You are correct. That's one of the reasons for 4-wire range and dryer circuits.

 

[gar]

090730-0822 EST

philly:

You are partially correct.

I was thinking that because there is no voltage potential on the neutral in relationship to ground, there would be no new ground current introduced through the body.

If there is current flowing in the neutral and since there is resistance in the neutral, then depending where you pick two points on the neutral the magnitude of the voltage drop between those points is defined by ohms law.

Thus, if non-zero current flows in the neutral, and the two points on the neutral are not the same, then there is a non-zero voltage drop between those points.

Then of course the current thru your body connected between the two points of contact is defined by the voltage drop and your body resistance.

The remainder of your paragraph is correct.

.

 

[mivey]

What gar said plus get away from thinking that current will push its way onto you. The voltage pushes the current.

 

[al hildenbrand]

Philly,

Having read these first responses to your question, and having given some thought to the "touch voltage" of a current carrying neutral, I pose that you consider a simplified circuit.

Consider a simple temporary construction power service that one might find at a site for a new single family dwelling. A small circuit breaker panel is placed within 15 feet of a power company pole or pad mount transformer. The service is grounded with a driven ground rod.

Only you, the electrician, are on site today, and you have 150 foot of #12 gauge extension cord (in good condition) running to your drill. It's Friday afternoon at 3:30 and it's raining, it's been a steady soft slow rain all day. You're soaked. You are standing in the mud drilling from the outside to the inside of this dwelling under construction.

Here's the exercise: The ground rod end (in the temp service) of the cord is at zero volts. The current in the motor is 10 (ten) amps.

What is the voltage rise in the neutral from the receptacle at the temporary service to the plug at the drill?

 

[steve66]

I had a discussion recently about what would happen if someone were to touch a neutral conductor (grounded neutral) that had current flowing through it.

I was thinking that because there is no voltage potential on the neutral in relationship to ground, there would be no new ground currnent introduced through the body. However because there is current flowing through the neutral your body would set up a current divider for this current with the neutral wire connected back to the source, and a small amount of current would go through your body. The amount of current would be dictated strictly by the amount of neutral current determined by upstream loads, and by the impedance ratio between your body and the neutral path returning back to the source.

Am I looking at this correctly?

That's pretty much the case as long as the neutral wire has a good connection back to complete the circuit.

It's also important to remember as soon at that connection is gone, the neutral wire becomes a "hot" wire.

I've seen a journeymen splicing neutral wires on live circuits. He didn't realize (and neither did I at the time) that as soon as he cut the neutral wire (to install a wire nut) that one of those ends suddenly had 120 Volts on it.

Steve

 

[al hildenbrand]

. . . as soon at that connection is gone, the neutral wire becomes a "hot" wire.

:smile:

You're pointing in the direction that I was starting to go, as well. . . that is, when something goes wrong.

My diagram shows a ground fault, which could be the hot to neutral fault, as well.

In this simple circuit, the return wire in either case is a #12 copper.

Since the person is holding the conductive metal body of the tool, which suddenly has a hot against it, the interesting thing is to built a model of the circuit established.

There are several resistances and parallel paths. . . .

 

[glene77is]

:smile:

My diagram shows a ground fault, which could be the hot to neutral fault, as well.
.

Al,

Great Guns!

Terrific example of a common and dangerous scenario!
We lost a good man under a house, due to a similar scenario.
He could not breath due to small electrical leakage currents through his body.
It's real.

Excellent technique for describing a problem,
prior to solution!

Start with this generalization,
that 125 Volts applied to the accepted 25 Ohm ground earth path
will only pass about 5 Amps.
We all know that will not trip the circuit breaker.
Thus the need for a GFCI receptical.
GFCI recepticals also trip with a fault of Neutral to Ground!

Try 5,000 Ohms for human body resistance,
considering salty sweat on skin, and damp boots, and leather soles.


I am sure there will be good comments
and math calculations for this thread.

:smile:

 

[philly]

That's pretty much the case as long as the neutral wire has a good connection back to complete the circuit.

It's also important to remember as soon at that connection is gone, the neutral wire becomes a "hot" wire.

I've seen a journeymen splicing neutral wires on live circuits. He didn't realize (and neither did I at the time) that as soon as he cut the neutral wire (to install a wire nut) that one of those ends suddenly had 120 Volts on it.

Steve

Is this because when you cut the neutral wire the one end connected to the load is no longer grounded. Your body is now a resistance in the path of the hot wire to ground. The voltage will drop across the connected load resistance and your body according to a voltage divider. Since your body will have a much higher resistance than the load the most if not all of the 120V will be present at the end of the wire that is connected to the load and will be dropped across your body.

What would happen if the transformer was not grounded? There would be now way for the current to return to its source after passing through your body. What would happen in this case? Would there be no current flow, but only a voltage shock?

Even if you cut the neutral and then somehow put your body in between the neutral wire from the load and the other side of the cut neutral wire returning to the source, you would still have 120V across your body, because your body would now be in series with the load and current would return in series on the neutral with the voltage across you body being determined again by a voltaeg divider?

 

[LarryFine]

Terrific example of a common and dangerous scenario!

And it also illustrates what's wrong with an inspector's advice in another thread about using the neutral to bond a grounding receptacle on a non-EGC-equipped circuit.

"Danger, Will Robinson, danger!"

 

[steve66]

Since your body will have a much higher resistance than the load the most if not all of the 120V will be present at the end of the wire that is connected to the load and will be dropped across your body.

Exactly. Assume our circuit is running a few incandescent lights, or a few fluorescent ballasts. If you open the neutral, or if your body is the only path for current (either back to ground, or back to the neutral bus) those lamps or ballasts are essentially short circuits. You might as well grab a hot wire.

What would happen if the transformer was not grounded? There would be now way for the current to return to its source after passing through your body. What would happen in this case? Would there be no current flow, but only a voltage shock?

If its not grounded, you won't get shocked unless you complete the circuit back to the transfomer. Hospitals sometimes use ungrounded systems in their operating rooms for this reason.

Even if you cut the neutral and then somehow put your body in between the neutral wire from the load and the other side of the cut neutral wire returning to the source, you would still have 120V across your body, because your body would now be in series with the load and current would return in series on the neutral with the voltage across you body being determined again by a voltaeg divider?

Yes, that would probably give you quite a shock. You might as well grab a hot wire with one hand and the neutral with the other. Again, the resistance of the loads in most branch circuits are going to be about zero compared to the resistance of your body. And by the voltage divider formula, the higher resistance gets the higher voltage across it.

Steve

 

[al hildenbrand]

What would happen if the transformer was not grounded? There would be now way for the current to return to its source after passing through your body. What would happen in this case? Would there be no current flow, but only a voltage shock?

With no ground at the transformer (source), there is no circuit.

With no circuit, there is no path along which potential (voltage) falls.

There may be a capacitive coupling that a voltmeter with extremely high input impedance will be able to measure, without discharging, but that won't have enough energy to provide, as you say, a "voltage shock".

 

[infinity]

Exactly. Assume our circuit is running a few incandescent lights, or a few fluorescent ballasts. If you open the neutral, or if your body is the only path for current (either back to ground, or back to the neutral bus) those lamps or ballasts are essentially short circuits. You might as well grab a hot wire.


Steve

If those loads were essentially short circuits and they were drawing say 1 amp of current how would that compare to grabbing the hot wire as you suggest? The way I see it that load will limit the current flow to whatever it was drawing before you became part of the circuit. Human body from hot to ground is only limited by the resistance of the body, wire and the size of the OCPD. Human body across a 1 amp load can't draw more than 1 amp.

 

[al hildenbrand]

. . . if you cut the neutral and then somehow put your body in between the neutral wire from the load . . . ?

That's an interesting experiment, but your original question concerns what I would term the "touch voltage" of a load bearing neutral, which requires an intact circuit.

The intact circuit leans in a direction towards the simple circuit that I am illustrating in my diagram.

The circuit is intact. The tool malfunctions internally and the hot (120 V) comes in contact with the metal body of the motor (which is solidly bonded to the Equipment Grounding Conductor).

The person has one hand solidly gripping the metal body.

As the hot to EGC current flows, what is the "touch voltage" at the metal drill body?

 

[gar]

090730-1519 EST

philly:

Al provided a question earlier assuming 150 ft of #12 wire and 10 A thru the neutral. Effectively he asked what is the voltage drop on that neutral? Here is the answer --- 1000 ft of #12 copper is about 1.6 ohms, 150 ft is 0.24 ohms, 10 A and 0.24 ohms is 2.4 V. Under certain conditions 2.4 V can kill you.

To add to Al's last question. If your bare feet are on wet earth, your bare hands are holding the metal case of an electric drill, the extension cord is of most any length, all three wires in the cord are the same material and diameter, and there is a direct dead short between the hot wire and the drill housing, then the approximate peak voltage between your feet and hands is about 0.5*120*1.414 = 85 V. You will probably be dead.

.

 

[don_resqcapt19]

If its not grounded, you won't get shocked unless you complete the circuit back to the transfomer. Hospitals sometimes use ungrounded systems in their operating rooms for this reason.
Steve

Larger systems often have enough capacitive coupling to create a shock hazard on ungrounded systems. The hospital systems are small and carefully constructed to limit this problem.

 

[don_resqcapt19]

090730-1519 EST
...To add to Al's last question. If your bare feet are on wet earth, your bare hands are holding the metal case of an electric drill, the extension cord is of most any length, all three wires in the cord are the same material and diameter, and there is a direct dead short between the hot wire and the drill housing, then the approximate peak voltage between your feet and hands is about 0.5*120*1.414 = 85 V. You will probably be dead. ...

I don't understand your calculation here. The voltage between the case and the earth should equal the voltage drop on the EGC. That would require a voltage drop calculation using the fault current.

 

[steve66]

If those loads were essentially short circuits and they were drawing say 1 amp of current how would that compare to grabbing the hot wire as you suggest? The way I see it that load will limit the current flow to whatever it was drawing before you became part of the circuit. Human body from hot to ground is only limited by the resistance of the body, wire and the size of the OCPD. Human body across a 1 amp load can't draw more than 1 amp.

Yes, I would agree. The loads would never pass more than 1 amp. I guess I didn't make it very clear what I meant by "short circuit".

But at the few milliamps it takes to kill someone, the loads only drop a few millivolts. That's what I meant when I said they look like a short circuit. At the load, the neutral wire and the hot wire have the same voltage or "touch potential". Almost the entire applied 120 volts appears across the person who is unfortunate enough to complete the circuit.

I hope I'm not hijacking this thread since we started with a connected neutral. I just wanted to emphasize how different the situation becomes if we break the neutral.

 

[PetrosA]

This may have appeared here already, but if not, make sure you have a strong stomach before watching. This guy shows what happens when the human body is used to complete a circuit.

ELECTROCUTION SHOWN

 

[gar]

090730-1905 EST

don:

Here is the theory of the calculation.

Ideal voltage source. A length of wire of the same diameter and material thru the entire length. Short this wire across the voltage source. It does not matter what is the resistivity of the wire, its length, and diameter. The voltage drop from either end to the midpoint is 1/2 the source voltage.

The real world. A reasonably stiff voltage source. A branch circuit where the hot wire is the same diameter as the EGC, and obviously both wires are the same length. A dead short from HOT to EGC and the end or anywhere along the circuit this midpoint will have half of the source voltage.

Because the source has internal impedance the voltage into the branch circuit will be somewhat lower than the open circuit voltage. Thus, the midpoint voltage will be somewhat lower than 1/2 the open circuit source voltage. Also if the dead short is really not a very low resistance compared to the wire resistance, then the voltage will be somewhat lower yet.

However, as a rough approximation my 1/2 voltage is not too bad relative to illustrating the problem.

The voltage is worse on the ECG when the EGC wire is smaller than the current carrying conductors. This is my case at home with 1965 NM with ground. My EGC is about #15 on #12 cable.

.