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03-22-2012, 02:17 PM   #1
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## physics behind what happens when neutral is not connected?

i am trying to understand the physics behind what happens when a circuit end point (fixture or outlet) is and IS NOT connected back to the panel's neutral bus.

can anyone offer a simple explanation, preferably a diagram?

thanks

03-22-2012, 02:49 PM   #2
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Electricity follows the path of least resistance.

If we assume that a neutral is not connected in your panel....assuming the light or what ever is being powered is on, that neutral will have what ever the applied voltage is on it....until something connects it to ground or neutral.

So....if you disconnect a neutral in your panel...then turn on the light...that neutral will have about 120Vac on it.

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 03-22-2012, 03:10 PM #3 Member   Join Date: Jan 2012 Location: IL Posts: 929 Rewards Points: 826 As the dawg mentions electricity has to flow from one place to another. If the neutral (for that branch) is disconnected then the circuit is not complete and no current can flow. This is not always the case though. In a MWBC a shared neutral is used for two circuits of opposite phases. In this case if the neutral goes open then the circuit will still be live and devices will work although they could get damaged. When neutral goes open in this case the neutral wire will float to a voltage dependent on how much load is on each phase of the MWBC. If the loads are exactly matched then the neutral will float very close to ground. If they are mismatched the voltage will move depending on the mismatch. The lighter loaded phase will see > 125VAC the other (the heavier loaded) will see <125VAC. These voltage swings can damage electrical devices in the home. A similar problem can happen if the panel neutral is open. Then the entiure panel neutral can float and the same shift from Phase A to Phase B. Bottom line open neutral is never good. At best things simply don't work, at worst devices get permanently damaged.
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 03-22-2012, 03:31 PM #4 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,678 Rewards Points: 4,936 Unfortunately you are dealing with alternating current, which is not simple to wrap your head around. People go to school for long periods of time to understand how alternating current really works, and many people have built in misunderstandings about electricity which makes it really hard to explain something like this simply. But let me give it a try. First off, in a normal, operating alternating current circuit, the electrons that carry the charge move back and forth in the wire 60 times each second. Think of it like water sloshing back and forth in a canal. All of the electrons essentially move in unison, back and forth. If you measure the voltage potential on the wire, on a 120 volt circuit you will measure +120 volts on the hot leg (the one that comes from the panel). On the neutral leg, you will read close to 0 volts. The energy of the circuit is dissipated by flow through the device, whether it be a light bulb, motor or whatever. You are going to measure the same voltage whether the circuit is operating or not, assuming normal connections. So what happens if the neutral disconnects at the panel? You still measure 120 volts on the hot leg, whether the device is switched on or not. If the device is switched off, you do not get a reliable voltage reading on the neutral leg, because the neutral is now floating (it is not connected to ground, which is taken to be 0 volts potential). If you switch the device on, the device does not work, because there is no path for the electrons to follow, since a circuit requires a closed loop to operate from the source, through the device, back to the source. With the device switched on, and an open (disconnected) neutral, you will measure 120 volts on the hot leg, and you will get an unreliable reading on the neutral. By the way, an open neutral is dangerous precisely because the neutral is not connected to ground at the panel. If you touch the neutral wire itself, you have now connected the circuit to ground through your body, and you can get a shock. How bad a shock depends on the relative resistance of you versus the device the circuit goes through, since you will effectively be in series with the device (the load). This is a simplified version of reality for a number of reasons, but it should be a starting point to understanding AC electric circuits.
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03-22-2012, 04:22 PM   #5
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Quote:
 Originally Posted by Daniel Holzman If you touch the neutral wire itself, you have now connected the circuit to ground through your body, and you can get a shock. How bad a shock depends on the relative resistance of you versus the device the circuit goes through, since you will effectively be in series with the device (the load).
Not really. Probably more a factor of how much resistance you have to ground (barefoot on wet cement is very bad, work boots on dry plywood is better). It only takes a few mA to get your heart racing. This is why GFCI's are set to trip at 5mA. The source impedance (resistance of the supply side) is no doubt small compared to these other factors that it doesn't matter much. In any event I wouldn't try an experiment to prove or disprove this.

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 03-22-2012, 04:28 PM #6 Member   Join Date: Apr 2007 Posts: 2,316 Rewards Points: 1,062 i knew this would turn out an interesting thread with some inevitable conflicts...
03-22-2012, 04:30 PM   #7
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Quote:
 Originally Posted by curiousB Not really. Probably more a factor of how much resistance you have to ground (barefoot on wet cement is very bad, work boots on dry plywood is better). It only takes a few mA to get your heart racing. This is why GFCI's are set to trip at 5mA. The source impedance (resistance of the supply side) is no doubt small compared to these other factors that it doesn't matter much. In any event I wouldn't try an experiment to prove or disprove this.
Yep. Almost any load (even a night light bulb) will pass enough current to kill you. The difference between putting yourself in series with a table lamp versus a space heater is pretty much irrelevant.

 03-22-2012, 04:32 PM #8 Member   Join Date: Apr 2007 Posts: 2,316 Rewards Points: 1,062 can you get killed by a car battery?
03-22-2012, 05:11 PM   #9
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Quote:
 Originally Posted by amakarevic can you get killed by a car battery?
Not through your skin. Skin resistance is far too high for 12V to push enough current to cause injury. In fact, unless you are very wet you usually can't even feel 12V at all. With very wet hands I have occasionally noticed a barely perceptible tingle from 12V. If your skin is bypassed somehow though, like if you had a wire in your mouth and another... somewhere... else... then a 12V battery could definitely cause injury or death. It's all about resistance.

To pass enough current to potentially cause ventricular fibrillation (about 50mA), a resistance of 240 ohms or less is needed at 12V. This is roughly in the same range as the internal resistance of the body from one limb to another (a few hundred ohms). Skin resistance adds thousands of ohms to this. Wet skin is a few kohms, and dry skin varies from tens of thousands to hundreds of thousands of ohms. My dry skin resistance from hand to hand is usually about 300K. So that would pass 0.04mA at 12V - definitely not perceptible. My wet fingers are usually 10-30K, which will pass 0.4-1.2mA, which is right around the lower limit of perception.

The internal resistance of an adult chest is generally assumed to be 70 ohms by engineers of defibrillators. This value reflects the impedance that is seen when a several-thousand-volt potential is applied from one gel-coupled defibrillator pad to another. This relies at least somewhat on arcing through the skin to bypass the extra resistance. So a 12V battery applied to electrodes on your chest would probably not achieve the same impedance - but if there were electrodes under your skin (like needles, or some type of implanted equipment), the result could be very bad. There are anecdotal reports of people being electrocuted by very small voltages when applied to skin-penetrating electrodes. This is usually in a medical context or, in one case, someone allegedly intentionally trying to measure their body resistance with an ohmmeter and some needles.

Last edited by mpoulton; 03-22-2012 at 05:24 PM.

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03-22-2012, 05:14 PM   #10
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Quote:
 Originally Posted by amakarevic can you get killed by a car battery?

12 Volts will not drive enough current through your relatively high resistance body to electrocute you.
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03-22-2012, 05:26 PM   #11
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Quote:
 Originally Posted by amakarevic can you get killed by a car battery?
By shock probably not, blow it up while boosting a car quite possible. Take the battery out and drop it on your head from a high distance, likely you would die.

03-22-2012, 05:33 PM   #12
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Quote:
 Originally Posted by amakarevic i am trying to understand the physics behind what happens when a circuit end point (fixture or outlet) is and IS NOT connected back to the panel's neutral bus.

Pretty much the same as when you turn off a switch to a light. Only imagine the switch was on the neutral side instead.

03-22-2012, 11:46 PM   #13

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Quote:
 Originally Posted by amakarevic i am trying to understand the physics behind what happens when a circuit end point (fixture or outlet) is and IS NOT connected back to the panel's neutral bus. can anyone offer a simple explanation, preferably a diagram? thanks
I think the first thing that needs to be understood .. what is the conductor in question ?

Your question is asking what happens (physics) when the white wire of a 120 volt branch is disconnected at the panel where it originates.

Up until 2008 the NEC never defined a neutral conductor. The white wire in your question has been referred to in all the replies so far as a neutral.

It is not a neutral conductor.

Prior to the 2007 cmp task force undertaking the charge of defining a neutral conductor it was generally meant to be any conductor carrying the unbalanced current between two or more ungrounded (hot) conductors. So a prime example would be your service neutral in a 3 wire 120/240 volt 60hz system.

The formal defintion finalized in 2008 is as follows and it takes two definitons and an FPN to really seal the deal.

So what this all is saying is the white wire in a 2 wire with ground 120 volt branch circuit is not a neutral but is simply a 'grounded conductor' that carries all the current of the branch circuit.

Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions

Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wiresystem, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system.

Informational Note: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point is zero.

Last edited by Stubbie; 03-23-2012 at 12:04 AM.

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