Shocked by Neutral vs. Ungrounded

[rojodo3030]

Hey guys. I have heard a lot of times in the trades that if you break a joint on the neutral wire and you touch the neutral wire that comes from a load you will get shocked. This makes logical sense because there is voltage on it and now you are in series with the load. However, a lot of guys also claim the neutral wire will shock you more than the hot wire. This doesn't make sense to me. Here is the scenario I envision so we can get specific.

Scenario one: you touch an ungrounded conductor with 120v with one hand and a metal junction box with the other. Boom, you get shocked.

Scenario two: you touch the neutral after a load (let's say a lightbulb, with 100 ohms of resistance) with one hand and a metal junction box with the other. The neutral is disconnected from the panel so there is 120v on it (and some voltage drop from the load).

In Scenario one you get shocked by the full 120 volts which results in a bigger shock.

In Scenario two you get shocked by 120 volts - the voltage drop of the load, which results in a slightly smaller shock.

So why do I keep hearing the neutral shock is more dangerous? Is this just a common misunderstanding? Or maybe it's a lie passed through the trades that makes people more cautious when doing work? Or maybe there is something I don't know.

 

[roger]

It's a myth.

 

[hillbilly1]

It just appears to be worse because your not expecting it from the neutral.

 

[infinity]

As Roger said it's a myth. If you end up in series with a load and the neutral and say that the load is 1/2 amp that is the maximum current that can flow through your body and the load. Get in series between a hot leg and ground and well do the math. Ouch.

 

[winnie]

I suppose it is possible that the impedance of the load could reduce the voltage and somehow lead to the person staying in contact with the circuit for a longer period of time.

My strong hunch is 'myth', but I see how this might be wrong.

 

[kwired]

As Roger said it's a myth. If you end up in series with a load and the neutral and say that the load is 1/2 amp that is the maximum current that can flow through your body and the load. Get in series between a hot leg and ground and well do the math. Ouch.

1/2 amp is a lot through a body and likely would result in burns. just a few milliamps is a pretty good jolt

 

[infinity]

1/2 amp is a lot through a body and likely would result in burns. just a few milliamps is a pretty good jolt

Sure it could kill you but it's not the same as if you got between the hot leg and the neutral or ground with only a 20 amp circuit breaker in the circuit. In my example the 1/2 an amp is in series with the resistance of your body so the actual current will be much less.

Back in the day when I as an apprentice we used to change fourescent ballasts live. My journeyman always said (incorrectly) that getting hit off of the open ballast neutral was worse than getting hit off of the hot leg to ground. Those ballastshad a current of about 1/10 of an amp which is the maximum current that you could get hit with. Factor in your body's resistance and the current was considerably less so hopefully you might not die.

 

[jaggedben]

I suppose it is possible that the impedance of the load could reduce the voltage and somehow lead to the person staying in contact with the circuit for a longer period of time.

My strong hunch is 'myth', but I see how this might be wrong.

Unless a human body is going to trip a non-GFCI breaker, which is extremely unlikely at common voltages, I think it's a moot point. Don't overthink it. It's a myth.

 

[jaggedben]

Sure it could kill you but it's not the same as if you got between the hot leg and the neutral or ground with only a 20 amp circuit breaker in the circuit. In my example the 1/2 an amp is in series with the resistance of your body so the actual current will be much less.

Back in the day when I as an apprentice we used to change fourescent ballasts live. My journeyman always said (incorrectly) that getting hit off of the open ballast neutral was worse than getting hit off of the hot leg to ground. Those ballastshad a current of about 1/10 of an amp which is the maximum current that you could get hit with. Factor in your body's resistance and the current was considerably less so hopefully you might not die.

I'm going to assume 277V here for the 'worser case' and posit a (very low) 500ohm body resistance for the sake of argument. And we'll simplify impedance to resistance and say your 1/10 amp ballast is 2770 ohms. So the difference between the two scenarios is 554ma from the hot at 500ohm or 84ma from the neutral at 3270ohm. This difference may be significant but an important point already is that the difference bears no resemblance to the OCPD rating vs. the rated amps of the load.

But now let's also posit a more realistic body resistance and compare, say 10,000 ohms, which still may be on the low side. Now the relative amps you get shocked with are 27ma from the hot and 21ma from the neutral. So now the ballast is really not very relevant anymore whatsoever. And this whole exercise shows that body resistance (which will vary) is enough of a superseding factor that when it comes to saftey training we should just ignore the load in series completely.

 

[don_resqcapt19]

Sure it could kill you but it's not the same as if you got between the hot leg and the neutral or ground with only a 20 amp circuit breaker in the circuit. In my example the 1/2 an amp is in series with the resistance of your body so the actual current will be much less.

Back in the day when I as an apprentice we used to change fourescent ballasts live. My journeyman always said (incorrectly) that getting hit off of the open ballast neutral was worse than getting hit off of the hot leg to ground. Those ballastshad a current of about 1/10 of an amp which is the maximum current that you could get hit with. Factor in your body's resistance and the current was considerably less so hopefully you might not die.

In both cases your body acts as a current limiting resistor and the current flow through your body is the same.

 

[kwired]

Sure it could kill you but it's not the same as if you got between the hot leg and the neutral or ground with only a 20 amp circuit breaker in the circuit. In my example the 1/2 an amp is in series with the resistance of your body so the actual current will be much less.

Back in the day when I as an apprentice we used to change fourescent ballasts live. My journeyman always said (incorrectly) that getting hit off of the open ballast neutral was worse than getting hit off of the hot leg to ground. Those ballastshad a current of about 1/10 of an amp which is the maximum current that you could get hit with. Factor in your body's resistance and the current was considerably less so hopefully you might not die.

If you put yourself across 120 volts, then resistance of your body is high enough that you are normally the current limiting component. To get a half amp of current to flow chances are you will be needing to raise the voltage applied.

It doesn't matter much whether the intended load on the open neutral were half amp load or a 1000 amp load, you get across that open circuit the effect on you is going to be nearly the same either way. Sort of like what hurts more getting across the output terminals of a 120 volt 50 VA control transformer or across 120 volt terminals of a 500 kVA transformer. Your body is still the current limiting factor in both cases and both cases you are across 120 volts. If a human body had a really low resistance then maybe one would have much more effect than the other but we would be talking about like less than one ohm before this is a factor, or with being in series with a load probably need to be at least 100 ohms or less and that is probably being somewhat conservative. Normally a person will be in the thousands of ohms at very least.

 

[rojodo3030]

I suppose it is possible that the impedance of the load could reduce the voltage and somehow lead to the person staying in contact with the circuit for a longer period of time.

My strong hunch is 'myth', but I see how this might be wrong.

Higher voltages grab you. You don't see someone being held hostage by 120 but you do with 480. That is where I was confused about what others were saying. A neutral shock vs a hot leg is slightly better due to less voltage, therefore less amps going through your body.

 

[rojodo3030]

1/2 amp is a lot through a body and likely would result in burns. just a few milliamps is a pretty good jolt

1/2 an amp is death. It only takes 7 milliamps across your heart to kill you.

 

[don_resqcapt19]

Higher voltages grab you. You don't see someone being held hostage by 120 but you do with 480. That is where I was confused about what others were saying. A neutral shock vs a hot leg is slightly better due to less voltage, therefore less amps going through your body.

The reason that UL 943 selected the 5mA trip for GFCIs is because about 95% of humans can let go on the energized conductor when the current flow through the person is 5mA or less. 120 volts can prevent you from letting go.

You might not even be able to measure that slight drop in voltage, even using lab quality meters. The only lessening of the voltage would be from the voltage drop caused by the current through your body.

 

[jaggedben]

1/2 an amp is death. It only takes 7 milliamps across your heart to kill you.

And the difference between getting shocked by a hot or an open neutral will never correspond to that kind of difference in amps. If you live or die it will be for other reasons than which color wire you got shocked by.

 

[kwired]

Higher voltages grab you. You don't see someone being held hostage by 120 but you do with 480. That is where I was confused about what others were saying. A neutral shock vs a hot leg is slightly better due to less voltage, therefore less amps going through your body.

Many are misled by 120 volts not being very dangerous simply because they have had shock experiences and survived those with no troubles. I have no support data to refer to at the moment but is very likely more electrocutions involved 120 volts than just about any other voltage.

120 volts is often survivable because of insulating value of shoes clothing or surfaces one is in contact with are not very conductive.

Now stand barefooted on wet grounded surface or lay down in a crawls space or under a mobile home and contact something with 120 volts potential to earth and because of the surface contact area you have much lesser resistance to ground than if standing on your two feet with shoes on or immersed in a pool or other body of water you have even less resistance and risk of death is increased dramatically.

Long time ago I remember telling one of the guys on a construction site that door bell button wire wasn't going to be a problem, and it generally isn't at 16VAC. I can touch both terminals at same time most in most any situation and never feel a thing. But he happened to be cleaning brick they were finishing up and using an acid based cleaner. It increased conductivity enough he got a pretty good jolt from the door bell button wire.

 

[busman]

I hate to be the outlier, but if the load is inductive, wouldn't it push the full load current, at least instantaneously, regardless of the person's resistance? Current CANNOT change instantaneously thru an inductor, the voltage goes up to push the same current.

Mark

 

[LarryFine]

Perhaps, if you're making contact at the moment of opening the circuit.

 

[kwired]

I hate to be the outlier, but if the load is inductive, wouldn't it push the full load current, at least instantaneously, regardless of the person's resistance? Current CANNOT change instantaneously thru an inductor, the voltage goes up to push the same current.

Mark

There is no inductance when the circuit is open. You end up being in series with whatever resistance the winding has although it will likely be pretty low resistance and since you are a high resistance the winding ends up with a pretty low voltage drop across it while there is current flowing

 

[busman]

There is no inductance when the circuit is open. You end up being in series with whatever resistance the winding has although it will likely be pretty low resistance and since you are a high resistance the winding ends up with a pretty low voltage drop across it while there is current flowing

Unless I learned it wrong in school, current CANNOT change instantaneously thru an inductor, so if it is carrying 10 amps to a motor and you separate the neutrals and have one lead in each hand, you will have 10 amps going thru you no matter your resistance as the coil will up the voltage to make up for resistance. I believe this is the reason that motors spark at the plug end when unplugged under load. Basically the coil bumps the voltage as much as necessary to bridge the air gap and make a spark. Please tell me if I have this wrong.

Mark