Neutral current with switch off

Status
Not open for further replies.
M

Mustang125

Member
Location
New Hampshire
Occupation
Master Electrician, Project Manager and Estimator
I apologize for the bad one like but a picture is worth 1000 words.
Basically if you have multiple lights on the same circuit but controlled by different switches, can you still get voltage on the neutral in the room with the light switch off?
Neutral amps.jpg
 
Absolutely you would measure some voltage differential from ground.

In your example there will be some voltage drop on the neutral legs that are carrying current - I've circled that part in blue below. Assuming that there was a bond to ground at the point 4A, then whatever IR losses were in the wires I colored in blue is what would be present on the part you are talking about touching (measured to ground).

1621014065473.png

From a practical standpoint, how much actual voltage was present would depend on cable lengths, where the service bond was, what the load currents were and the size(s) of wires in the circuit(s).

If you are worried about measuring (for example) 120 VAC - and it's wired properly, then no. But could you measure a few tenths (or a few volts if the loads were big and the lengths were long), then absolutely.
 
Last edited:
the real core of the question is if you shut the switch off in the middle room to work on the light, if your grounded could you get electrocuted by touching the neutral wire at that light? (where I point and ask, "what will happen if you touch here")
 
that was kinda my thought also but assuming "The person" is a path of "least resistance" over the say 200' of wire back to ground?
 
Yes, there will be voltage, up to few volts if the load is large enough and noodle long enough from the panel. Enough to harm at 1000 volts body impedance? None that I know of.
 
Is there a way to “prove” this with theory?
This is a pretty common real world practice of shutting the switch off and working on a light and leaving the rest of the circuit on in order to disturb less offices but I have heard of people getting hit on the neutral at the off light, they always say “it’s because there are other lights on the circuit” but I never understood this. Because to me electricity is lazy and is not going to back track to a room to go thru you to go to say building steel to get back. But I’d love to be able to “prove” it
 
Spencer125 said:
Is there a way to “prove” this with theory?
This is a pretty common real world practice of shutting the switch off and working on a light and leaving the rest of the circuit on in order to disturb less offices but I have heard of people getting hit on the neutral at the off light, they always say “it’s because there are other lights on the circuit” but I never understood this. Because to me electricity is lazy and is not going to back track to a room to go thru you to go to say building steel to get back. But I’d love to be able to “prove” it
Click to expand...


Electricity is the exact opposite of lazy. Its takes all paths back to the source.

Look into voltage drop across a resistor and resistor and resisitive dividers.


I used to build test setups with resistors and be able to measure the voltage drop across them.
 
Spencer125 said:
Is there a way to “prove” this with theory?
This is a pretty common real world practice of shutting the switch off and working on a light and leaving the rest of the circuit on in order to disturb less offices but I have heard of people getting hit on the neutral at the off light, they always say “it’s because there are other lights on the circuit” but I never understood this. Because to me electricity is lazy and is not going to back track to a room to go thru you to go to say building steel to get back. But I’d love to be able to “prove” it
Click to expand...


For example, going by chapter 9 Table 9 of the NEC 3.31mm2, 12 AWG copper, has an AC resistance of 2 ohms per 1000 feet.


A 16 amp load will have resistance of a resistance of 17.3 ohms at 277 volts.


A 250 foot circuit will have a resistance of 0.5 ohms on the hot, and a resistance of 0.5 ohms on the neutral.

Adding 17.3 ohms of load, and 0.5 ohms on the hot we get 17.8 ohms from the circuit breaker panel to the neutral of the load at the far end. This forms part one of a resistive divider. The second part of the divider is the neutral going back to the panel which has a resistance of 0.5 ohms.

Using this calculator:



We get 7.5 volts drop across the neutral conductor between the panel and load. Those 7.5 volts will try find and take other routes back to the service if presented such as the human body.


Assuming body resistance of 1000 ohms we get 7.5 millamps of current which may be perceived. At 200 ohms body impedance (sweaty, grounded) we get 37 milliamps, which can not only be painful, but also could stop breathing worse case:

1621042590026.png


This alone is why the neutral should always be treated as live. Add to the fact that splices can open or go high resistance whereby the voltage drop of the neutral goes up.
 
1 megohm of Body resistance

dripping sweat can be as low as 400 ohms. -- still talking miles of wire though.
 
junkhound said:
1 megohm of Body resistance

dripping sweat can be as low as 400 ohms. -- still talking miles of wire though.
Click to expand...


Yup, I agree- IMHO it can be as low as 400 ohms.

I found a study listed on NCBI that has a value as low as 300 ohms given the right conditions:

Wet Locations Body Resistance

What is the typical body resistance in wet locations? Dry locations? I'm at around 200 and 1000 ohms respectively?
forums.mikeholt.com
 
Sorry for my ignorance on this subject. But I’m still having trouble with this. Based on everything you have said it seems that a neutral conductor at a light fixture always has a potential for shock, because if shutting off the switch in that room (isolating the load your servicing)doesn’t limit the potential then shutting off the breaker really wouldn’t either? All neutral conductors on connected in the panel and there would be no way to isolate it. So are you saying there is technically no 100% safe way to work on the neutral conductor at that light? Other than shutting of the main for the whole house to eliminate all loads?
 
Using your first diagram, what everyone is saying, is that, yes, the potential for a shock exists.

How much of a shock depends on a lot of real world factors.

Electricity does not take the path of least resistance. It takes all paths; voltage, and therefore current, in strict mathematical proportion to resistance.

Which is why, yes you could potentially be shocked in your original scenario. In actuality, it’s very rare.
 
ptonsparky said:
Years ago I heard an apprentices tool belt jingle, a few moments later he asked me why he got bit off the white wire. He learned about open neutrals in one quick lesson.
Click to expand...
Had one a few years back that apprentice got hit on a "breaker off" switch change out. It had 30V to ground on a switch with the breaker off. Found the neutrals of 2 circuits tied together in an attic J, one on a live lighting circuit and one for the light in the room that we were working on. Shut off light switch in the other live room and the voltage disappears so definitely a backfeed on the neutral. Don't know what the voltage might have been if there wasn't a couple of bulbs inline that it had to back feed through. This would be the same case if you had a MWBC, and shows example of electricity takes all available paths.
I wonder how a GFCI breaker would see the split neutral current return at the breaker? Would it be tripping seeing less than full return on the associated neutral?
 
If you use your meter and read zero volts from hot to neutral with the switch off does this prove 0 potential for shock or once you introduced your body to the circuit you have now added a path and voltage could now be there sense your reading?
 
Status
Not open for further replies.
Top