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#32138 12/16/03 07:06 PM
Joined: Oct 2002
Posts: 830
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Could someone refresh me on the theory, that when a main neutral is lost on a 120/240 volt circuit, it throws 240 volts on one leg?? Thanks, I've run across this several times in the past.. Steve Has something to do with electricity seeking ground, right??

#32139 12/16/03 08:04 PM
Joined: Sep 2001
Posts: 806
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Member
Opening the neutral connection effectively places the 2 halves of the panel in series with each other, across the 240V line, and the voltages across each half divide according to Ohm's law. If the loads on the 2 halves were perfectly balanced, each side would see 120V, and there would be no detectable problem. If one phase is more heavily loaded, it will drop LESS voltage, with the difference appearing across the opposite side.

Worst case would be a small electronic load (VCR?) on one side of the panel, and a space heater on the other side. The VCR is across 240V, with only the low resistance of the heater in series with it. Unless designed to take that voltage (some are), the VCR is toast VERY quickly.

A sure sign of an open neutral somewhere are lamps that get brighter when larger loads cycle on.

#32140 12/16/03 08:07 PM
Joined: Oct 2000
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Hopefully someone can offer a more technical answer here, but basically the voltage on each 'leg' will depend on the resistance/impedance of the connected load on that 'leg' at that moment.

Both Voltages could be equal if both loads were perfectly balanced, but Murphy's Law says that one will be high and one low.

Bill


Bill
#32141 12/16/03 08:09 PM
Joined: Oct 2000
Posts: 4,035
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NJ,

I've got to learn to type faster.
Your post wasn't there when I started typing.
Good response.

[Linked Image]
Bill

[This message has been edited by Bill Addiss (edited 12-16-2003).]


Bill
#32142 12/16/03 08:13 PM
Joined: May 2002
Posts: 1,716
R
Member
Steve, first of all it will not put 240v on one leg, it will be E = I x R on each load in the circuit, which is now a 240v series circuit.


Now, electricity doesn't seek ground, (earth) it seeks it's source.

It can however use ground (earth) as a conductor.

Roger

#32143 12/16/03 09:42 PM
Joined: Oct 2000
Posts: 5,389
S
Member
sometimes a pix helps....
[Linked Image]

#32144 12/16/03 09:49 PM
Joined: Dec 2000
Posts: 4,287
Member
Any circuit that shares a "NOODLE" [Linked Image] with the other will be a series circuit.
The circuits would be just fine if they were equally balanced, but they aren't usually, therefore the voltages "float", according to the load.
I'm crappy at explaining this...
Help from Scott T??

#32145 12/16/03 10:10 PM
Joined: Dec 2003
Posts: 524
Member
y'know, I had this happen...not a week ago...I just got finished making my splices in a j-box in the attic...(of course,where else would I be...?)and I had brought up a 14-3 nm from the basement, to the attic for some receptacles and lighting...the 2 phases were then energized and on finishing, I realized I didn't like the way the noodles were spliced so I untwisted them,and to my surprise...the temporary pig-tail lampholders I had strung up had SUPERNOVA'd...I put the "tidy whities" back together and the lamps went back to normal again...I really got scared...I thought somethin' was really awry here, and I called a buddy o'mine who told me to "calm down" and that "that phenomenon is a normal occurance"...phew, I thought somethin' was wrong with my wiring and the sheet-rocker just got done w/ his 1st coat of spackle...I had split the neutral between the receptacles and the lighting....I'd never run into this until recently....boy did I feel stooopid!!!...I also came up on the reverse of this....after losing the noodle on a 2-wire circuit,that I was troubleshooting, I put my DMM across the hot and neutral and read about 78 volts...I guess thats because the current was reading through the lamp filaments and the resistance thru it lowered the actual 110-120volts supplied to it....I eventually found the bad neutral splice... [Linked Image] [Linked Image]

[This message has been edited by Attic Rat (edited 12-16-2003).]

[This message has been edited by Attic Rat (edited 12-16-2003).]


.."if it ain't fixed,don't break it...call a Licensed Electrician"
#32146 12/16/03 11:11 PM
Joined: May 2002
Posts: 1,716
R
Member
I know this is primitive and simple but here goes. Don't pay attention to the arrows.

[Linked Image]

The circuit has 5 amps flowing.
If we look at each load and use E = I x R

The 36 ohm load would see 180 volts

5 x 36 = 180 volts

The 12 ohm load would see 60 volts

5 x 12 = 60 volts

180 + 60 = 240 volts.

I will practice before I post any more drawings. [Linked Image] [Linked Image]

Roger

#32147 12/17/03 11:57 AM
Joined: Aug 2001
Posts: 7,520
P
Member
Just to add that the situation will become a little more complex if a parallel neutral path exists.

If you lost the incoming service neutral, there may still be a circuit back to the transformer neutral via the grounding electrode, water pipes, etc.

The resistance of this path will be higher than that of the service neutral. The greater this resistance, the more the imbalance between the two sides of the circuit will be.

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