broken splice caused 120v to jump to 158v?

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Tibz78

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While working on replacing receptacles in a dormitory hallway I noticed the fan that was plugged
into the receptacle downstream started to spin very fast when I broke the splice of the neutral.
I had to work on these live because some of the circuits were tied into rooms that were occupied.
the voltage with the neutrals together was 119v, neutrals separated it jumped to 158v.
This is an old building built in the 40's, so I know some of the wiring isn't up to par.
Im the first licensed electrician on campus and I've noticed panels packed full of wires.
What would cause that voltage to jump like that? Any thoughts?
 
You broke the neutral on a multiwire branch circuit . You are lucky you didn't fry the fan. When you break the neutral on a MWBC one phase will shoot up while the other will drop. Unfortunately you end up putting 240V thru the system
 
Follow this diagram and see what happens to Load 2 when the neutral gets broken from a MWBC

ry%3D480
 
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Tibz78 said:
I had to work on these live because some of the circuits were tied into rooms that were occupied.
Click to expand...
That is no reason to work on a live circuit. There are very few instances that working live circuits is allowed for anyone. Who was making you do this?

Roger
 
Dennis Alwon said:
Follow this diagram and see what happens to Load 2 when the neutral gets broken from a MWBC
Click to expand...
240V drop across Load 2 would only occur if Load 1 had extremely low current impedance (such that the voltage drop across Load 1 was 0V).

The actual voltage drops across Loads 1 and 2 would be inversely proportional to their impedances, and sum to 240V (assuming a 120/240V 1? 3W circuit).
 
Tibz78 said:
While working on replacing receptacles in a dormitory hallway I noticed the fan that was plugged
into the receptacle downstream started to spin very fast when I broke the splice of the neutral.
I had to work on these live because some of the circuits were tied into rooms that were occupied.
the voltage with the neutrals together was 119v, neutrals separated it jumped to 158v.
This is an old building built in the 40's, so I know some of the wiring isn't up to par.
Im the first licensed electrician on campus and I've noticed panels packed full of wires.
What would cause that voltage to jump like that? Any thoughts?
Click to expand...

with an open neutral, a group of appliances on a line will form a series circuit. Voltages will adjust according to resistance on the line.
If the exact same appliances were on each leg, you wouldn't have noticed a difference.
 
Tibz78 said:
While working on replacing receptacles in a dormitory hallway I noticed the fan that was plugged
into the receptacle downstream started to spin very fast when I broke the splice of the neutral.
I had to work on these live because some of the circuits were tied into rooms that were occupied.
the voltage with the neutrals together was 119v, neutrals separated it jumped to 158v.
This is an old building built in the 40's, so I know some of the wiring isn't up to par.
Im the first licensed electrician on campus and I've noticed panels packed full of wires.
What would cause that voltage to jump like that? Any thoughts?
Click to expand...

Something I made up to explain thisView attachment circuit diagram.pdf
 
Here's a simple to the point illustration from Ed MacLaren.

3wire3.gif


Roger
 
thanks everyone for the information

thanks everyone for the information

Thanks for the information everyone provided.
Im currently going through the process of installing breaker ties and working on shut down procedures acording to NFPA 70. Im the first qualified personal ever hired on this campus so I have the pain staking task of documenting and fixing numorous violations. Believe me most of my findings fall on deaf ears but I just have to fix it so nobody gets hurt. When I first came on I was in an argument with my boss due to the fact that one of the street lights had touch voltage and he said to just install a ground rod and attach it to the pole! After much debate I did it the right way, ran an egc and bonded everything back to the panel, also replaced the ballast.
Im dealing with maintenance personal who don't realize how little amperage can kill you!
Thanks again. I'll post some pics so you can see how bad it is.
 
TIBZ- Remember this is the part of the reason that the neutrals should be pigtailed and not use the device as a means of connecting the neutrals together
 
Dennis Alwon said:
You are lucky you didn't fry the fan.
Click to expand...

Lucky he did not fry the fan, what he may not know is what else was in the MWBC circuit that had low voltage across it, or what else may have had high voltage across it and was not discovered at the time. In order for the fan to see higher voltage it had to be in series with something else to the other 208 or 240 volt conductor, otherwise it would have just quit operating when circuit was opened.
 
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