My background is civil / structural, and occasionally I run into electrical items or questions. I called a UPS manufacturer with a question (after the UPS failed), they answered quickly and got off of the phone. I did not fully understand and have been unable to get them back on the phone. Electricians were working in the area; on other equiment; and the only common point that they have is the main power distribution panel, where they both have breakers (the new equip. & the existing equip.). The manufacturer said they must have dropped the neutral, or lost the neutral? What does this mean? It must have been a one time thing, because everything is running and operating properly now. Thanks in advance for any answers!
Dropped neutral?
- Thread starter gene7
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barbeer
Senior Member
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- Pinellas County, FL
Dropping a neutral means the grounded conductor of an ac system would somehow be caused to no longer be electrically and physically connected to the system. Weird things and bad things can happen under that condition.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
You didn't mention the voltage of the power system. I will guess that it is the 240/120 volt system that is commonly used in small commercial facilities. It may be a 120/208 volt system, if the building is larger. Both are subject to the same unusual behaviors, if the neutral is "dropped" or "lost." That term means that the neutral wire has been disconnected at some point from the original source to your load. Many things can happen in such an event, and very few of them are good things.
In a 240/120 volt system, for example, the voltage between the two "hot" wires is 240 volts, and the voltage between either hot wire and the neutral is 120 volts. If loads are generally balanced between the two phases, then current will mostly flow out (from the source) on one hot wire, and back to the source on the other hot wire, and the neutral wire will be carrying very little current. Indeed, the neutral wire will only carry the unbalanced current, based on how much load is connected to each of the two phases.
But if the neutral wire becomes disconnected, the only remaining path is from one hot wire to the other hot wire. That will likely cause the voltage seen by the loads on one side to be different from the voltage seen by loads on the other side. If the loads are very much not in balance, then the difference in voltage can be severe enough to either burn out something (on the higher voltage side) or result in an undervoltage trip of equipment on the other side.
It is likely that what you saw was an undervoltage trip of your UPS. Once the neutral wire was reconnected, the source of the problem will have disappeared.
In a 240/120 volt system, for example, the voltage between the two "hot" wires is 240 volts, and the voltage between either hot wire and the neutral is 120 volts. If loads are generally balanced between the two phases, then current will mostly flow out (from the source) on one hot wire, and back to the source on the other hot wire, and the neutral wire will be carrying very little current. Indeed, the neutral wire will only carry the unbalanced current, based on how much load is connected to each of the two phases.
But if the neutral wire becomes disconnected, the only remaining path is from one hot wire to the other hot wire. That will likely cause the voltage seen by the loads on one side to be different from the voltage seen by loads on the other side. If the loads are very much not in balance, then the difference in voltage can be severe enough to either burn out something (on the higher voltage side) or result in an undervoltage trip of equipment on the other side.
It is likely that what you saw was an undervoltage trip of your UPS. Once the neutral wire was reconnected, the source of the problem will have disappeared.
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