Floating Neutral?

Doc13067

Doc13067

Senior Member
Location
Bucks County/Philadelphia, PA USA
I recently came across something I was not able to figure out and only had a limited amount of time. I still may have to go back so I wanted to get some opinions. I got called to a mobile home because the occupants claimed power was out in a lot of rooms. When I got there, I found that phase to phase I had 240 V but from A to neutral I had 170 and from B to neutral I had about 70. When I shut everything off it balanced out with 120 on each phase to neutral. Then I turned the branch circuits on one at a time and each time the voltage was imbalanced. I would turn the breaker back off and the balance would return. When I was done, there were four single pole circuits that had to stay off in order for the voltage to stay balanced. Is this a loose neutral connection at the transformer that causes it not to get noticed until a load is put on the neutral? I was taking my measurements from the line side of the meter and I found out whatever condition existed. It existed from the indoor subpanel bus bar to the line side of the meter which is the last place I can get I’m reading from, but I felt that since the conditions were exactly the same between the bus bar and the line side of the meter the problem probably does not exist between those two points so it’s either on the branch circuits or ahead of the meter socket and I am hoping against hope that the problem is not somewhere inside the home. Does it make sense to think that it was one neutral connection at the transformer causing the issue?
 
Doc13067 said:
Is this a loose neutral connection at the transformer that causes it not to get noticed until a load is put on the neutral? I was taking my measurements from the line side of the meter and I found out whatever condition existed.
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Was this an overhead service? The problem could be at the drop connection. If you check for an open neutral at the line side of the drop that would tell you whether or not it's at the transformer.
 
When you get the neutral issue fixed, check and make sure the MH is connected to an equipment grounding conductor. Many are not. If the problem exists at the line side of the disconnect, like others have said, it is probably on the utility side. Call the utility and have them check their side, and while they are there, pull the meter and inspect the connections there.
 
Doc13067 said:
Is this a loose neutral connection at the transformer that causes it not to get noticed until a load is put on the neutral? I was taking my measurements from the line side of the meter and I found out whatever condition existed.
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Anywhere from the transformer terminal to the meter terminal.

Most likely the neutral conductor underground.
 
Doc13067 said:
Ok great. And the reason I’m only seeing this with single pole circuits on is because the neutral is connected enough to give 120 volts but probably not enough for any significant amperage to flow on it, right?
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It's because 1p circuits put load on the neutral, while (most) 2p loads don't.
 
Yes, you have a "weak" neutral. With no current flowing on it, there is no problem and the voltage is balanced.

As soon as there is current flow, the weak neutral can no longer effectively keep the "zero" point at zero. Line to Line will still be 240, but L to N will start to swing away from 120. This is a terrible situation for many 120v loads. Lights burn out, appliances get damaged.

As mentioned above, your problem is upstream of the Line side of the meter, so it's a POCO problem.

If you do get the POCO onsite – a classic failure on their part is to pull the meter and measure voltages. And they declare all is good on their side and leave. Naturally, with no current flowing, the weak neutral problem is not "exposed". Either show them some of your testing, or ask them to load the service themselves. A good lineman will have a “Beast of Burden” tester on the truck and know how to use it.

Good luck.
 
Doc13067 said:
Ok great. And the reason I’m only seeing this with single pole circuits on is because the neutral is connected enough to give 120 volts but probably not enough for any significant amperage to flow on it, right?
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Every time you add or remove a 120 volt load you change the resistance as indicated in Roger's drawing. That is going to change voltage seen across items on the associated conductors. The closer you get to having equal load on each leg, the more it acts like things are normal as voltage will be closer to same on both sides of neutral. An intact neutral back to the source stabilizes voltage of each leg to about 120 even if load is unbalanced.
roger said:
It's because the service is a Multi Wire Circuit and loosing the neutral it's becoming a series circuit.

View attachment 2578461
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MD Automation said:
A good lineman will have a “Beast of Burden” tester on the truck and know how to use it.
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In absence of such tester all you need is a simple space heater, heat gun or similar load. Make up some sort of pigtail with alligator clips to connect to where you want to test. I suggest shutting main off and testing supply side so you don't end up unbalancing voltage and end up putting say 200 volts across items that can't handle that sort of voltage and destroying them, if they aren't already damaged from the conditions they been exposed to. If you are connected to incoming service conductors and turn on that heater, and check voltage, it should only have a minimal drop, say a volt or two. If it drops 30-40 or even 100 volts neutral is bad somewhere between your test location and the source.
 
Doc13067 said:
I found that phase to phase I had 240 V but from A to neutral I had 170 and from B to neutral I had about 70. When I shut everything off it balanced out with 120 on each phase to neutral. Then I turned the branch circuits on one at a time and each time the voltage was imbalanced. I would turn the breaker back off and the balance would return. When I was done, there were four single pole circuits that had to stay off in order for the voltage to stay balanced.
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When I encounter this I leave the main breaker off and call the utility as the over voltage could fry equipment, its not really safe to leave anything on.
Side note I have seen the under voltage make all the GFCI's and AFCI's in a house trip and people think they have a breaker issue when its a really a utility problem.
Doc13067 said:
Does it make sense to think that it was one neutral connection at the transformer causing the issue?
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Yes, utilities use the cheapest wire that will still do the job so it often is the culprit.
 
Georgia Power has a lot of underground neutrals fail, don’t know if it was bad installs or cheap wire. Up here with the EMC’s, it’s usually bad connections at the transformer, or meter.
 
I'd kill the main breaker right away. The way the 'customer service' works now I have the customer call and report a 'power outage' and thats it. Do whatever you need to do to get a lineman out there.
Leave the power off until you get your POCO out there and work with your local lineman, I got to know a local POCO guy here due to dealing with this kinda 'service work'.
He said they often use the old cheap alloy's of AL wire that the NEC no longer allows, in sizes the NEC would never allow, he tests service laterals with a funny device that looks to be a suitcase of hairdryers I wish I had a photo of it it looks custom made, all the testing is done with the meter out and the main off.
He also has a really nice expensive crimp tool and a new crimp usually fixes it.
 
You need to be onsite when the POCO comes. My experience is they will come and do a quick survey and declare all is good. Make them pull the meter so you can check the connections, then have them physically check their connections from meter to transformer.
I had a few where they came with no one there and declared all was well. They did this a couple of times until I was called in. I waited on them to come and made them pull the meter, and do all their checks. They found a splice point in the underground had failed.

Moral of the story, don't take POCO's word for it unless you are there!
 
pipe_bender said:
He said they often use the old cheap alloy's of AL wire that the NEC no longer allows, in sizes the NEC would never allow,
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Most POCO's do still use aluminum alloys that pre date the AA8000 series that NEC requires. NEC doesn't apply to them, many those connections are still good if done properly and particularly if using high pressure crimped devices. Ampacity of a conductor is different out in free air or underground where ambient usually won't get very high than it is for typical indoor installations. If something out on the pole burns up it isn't seen as being as much of a threat as if you have similar circumstance within a building, so they do have some different practices than what NEC requires because of this kind of thing as well.
 
More than once I've had to tell the POCO guys to check the voltages with the main breaker ON.

Years ago, a customer had open-neutral syndrome, and the POCO swore I was wrong . . . until I told them to go back back and test it with the main breaker on. :rolleyes:

They repaired the underground break (aluminum oxide about 2' long) the same day.
 
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