Dropped Neutral

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Benji78

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Long Beach, California
I have a 120/208 3 phase panel and there is a neutral dropped in one or many of the branch circuits and it is affecting the feeders and the entire panel ill explain.
With all of the branch circuits in the off postion the feeders read normal: 120 from phase to N and 210 between phases, I can turn about 75% of the panel on without the voltage being affected and then when I turn on any of the remaining 25% I get A phase with 20v, B&C with 200v all to neutral on the feeders for the panel which looks like a dropped neutral exept it is effecting 3 groups of circuits (Black,Red,Blue, White) 5 breakers total, 2 from 2 groupings and 1 from another grouping. I tried separating each boat and had mild success because as soon as I started connecting the wires down the line the problem reappeared, even as I went futher and further into the circuit. Also I could turn on an A&B phase from one group and turn on a C phase in another group.
The funny thing is as I was troubleshooting the problem magically vanished and the system returned to normal. I need to mention it is in a multi-tennant complex with about 30 small wharehouse units, and when it came back on the alarm in the unit next door sounded. Any chance this has to do with mixing circuits from different panels?
Thanks for taking the time to read this post and any knowledge to shed some light on this would be greatly appreciated!
 
Check out the neutral in the feeder or service. This sounds like you have lost a neutral upstream from the panel.

Chris
 
I don't believe your problem is past the panel you are in, but before it. I agree with Raider, check out your service or feeder neutral.
 
111228-1315 EST

Benji78:

At your main panel with all breakers off measure the voltages between phases and neutral as you have done. Also measure from neutral to earth (a water pipe or a screwdriver in the earth). The first part you have already done.

Still with all breakers off connect a 1500 W portable heater from a phase line to neutral. What happens to the above 4 voltages? A 100 W light bulb might be sufficient. With a high resistance neutral between the main panel and the supply transformer you will see a shift in the line to neutral voltages. With a single load load from any one of the phase lines to neutral and a high resistance neutral you should see a moderate to large shift of the voltage from neutral to earth, and correspondingly an unbalance from phase lines to neutral.

The magnitude of the high resistance in the neutral in combination with the test load will determine the amount of unbalance, or neutral to earth voltage. If you know the test load resistance and the unloaded line to neutral voltage and only load with this one test resistance then the neutral to earth voltage will allow you to estimate the value of the neutral high resistance. Suppose the test resistance is 10 ohms and with this test load the neutral to earth voltage is 60 volts with a source voltage of 120 V, then the neutral resistance is about 10 ohms.

.
 
if this is a separately derived system, check also the bond between the neutral and the ground at the transformer or first disconnect.
 
iMuse97 said:
if this is a separately derived system, check also the bond between the neutral and the ground at the transformer or first disconnect.
Click to expand...

How would that affect the phase to neutral readings?

I agree t is a neutral issue upstream of the panel or in the termination connection in the panel.
 
Compton-20111228-00164.jpg

So here are my findings:
We had access to the tennant next door and found a pull can in the feeder run for the unit Im working on, as soon as the pull can was opened there was a neutral with a polaris lug that was completely melted and the connection fell apart as soon as I touched it. I cut back the the wire to fresh copper and re-terminated with a new polaris lug, then tightened the lug at the switchgear and the panel.
It looks like maybe the tennant before had some really unballanced loads, Im just speculating because this guy is running a few computers and some office equipment, I cant really see what he has running doing this to his system. Any thoughts?

Again a big thanks to all that have responded I appreciate the feedback it all helped!
 
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