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"Real electrician question"

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Bro in law is busy, can't call.  here's the deal:



Hotel Ballroom.  Ac outlets all around, total of six circuits either in the ballroom or in the back hall behind the room, all 120.



The panel that feeds them is spread between these and some kitchen 208 AND 240 stuff.  All behind a 75 (?) KVA Delta Y box.  Pretty spread out, but nothing really odd at all.



 



 



Problem.  Every circuit in that ballroom is showing 70-SEVENTY! volts neutral to ground.



 



I am going to have a helluva time using this for video and audio gear.  I don't have the chance to start looking around yet, and I may not for a while, but I have to find a problem to point out to get these building owners off their duffs and on the phone.



Where do you start looking?  What little I have been able to do-that transformer is humming like a banshee.  Bad loading?



Hepl, I have to be able to sound half intelligent on the phone tomorrow.


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correct.  I am interested in how a "pro" goes about troubleshooting.  Where to start n all.


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probably lost the neutral somewhere along the way, if its the whole circuit i would start in the panel.

 

something is powered on the circuit that is back feeding into the neutral with no path back to the panel.

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Quote:

Originally Posted by Plug and teaser View Post

 

3 Phase stuff loses me every time....headscratch.gif

 

Three phase I get, its Delta Y that pisses me off.  One of the biggest causes of noise in an electronic system is an unbalanced transformer, and a delta is unbalanced by design

Quote:

Originally Posted by Plug and teaser View Post

 

Just curious...if he has 70 volts from the neutral to ground, what does he have from the hot side of the plug to ground ?

 

120, all good there

 

 

Quote:

Originally Posted by ted527 View Post

 

probably lost the neutral somewhere along the way, if its the whole circuit i would start in the panel.

 

something is powered on the circuit that is back feeding into the neutral with no path back to the panel.

 

its six or seven circuits out of many.  I haven't had the opportunity to look at anything else on the panel.

 

If I get the chance to open the panel with meter in hand, what would i look for-keep in mind I am not diagnosing and repairing here.  i just have to be able to point the owner toward the cause with some credibility to get him off his arse and do something (plus brownie points for moi-"gee that guy knows his poop")

 

 

 

 

 

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first thing i would do is open the panel and make sure EVERY neutral in there is tight.

 

with a meter go from neutral bars to ground and see if you read anything, if it is a sub panel neutrals and grounds

should be separated so it could be a problem in the supply to the panel.

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Be careful! Nothing is worth killing yourself over...when in doubt, call a licensed electrician.

 

If only a few branch circuits on the entire panel show 70V N-G, looks like the neutral on these branch circuits is floating (check the N-bus to G-bus reading in the panel first). Like ted said, check to make sure the neutrals are torqued down properly (my guess this is the problem). Are the circuits with the 70V N-G sharing the neutral?

 

 

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Since there can be 3 hot 120VAC wires, with the usual wire colors black, red, blue.

 

I have never heard of 70VAC on a neutral, although I have seen many times under 20VAC but mostly a reading of less than 5VAC.

 

I have also miss-read my meter and been surprised at the very high "millivolt" readings with double or triple digits like 0.70VAC

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Any industrial type stuff near the hotel?

 

This is all conjecture on my part but if you can lay your hands on a oscilloscope and view what the AC looks like you may be able to determine what is going on. Since the hotel is using a poly phase or delta Y transformer to supply 3 phase to various circuits within the hotel and on those circuits more than likely have a non-linear loads on them the system may develop harmonics and cause a basic voltmeter to read inaccurately including what's across neutral and ground.

 

With a scope, if you're getting a wave form something like this rather than a pure sinewave the voltmeter will most likely read incorrectly even between neutral and gnd but 70VAC is sort of high even for that.

 

305

 

I work in a shipyard. The lab I work in has a 200KVA polyphase Liebert power conditioner that supplies power to over 200 computers and other ship board type loads which has been detecting a ground fault for as long as I can remember. It is fed by a dedicated polyphase transformer within the plant that is served by 3phase as well. There is no ground fault in the lab but the power conditioner is thinking that one exists. Our facilities electricians and engineers concur that its harmonics created within the plant power system and assure us grounding is OK. I don't know what the V is but there is an imbalance between neutral and ground when using a meter and an o'scope reveals anything but a pure sine wave on the lab AC circuits. Maybe the hotel has something like this situation. Anyway, it's just a guess. I'd start with an o'scope and look for AC waveform distortion to start and maybe try to rule that out.

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