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#18246 - 12/07/02 05:56 AM HELP!!!!
sparky806 Offline
Member

Registered: 12/07/02
Posts: 59
Loc: Shawnee, KS, USA
I am working on a 10 story hotel that was struck by lightning. I have replaced all service equipment,sub-panels, feeders, etc. The local utility has replaced their pad-mount xfrmr. The building is fed with two 2,000 amp nema 3R Siemans switches with GFI and shunt trips. These each feed a 2,000 amp MDP, one for heating/cooling load, the other for lighting and general use. The problems I am haveing involve the grounded conductor on the heat panel. Panel is fed from switch with 5 sets of 500mcm with a 250mcm ground in each set. While checking nuetral current, we discovered that each nuetral conductor had a different load, with a difference of up to 80 amps. Further checking revealed that the nuetral current at the main switch is non existent, however I am showing about 20 amps at the ground rod, and about 15 amps at the water ground. This is going on while pulling roughly 1,400 amp on the service. Service is 480/277wye, all equipment is new, and grounding is well above NEC requirements. My question is...if I have no current on the nuetral, and only 15-20 amps on the ground, WHERE is the nuetral current going. I have had 3 master electricians and 2 electrical engineers on this for a week, we have checked and rechecked everyting we can think of, and have gotten nowhere. Any suggestions?
Richard

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#18247 - 12/07/02 06:57 AM Re: HELP!!!!
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
What is the expected neutral current if you were to measure each phase and calculate the expected neutral current? Is it possible there is an accidental n-g bond and the leakage is traveling through the ground? I realize you said that the ground rod is not carrying all of the neutral current, but you said the grounding is more than NEC. If you bonded structural steel, fuel tank, etc, they might be acting as your grounding electrodes and a single measurmeent at the ground rod will not suffice to track down the "missing current".
_________________________
Ron

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#18248 - 12/07/02 07:38 AM Re: HELP!!!!
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
Richard
What are the different neutral currents that you are recording. Should this heat panel even have that much neutral current in the first place. Does the other panel check out as normal in respect to line and neutral currents. Have you rechecked that each phase neutral and ground is in each conduit and one didn't get crossed along the way. Is it possible that you have some other load downstream, like a transformer or a motor that may have been damaged and faulted in ths strike? Have you switched the branch circuits off to see if you can isolate the problem? Some of these are obvious questions I am sure your crew has already checked. If you provide a litle more info some of the other guys may be able to offer some real help.
Good Luck
Watthead

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#18249 - 12/08/02 08:28 AM Re: HELP!!!!
sparky806 Offline
Member

Registered: 12/07/02
Posts: 59
Loc: Shawnee, KS, USA
Ron,
A=1372amps, B=927amps, C=1646amps. N should be approximitly 90amps. I have 20amps at ground rod, 15amps at water ground. That leaves 55amps not going to nuetral or ground.

I do have the correct nuetral current at the MDP, but when measured at the load side of the main switch (keep in mind, I am measureing oppisite ends of the same conductor, less than 10 feet long), I have a total of less than 10amps.

Thia would normally indicate to me that I have a poor or open nuetral and it is going to ground. These are new conductors and have been megged with an Amprobe AMB-5D megger. I am useing a Fluke 41B power/harmonics analyzer for all other readings. I have rechecked current/voltage reading with 2 other meters to verify my readings. All show the same thing.

In response to Watthead, nuetral currents vary as much as 80amps between parralelled conductors. This panel feeds 165 P-Tacs (room heat/cool units), so a balanced load is not possible. We megged and ohmed out all conductors to verify nothing was crossed. All step down transformers in the building have been replace, all motors have been megged and replaced or repaired as needed. We have turned off each circuit from MDP and cannot isolate it to any one circuit.

Saturday, I found another problem (or clue).
While checking feeders to a 600 amp sub-panel, I found 60volts on my phase conductors while they were disconnected at both ends. when checked from a hot to any one of these conductors, I read 280volts. Conductors still disconnected and have megged above 10,000 meg-ohms.

This would be an easy problem to TS on a branch circuit, but I have never seen an entire system do this. If I don't solve this soon, I may start looking for a mental health forum. Thanks,
Richard

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#18250 - 12/08/02 09:03 AM Re: HELP!!!!
RoyGBiv Offline
Junior Member

Registered: 12/08/02
Posts: 4
Loc: Chicago, IL, USA
Hi,

Where are you located?

It sounds like you might have a floating ground involved (could be from your service provider" and it is also possible that your electrical conduit is HOT which is giving you the 60V "induced voltage" readings on your conductors when they are disconnected. Try driving a ground rod OFFSITE and isolated from the building structure to see if you get any voltage from your conduits to the New Ground Rod?

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#18251 - 12/08/02 09:20 AM Re: HELP!!!!
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Sparky,
With those phase currents, I come up with a calculated grounded conductor currrent of 628.5 amps. The neutral current on this system is the square root of (A^2 + B^2 + C^2) - ((A x B)+ (A x C) + (B x C). A^2 = a squared
 Quote:
I do have the correct nuetral current at the MDP, but when measured at the load side of the main switch (keep in mind, I am measureing oppisite ends of the same conductor, less than 10 feet long), I have a total of less than 10amps.

This would indicate a parallel path for the grounded conductor current. Have you measured the current on the main bonding jumper?
Is there any way you can post a oneline of the system so that we can look at it?
Don


[This message has been edited by resqcapt19 (edited 12-08-2002).]
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Don(resqcapt19)

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#18252 - 12/08/02 09:29 AM Re: HELP!!!!
sparky806 Offline
Member

Registered: 12/07/02
Posts: 59
Loc: Shawnee, KS, USA
I'm in the Kansas City MO. area. The conduit being hot is a concern considering the water is bonded to the ground system. I have discussed the safty aspects of this with Hotel managment and the EE's on site, but they don't think it justifies closeing a 200 room hotel. I will try the new ground rod monday morning. This is a 30 year old building useing the emt for an equipment ground. I have pulled grounds to everything that I have replaced, but I still have miles of ungrounded/unbonded conduit.

BY the way, I have kept the hotel open and running since the lightning strike on Augest 17 '02. We ran on a 2meggawatt gen-set for 8 weeks useing 800-1000 gallons of diesel fuel a day. Current damage estimates are 1.3 to 1.5 million dollars. Profit will be about 45%. Not a bad job for a little ol' service company that usually does about 4 mill. a year in gross sales.

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#18253 - 12/08/02 09:40 AM Re: HELP!!!!
sparky806 Offline
Member

Registered: 12/07/02
Posts: 59
Loc: Shawnee, KS, USA
I can post all my Fluke 41B reading monday evening. I will also post damage photos and some pics of the new system. I also have a testing co. coming this week to do a fall of potential test and Hi-Pot all of my conductors. I will post those results as soon as I have them.

Resqcapt19....I believe you are correct on the nuetral current. Been a long time since I had to do that calculation.

I suppose there could have been a grounding grid of some type installed when the building went up that I am unaware of.

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#18254 - 12/08/02 09:55 AM Re: HELP!!!!
RoyGBiv Offline
Junior Member

Registered: 12/08/02
Posts: 4
Loc: Chicago, IL, USA
I had a similar problem in a manufacturing facility with over 1 million square feet of space. The problem was isolated in one area of the building and was there for over 25 years. It was determined that the fault was with the utility company but I had some reservations on this theory even though it was completely feasable. After numerous attempts to locate the problem, the maintenance electricians finally gave up trying to locate the problem. The problem also involved 480v electric heaters. We had several shutdown periods where I tried in my spare time to locate the problem, but did not have quite enough time to open everything up and put everything back together before we started up the plant again and then there was also the problem of the mysterious voltages and amperages going away and returning to normal conditions.

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#18255 - 12/08/02 05:02 PM Re: HELP!!!!
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Small aside — for sets of paralleled conductors, the shorter the conductor run, the more difficult it is for shared currents to divide equally.

Now the long shot... The utility padmount transformer is described as having a 480Y/277V grounded-wye secondary. Are the primary windings of that transformer {served from the utility distribution system} also a grounded-wye interconnection, as opposed to ungrounded-wye or delta?




[This message has been edited by Bjarney (edited 12-08-2002).]

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