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Floating Neutral

10/11/2009 7:51 PM

The neutral of a hotel electrical system im managing floats intermittently. Though the occurence is rare, the damage it causes is very costly. We have already identified the problem area but since the cables are buried, finding the source of the anomaly will take a lot of work. In the meantime i would like to protect the establishment. I thinking of setting up a by-pass by installing an isolation transformer. What do you think?

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#1

Re: Floating Neutral

10/11/2009 9:26 PM

Hi Shang,

Ground the neutral to a good earth pit at the site.

Rthe16th

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#2

Re: Floating Neutral

10/11/2009 9:34 PM

Install a temporary neutral from the transformer to the main switch or where ever it needs to go.

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#3

Re: Floating Neutral

10/12/2009 11:17 AM

All considered and noted...thanks guys

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#4
In reply to #3

Re: Floating Neutral

10/12/2009 10:33 PM

A isolation transformer will be costly depending on size of load. Space may be a problem. A new Neutral from the transformer to the panel or switch will work. Neutral to be same as in the original cable or sized accordingly for load per code requirements.

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#5
In reply to #4

Re: Floating Neutral

10/12/2009 10:58 PM

There is a chance that a solid ground at both ends of the buried conductor might discover a partial short and this might draw more with time, especially if it has done costly damage in the past. Thus this double ground will probably protect equipment, but runs a risk of provoking a hard fault in the buried cable, which will require it's repair.

in any event, it is probably wise to call the power company if they provided the hookup, so they will fix it free. If it is your hookup they might cut power right away until you repair it due to safety risks. It is indeed a risk.

There is another possibility. See if you can get buy/borrow an infrared pyrometer and inspect the entire current path for hot connections. This is especially true if there is any aluminium wire in the system as it is notorious for using differential expansion to progressively infiltrate a junction with non conducting Al2O3, as it heats it gets worse.

The pyrometer allows you to make non contact temperature measurements. This one is $300 to 400 US$. explore their catalog, they might have cheaper ones

http://www.omega.com/ppt/pptsc.asp?ref=OS201_202_203_205

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#6
In reply to #3

Re: Floating Neutral

10/12/2009 11:24 PM

Depending on the depth of the mains cable and material above the underground mains, surely the circuit can be isolated for long enough for a RF trace to be used to find the open circuit. You really have no option, as the cost of problems that this can induce can soon over ride the cost of excavating and repairing. I should think that if there were any serious occurrences happening because of the neutral problem, you would be legally in a mess and the supply authority and/or safety body in you state/country would probably take severe action. The voltage between phases, without the neutral, will at times become very high, depending on the out of ballance current value and high enough to destroy any expensive quipment. If you have a neutral fault right now, you may soon have a phase fault any how, because you do not know why you have the neutral fault. This unknown would make the Isolation transformer an unlikly solution, apart from the cost. Get moving and act now, by either running new mains or a new neutral. Your local laws may prohibit the neutral being rerun seprate to the active phases, but then who ever is responsible would probably have that knowledge.

Bill Pilgrim, Queensland, Australia

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#7
In reply to #6

Re: Floating Neutral

10/13/2009 12:35 AM

This is retired ch engineer from hotels for over 30 years behind. regarding your floating neutral i suggest a simple suggestion set up weekly checking exercise for neutral connections in all your switch boards and switches and tighten the nuts of neutral as well as phase connections probably you would get over this problems because these connecting nuts have a nasty habit of turning loose over a period of time every time you operate the switch the nut will loosen itself slightly.good luck

crm

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#8

Re: Floating Neutral

10/13/2009 12:41 AM

I am in agreement with all.

I do wonder why the neutral would intermittently float?

Is it possible there is a phase draw event that overloads a leg so that the floating neutral goes hot, and the ground has been long broken?

Though I only ran temporary tie ins for sets single or three phase I did attempt to balance loads and it does seem that you can get away with backfeeds on the neutral without a proper ground as long as you do have phase balance.

Certainly for a facility you need a good ground independent of the power company supply, which often shares grounding for their system, but not necessarily your facility.

Though I am not a very good electrician, I did become aware of the Megger Meter, and wonder if it is the tool you need to figure where the break of the ground line is.

P.S. Some of the Veterans may remember our friend Sparkstation. I miss him for he would have been quick to answer this question with certainty that I would defer to.

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#9

Re: Floating Neutral

10/13/2009 3:45 AM

Having spent several years as a trouble shooter for a large power company I can assure you that there is nothing uncommon about a floating neutral. As several people have stated you should clean and tighten all connections on your neutral immediately. That usually takes care of the problem. If you are near the ocean floating neutrals are the number 1 trouble call as the salt air corrosion is murder on the connections.

You don't say what kind of buried cable you have. As old as the installation is it is possible that you have insulated conductors with an exposed copper clad, concentrically wrapped neutral. These type neutrals will react to the PH of the soil and will corrode and break down. If this is the case you can install a good earth ground, I'd go for 10 Ohms or less, at each end of your cable run and connect all of your neutrals to the new ground. This solves part of your problem. I'd then install an appropriately sized PVC conduit and pull in new cable (hot legs and neutral). Again - - based on 35+ years of experience, if your neutral has deterioated to the point of not being servicable then your conductor is not far behind in the race to failure.

Don't dally too long. This problem is doing a heck of a lot of damage to your motors and compressors in addition to blowing light bulbs and creating a fire hazard. Hope this helps.

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#10

Re: Floating Neutral

10/13/2009 11:34 AM

I like the answers you have received thus far. The neutral is established at the transformer. While we do bond the neutral to the ground at the main overcurrent device(s) "service", if we loose the wire connection then all we have is a connection through the earth which is high impedance. Improving the ground will probably do little to help in this situation. It seems strange that an underground cable would open and close again. I tend to agree it could be loose terminations. Torque the terminations, use anti-oxident on aluminum, balance loads if possible. Good luck

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#11

Re: Floating Neutral

10/14/2009 11:55 AM

hello guys,

Sorry for the late reply, an AVR trouble got my attention. What we have here is a common neutral -ground system, and the installation is just more or less a year old and very far from the sea. Im really trying to speed up the repair but since major civil and mechanical works will be involve, the approving process takes time, not to mention the project itself. so since we are trying to provide a solution to every aspect of the problem, we thought of setting up a neutral line by-pass using an isolation transformer. Our goal is to protect the establishment while the long-term solution is still on the works, which as i have mention will take a long time to implement and to accomplish. The isolation transformer is quite costly, but so is the damage the problem is causing. The anomaly as we have analyzed has a lot to do with the defective grounding system. Thus laying out another neutral line might just inherit the problem at hand. Technically, is the plan good enough or do you know of a better solution?

Thanks Guys...your thought are very much appreciated

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