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I am not an electric engineer so I can only explain\understand in layman language. I am facing a persisting problem in my house. I have 3 phase and one neutral till my main electric board through which each phase is distributed to different part of my house, if I divide my house in three parts, distinct phase is going to each part. Sometimes my neutral wire shows full voltage as of the phase, due to which I lost a number of electric appliances. I have an earth(ground) connection as well. Called a number of electric guys, no one was able to rectify it. I need help. Appreciate any help or suggestion.

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migrated from electronics.stackexchange.com Oct 1 '13 at 20:56

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

    
Your question title says "current" and in your question you say "voltage". Can you put this right or clarify what you mean? –  Andy aka Sep 25 '13 at 8:37
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Domestic electrical arrangements vary from country to country and from region to region - it may help if you can specify a geographic location. I suggest you explain what you have measured and how you measured it - did you measure voltage between earth and neutral at an outlet/socket using a CatII multimeter set to AC volts? –  RedGrittyBrick Sep 25 '13 at 8:42
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If your neutral is furnished from the supply, verify that it has not become open. The neutral should carry any phase load imbalance. Have an electrician connect your earth ground to your power box, and verify it to be a adequate ground for the job. –  Optionparty Sep 25 '13 at 9:21
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@OptionParty: I would add that in addition to testing the neutral conductor, the earthing point may itself need to be tested to see if it still provides an adequately low resistance to earth. –  Li-aung Yip Sep 25 '13 at 10:17
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You should be seeing a current on the neutral, if you are running any lights or appliances are all on that circuit. Are you sure you didn't mean "voltage"? –  BlueRaja - Danny Pflughoeft Oct 2 '13 at 20:10
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3 Answers 3

Three Phase Power

In three phase systems, there are three "hot" lines (L1, L2, L3). Often there will also be a neutral (N) and a ground (G). The neutral and ground should be bonded together at your service entry). The three lines are all 120 degrees apart from each other. Loads can be attached in either a wye or a delta configuration. In residential applications, often only two of the three phases are supplied (and different houses will get different pairs of phases as to balance them).

Delta

In a delta configuration, loads are attached between phases (and a neutral is not needed). This configuration is common for large motors and in industrial settings. In some delta configurations, a terminal on the transformer's secondary is grounded and provides a neutral. The ground terminal would be either one of the three lines or a center tap on the coil between two lines (creating a high-leg delta configuration since one of the lines is at a much higher potential (with respect to ground) than the other two).

Wye

In a wye configuration, loads are connected between a line and the neutral. Based on the question, I believe that this is the configuration being used. The power company supplies the three phases and a neutral, and the customer supplies the ground. Normally, the neutral is connected to the ground (which is bonded to metal rods in the earth, water pipes, etc...). If the load is properly balanced (meaning that there are equal currents flowing on each phase), the neutral currents will cancel out to be zero and the neutral would be unused.

However, it is rare that the three lines will be exactly balanced, so there would be a neutral current flowing based on differences of currents in the three phases.

The Root Cause

My hypothesis is that the neutral in the building is not properly connected to the power company's transformer. Without a good neutral connection, the neutral voltage is not held to earth potential (the ground connection usually has 1-20 ohms resistance to the earth). The neutral voltage will drift towards whichever line is the most loaded (as it forms a voltage divider). For example, if L1 has a large load and L2/L3 are lightly loaded, the neutral voltage will be pulled towards L1, causing the L2-N and L3-N voltages to became much larger than their nominal voltage.

So, the fix would be to repair the neutral connection between the building's breaker panel and the power company's transformer. This may be a bad connection of the neutral in the breaker panel, or a failing transformer. Repairing this could be dangerous because the problem might be in a section of cable that cannot be easily turned off (if the break is before the building's main breaker). Working with the power company to turn off your service or check their transformer will likely be needed.

Split-phase Systems

This problem has an analogue in the split-phase system which is common in the United States, and there are related questions on this site:

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If you are testing current, the hot and neutral lines will show the same since they both part of the same loop.

My guess hypothesis is that you are sharing a neutral line between phases and that you will probably not be able to detect with a regular meter. Each hot line from the individual breakers (breaker set for dual pole/pairs/etc) needs to have it's own neutral line; each branch circuit really but trying to explain it instead of throwing around electrical terms, that is how you ensure each neutral line is properly isolated. All the neutrals should join together on a ground bus/bar in the breaker and be the same or joined to the ground bus/bar for the ground lines and all properly run to an outside ground stake or ground utility line.

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I think that if you have 3 phase and 1 neutral wire then it is completely natural* to have voltage on neutral wire due to any kind of load imbalance. So I'm wondering:
1-why you need 3 phase in your house(cause this is used for industrial purposes)?
2-how are you going to balance the load?

Now even if you have a very good earth connection that means if one phase goes off, then the earth wound consume tremendous amount of power for a long time that will probably cause damage to devices or fuse welding.

*note:by "natural" I do not mean that it is ok to have full phase voltage on neutral wire but I mean that it is unavoidable and it may happen if you have load imbalance.

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Having a low voltage on the neutral (a few volts) is normal. Having full phase voltage on the neutral is not normal. –  Li-aung Yip Sep 25 '13 at 16:59
    
@Li-aungYip agreed. Did I say full phase voltage is normal? maybe the word "natural" was a little misleading, but it does not mean "normal". –  Amir Noori Sep 25 '13 at 18:09
    
The original question was about "Sometimes my neutral wire shows full voltage as of the phase". When read in conjunction with the question, your answer suggests that this is 'completely natural'. You might consider editing to clarify this. –  Li-aung Yip Sep 25 '13 at 18:16
    
@Li-aungYip is it clear now? Thanks for noticing. –  Amir Noori Sep 25 '13 at 18:22
    
It is somewhat more clear now. I have removed my downvote. –  Li-aung Yip Sep 25 '13 at 18:24
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