Why am I seeing voltage on the neutral wire from my 3 phase power supply?

Question

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.

Answer 1

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:

• Why do my bulbs glow brighter when the microwave is running?
• How can I determine why my lights are flickering?
• Can my apartment's electricity harm my computer? Is there an easy way to test the electricity?

Answer 2

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.

Answer 3

I'm not sure how your installation was made, but since you have 3 different phases distributed through the house and you have voltage in the neutral what makes sense to me is if you are using the same neutral for different phases. I'll illustrate it:

If you have the same equivalent resistances (or same amount of power draw) being used through the 3 sectors of the house you the neutral will be equal to ground (very near).

If you don't have nothing connected, the neutral should be at floating point (near cero)

If the resistances of the house (or power draw) aren't the same at some given time, the voltages will add as an AC Voltage in the neutral line. Resulting in lower voltage in one sector of the house and over voltage on the other. Normally you have over voltage in the sectors of less power consumption (TVs, PCs) and lower voltage in sectors of greater power consumption (boilers, etc).

What you need to do (or I would do) is make the neutral lines for each phase independent, using ground rods.

ps: I'm not an expert.

Answer 4

Simple if there is break in neutral you could find voltage appearing in it which could be fatal to the equipment and appliances

Answer 5

If you have a wye (4-wire system) configuration coming into the house, the fourth wire is the neutral or return (has nothing to do with ground!). If appliances are drawing power from only one phase, the same current showing as coming through the phase's hot will show at the neutral (return) and zero amps should register at the other phases' hot. Without knowing more specific circumstances of the power company distribution system involved here, phase balancing should not your concern as your full load in one phase respect to zero in the others probably represents less than one hundred-thousands of the phase's winding capacity of the power company generator, thus very little chance of additional vibration. In a wye configuration all 4 wires are coming from the power company transformer, where the three phases' hots are needed for three-phase loads and any two consisting of one phase's hot and the return are required for single-phase loads. In the USA, the neutral needs to be grounded at the panel in order to provide a return for GFCI (Ground-Fault Circuit Interruption) protected outlets.

Rodolfo Electrical engineer

Answer 6

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.