Neutral Ground Voltage

It is really simple - The Line voltage is the voltage between phase and the neutral, so phase V = Neutral + Line voltage (phasor sum) . In case the neutral voltage is high you have mainly problem with a few components especially micro chips, computers etc may burn-off- since they are connected between phase and neutral and the Neutral voltage phasors may create abnormal heating etc (I had a TV destroyed at home and a few computers in office - electronics part) due to the neutral voltage problem The floating neutral voltage will not usually be high enough to cause a safety hazard.

Hi,

the voltage measured between earth and neutral is generally the result of the voltage drop across the neutral wire back to the transformer. If it is too high ( ref local codes) then have a larger section cable installed. It is unlikely that the N-E voltage should give you any problems as there should be no connection between them at your equipment.

Regards

Chas

For me you answer was good and worth a GA.

What many people forget is that on a 3 phase system with neutral and earth, the lower voltage loads are all phase to neutral, 110 volts in the USA and 230 Volts in most of europe.

If for example you have a 10 amp load on each phase (in Germany you are allowed to draw up to 16 amps per phase), then you have !30! amps running through the neutral wire and it is probably the same thickness as the other conductors! Whereas it really should have 3 x the current carrying capacity of the 3 phase connections......or 3 x the cross sectional area.....

This high current over the neutral wire causes a voltage drop over that wire and the "neutral point" wanders from the switch box, up the wire, therefore showing a voltage difference between neutral and ground at the switch box.

I have run subsideray neutrals cables in parallel to reduce the effect, this is allowed in Germany under the power regs here. Whether it is allowed anywhere else I could not say.....sorry. If it is not allowed, it should be!!! Or the cable should be made with an extra thick neutral wire in the first place!!

That seems at odds with the reason for using 3-phase systems in the first instance.

As the phases are shifted 120-degrees, 10-amps in all three phases simultaneously would ideally correspond to zero current in the neutral. Even with quite poor power factors, the worst case would be with just two phases active; and so long as both currents are within allowed limits, the current in neutral should not exceed about 12-amps. So perhaps it could be marginally thicker than the others, but not a great deal.
It's worth looking at a 3-phase power transmission system - the size of any ground wire is negligible. I admit the practical reasons for this are slightly different, but the basic principle is the same.

Have a safe and happy break

Fyz

FYZ,

your maths are wrong. the worst case scenario is when only one line is active when I1 = IN.

As I2is at 120º to I1 the resultant current in the neutral will be about 5.1A in the case you cited.

regards

chas

You do not seem to understand that domestic loads are generally single phase to ground!!!

Phase to phase in the USA gives about 220 volts and 380 volts on a 240 volt system.....

Now do you understand?

When it comes to single phase linear loads on a 3 phase system, Fyz is correct. The currents cancel each other out in the neutral such that the neutral only carries the remaining unbalanced current.

You and Capblanc are on the money with respect to the voltage between neutral and ground. Unless there is an unusually high reading, which may indicate a poor neutral connection somewhere in the system.

What would be considered as unusually high? I could not give a good concrete answer for that. If the NEC (US) allows 5% total voltage drop, then anything over 2.5% line to neutral voltage would be suspect.

Andy; if you have a load of 10 amps on each phase of a wye or star system there would zero amps on the neutral, the neutral carries the unbalance. how ever with nonlinear power supplies can cause a 3rd harmonic with i suggest doubling the neutral size.perry

ONCE AGAIN THE QUESTION IS NOT VERY SPECIFIC.

YOUR VOLTAGE READING FROM GROUND TO NEUTRAL SHOULD BE ZERO. IF IT IS NOT ZERO IT USUALLY INDICATES A GROUNDING/BONDING PROBLEM. THERE IS NO ACCEPTABLE N-G VOLTAGE.

THERE ARE PROBLEMS WITH NEW ELECTRONIC POWER SUPPLIES, THEY HAVE A TENDENCY TO CREATE HARMONICS. THIS MAY SHOW UP ON A NUETRAL AS VOLTAGE DUE TO THE PHASE SHIFT.

SO CHECK YOUR SYSTEM TO SEE IF YOU HAVE ANYTHING CREATING A PHASE SHIFT. THEN CHECK YOUR GROUNDING FOR IMPROPER INSTALLATION.

IF THAT DOESN'T WORK CHECK YOUR GROUNDING FOR IMPROPER INSTALLATION.

IF THAT DOESN'T WORK CHECK YOUR GROUNDING FOR IMPROPER INSTALLATION.

"THERE IS NO ACCEPTABLE N-G VOLTAGE."

Not true and not possible in a real system when current flows.

A few volts will always appear when some current (below rated) is flowing in the neutral. But it should be below 10-20V.

Above this, there is a problem with system grounding and there could be consequences.

What is supposedly acceptable and what happens is apparently two different things. In all my cases there were always some volts running around neutral to ground. Typically in mature grids as in NYC at least three volts showed up in float, or to ground. I did see floating turn neutral to 80 amp hot, during a shoot in Pennsylvania somewhere. Such events recommend leg phase balance feeds for a job circuit. So basically I am agreed from what I've seen and touched or had touch me, that rarely are in ac the neutral actually without any current, and accept that even great grounds still leave some backfeed running around. Typically it is in electricity the case that grounding is not perfect, though some grids are better than others. In the meantime to stay alive I recommend not becoming part of the ground.

If you read a previous post of mine, you would understand that t

although a possible grounding problem can be the problem, on a correctly grounded system, you can still get this problem when heavy single phase currents are drawn, due to the fact that the neutral wire is often of the same thickness as the other 3 phases....

For example, you feed a fusebox for single phase loads via a 3 phase cable that is rated for 15 amps. You fuse each phase for 10 amps and use the 10 amps. That means that 30 amps will be flowing via the neutral wire........twice its rated current!!!

The problem is that all current "returns" are via the neutral...........it should be designed to be of a much thicker copper than the other wiring. Due to voltage drop, the neutral will "creep" away from ground over the length of the cable, so if you measure at that fusebox from neutral to ground, you will see a voltage difference......

Hi Andy,

I don't mean to be rude but I think you have a misconception about three phase power and it's principles. If you goto http://en.wikipedia.org/wiki/3_phase_power there is a good explanation.

regards

Chas

Hi All,

This voltage will be due to Neutral problems and not earth faults. Either it is the voltage drop due to single phase / imbalance current over the neutral or it is a disconnected neutral. I would consider (on a 400V/230V system) a drop of 10V to be on the limit. You must also remember that this voltage drop can actually INCREASE the voltage at your board!

As an example of this take a street where each house in sequence is fed from a different phase. House nº 1 and 4 both have a full house of electrics, heating, cooking, etc. they will cause the Neutral to rise toward L1. Let's assume a voltage standard of 230V +5%-10% giving us 241V max 207V min. The supply authority would might put out the max voltage possible from it's transformer, where the neutral is grounded. If the loadings work out at max then those houses on L1 get 207 V. Thus we could have (^34V/₂) 17V dropped across line and neutral. So if houses 2 & 3 are only using electric for TV and lights and thus assumed negligible voltage drop, they will receive not 230 V but, in this instance 242.5V i.e. outside spec! This is an extreme example and would never normally be this bad.

Now use the same scenario and look what happens if, at the bottom of the street, the neutral connection burns out. Now the heavy loads will cause the neutral wire down-stream of the disconnection to appear shorted to L1 and so L2 &L3 will receive almost the full 400V, with the result that TV, microwave, washing machine, lights, etc will burn out, even in some cases when they are in standby mode.

The earth wire will sit nicely, as it should, equidistant between the phases. If however a consumer is using the earth wire for return then it could have a voltage drop across it, but that assumes the protection arrangements are not working / installed properly.

All of the above can be applied to US208 3ph and even 120-0-120 two phase where it is even easier to understand.

I hope this clears things a little.

Kind Regards Chas

Adrian,

Andy Germany makes a good point that an unbalanced electrical load can cause this issue to occur. You didn't elaborate much on your system, but if you are working on an electrical distribution panel (fuse or breaker box), then it is possible for you to have one leg unbalanced enough to create problems with heat dissipation in the neutral. If this is the case, then you should be able to determine which leg is receiving too much load with an ammeter.

To get around this issue you could increase the size of your neutral, or you can find which leg is drawing too much current and move some of your more heavily loaded single-phase breakers onto the lower loaded leg(s) in order to balance the system as much as possible. This will minimize the N-G voltage.

This is a lot of work, and it is not common to encounter a panel that is so badly unbalanced that it is necessary to re-balance it, but I have encountered a case in the field where the electrician who wired the panel ended up with almost all of the lighting for a small industrial facility on one phase of a 120/240v electrical panel. This caused part of the copper bus in the panel to melt, and the panel had to be replaced (and rebalanced).

Hi All,

Thank you very much for your opinion let finish this debate

I have got so much comment from all of you, and I think I totally agree with Capblanc explanation that the Neutral ground voltage is only voltage drop issue. Of course the solution is increasing teh neutral size or using insulation transformer . No danger if get the voltage drop aroun 5 % percent of our normal voltage.

Now let open a new topic

Some one just saying that it could be a grounding problem... if our grounding resintance found good (below 1 Ohm) , what is the our next action to check our grounding system /to verify whether we have a bad grounding system or good grounding system

Is there any corelation between neutral ground voltage with earth resistance?

Cheer

Adrian

Hi Adrian,

If the system is working correctly there is no current flowing in the Earth wire and so the earth resistance has no V drop, thus no correlation to N-E voltage. However there will be some current in the earth return which will draw the E voltage toward the leaky phase. This may or may not be the heavily loaded phase toward which the N will be drawn.

kind regards

Chas

Hello Capblanc,

From reading the solutions that you have offered I have to conclude that I am missing some information on how European distribution systems are bonded. Where are the neutral and ground wires for service entering a structure normally tied together - at the step-down transformer or in the structure's electrical panel?

Hi EE

I´m working in the marine sector so our codes are somewhat different. I would however expect the stepdown Tx to have its output neutral grounded.

Chas

Ok, I am not an expert with marine techniques, but bonding at the transformer is a departure from US standards. In the US the neutral to ground bond is always located in the first distribution panel after the transformer (at least for land-lovers), so length of the wire is usually under 500 feet and conductor resistance is usually not an issue.

On the other hand, a step-down transformer may be several hundred or even thousands of feet away from where the neutral and ground conductors are tied together, so there is potentially enough resistance in the wire to make it act as a load in the circuit thus creating a difference in electrical potential (voltage). So in this case a bigger neutral conductor would be less resistive, so the voltage would be lessened.

If this were a problem then the solution that I would probably go to would be to install an isolating transformer to create a new neutral point.

Hi Adrian,

I think that I can give a reasonable explanation for why there seems to be so much confusion between US and Europeans on the topics that have sprung up in this thread. Until 2002 the National Electric Code in the US permitted the use of undersized neutral wires. The conductor size for the neutral wire could be one or two gauge sizes smaller than the "current carrying conductors" (hot wires) and pass inspection. The reason for this is explained in the physics lessons above. So, as you see, increasing the size of the neutral wire is not a common solution for a problem in the US.

I would dare to say that the reasoning for changing the US electric code in 2002 probably had something to do with situations like the one that I described above. The current NEC standard requires the neutral to be at least the same size as the current carrying conductors.

Adrian,

Effects of N- G Voltages -

1. It is dangerous to human under short circuit fault conditions.

2. N - G voltages affect the performances of Electronic Apllinnces . .. causing neusance operations , Damages to PLC equipments , memory losses of PCs etc...

3. theoritically only you can bring that voltages to zero. threshold limit is 5 V Maximum. Some sophisticated equipments require only 1 Volt.

You can reduce the N - G Volts by increasing the no. of Earth electrodes, there by reducing earth resistances.

If reducing earth resistance reduces N-E voltages then current must be flowing in the E wire. (unless you have some wizard way of explaining it!)

Chas

The only way to make it zero is to disconnect the supply from the load so that there is no current flowing in the load circuits.

Otherwise, when there is a load, there is a <...neutral-Ground Voltage...>. Its value is determined by the current flowing in the neutral and its size/length.

<...can it be danger...>

Loose or broken neutral wires between the source and a single phase consumer present the risk, in the limit, of the full phase voltage appearing on the neutral when no load is being drawn. The absolute value of the <...danger...> can only be determined by local circumstances.

<...maximum acceptable...>

This is determined by the local electrical standard, which in the UK would be British Standard 7671, again.