Voltage check for 3 phase gen to ground and neutral

[Biggy69]

While checking the output of a gen to ground, voltage was 120, 120,then 104. Checking the same connector to neutral resulted in 120,120,120. What could cause the difference in the reading? BTW this has happened multiple times.

 

[DaveWB]

One cause for your strange readings could be that your neutral is not earthed properly. Have your checked the resistance from neutral to earth?
DaveWB

 

[waross]

Do you have a load on the set when you measure the voltages? When you make the voltage check from the 104 line to ground is your meter positioned where it may be influenced by a magnetic field? What kind of connection do you have from ground to neutral? Does your generator have the neutral grounded in the transfer switch some distance away? Are you reading the voltage with a portable meter or panel meters or a combination. Have you checked for ground current?
A combination of a distorted wave form and a D'arsonval meter movement can give strange results. I have twice seen unloaded Wye secondary transformer banks where the line to line voltages were obviously not the right ratio to the line to neutral voltages. I one instance we were able to put an oscilloscope on the circuit and the distorted waveform that was causing the erroneous readings with the D'arsonval meter were obvious. In both cases when a load was applied, the voltages corrected themselves.
In your case, it would have to be something that didn't effect the phases equally.
yours

 

[mikey70]

Keep in mind there are third harmonic currents in the neutral of any three phase generator, even if the load is balanced. Add in any unbalance and you can have strange currents and voltages in the nuetral. You didn't say if the neutral is resistance grounded but I assumed it was.
See if you can get an oscilloscope and take a look at what you have, scopes are typically underutilized in the power industry, distorted waveforms can make meters and relays do strange things, not to mention actually changing the rms values. With the scope, check the peak to peak voltage readings and waveshape maybe it will show something different in c phase.

 

[buzzp]

Obviously there is a voltage drop between the point you measured to ground and point you measured the neutral. So you have some neutral current flowing causing a voltage drop. This could be because of harmonics on the neutral (harmonic currents flow in the neutral), improperly grounded/bonded system, or be because of higher resistance ground fault. If you measure between ground and neutral you will see about 14 volts.
If you don't have a scope to validate the harmonic idea, then try taking the measurements at different times, ie, when certain equipment is running or not running and different times of the day. Is this a home generator or are you in a commericial or other building?