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Neetral of the generator disconnected accidently, due to this L-N voltage become L-L voltage. And some equipment damage. The generator was not properly earthed. Is it due to that? Pls give advice and cause?

I doubt that the L-N voltage became L-L. Its still the same voltage, just not earthed. I suppose if you had something referenced to earth (like a MOV) it might damage that.

Responding to Shiju's 12-Jan (21:24) query... inadvertent disconnection of the generator's neutral-to-earth connection resulting in isolation of the neutral path between the generator and the load, could have led to a very dangerous phenomenon referred to as "Neutral Displacement!" Following are several observations based on the presumption that phase-to-phase voltages are balanced:

o The three phase voltages form an equilateral triangle. Electrically, the system neutral is coincident with the centroid of that triangle. Typically, it is connected to an earth-plane at the source of supply. That is, there is equipotential between each phase and the neutral path from the generator to the load!

o If the phase-to-neutral loads are balanced when the path is interrupted, the voltage triangle remains intact, and there is no displacement of the centroid! This can be confirmed if none of the phase-to-phase loads were “damaged!”

o If the phase-to-neutral loads are unbalanced (normal expectation) when the neutral path is interrupted, the voltage triangle remains intact. Again, phase-to-phase loads are left undamaged. But, the centroid will be displaced, altering the voltage appearing across phase-to-neutral loads!

o In severe cases displacement can put the centroid outside of the voltage triangle exposing phase-to-neutral loads to extreme overvoltage!

o The degree of displacement is influenced by the degree of unbalance in the phase-to-neutral loads!

o Alternatively, if the phase-to-earth capacitance (always present) is unbalanced, then, it too will result in the centroid being displaced. However, in this case the phase-to-earth voltages also exhibit different phase-to-earth voltages!

o It would be very helpful if you could provide more forensic detail than just the word "damage!"

If you need additional infotmation, please contact me!

Regards, Phil Corso (cepsicon@aol.com)

Equipment damage occured because. when the neutral was disconnected, your single phase equipment was then connected across two phases, which i'm sure it wasn't designed for!

Even with proper earthing the damage would still have occured.