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Loss of neutral in star connected system

Loss of neutral in star connected system

Loss of neutral in star connected system

(OP)
Gentlemen (and ladies),

I would be grateful if you could help me to ascertain the phase to phase voltages which will result within a three phase star connected network should the neutral conductor become disconnected from the star point of the transformer. This occurred in a hospital recently and damage resulted (Light fittings burnt out etc). Obviously the load on each phase will determine the voltage but are there any other factors that need to be considered ?


RE: Loss of neutral in star connected system

There is no unique voltage that develops in such a situation. Most of your single-phase loads are probably (surely) connected phase-to-neutral and if the neutral is broken you will have combinations of single-phase loads connected in series between the phases. A worst case scenario would be a low wattage load such as an analysis instrument or computer from one phase and a heavy load (heater or high-powred lamps) connected to another phase. Loss of neutral will then place almost 1.732*nominal voltage across the light load and probably destroy it.

The voltage developed across a load can be anything between close to zero volts to sqrt(3)*phase-neutral voltage.

Theoretically, there can be resonance phenomena if you have highly inductive loads connected from one phase to neutral and highly capacitive loads from another phase to neutral. Loss of neutral will then put L and C in series and you may have a resonance amplification that drives the voltage well above phase-phase voltage. But such a situation is not very likely to occur outside the school-books.  

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