Floating Neutral in Power Distribution

The floating neutral in Power distribution can play havoc. The household appliance/gadgets operate at 230-250 volts, which is the line voltage (Phase to Neutral voltage) of a distribution system. A 3 phase Distribution Transformer of say 11kV/0.433 k V,.Dy11, having winding of Delta in Pry and Star in secondary side (generally Dy11 group). Its secondary phase to phase voltage will be 433 V and Phase -Neutral voltage will be 433/√3 = 250 V.
Due to floating neutral, the phase to phase voltages keep on changing with different loads.

If the neutral is solidly grounded and any of the phases get earthed say Y ph (ie at 0- potential) the voltage of healthy Phase to neutral shall remain as it is that is it will be 250 volts only. The gadgets/appliances connected to those phases and neutral shall not get damaged, whereas if the neutral has got disconnected from earth and one of the phases get earthed say Y phase gets earthed, the voltages of the remaining phases ie R and B phase to neutral would get raised to Phase to phase voltage ie 433Volts.The house hold appliances (rated for 230-250 v) connected between these phases to neutral would be burnt / get damaged causing loss to the consumer.

I have narrated with ref to distribution system having 3 phase and single phase system with Neutral earthed at 430 volts side and a distribution line containing 5 conductors strung on the LT poles and laid over the area where LT supply is to be served to consumers. Out of the 5 conductors, 3 are R,Y,B, phase conductors, 4 th is Neutral conductor and the 5 th is for Street lighting etc.

Single Phase L&F consumer is served with a the suitable phase and Neutral (250V single Phase LT supply). Some consumers demand LT- 3 Phase supply (433 Volts with Neutral) at their residences. It is mandatory on the part of the consumer to have 3 Phase supply if his load exceeds 2KW (as per tariff).He may use the 3 ph supply for3 phase equipments etc. {Generally household appliances, fridge. ACs etc operate on single phase supply only that is Phase to neutral supply(250 volts) }.

The supply in MP (India) is served to the LT L&F consumers through over-headlines only (Economical Proposition) by making Jumpers etc. The 3 Phase supply is generally distributed phase wise with neutral to different rooms /floors of the consumer.

Now suppose the neutral jumper gets disconnected from the pole or anywhere in between the Distribution Transformer and the Pole from where the supply is served, the condition of floating Neutral shall occur. Normally Phase to Neutral supply is available and LT appliances will function without neutral being earthed .It will go on un-noticed may be for long time. While this condition is prevailing and one of the phases get earthed, that Phase will go down to zero potential and the Neutral too will be at the same potential as that Phase. Thus the supply from the remaining phases to neutral shall become 430 volts instead of 250 volts. This over voltage condition shall damage the appliances.

The notion that if the phase conductor is earthed, shall get Short Circuited is not correct as without neutral solidly grounded there is no path for the current to flow and create Short circuit.

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