415v between phases

[ThrownRazor25]

Been at a call out in a block of flats where on ocasions there has been a power surge.On further investigation the culprit being a slack neutral on the main incomer coming into the building.

Now i understand that the loss of a neutral can create a voltage of 415V but can anyone give me a proper reason behind this as gaffers and the likes at my work just say "thats what happens"

It was a TNCS system and one of the phases blew last week under the pressure.

thanks

 

[stef]

Cos the three phases are fed from the secondary winding (star) of the sub tranny. If the neutral goes on the main incomer any load between any phase will then back feed 415, mainly when a transformer is connectd between phase and neutral. Sounds a bit implausible, but if you draw the three phase circuit as it is it will become clear.

 

[stef]

Better way of saying this is without a neutral you have two phases in series then, the resistance load is too low therfore the breaker trips or the load blow up. Also bare in mind that with three phases at no point in time do you get 415 V on all three phases at the same time as they are 120 degrees shifted.

 

[Chr!s]

I'd say the supply is PNB, so star point at intake.

I'd also assume you mean Line voltage, as you say loss of star point could produce some high Line voltages.

 

[Chr!s]

The other possibility is where the loading unbalanced, there will be greater volt drops across different phases, shifting the star point, this will produce high phase voltages, Tony posted a good diagram, if your lucky he will post it for you.

 

[Knobhead]

These phase vector drawings show what happens when the neutral becomes disconnected. If the currents are equal the voltage will be equal. Out of balance and the fun and games start.

Balanced
View attachment 14297
Out of balance.
View attachment 14298

 

[ackbarthestar]

Just to throw this one into the mix.
I've measured a voltage swinging between 0V and 580V on an PNB system where the neutral had failed !