nutral

in three phase distribution line if the nutral is broken whats happen to the voltage?

Hi,
if your end-load (ie, 3-phase motor, single-phase motor .... Etc) is balanced then; nothing will happen, simply because in this case the neutral point will be a zero-potential point .

In three-phase balanced system (ie, three phase motors in factories), the power connection will be just by connecting the three phases, ----> and the neutral will not be connected.

A neutral line will be connected only in a single-phase system (just as your home applications ie; fans,tv,dvd and so on) and will not be used in a 3-phase connection.

Notice that the over-whelming majority of the world end-loads are balanced so you will not have a problem as an end-user. The problem will happen only if the load is un-balanced.

Note: distribution grids from power utilities are not giving the same loads for the three phases, so then they are not balanced exactly, and have some differences between them, but the neutral in this case is a lot of times connected through earth and not through a wire. :)