Due to unbalance of load

if the three phase 
load is unbalanced (ie) 
the impedance me 
three phases are not 
equal, a circulating 
current will be 
produced, hence this 
current should flow 
through the neutral. 
otherwise the 
winding will damage.

Balanced loading in 3 phase system is only achieved in 
factory settings, in distribution system loads keep on 
flactuating during the day hence loading keeps varying 
throughout the day

if the three phase load is unbalanced, there will be a 
voltage between the common point (star system) and earthed 
ground( system ground), known as Neutral shift voltage. So 
if you are short circuiting these, there will be a current.

Ir+Iy+Ib=Io not Zero for unbalanced loads.

in electrial system we can sy that neutral is the return 
path of the elctivity as if there is any immbalance of the 
supply or unbalanced load then some current flows through 
the neutal , and if the system is balanced then only no 
current flows through the neutral , in single phase supply 
there is no current in neutral until there is disbalance 
from incoming side or the equipment is faulty.

In single phase systems current flows thro the neutral as 
it is the return path
In three phase systems w.r.t individual phases there is 
neutral current  .(Ir0,Iy0,Ib0)
For balanced system  Ir0+Iy0+Ib0=0
For unbalanced system  Ir0+Iy0+Ib0 not equal to zero

Due to unbalance load on 3Ph .