in electrical distribution system .100 kva 11/.4 kv distribution transformer. occurring a fault. its LT side produce phase in neural wire also. tell me what is fault?

L-N ,2L-N. or 3L-N fault/short cct ? Is it from the distribution line or internal to transformer?
Technically speaking,you may have to isolate the load and conduct comprehensive test on your transformer viz: Continuity,Resistance,Insulation Resistance,Polarization index,DDF,Earth/ground tests etc to determine if your transformer is healthy. Then check the distribution line for faults as outlined above. It may be a starting point.

ya thanx for knowledge. now i want describe that fault.
in this case my LT line is faulty.which is connected on load side. here LT side Phase and neutal wire are joint at a point.
so that occure this

LT side phase and neutral are joint plus neutral is disconnected from transformer.

maybe you can check whether there is any effect from harmonic or reactive power.

Find out the cause.Is it overhead or underground? Check the impact of the fault on your existing installation and fuse. It may affect fuses even though they may not have blown. Investigate the cause and analyze it completely and use out come to mitigate future occurrence.