why does my electric transformer burns out?

[bewildered]

its a transformer which lowers from 240v to 12v

[joregare]

Transformers most likely burn out due to the heat wear effect on the coil's insulation. When the insulation is locally burn, the coil short-circuits and the transformer is gone.One particular reason explaining why this is happening frequently, may be that the power rating of your transformer is smaller than the power demand you are drawing from it. This will cause your transformer to overheat and live shorter. The solution: increase transformer power or reduce power draw.

[jacartamonkey]

a second possibility is that the applied voltage (primary) voltage is too high