Why triac output goes higher than line 230-240 voltage in AC dimmer circuit?
Line voltage is 240 volt and output voltage goes till 328 volt. Is
  this normal working of triac BT139 or I am doing something wrong?




If the RMS voltage is (say) 1 volt then the peak voltage will be 1.4142 volts. If the RMS voltage is 230 volts then the peak voltage will be 1.4142 x 230 volts = 325 volts and the peak-to-peak voltage will be 650 volts.

Picture from here