Break over  voltage of Finger Voltage 

The  minimum  forward voltage  when the gate  is opened  at which SCR  starts  conducting  heavily is called  the break over  voltage. To  specify  this value  the gate is  in the  open circuit  condition and the  junction temperature is at  its maximum  permitted value  although VBO is still a function of dv/ dt. Though the SCR  can be tumed on with  gate open by making  the supply  voltage  equal to break over  voltage, but  in practice the SCR is prated with supply  voltage  less than  break over  voltage  and it is  tumed on by applying  a small  to the gate. Commercially  available  SCR s have break over  voltage  from  about 50V to 500V.