SOLUTION

Peak Inverse Voltage (PIV):-  In the true sense of the word the, term PIV means ‘Peak Inverse Voltage’ and denotes the reverse voltage capability of the diode. But, in general, it is taken as the maximum reverse voltage that occurs across a diode in a rectifying circuit. It is defined as the maximum reverse voltage across the diode terminals when it is not conducting. 

In the figure the diode D₁ is in the non-conducting state, then the voltage across it is (e_A − e_B) or ( e_A  − e_C ).

Since (e_A − e_B) is the line to line voltage in the star connection; it is equal to √3 times the phase voltage. The maximum value of this line-to-line voltage is the PIV, i.e.,

PIV = Maximum of (e_A − e_B) = √3Em