Solution

f=50Hz
$$I=I_{0}\sin Wt$$
peak value $$I=\displaystyle\frac{I_{0}}{\sqrt{2}}$$
$$\displaystyle\frac{I_{0}}{\sqrt{2}}$$=$$I=I_{0}\sin Wt$$
$$\displaystyle\Rightarrow \frac{1}{\sqrt{2}}=\sin Wt=\sin \frac{\pi }{4}$$
$$\displaystyle\Rightarrow \frac{\pi }{4}=Wt$$
or, $$\displaystyle t=\frac{\pi }{400}=\frac{\pi }{4\times 2\pi f}=\frac{1}{8f}=\frac{1}{8\times 50}=0.0025=2.5ms$$