Why polarisation Index is taken 10min/1min ? why not more than 10 minutes ?

 Becouse the electrical insulation is hygroscopic in nature and presence of moisture will be there either in low quantity or in excess. Water molecules are very polar. When an electric field is applied across the insulation start absorbing electrons from the hydrogen molecules causing ionization of hydrogen. In other words, the molecules constituting water align in the electric field, just as magnetic field. The energy required to align the molecules comes from the current in the DC test voltage supply. This current is called polarization current. The water becomes completely polarized when the absorption of electron from hydrogen merging with oxygen is completed. Once the molecules are all aligned, the current stops. The approximate time for complete polarization is minutes. That is why the IR is measured after minutes of applying voltage