HI-POTENTIAL TEST....

Q. What is the need of doing hi-pot test?

A. We are checking that the cable is capable of withstanding of high voltage are not.We also say as we are checking dielectric strength of insulation medium for high voltages.

Q. hi-pot test is limited times test,we should not do for more times, why?

A. As we do it many times ,the properties of insulation medium changes and life time decreases.

Q. How much voltage should apply?

A. assume the cable rated voltage as Vvolts(ac rms voltage)

        The amount of DC voltage applied to cable = 80% * ( 2* V + 1) * √ 2 

Q. Why we should apply only 80% of voltage ,in bus bar hi-pot we apply 100% voltage why?

a.  Already at factory they will do hi-pot test at 100% dc voltage,in order to protect life of  insulation medium  we will reduce applied voltage,if we do same test for third time we will decrease to 60% voltage.

                      In bus bar,the insulation medium is air,so it will replaced by fresh air so no problem for bus bars.but in case of cables the insulation medium is constant,so its property will change.

Q. Why we applying dc voltage only for cables but in case of bus bars we can apply ac voltage?

A. The distance between the cores is very small so capacitance between cores are high,so they will take high charging currents in case of ac voltage then we cannot get exact leakage current so we calculated insulation resistance will come as low value.

                            Another main important point is as capacitance is high, ferranti effect will occur so cable will get damaged at other end of cable.

PROCEDURE:

 1.Remove all external connections before doing test.

 2.Connect the remaining cores and cable gland to earth.

 3.Confirm that no other person is in contact with cable.

 4.Apply the dc voltage with using hi-pot kit.

 5.Note down the leakage current.

 6.Discharge the charge after the test.

 7.Repeat the above steps to all cores.

Q. Why we should do megger after the hi-pot?

A. To identify the insulation is same as before hi-pot or not.