When load increases, or power factor changes(falls) of the load connected, load current increases considerably, increases the secondary winding drop(Is(Rs+jXs), the terminal voltage reduces considerably(as VAR supplied by transformer cannot changed), with same secondary induced EMF(E2). to keep terminal voltage with in the limit of regulation(E2-V)/V%), the induced EMF of primary(E1) or secondary(E2) should be increased, and is achieved by increasing few secondary turns, with same primary or secondary EMF(E1orE2) as needed(E1/E2=N1/N2) by tap changing device.Generally It is installed on HV/ load side as it has to handle less current for same VA rating of power transformer. Though it is online and contact on lower turns is made open, only after the contact of higher turns is made on. Yet as induced V=L(didt), by accident, mistake is made, due to less current(HV/load side), and hence the induced voltage(spark) would be less dangerous by elliminating core inductance.

On HV side ----> Less current so less spark as compared to LV side

                           Also HV side is exposed to the outer atmosphere so it is easy for                                      tapping.

on HV side voltage is high but current is less but in the LV side voltage is less and current is high. If we connect Tap changer on the LV side then sparking will take place. Better controlled output can be determined when we connect tap changer in LV. The HV winding generally wound over LV winding hence it is easier to access the HV winding turns instead of LV winding.