Manufacturers perform routine tests (ratio, winding resistance, insulation resistance, tan-delta, impedance, no-load/load losses, induced and applied voltage withstand), type tests (lightning and switching impulse, temperature rise), and sometimes special tests (partial discharge, FRA, sound level, OLTC mechanical tests). Oil tests (BDV, moisture, DGA baseline) and leak checks are also carried out. Test reports are provided to utilities as part of FAT documentation before shipment.