DO Fuse blowing?

It is not clear weather your incoming is overhead lines or underground cables.The reason for the DO fuse blowing out is different in either case.If it is overhead the most probable reason may be an intermittent earth fault caused due to some tree branch touching the lines . The second probability may be a cracked insulator .Check up if the fuse blow out happens during wind or rain or under normal weather conditions. A through checking of the OH lines are required for ascertaining the cause of fault. If it is underground cable take an IR test of cable and check up the end terminations.

If your load carrys any harmonics high frquency tend to flow in outer periphery of the fuse conductor due to internal inductance effect .This is also known as skineffect .suppose your full load current is 50 amps fuse rating should be 75 to 100 amps .

our full load current is around 80A and we are using 200A fuse.

We are having underground cable system. Recently we have founded one cracked insulator on one of the incomer and we replaced that. even then also the problem is persisting still.

Seems like your 2 feeders are bringing in two different VOLTS and at a considerable phase difference.

Must minimize both for happy load- sharing!

It is essential to know weather both your incomers are charged simultaneously but load is put on one feeder only at a time and second feeder remaining as stand by to be charged on fault on first feeder.Or both the feeders are independently used with bus coupler system or both feeders are connected in parallel (normally two incomers are not made parallel unless they are from the same transformer). The knowledge of the system is essential to arrive at possible defect. Since your incomer DO fuse is getting fused ,there is a probability of cable end termination getting faulty. cable end termination kits will fail if the cable is XLPE and the joint is not done properly.

I faced similar problem particularly when the system restores after a power failure. It was due to an high inductive baterry charger which starts on immediate restortion and it takes a few minutes get stabilized to its normal operating current. The inrush current caused the fuse to blow.

Another reason may be the cable joint or parts of HT breakers if any. It is also worth checking the healthiness of the transformer and it's switching/ protection devices on the LT side.

If it is open area, high velocity of wind also causes the DO fuse to fall and at the moment of fall the fuse blows off.

Kindly maintain a log of the fuses that is blowing and the time to assess it better.