Had a 15 amp ( 480 V ) tank immersion heater short to ground . This tripped the Main 1200 amp bolt- loc switch that feeds 3 panels before the heater . We reset the ground -fault on the 1200 amp Bolt-Loc switch and powered everything back up . The heaters are protected by JTD fuses . We know the Bolt-Loc saw the short to ground and tripped .Questioning why would the fuse's not open on this short to ground ? Would we want to use faster acting fuses for the heaters ? Also would we not want a coordination study done so a ground fault does not open all the way back to the 1200 amp bolt-loc switch ?