The battery disconnect is required by code, but frequently not installed. This is a very important piece of equipment as the equipment that comes after this disconnect generally has not been designed to provide a safe working environment when servicing the equipment. Batteries can have short circuit capacities of several thousand amps, (consider a normal arc welder operating at just 100 or 200 amps), in other words, you can get some very strong and severe arcing if something goes wrong. The battery disconnect allows the power from the batteries to be disconnected prior to working in the DC breaker box. Again, the cables in and out of the battery disconnect need to be in conduit or code compliant cable suitable for the purpose. Fuses must be rated for the appropriate DC voltage and interrupting capacity.