Fuse is a device which opens the circuit when the current passing through it exceeds its continuous current rating. Hence fuse can provide overcurrent protection to the installation.

Fuse switch disconnector is a device which has fuse in series with a switch. Now a switch is a device which is used to make the circuit, carry rated current, and break the circuit.

Hence the main difference between the two is – Fuse switch disconnector can make, carry, and break the circuit along with providing overcurrent protection to the installation; while fuse can only provide overcurrent protection.

A switch fuse unit is a (single or 3-phase) isolator switch and corresponding set of fuses which are interconnected. The switch and fuse(s) are typically installed in a common cabinet. They are electrically connected but there is no mechanical interlocking between them. If a fuse blows, the fuse cannot be isolated from both supply and load, which can make fuse replacement a hazardous procedure.

A fuse switch unit is also an isolator switch and fuse set. However the fuse carriers are part of the breaking bridge in the switch. So when the isolator switch is set to OFF position, the fuse carriers are removed and isolated from both line and load. This makes fuse replacement a much safer procedure.