Fuse Switch Disconnector: Function, Selection, Applications

What is a fuse switch disconnector?

The fuse switch disconnector is a switching device for occasional manual switching/isolation of loads and distribution boards. It is capable of making, carrying, and breaking the specified rated current (including a defined overload). It protects downstream electrical loads, safely disconnecting all poles from the power supply under load.

The fuse switch disconnector has combined properties of a load switch and disconnector in addition to the protection offered by fuses. It consists of three switchblades each constituting a double break per phase. These blades are not continuous throughout their length, but each has a gap in the center that is bridged by the fuse cartridge. This arrangement has disadvantages in that when changing a fuse the electrical circuit to which it is connected may still be live.

The fuse switch disconnector performs an isolation function. It also protects the circuit against overcurrents. Overload and short-circuit protection are provided for downstream devices/loads by inserting LV HRC fuses, sizes 000 to 3, depending on the type of mounting. The HRC fuse has a high short-circuit breaking capacity. Besides, opening times against short circuit is also short. When the HRC fuse blows, it must be replaced with a new one.

The easiest and cheapest way to combine fuse holders and disconnectors is the fuse switch disconnector. With these devices, the melting fuse simultaneously takes over the function of the switching element. When removing the fuse carrier, which also contains the fuses, a visibly open isolating distance is created, which allows safe maintenance work. The melting fuses can be easily removed and replaced.

For these reasons, a fuse switch disconnector is in practice only operated by trained, qualified staff, and, depending on local regulations, protective clothing is worn.

Fuse switch disconnectors have only a limited switching capacity. They are therefore particularly suitable for applications where the switching function is only occasionally required.

Advantages of fuse switch disconnectors

The benefits of the switch disconnector are:

• Has an isolation function.
• Withstands high short circuits.
• Opens and closes manually or automatically.
• Opens and closes manually on the panel with rotary handles.
• Switches high currents.
• Switches under load.
• Has 2, 3, and 4 pole versions.
• Allows checking the state of the fuses through a transparent cover.

Selection of fuse switch disconnectors

The switch-disconnectors are manufactured to use as conductor elements (link) the blade contact fuses. (NH-type fuse) The switch-disconnector model to be used is directly linked to the dimension of the used fuse. Therefore, first, a fuse housing (NH000 up to NH3) must be selected to define which fuse-switch-disconnector will be used.

Secondly, you must check if the switch disconnector is capable to carry circuit-rated current. The rated current of the load must be lower than or equal to the maximum current of the switch disconnector. The rated current of the fuse can be higher than the rated current of the switch-disconnector due to the criteria to fuse dimensioning.

Fuse switch disconnector accessories

Some accessories can be mounted on fuse switch disconnectors such as:

• Mechanical fuse monitoring
• Electronic fuse monitoring
• Saddle and prism terminals
• 3-phase busbars
• Auxiliary contacts
• Terminal covers
• Padlocks and sealings

Fuse switch disconnector applications

• Machine switching (on/off) (HVAC, Crane, Pump, Conveyor)
• Power distribution
• Low voltage switchboard
• Motor control centers (Isolating and protecting motor starters)
• Security applications
• Power plants
• UPS and telecom applications
• Solar applications

Circuit diagram