Low Voltage Fuses

Fuses are used to interrupt electric circuit in the event of heavy flow of current to short-circuits or earth faults in a system.  In low voltage system, fuses are widely used as they are cheap, reliable and can be easily replaced.

Low voltage fuses can be classified into
  • Cartridge Fuses where the fuse element or the fuselink is located in a cartridge and
  • Wirable Fuses where the fuselink is a wire that is wired over a ceramic fuse carriers
In cartridge fuses, the fuselink consists of wires which have low resistivity.
  The material used for the fuselinks needs to have low melting and vaporising temperatures and low specific heat.  This ensures that the fuselink acts quickly, melts and interrupts the circuit.  The vast majority of fuselinks used today are alloys which use either copper or silver as the main ingredient in their construction. 

The fuselink is enclosed in a filling material which is usually quartz.  The function of the filling material is to absorb the energy released during fuse operation and to extinguish the arc which may occur.

The cartridge fuse is capped on either side of fuselink bodies. The fuselink bodies are cylindrical in shape and are usually made of ceramic.  The caps serve to  contain the filling material.  They must be robust, have good conductivity and be able to absorb the energy which may be released during fuse operation. 

Cartridge fuses are widely used in industrial low voltage systems. The cartridge fuse is fitted in a fuse holder.  The fuse holder consists of a base to which are connected the incoming and outgoing wires of the circuit.  Mounted on the base is the fuse carrier which holds the fuse.  The fuse carrier is designed in a manner that the fuse can never be touched when the circuit is live.

Rewirable fuses consist of a fuse carrier which carry a piece of wire which acts as the fuse link.  This kind of fuse is not fully enclosed.  These fuses are amongst the earliest fuses. There is a risk of fitting a fuse wire of higher rating in the fuse holder.  They are now being replaced with the cartridge fuses.

In three phase circuits, the use of fuses entails the risk of single phasing.  That is, if the fuse of one phase blows, the motor may be exposed to single phasing which may cause heating and damage the motor.  For these reasons, three phase fuse units are designed to isolate all phases when the fuse on one of the phases is blown.