Types of Fuses

1.    Re-wirable semi-enclosed

2.    Totally enclosed cartridge fuse

3.    High rupturing HRC fuse

4.    Drop-out fuse

5.    Expulsion fuse etc.

Re-wirable Fuse/Semi-enclosed fuse(Kit-Kat)

                                                 

It consists of a (i) base and (ii) fuse carrier. The base is made up of porcelain and carries fixed contacts to which incoming and outgoing phase wires and connected . The fuse carrier is also made up of porcelain and carries fixed contacts to which fuse element is connected (tinned copper). The fuse carrier can be inserted or taken out of the base when desired. The fuse is used for protection on low voltage circuits.

When fault occurs fuse element melts and circuit is disconnected. The fuse element can be replaced easily.

Advantage:

1.    Easy replacement

2.    Cost of replacement is less

Disadvantages:

1.    Improper material of fuse element leads to malfunctioning. Low braking capacity so cannot be used for high fault level.

2.    Properties affected by ambient conditions, accurate calibration of fuse wire is not possible because fusing current very much depends upon length of wire.

3.    Loose connection produces heat.

HRC FUSES

It is consists of  a heat resisting ceramic body. The fuse elements are fitted inside the body. The ends of the fuse elements are connected to the metal end caps. The metal caps are screwed to the ceramic body. End contacts are welded to the metal end caps. These end contacts can be bolted on the panel. The fuse body is filled with powdered pure quartz. The fuse element of silver or copper with special shape. The fuse body is filled with powdered quartz./ Cartridge fuse is similar to HRC fuse fully enclosed type.

                                               

Operation: When current increases above permissible limits heat is generated and temperature rises and tin bridge melts producing break is circuit. Thus arc is produced. This arc is immediately spread over the neighboring elements and they also melt. The vapor of metal reacts chemically with quartz powder and produces a insulating substance which helps in quenching the arc.

                                                 

As prospective current increases, pre-arcing time  decreases. Further the characteristic becomes asymptotic and there is a minimum current below which fuse does not operate.

Advantages and Disadvantages of HRC Fuse

Advantage:

1.    Capability of clearing high as well as low fault currents.

2.    Minimum maintenance

3.    No deterioration with age (completely closed0

4.    Operation is with high sped (fast0.

5.    Provide reliable discrimination.

6.    For equal breaking capacity these are cheaper than other types.

7.    Inverse time current characteristic.

8.    Consistent in performances.

Disadvantages:

1.    They are to be replaced after each operation.

2.    Heat produced by the arc may affect the dissociated switches and contacts.

3.    Interlocking is not possible.

HR Fuse With Tripping Device

Sometimes HRC is provided with tripping device. When fuse blows out due to fault conditions the tripping device operates and it causes tripping of CB figure shows details. It consists of ceramic body and end cover cap fixed rigidly. These are connected by number of silver fuse elements. At one end, there is a plunger which hits the tripping arrangement of CB under fault conditions and CB operates.

                                                 

The plunger is connected electrically through a fusible link, chemical charge and a tungsten wire to the other end of cap as shown. When fault occurs the silver fuse elements first blow out, then current is transferred to tungsten link which fuses and causes chemical charge to be detonated. This forces the plunger outwards to operate CB.