Advantages of HRC with Tripping Device Over HRC Without Tripping Type

1.    In case of single phase fault the plunger operates and CB rips all the 3 phases thus avoids single phasing.

2.    Relatively inexpensive CB is required.

3.    Capable of dealing with small fault currents itself. This avoids replacing

HRC Fuse with Indicator

                                     

Figure shows an HRC fuse developed by General Electric Company. The cylindrical body made of ceramic material is closed by metal end-caps and the copper terminal-tags are electro-tinned. The fuse element made of pure silver is surrounded by silica as the arc quenching medium. In  order to increase the breaking capacity of the fuse, two or more widely separated fuse elements are used in parallel. The breaking capacity is increased due to a greater surface area of the fuse element in contact with the silica filler. An indicating device consisting of a fine resistance wire connected in parallel with the fuse element and led through a small quantity of mild explosive held in a pocket in the side of the fuse and covered by a label is also provided. It indicates whether the fuse has blown out or not. When the fuse operates on occurrence of fault, the fine wire is automatically fused resulting in the combustion of the explosive material. The combustion of the explosive material chars the label, and thus indicates that the fuse has blown out.

Applications of HRC fuse

1.    Protection L.V. distribution systems against over load and short circuits.

2.    Protection of bus-bars.

3.    Protection of cables.

4.    Protection of motors.

5.    Back up protection to circuit breakers.

6.    Protection of semi-conductor devices.

DROP OUT FUSE LINK

This is also an expulsion type high voltage fuse with one pole in closed position. When the fuse element gets fused on the occurrence of a fault, the fuse element carrying tube drops down under gravity about its lower hinged support, and hence the operation of the fuse can be readily spotted from distance. As the fuse element carrying tube drops down on operation of the fuse, it provides additional isolation. The blown fuse element is replaced by lifting the complete tube from the hinge and bringing it down by means of a special insulated rod. After replacing the fuse element, the tube is replaced in the hinge and the device is closed in a way similar to closing of isolators. These fuses are extensively used for protection of outdoor transformers, and the fuse isolator combination is generally pole mounted.

EXPULSION FUSE

 In an expulsion type fuse, the arc caused by operation of the fuse is extinguished by expulsion of gases produced by the arc.

As expulsion fuse contains a hollow tube made of synthetic-resin-bonded paper in which the fuse element is placed and the ends of the element are connected to suitable fittings at each end. The length of the tube is generally longer than the conventional enclosed fuses. On the occurrence of a fault, the arc produced by fusing of the fuse element causes decomposition of the inner coating of the tube resulting in the formation of gases which assist are extinction. Such fuses are developed for 11 kV, 250 MVA and are commonly used for protection of distribution transformers, overhead lines and cable terminating with overhead lines.

DISCRIMINATION BETWEEN TWO FUSES

In order to understand this type of discriminations consider a radial circuit shown in the figure. The power to the radial line is being fed at the left end. The fuse F₁ which is nearer the feeder is called the major fuse, and the fuse F₂ which is far from the feeder is called the minor fuse. If a fault occurs at the far end of the feeder, i.e. at point F, the fault current will flow through both the fuses F₁ and F₂. If the fuses used do not have discriminative character, there is a likelihood that the major fuse F₁ is blow out and thus supply to the whole line will be interrupted, although there is no fault between F₁ and F₂. Therefore, when a fault occurs beyond F₂, only F₂ should operate and F1 should remain unaffected. This is called proper discrimination. For proper discrimination in this case, the pre-arcing time of the major fuse F₁ must be grater than the total operating time of the minor fuse F₂.

DISCRIMINATION BETWEEN FUSES AND OVER-CURRENT PROTECTIVE DEVICES

In motor circuits, fuses provide short-circuit protection and the over-current relay provides over-current (overload) protection. The characteristics of the fuses and over-current relay are coordinated in such a way that the over-current relay operates for currents within the breaking capacity of the circuit breaker 9or the contactor), and the fuses operate for faults of larger current. For this purpose, the characteristic of the over-current protective device should be below the characteristic of the fuse, as shown in the figure. The fuse is so selected that the intersection of the characteristics of these two protective devices must take place at a point (A) corresponding to six times the full-load current, keeping in view that the protective devices do not operate unduly during starting. In this case, the fuse provides back-up protection to the motor and is connected on the supply side.

COMPARISON OF FUSE WITH CIRCUIT BREAKER

Points	Fuse	Circuit Breaker
1.     Function	It performs both detection and interruption function.	It performs interruption function only. The detection of fault is made by relay system.
2.     Operation	Inherently completely automatic.	Requires equipments (e.g. relays) for automatic operation.
3.     Breaking capacity	Small	Very large
4.     Operating time	Very small(0.002 sec or so)	Comparatively large (0.1 to 0.2 sec)
5.     Replacement	Requires replacement after every interruption	Bi replacement after operation

CATEGORIES AND CLASSIFICATION OF FUSES

Class P            -           fusing factor less than 1.25

Class Q           -           fusing factor less than 1.75

Class K            -           fusing factor less than 1.75

FACTORS FOR SELECTING FUSE

Following factors should be considered while selecting a fuse. Reliable fuse should be used such that it will blow out under abnormal conditions only. It should remain intact under short period over-loads or during switching surges.

Following points should be considered for selection:

a)    Nature of Load:

1.    Normal current.

2.    Starting circuit.

3.    Permissible overload.

4.    Whether it is a steady load or fluctuating load.

b)    Nature of Protection Required:

1.    Overload or short circuit protection.

2.    Quick opening or slow opening.

3.    Cut-off value desired.

c)    Grading and Discrimination Between other Fuses and CBs in the Circuit

Fluctuating loads: The current reaches to a high value in short durations. So the fuses should not blow under transient overloads. So current/time characteristic of fuse should be always above transient current characteristic of load with an enough margin.

Fuses of class Q having fusing factor greater than 1.75 are suitable for fluctuating loads.

Switching surges: Caused by switching of transformers, motors, capacitors, fluorescent lighting etc so rating of fuse for this application should be 20 to 25% more that F.V. value of current.

For steady load: Class P fuses are used.

ISOLATORS (ISOLATING SWITCHES)

When carrying out inspection of repairs in a sub-station or charged line repairs it is essential to disconnect reliably that section of line on which the work to be carried out. This disconnecting device is a isolator. As the name suggests it isolates the part of the section from the live lines. It assures the safety to the team of electricians and line-mans who are expected to work on such sections. The isolator makes a visible break without any arcing/sparking.

So isolator is defined as a device used to open or to close a circuit either when negligible current is interrupted or when no significant charge in the voltage across the terminals of each pole of isolator will result from the operation.

Vertical Break Type Isolator

 The contact arm is divided into two parts:

i)       Male contact i.e. moving contact.

ii)    Female contact i.e. fixed contact.

The moving contact is opened by a operating mechanism. The contacts are supported on the solid core insulators. Operating blade is supported on the operating rod insulator. All the insulators are fixed on the base. Earthing switches are also fixed on the isolators and linked with the isolators. Isolators, earth switches and circuit breakers are interlocked to avoid mal-operation to protect the equipment and provide safety to the operating staff. The operation of isolator may be manual and or with electrical/pneumatic mechanism.

Uses: Outdoor applications including isolation of circuit breaker, transformer banks and surge arrestors and line sectioning.

Horizontal Break [Centre Rotating Double Break] Type Isolator

Construction: Fixed female contacts of metal are supported on the stack of insulators. Male contact rod is connected to the central insulating support which can be moved to have a horizontal moment of the male contact for opening or closing into the female contacts. The moment of the rod and its support may be done by manually or manually but electricity or pneumatically by then operators. The operating mechanism disengage or engage the rod in the female contacts for closing or opening operation of isolator. The male contact’s moment is horizontal and hence this type of isolator is named as horizontal break isolator. The assembly is supported on the galvanized steel channel or steel frame. The moving male contact can be moved through 90⁰.

Use: This type is used indoor or outdoor equipment of capacity above 245 kV.

Pantograph Type Isolator

This type is for 245 kV 1200 Amp and occupy less space easy for operation.

The supporting insulator is mounted on the concrete foundation pad. To this foundation the operating mechanism is also fitted. A solid rotary insulator is moved for isolator operation (make or break operation). The moment of the pantograph for make-break of blade is shown by dots and dotted line. On closed position the two arms of the pantograph close on the overhead station bus bar giving a grip. The current is carried by the upper bus bar to the lower bus bar through the conducting arm of the pantograph.

On the opening position the pantograph arms collapse in the vertical plane and vertical isolation is obtained between the line terminal and pantograph upper terminal.