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Monday, 13 April 2015

What are HRC Fuses and how they are superior to ordinary fuse?

A fuse is a small piece of wire connected in between two terminals mounted on an insulated base. It is the cheapest and simplest form of protection and is used for protecting low voltage equipment against overloads and short circuits. Although fuses such as drop-out fuses are employed for the protection of high voltage equipments also. For example; protecting a 33/11 kV transformer using drop-out fuse at the HV side.

A fuse is connected in series with the electrical circuit and is supposed to carry the normal working current of the circuit safely. During overloads or short-circuits, the current in the circuit and through the fuse increases, depending on the type of the fault. This increase in current causes the fuse element to heat up to the melting point. 

Thus, one can say that a fuse is a protective element in the power circuit which essentially consists of a fusible element in the form of metal conductor of specially selected small cross-sectional area.     

How the normal fuse works?

The heat developed in the fuse element during normal operation is readily dissipated and therefore, the fuse element remains at a temperature below its melting point. However during abnormal conditions, the heat generated due to the heavy currents cannot be dissipated fast enough and the fusible element melts and eventually breaks and protects the circuit from further damage.

The time needed for the blowing out of fuse depends on the magnitude of the abnormal current. The fuse has inverse time-current characteristics i.e. larger the current, smaller is the time required to operate.

What are the materials used in a normal fuse element?

The materials used for the fuse element must be of low melting point, high conductivity, low ohmic losses, low cost and free from deterioration. The materials normally used as fuse element are tin, lead, silver, zinc, aluminum, copper etc. For small values of current alloy of lead and tin (37% lead and 63% tin) is used. When the circuit current is expected to be more than 15 A, the diameter of the fuse wire will be large and after fusing the metal released will be excessive if alloy of lead and tin is used as fuse element and hence copper or silver is used.

What is HRC Fuse and why they are superior to ordinary fuse?

The protective capacity of simple re-wirable fuse is uncertain. The fuse wire is subjected to deterioration due to oxidation through the continuous heating up of the fuse element. In a modern distribution system, it is essential that fuses should have a definite known breaking capacity and this breaking capacity should have a high value. The answer is High Rupturing Capacity (HRC) fuses.

An HRC fuse consists of a ceramic body and brass end caps to which are welded fusible silver (or bimetallic) current carrying elements. The space within the cartridge surrounding the fuse element is completely packed with a filling power usually Quartz. Quartz has excellent arc extinguishing properties.

The operation of HRC fuse consists of the folowing steps:
1.       Overcurrent or short circuit in the network causes melting and vaporisation of the fusing (silver) element.  
2.       During arcing, the fusion of silver vapor and the quartz filling powder takes place.
3.       A high resistance is created as a result of this fusion.

Characteristics of HRC fuse:

The HRC fuse elements are accurately shaped and designed for consistency and reliability. HRC fuse link operates within very close band to the time-current values given in Indian and International standards and with a very low let through energy and cut-off current at short circuit. When an HRC fuse interrupts a heavy fault, it reveals an ability to limit the short circuit current. This ability is referred as a “cut-off” and has the effect of reducing the magnetic and thermal stresses both in the system and within the fuse itself under fault conditions. Because of the “cut-off”, the operating time of an HRC fuse is as low as 1/4th of a cycle. The fuse gives satisfactory performance under both long term cyclic overload conditions and repeated transient overload conditions such as motor starting surges or inrush currents of transformers or capacitors. These fuse links are designed to very low power loss for energy saving and cool running.

All the characteristics of HRC fuse are maintained throughout its life. An HRC fuse do not falsely operate when carrying normal currents, which commonly happens in a re-wirable fuse because of oxidation and the reduction of the cross-sectional area. This is avoided in HRC fuses by hermetically sealing the fuse element within the ceramic body.

Applications of HRC fuses:

HRC fuses are ideal for low and medium voltage circuits of industrial and general nature for protection of motors and other equipments. They are also used for the protection of distribution transformers. A circuit breaker of low rupturing capacity may sometimes be used along with an HRC fuse.   These fuse links come in the capacity range from 6 A to 800 A in different sizes with a breaking capacity of more than 80 kA at 415 V, 50 Hz AC.