HRC FUSE

A fuse is a small piece of wire connected between two terminals mounted on the insulated base and is connected in series with the circuit. The fuse is the cheapest and simplest form of protection and is used for protecting types of equipment against fault and/or short circuits. The fuse is expected to carry the normal working current safely without overheating and during the fault or short circuits, it gets heated up to melting point rapidly. The materials used normally are tin, lead, silver, zinc, aluminium, copper etc.

FUSE:

A fuse is a small piece of the conductive element, which is inserted in an electrical circuit, and is melted when the excessive current flow in the circuit is due to a short circuit or any kind of fault.

Fuse element or Fuse Wire:

It is that part of the fuse which melts when excessive current flows in the circuit and thus isolates the equipment from any kind of fault or short circuit.

Fuse Rating:

It is the maximum value of current flow through fuse wire in which the fuse work in normal operating condition and the fuse does not melt it. This value of the fuse is less than the minimum fusing current.

Fusing Factor:

This is the ratio of minimum fusing current to the current rating of the fusing element, i.e.,
Fusing factor = Minimum fusing current /Fuse rating
The fusing factor is always greater than unity.

Prospective Current:

It is defined as the RMS value of current which would flow in a circuit immediately following the fuse when a short circuit occurs assuming that the fuse has been replaced by a link of negligible resistance.

Melting Time or Pre-arcing Time:

The time is taken from the instant the current that causes a break in the fuse wire starts flowing, to the instant the arc is initiated.

Arcing Time:

The time is taken from the instant of arc initiation to the instant of an arc being extinguished.

Total Operating Time:

It is the sum of the pre-arcing and the arcing time

HRC FUSE

• HRC stands for High Rupturing Capacity. HRC Fuse has a high rupturing capacity.
• Because of its high current rupturing capacity, a special method for extinguishing the arc is required in the design of the HRC Fuse.
• This type of fuse contains a fuse wire in it, which carries the short circuit current safely for a given time interval.
• During this interval, if the fault is eliminated, then it does not off otherwise it will melt and remove the circuit from the electrical supply hence, the circuit remains safe.
• The common material, which is used to make an HRC fuse is glass, but this is not always the case. Other chemical compounds are also used in HRC fuse manufacturing and construction based on different factors.

Construction of HRC Fuse:

• The HRC fuse consists of a heat resistance ceramic body usually of steatite, pure silver element, clean silica quartz, asbestos washers, porcelain plugs, brass end-caps and copper tags shown in fig.
• The space within the body surrounded by the element is completely filled by the filling powder which generally consists of chalk, plaster of Paris, marble, dust etc.
• This powder act as arc quenching and cooling medium.
• The brass end-caps and copper tags are electro-tinned.
• The metal end-caps are screwed to the ceramic body by means of special forged screws to withstand the pressure developed under short circuit conditions.
• The contacts are welded to the end caps.

Fuse Operation:

• Under normal working conditions, the fuse element is at a temperature below its melting point. Therefore it carries normal current within overheating.
• When any kind of fault or short-circuit occurs then the current increases and therefore the heating effect of the current temperature also increased proportionally to the square of the current.
• Due to that phenomenon, the HRC fuse element melt before the fault current reached its own peak value.
• The heat produced in the process vaporized the melted silver element.
• The chemical reaction between the silver vapor and filling powder results in the formation of a high-resistance substance with helps in quenching the arc.

Cut-off Current region of HRC fuse:

• When an HRC fuse interrupts a heavy fault it exhibits an ability to limit the short circuit current. This ability is referred to as a ‘cut-off’.
• It has the effect of reducing the magnetic and thermal stresses both in the system and within the fuse itself under fault conditions.
• The cut-off is, in fact, one of the main reasons why the HRC fuse is so successful as a protective device and it is at times preferred over the circuit breaker of low ratings.
• Due to this property of the HRC fuse, the operating time is as low as 1/4th of a cycle. The maximum to which the fault current reaches before the fuse melts is called the cut-off current.

Properties(Advantage) of HRC Fuses

The designs of HRC fuses have overcome the disadvantages of conventional rewireable fuses and their properties are described as follows

(i) Fast Operation:

The HRC fuse interrupts the short circuit current long before its maximum value is attained which is not true in the case of CBS. This property of the HRC fuse reduces both thermal and magnetic stresses on the equipment to be protected and the short circuit fault is interrupted well within the first quarter of a cycle.

(ii) High Rupturing Capacity:

The rupturing capacity of a fuse is expressed in terms of MVA and is equal to the product of service voltage and the r.m.s. value of the prospective current it can handle capacity.

(iii) Low-temperature Operation:

This is required to eliminate the deterioration of the fuses and to prevent overheating of associated contacts. This is achieved by employing fabricated elements of pure silver which are specially designed to give a low-temperature rise when carrying their full-rated current.

(iv) Low Cost:

It is known that because of the cut-off characteristics of the HRC fuse, for the same rupturing capacity the actual current to be interrupted by an HRC fuse is much less as compared to any other interrupting device and hence it is less expensive as compared to other interrupting devices.

(v) High speed of operation.
(vi) Not required maintenance.

The disadvantage of HRC fuse:

(i) They have to replace after each operation.
(ii) Heat produced by the arc may affect the associated switches.

Applications of HRC Fuses:

(i) protection of cables,
(ii) protection of bus bars,
(iii) protection of industrial distribution system,
(iv) contractor gear for motor control,
(v) earth faults—both of low and high magnitude,
(vi) semiconductor rectifiers and
(vii) aircraft.