HRC Fuse: Construction, Operation, Characteristics, and Applications

HRC Fuse Construction, Operation, Characteristics, and Applications

The HRC fuse is the simplest type of protective device. Generally, it is used to protect household equipment.

The fuse consists of a small piece of a metal element called a fuse element. When excessive current flows through the circuit, the fuse element melts, and the current is interrupted. Hence, the circuit is disconnected from the supply.

Therefore, it is used to protect a circuit from a dangerous current.

High Rupturing Capacity (HRC) fuse is a cartridge type fuse used for distribution purposes.

Construction

The outer body is made up of a heat-resisting ceramic material. The metal end caps are connected with the ceramic body. Generally, brass is used for the end caps.

A fuse element is welded between the end caps. The construction of the HRC fuse is shown in the figure below.

Construction of HRC Fuse
Construction of HRC Fuse

The shape of the fuse is cylindrical. A filling powder is filled surrounding the fuse element.

Generally, quartz send, POP (Plaster of Paris), or marble dust is used as a filling powder.

The material used for the filling powder is selected in such a way that its chemical reaction with silver vapours forms a very high resistance substance. This helps in arc quenching and acts as a cooling medium.

The filling powder has the characteristic of absorbing the heat at a very high rate, and it does not evolve appreciable gas.

Operation

In normal conditions, the current flowing through the fuse element is rated or underrated current. Therefore, the temperature of the element is well below its melting point.

Therefore, in normal conditions, the fuse carries current without overheating.

In abnormal conditions, the current flowing through the fuse increases. And high current produced temperature above the melting point of the fuse element.

So, the fuse element melts before the fault current reaches its peak value.

The silver vaporizes after melting. A high resistance substance forms between the silver vapour and filling powder by chemical reaction. This helps to quench the arc quickly.

The steps for the operation of the fuse is as below;

  • Occurrence of fault or short circuit
  • Increase the current flowing through the fuse element
  • Melting silver element
  • Vaporization of silver element
  • Fusion of silver vapour and form a high resistance substance
  • Extinction of arc

Characteristics of HRC Fuse

The characteristics HRC fuse are;

  • Cut-off characteristics
  • Time-current characteristics
  • I2t characteristics

Cut-off Characteristics

According to the design of a fuse element, when excessive current starts flowing, the fuse element starts melting at one or more points.

The fault current has a significant positive peak. But the fuse blows before reaching its peak.

The RMS value of the first cycle of fault current is known as prospective current. And the current at which the fuse melts and arc starts is called the cut-off current.

The voltage reduces momentarily when the fault occurs. And when the fuse melts, the arc voltage reaches several times more than the supply voltage. This depends on the cross-section and length of the fuse.

The cut-off characteristics of the HRC fuse are shown in the figure below.

Cut-off Characteristics
Cut-off Characteristics

In the above figure, the current and voltage waveforms. The cut-off value depends on the standard current rating of fuse, asymmetry of the current waveform, and the prospective current.

The breaking capacity of the HRC fuse is represented by its normal service voltage and the RMS value of the prospective current.

Time-current Characteristics

In an HRC fuse, the operating time decreases as the fault current value becomes high. Therefore, the HRC fuse shows inverse time-current characteristics.

The below figure shows typical time-current characteristics of an HRC fuse.

Time-current Characteristics
Time-current Characteristics

I2t Characteristics

The information about the heating effect due to the pre-arcing current and at the time of arc quenching can be obtained from I2t characteristics.

The below figure shows I2t characteristics showing curves of pre-arcing I2t and total I2t against fusing current.

I2t Characteristics
I2t Characteristics

The I2t characteristics indicate the amount of energy released, which will be passed to the protected equipment at fault current interruption.

Fuse Element of HRC Fuse

The fuse element is enclosed in fuse links. The fuse links need to change when it blows out.

Generally, the fuse element is made up of silver or copper with a special shape. In most cases, the fuse element has two or more sections joined with the help of tin joints.

The various shape of fuse elements used in the HRC fuse is shown in the figure below.

Fuse Element of HRC Fuse
Fuse Element of HRC Fuse

Selection of HRC Fuse

The below points need to be considered while selecting an HRC fuse for a specific application.

  • The normal current or rated current of the circuit.
  • Level of overcurrent protection required.
  • The voltage appearing across the fuse after its operation should not be greater than its rating.
  • Rupturing capacity must not be less than the current to be interrupted.
  • Discrimination is needed when it is used with other fuses.

Advantages

The advantages of the HRC fuse are listed below.

  • The operation is very fast.
  • It can avoid high current passes through the circuit.
  • It has inverse time-current characteristics.
  • The performance is consistent.
  • It provides reliable discrimination.
  • It does not deteriorate with high speed.
  • The cost of fuse is less compared to other protective devices.

Disadvantages

The disadvantages of the HRC fuse are listed below.

  • It must be replaced after each interruption.
  • It takes more time to replace.
  • Inter-locking is not possible.
  • It is subjected to high-temperature rise, and heat can affect the adjacent contacts and switches.

Applications

The applications of the HRC fuse are listed below.

  • It is used as backup protection for circuit breakers.
  • It protects low voltage distribution systems against overload and short-circuits conditions.
  • Protection of meshed feeder with the steady load.

Difference Between Fuse and Circuit breaker

The difference between fuse and circuit breaker is shown in the below table.

Criteria Fuse Circuit breaker
Working It detects and interrupts the fault current. A relay system does the detection. It only interrupts the fault current.
Operation The operation of a fuse is automatic. It does not operate automatically. The relay system is required to give a signal to operate.
Breaking capacity Breaking capacity is less. Large breaking capacity.
Arc interruption Arc interruption is simple. Arc interruption is complicated.
Operating time Small Large
Operating speed High Comparatively slow
Replacement It needs to replace after every interruption. No need to replace after every interruption.
Replacement time Replacement of fuse takes time. No need to replace.
Design Simple Complicated

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