HRC fuse full form

HRC fuse full form – High Rupturing Capacity Fuse

HRC fuse is a developed form of cartridge fuse which protects the circuit at high voltage and current and high load. Their rupture capacity is very high, up to 500 MVA at about 66 kV. That is why they are called high rupture capacity fuses. The best feature in this is that even at the time of instantaneous short circuit fault, it has the capacity to carry the highest current for a certain time.

If within this fixed time, the short circuit fault gets automatically rectified, then the fuse does not melt and get destroyed, but if the fault is not rectified within the fixed time, then the fuse melts and gets destroyed. In this way, the value of current in the circuit becomes zero and the circuit gets protected. Its structure is shown in the following picture.

This fuse is made of a cylindrical hollow tube made of insulating material like glass or any other heat resistant chemical substance in which the current carrying fuse element is filled with powdered filler material (sand, quartz) along with the wire of the arrester in the empty space around it and both the ends of the tube are completely sealed with a strong brass cap so that no air can enter it.

Working of HRC Fuse

In this, the silver wire melts as a fuse element. When a short circuit is caused by the fuse element,

When current flows, the fuse wire (band wire) gets vaporized and the arc starts increasing between the ends inside the fuse. Increasing the arc causes the fuse element to liquefy because the liquefaction takes place as a result of the chemical process of evaporated silver and the filling material. This liquefaction process suppresses the arc and along with the arc stopping, the current flow also becomes zero and the circuit breaks. In this way, HRC fuse protects the circuit from high current conditions.

Selection of HRC Fuse

HRC fuse is used in various places. But for each application different HRC fuse will be used, it is very important to select the fuse for that and the selection of fuse depends on the following things-

Selectivity should be high.

Rupturing capacity of the fuse should not be less than the breaking current.

The voltage available across the supply after operation must not exceed the rated voltage of the fuse.

The normal current of the circuit must flow through the fuse.

These fuses should have high overloading capacity.

Installation of fuse

In any connection, the fuse should be installed after the M.C.B. or Main Switch. Apart from this, a fuse should be installed before any shift load to protect that load.

Selection of Fuse

Selection of Fuse – To ensure that the fuse will provide correct and reliable protection of a section or element of an electrical circuit it must be selected according to its characteristics and ratings.

Rated Voltage

Rated Voltage – Fuses and their fusible elements should be selected so that their characteristic voltage rating is the same as the supply voltage of the circuit in which the fuse is to be installed.

Maximum Current Rupturing Capacity

Maximum Current Rupturing Capacity – This is the capacity of a fuse to safely extinguish the arc generated in the melting of elements in a fuse. The current breaking capacity of all its melting elements should be equal to or more than the maximum short circuit current that can flow in the circuit. This is considered as the capacity of the fuse to withstand the highest breaking current at which the circuit can be protected.

Rated Current

Rated Current – Fuse elements used in fuses must be selected for a rated current sufficient to ensure that such elements will not fuse when the circuit is carrying its continuous maximum load and when load current flows in the circuit for a short time. In order to satisfy these above requirements the following conditions must be observed to continue normal operation when as close as practicable to the continuous maximum load being carried.

Rated Current = K Times Sustained Maximum Load Current

”Where ‘K’ is a factor of safety with respect to the load and depends upon the nature of the load. Practically no load such as lighting load K is very high in case of motors in the range 1.1-1.2: The no load current draws a current much larger than the normal operating current during the starting period. The current rating of the fuse elements should be selected as per the other requirements.”

Protection of fuse

Selection of fuse with respect to Steady Load and Fluctuating Load is discussed below.

(1) Steady Load

When a fuse is required to give both overload and short circuit protection the constant current rating shall not be exceeded in excess of the full load current of the circuit to provide short circuit protection.

When the fuse is only required to provide short circuit operation a fuse of higher rating than the normal load current may be selected. As the heat generated inside the fuse will cool down, it will have a long life and there will be a good margin for some temporary overload without risk of degradation.

(2) Fluctuating Load

(1) Motor Load – H.R.C. Fuse is used to protect the motor, cable and starter from damage due to short circuit.

Protection against overhead must be provided by a separate device such as an overhead trip.

(ii) Switching of Transformers-Fluorescent Lighting

The fuse selected must have a rating that will withstand transient current surges and switching.

Usually a fuse is selected which has a nominal rating of 25 to 50% above the normal full load current to be protected.

(iii) Switching of Capacitor Circuit-

Switching of Capacitor Circuit- Similar selection is recommended for capacitor but here the trend should be towards higher percentage.

HRC fuse types-

When the fuse has to be started at high voltage then a major problem of corona occurs, therefore some special types of high voltage fuses have been developed which are as follows.

(1) High Voltage HRC Fuses –

HRC fuse element withstands short circuit or faults for specific interval of time and if the fault is cleared within that specific period then the fuse element is safe and does not melt or break. The fuse element is kept inside an airtight container. HRC fuses have time inverse characteristic i.e. if magnitude of fault current is high then their breakdown time is less and for low magnitude fault their setting time is more. When fault current passes through the fuse the fuse element melts and breaks. The enclosure in which fuse element is kept is filled with chemical powder. Vapour and powder of fuse element combine to form high resistance material which extinguishes the arc.

Cartridge Type H.V. Fuse –

The construction of fuse element is similar to low voltage fuse. In this type of fuse, the fuse element is ring shaped to eliminate corona effect. In some of the HRC fuses two fuse elements are used, these fuse elements are connected parallel to each other. One element is used for normal current flow and the other is used for short circuit current. For heavy resistance the fuse element is made of tungsten metal due to its high resistance property.

At high voltages up to 33 kV breaking capacity of about 8700A is used and high voltage cartridge fuses are available at 6.6 kV and 11 kV at the order of 200A and 50A at 33 kV.

Liquid Type HV HRC Fuse

Liquid type HV HRC fuse is mostly used in high voltage circuits. It is used for transformer protection or for circuits having voltage more than 400 ohms.

A fuse consists of a glass tube filled with carbon tetrachloride. The fuse element is contained inside the glass tube. One end of the tube is sealed and another is fixed at the end of the glass tube with the help of phosphorus bronze wire. When a fault occurs, the short circuit current passes through the fuse element. The fuse element melts and breaks.

The small amount of gas produced during fusion uses the liquid in the arc to fuse.

This type of fuse is used as a backup protector of the circuit breaker. HRC fuse is the backup protector of the circuit breaker, increasing short-circuit capability.

Metal Clad Fuses-

Metal Clad Fuses Oil immersed fuses provide an alternative to oil circuit breakers.

This type of fuse can be used for very high voltage circuits and can operate satisfactorily under short circuit conditions.

Advantages of HRC Fuse

To provide reliable protection at high voltage and current.
Its maintenance cost is minimum.
It is a cheaper protection device than other protection devices at high voltage.
There is a possibility of the HRC fuse getting oxidised and corroded.
Its breakdown current is very high (up to about 40 A).
They are designed for up to 500 MVA at 66 kV.
Its time-current characteristics are inverse.

Drawbacks of HRC Fuse

Along with the above advantages, there are some disadvantages in the use of HRC fuse which are as follows-

A new fuse unit has to be installed after each protection operation.
Due to excessive current and use at high load, the temperature increases.
Its price is higher in the fuse category.
Interlocking is not possible in this.

HRC fuse: top 4 types, working principle, and full form

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