High Rupturing Capacity (HRC) Fuses
A fuse is a protection device used to protect a device or a circuit from severe damage caused due to overcurrent or short circuit faults.
High rupturing capacity (HRC) fuses are totally enclosed fuses with definitely known, high breaking capacities which have been developed after intensive research by manufacturers and supply engineers.
An HRC fuse is a type of cartridge fuse, in which the fuse element is enclosed within a transparent capsule, usually made up of steatite–a ceramic material having good mechanical strength. Manufacturers are now using epoxy resins instead of ceramic material. The capsule is provided with two end caps. The fuse element is connected between end caps within the body. The whole setup is designed such that it can withstand the high pressure developed under short circuit conditions. Powdered quartz, which acts as an arc extinguishing agent, fills the space between the fuse element and the envelope.
Silver or copper is normally used as the fuse element due to its low specific resistance. The fuse element normally has two or more sections joint by means of tin joints. Tin has a lower melting point of 2400 C that is three times lesser than the melting point of silver (980o C). Hence the melting of tin joints prevents the fuse from attaining high temperatures during overload and short circuit conditions.
Process of Fusing
On the occurrence of a fault, the current flowing through the fuse element exceeds its predefined maximum value and the temperature of the fusing element rises and results in the following:
- Melting of silver elements (pre-arcing)
- Vaporization of the elements (Arcing)
- Fusion of silver vapor and filling powder
- Arc extinction
Normally the fuse elements are connected in the middle by a tin bridge. This tin bridge has a precise melting point of 230 oC. Once the temperature of the element rises above this temperature, the tin bridge starts melting. Thereby an arc is established between the molten ends of the fuse element. The temperature produced by the arc is sufficient for the sudden melting of the remaining fuse element. The silver vapor thus produced reacts with the quartz powder filling. The chemical reaction between silver vapor and filling powder establishes a high resistance between ends of the blown out fuse elements.
Gradually, this high resistance changes over into insulator and the current is cut off. A transient voltage is created within the fuse at the instant of fault current interruption. The temperature and the internal pressure of the fuse increase to a higher value.
HRC fuses are sometimes used as backup protection for the circuit breakers. The characteristic of the fuse and the circuit breaker is coordinated as such that all the faults within the range of the circuit breaker are cleared by it, whereas those beyond its range are cleared by the fuse.
Preferred rating of HRC fuses are 2, 4, 6, 10, 16, 25, 30, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630, 800, 1000, and 1250 amperes.
High rupturing capacity HRC fuses
High Capacity HRC Cartridge Fuses
This type of fuse was developed by General Electric Company. In this of fuse, the breaking capacity is increased by using two or more separate silver elements in parallel. The sizes of the elements are varied so that the elements fuse in a sequence, one after the other. The body is made up of cylindrical ceramic material and is closed by metallic end caps to which the fuse elements are fixed. The fuse elements are surrounded by silica, which acts as the arc quenching medium. An indicator, normally a fine resistance wire, is connected in parallel to the fuse element. In this type of fuse, the whole fault current is not shed immediately due to the usage of more than one fuse element. This construction avoids voltage transients in the circuit.
Advantages of HRC fuses
- When compared to other circuit interrupters of same capacity HRC fuses are the cheaper one.
- Simple and easy to install.
- No maintenance required.
- High breaking capacity.
- They are consistent in performance.
- Their inverse time characteristic makes them much suited for overload protection.
- They are capable of clearing high as well as low currents.
- Quick operation.
- They are capable clearing high as well as low currents.
- During severe faults, an HRC fuse breaks the circuit before the peak current of the fault is reached.
- These fuses once blown out cannot be reused.
- Causes over heating of adjacent contacts.
- The possibility of interlocking is greater.