Electrical Fuse | Safety at back-end

THE TERM SHORT CIRCUIT

Whatever the type of the load; It only works when it get connected across a potential difference(“A difference of voltage between the two terminals”).
Load in between the potential difference, Consumes current as per according to its capacity. We can say a load actually limits the source current.
“ If some how potential difference say phase & neutral directly get in contact with each other without any load. An uncontrollable high current rapidly flows across the circuit. The level of the current can be up to the extent such that it can damage the source which has generated it, That phenomenon is known as Short circuit. “
The short circuit can damage the total system including the source, load & Load wiring from which the high in rush current has flown. Short circuit is dangerous phenomenon; if not stop earlier; the situation can become more threatening.

THE TERM OVERLOAD

Whether the type it is, Machine, apparatus or device, All work efficiently & safely if operated under their safe limits. Load more than the capacity can reduces the useful life of the device. & in worst cases excess loading can damage the device completely.
” In terms of Electrical current, Overloading means current flow more than the rated current.”
In Both the cases short circuit & overload, Current flows more than the rated capacity. BUT both the scenarios are different from each other. 
  • In short circuit, Current flows rapidly while in overload; current flow at steady rate.
  • In short circuit, Current magnitude can be of multiple times the rated current which is much higher than the current in overload condition.
To limit the short circuit & overload, Electrical fuse is used before the connected load.

ELECTRICAL FUSE

“Electrical fuse is a thin wire connector which is having a specific voltage & current rating. It is specially designed to get blown out when current exceeds its rating.”
By getting blown out, It disconnect the circuit from power supply, So that faulty circuit get eliminated & rest of the circuit remains energized.
While fuse getting blown out; An arc is get generated. To diminish that arc, These fuses are equipped with arc extinguishing chamber .

Holders of cartridge fuses
Holders for cartridge fuses

WORKING MECHANISM OF FUSE

The current carrying capacity & resistance of the conductor depend upon the thickness & material composition of the conductor.
  • More the thickness; greater the current carrying capacity !
  • Material with less resistivity having a greater current carrying capacity !
That’s why conductor size & composition is been chosen By considering the continuous load current & possible short circuit current.

When conductor’s size is much lower than the required size for the current flowing through the circuit than conductor get heated up. The heat generation is directly proportional to I²R. If conductor size is much-much lowered than it get burnt out. That mechanism is utilized in Electrical fuses. Under the rated capacity, These fuses works fine. While when current exceeds the limit, It get blown out.

Line diagram | Fuse connected before the loads
Line diagram | Fuse connected before the loads

 

FUSE SYMBOL

Fuse symbols by IEC & IEEE
Fuse symbols by IEC & IEEE

 

RATING

Fuses are specially designed for particular voltage & current rating. The voltage level belongs to the insulation level of the non conductive part of fuse. While current rating is the rated current of the fuse under which fuse works perfectly without getting blown out.

CHARACTERISTICS 

To understand the behavior of the fuse, Important Characteristics been discussed below.
Usually the fuses are designed & tested at around 25°C. Ambient temperature  play an important role for fuse. A graph between the rated current & ambient temperature shows that efficiency of fuse goes on decreasing as ambient temperature increases.

Characteristics of Electrical fuse; % of rated current Vs Ambient Temperature
Characteristics of Electrical fuse; %rated current Vs Ambient Temperature
Short circuit is indefinite term; the magnitude level is unknown As it can damage the source.
Fuse do not get blow off under its rated current. But The behavior of fuse is entirely different when fault current is 2*rated current or when fault current is 4*rated current.

Electrical fuse follows inverse time characteristics which means more the fault current lower will the blow off time. Which means fuse get blown out more rapidly if the fault current is much higher. Which is clearly shown in a graph below :

Inverse time Characteristics of Electrical Fuse; % of rated Current vs Time
Inverse time Characteristics of Electrical Fuse; % of rated Current vs Time

DIFFERENT TYPES OF FUSES & ITS HOLDERS

Fuses with different sizes & different ratings are available in the market. Also these fuses have different type of holders. Some of the fuses in together with their holders are shown below:

Blade type HRS fuse 200Amps by L&T
Blade type HRS fuse 200Amps by L&T

 

Cylindrical Glass Cartridge Fuse with Holder
Cylindrical Glass Cartridge Fuse with Holder

 

Panel Mount Holder in which a cartridge fuse is fitted
Panel Mount Holder in which a cartridge fuse is fitted

SUMMMARY 

  • Electrical fuse is used as a protection from overload & short circuit.
  • Electrical fuse is thin wire connector which is having a rated current & voltage. It is specially designed to get blown out when current exceeds its rating.
  • While fuse getting blown out; An arc been generated. To diminish that arc, These fuses are equipped with arc extinguishing chamber.
  • Current carrying capacity of the conductor depend upon the thickness & material resistivity of the conductor that’s why fuse wire size & composition is chosen accordingly.
  • Electrical fuse follows inverse time characteristics.
  • There are multiple types of fuses with different shapes & sizes. Also these fuses have different types of holders.
Hope, You like the post. If you have any doubt, Suggestion or query please do comments !
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