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Electronic Components - Electric Fuse.

What is a fuse?

A fuse provides overcurrent protection of a load or source circuit as a low-resistance resistor. It requires a metal wire or strip that melts when too much current flows, which interrupts the circuit the fuse is in. Overloading, mismatched load, short circuits or device failure are common causes of too much current.

When a fuse blows, it interrupts the current preventing fire or overheating. Therefore, fuses are essential in electrical systems. Fuses do this by melting at a lower temperature than the wire. Once the fuse is melted, the current will not flow and therefore the fuse has broken the circuit.

Fuses come in a variety of sizes and styles. However, each type has a standard pakage to be interchangable. The element of a fuse is can be made of zinc, copper, silver, aluminum, or alloys to provide consistent characteristics.

Types of fuses.

  • Plug & Type S - The metal strip is visible through a window that melts when there is an overload.
  • Time Delay - These fuses have a spring-loaded strip that allows temporary overloads, but when there is a sustained overcurrent it will melt.
  • Cartridge - These fuses show no sign of overload. They must be tested to reveal whether they’ve blown. A cartridge fuse has a cylindrical body and can often have different shaped end caps to avoid being put in the wrong way.
  • Resettable - These fuses do not melt the element, they cut the current when it gets too high, but they will return back to a conductive state.
  • Blade Terminal - These fuses are secured by screws to a fuse holder. Some are also held by spring clips.
  • Axial - Axial fuses have Axial leads and therefore can be soldered to a thru-hole circuit board.
  • Surface Mount Fuses - These are like Axial fuses, but they have solder pads instead of leads.

Parameters of a Fuse:

  • Rated current - The maximum current that a fuse can consistently conduct without the element melting
  • Speed - The speed of a fuse blowing depends on how much current is flowing through, therefore speed is not fixed. However, there are fast-blow fuses, there are slow-blow fuses and time delay fuse, these will blow at varried speeds.
  • Breaking Capacity - The maximum current that can be safely interrupted by the fuse.
  • Rated Voltage - This rating is the amount voltage or lower voltage that the fuse can interrupt. If the voltage of the circuit is higher than the rating, plasma inside the fuse can conduct the voltage and the current will continue to flow.
  • Temperature derating - Surrounding temperature will change the fuse's operations. The element will melt slower at lower temperatures, while at high temperatures the element will melt quicker.

Most fuses are marked either on the body or end caps. These markings explain:

  • Ampere rating
  • Voltage rating
  • Manufacturer/part numbers or series
  • Approvals by standards agencies
  • Breaking capacity
Example of 3AG style ceramic fuse. Example of 3AG style glass fuse. Example of 3AG style axial leaded fuse. Example of pico fuse. Example of cartridge fuse.

Commonly used terms for fuses:

  • Ampere: The measurement of intensity of rate of electrons in an electric circuit. An ampere is the amount of current that will flow through a resistance of one ohm under a pressure of one volt.
  • Ampere Rating: The current carrying capacity of a fuse. When a fuse is subjected to a current above its rating it will open the circuit after a predetermined time.
  • Arcing Time: The amount of time from the instant the fuse link has melted until the overcurrent is interrupted or cleared.
  • Dual Element Fuse: Fuse with a special design that utilizes two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5 times the fuse current rating. The other element, the short circuit section operates on short circuits up to their interrupting rating.
  • High Speed Fuses: Fuses with no intentional time-delay in the overload range and designed to open as quickly as possible in the short circuit range. These fuses are often used to protect solid-state devices.
  • Interrupting Rating: The rating, which defines a fuse’s ability to safely interrupt, and clear short circuits. This rating is much greater than the ampere rating of a fuse. The NEC defines “interrupting rating” as the “highest current at rated voltage that an overcurrent protective device is intended to interrupt under stated conditions”.
  • Melting Time: The amount of time required to melt the fuse link during a specified overcurrent.
  • Overcurrent: A condition which exists on an electrical circuit when the normal load current is exceeded. Overcurrents take on two separate characteristics-overloads and short circuit.
  • Overload: Can be classified as an overcurrent which exceeds the normal full load current of a circuit. Also characteristic of this type of overcurrent is that it does not leave the normal current carrying the path of the circuit, meaning that it flows from the source, through the conductors, through the load, back through the conductors to the source again.
  • Resistive Load: An electrical load which is characteristic of not having any significant inrush current. When a resistive load is energized, the current rises instantly to its steady state value without first rising to a higher value.
  • Short Circuit: Can be classified as an overcurrent which exceeds the normal full load current of a circuit by a factor of many greater than designed. Also characteristic of this type of current is that it leaves the normal current carrying path of the circuit. It take a “short cut” around the load and back to the source.
  • Time-Delay Fuse: A fuse with a built in delay that allows temporary and harmless inrush currents to pass without opening, but it is designed to open on sustained overloads and short circuits.
  • Voltage Rating: The maximium value of system voltage in which a fuse can be used, yet safely interrupt an overcurrent. Exceeding the voltage rating of a fuse impairs its ability to clear an overload or short circuit safely.

Some of the style of fuses in stock at TEDSS.COM are:

Some of the brand names stocked at TEDSS.COM are:

  • Bel Fuse
  • Copper Bussman
  • Gould
  • Littelfuse
  • Schurter
  • Shawmut
  • Wickman