The main components of a standard fuse unit consist of the following items:
- Metal fuse element
- Set of contacts
- Support body
There are different types of fuses; they can be categorized as below:
- Voltage fuses
- One time use Fuses
- Resettable fuses
- Thermal Fuses
- SM Fuses
There are two types of voltage fuses it include:
- Low Voltage Fuses
- High voltage Fuses
Low voltage fuses can be further divided into:
- Rewireable/ Semi Enclosed Type
- Cartridge/ Totally enclosed type
This type of fuse is commonly used domestic (in the house, offices) wiring. It is also known as Kit-kat fuse. Rewireable fuses as its name implies can be rewired or restored quickly and easily after it have blown. Rewireable fuse is made up of a base which holds the fuse element carrier and a fuse element carrier (usually one or more strands of wires are used as fuse element) .Both the base and the fuse element carrier are made of porcelain. The main elements of fuse wires are lead, aluminum, tinned copper and tin-lead alloys.
Though rewireable fuse have the advantage of being easily be restored when it blows by simply replacing the fuse element in the fuse carrier without risk of electrical shock but have the following shortcomings;
• Small time lag
• No specific limiting feature
• Slow speed operations
• Low rupturing capacity
This type of fuse can also be called totally enclosed fuse because it is completely closed in a container. It has metal contacts on either side. Cartridge fuses are used to protect electrical appliances for example motors air-conditions, refrigerator, pumps etc, as well as home distribution panels.
This can be further classified into two which are;
A. D type
B. Link Type
A. D type Cartridge Fuse- This is a type of cartridge fuse that cannot be interchanged and it comes with a fuse base and cap, adapter ring and the cartridge. The fuse base has the cap screwed to it and the cartridge is pushed into it. The circuit becomes complete when the tip of the cartridge is in contact with the conductor. In this case, the main advantage we get is that of reliability.
B. Link type Cartridge Fuse– Link type cartridge fuse is further divided into two which are Knife blade type and bolted type.
i. Knife Blade Type- This kind of fuse is very easy to replace in a circuit without any risk of being shocked. For this purpose, special insulated fuse pullers are used. Knife blade is also known as spade or plug-in fuses. It is mostly used in automobiles for protection against short circuit.
ii. Bolted Type HRC : In this type, the conducting plates are bolted to the base of the fuse. There is also a presence of a switch through which the fuse can be removed without getting an electrical shock.
High Voltage Fuse
High voltage fuses are used in power transformers. This type of fuse uses silver, tin or copper as its fuse elements which are fusible.
One time use Fuses
These fuses enclose a metallic wire which burns out when there is an over current or short circuit. These types are cheap and are commonly used in nearly all electronic and electrical devices/systems.
Resettable fuse is a fuse which can be used several times without changing it. It opens the circuit when there is an over current or short circuit and after some time it connects the circuit again. Resettable fuse is also known as PPTC (polymeric positive temperature coefficient). Resettable fuse is used in electronic circuit where it is manually difficult to replace fuse. This fuse is usually used on a computer motherboard.
A thermal fuse works mainly as a temperature sensitive fuse in some devices. It is usually found in device like hair dryers. It contains a fusible temperature-sensitive alloy that holds a spring contact mechanism which is normally closed. This fuse can prevent a fire in a hair dryer. It does this when the surrounding temperature of the hair dryer gets too high as a result of this; the alloy melts and allows the spring contact mechanism to open the circuit.
SM fuse is simply surface mount fuse. It is usually used in mobile phones because of its small size.
Characteristics of a Fuse
- Current carrying Capacity of Fuse
- Breaking capacity
- I 2t value of Fuse
- Response Characteristic
- Rated voltage of Fuse
Current carrying Capacity of Fuse– As its name implies, is the certain amount of current it can easily carry without causing an interruption in a circuit. It is also the nominal amperage value of a fuse. This nominal amperage value is established by the manufacturer of the based on a controlled set of test conditions.
Breaking capacity: Breaking capacity is also known as interrupting rating or short circuit rating. This is the maximum value of current a fuse can safely blow without catastrophic failure like fire.
I2t (Ampere Square Second) value of Fuse: The I2 t terms related to fuse normally used in short circuit condition. It is the amount of energy which carries the fuse element when the electrical fault is cleared by fuse element.
Response Characteristic: The speed at which fuse blows, depend on the amount of current flowing through its wire. The higher the current flowing through the wire, faster will be the response time. Response characteristic can be classified into three general categories: very fast-acting or ultra fast fuses, fast-acting fuses and slow burn fuses. Fuses which respond quickly to the over current condition is called ultra fast fuses. They can be found in many semiconductor devices because semiconductor devices are easily damaged by over current. Slow burn fuse do not respond quickly to the over current condition, but blow after several seconds of over current occurrence. Such fuses are used in motor control electronics systems because motor takes a lot more current at starting than running.
Rated Voltage of a Fuse: The Voltage rating of a fuse must be greater than or equal to the voltage of the device used for it. Each fuse has maximum allowed voltage rating, for example, if a fuse is designed for 34 volts it cannot be used with 220 volts, different amount of isolation is required in different fuses working on different voltage levels.