Fuse Types || Description of all types

We have already discuss about fuse. As we know, fuse is required for the system to protect it from different system fault like short circuit fault, over-load on system. Here we are going to describe about fuse types that is used earlier and now popular also so that we can choose best for our system. Here we will discussed about Fuse Types || Description of all fuse types.  Mainly, we will focus on rewireable fuse/semi-enclosed fuse (also known as kitkat), cartridge fuse, drop out fuse, MCCB. Let's describe about their details description to know them easily. The fuse types are as follows:-

Fuse Types

Rewireable fuses/Semi-enclosed fuse

This fuse is mostly used in house wiring and small current circuits, also known as semi-enclosed. Sometimes  it is also known as kit-kat type fuse. It consists of a porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are connected by strands of wires which will melt when high amount of current is flow i.e. at overload condition and short circuit condition. The fuse wires can be made of lead, tinned copper, aluminum or an alloy of tin lead. Generally, the amount of current that lead to melt the wires is about twice the rated current. 

Standard ratings of this type of fuses are 6, 16, 32, 63 and 100 A. 

Advantages: 

• Easy to remove or replace without any danger of coming into contact with a live wire.
• Used for low and medium voltage 
• Rupturing current is less or small
• Negligible replacement cost 

Disadvantages: 

• Unreliable operation 
• No-current limiting feature 
• Lack of discrimination 
• Small time lag 
• Slow speed of operation 
• Risk of flame and fire

Cartridge Fuse

This is generally used in industrial area as a fuse because it has defined breaking capacity and also used in low voltage to medium voltage. Where large concentrations of power are concerned, as in the case of modern distribution systems, it is essential that fuses should have a definite known breaking capacity and also this breaking capacity should have a high value. High rupturing capacity cartridge fuse, commonly called HRC cartridge fuse have been designed and developed. 

The fuse elements are connected to two end caps of brass or copper inside the body of cartridge . The metal end caps are screwed to the ceramic body by means of special forced screws to withstand the pressure developed under short circuit condition and the end contacts are welded to the metal end caps. An indicating device is provided to distinguish the blown fuse. The space between the fuse element and the cartridge body is filled with powdered pure quartz which acts as an arc extinguishing agent. i.e. quartz powder avoids the oxidation of silver elements. Quartz sand is used as a filler because it can absorb heat at a very high rate and it does not involve appreciable amount of gas.  

Generally, fuse elements have two or more sections joined by means of a tin joint. Normally, its fusing factor for link fuses is 1.45.  The purpose of the tin joint is to limit the temperature of the fuse under small over load conditions. The melting point of silver is 980⁰C while that of pure tin is 240⁰C.

Advantages: 

• Simple and easy installation 
• High rupturing capacity 
• Low cost to compared to circuit breakers 
• High reliability 
• Non- deterioration

Disadvantages: 

• Over heating of adjacent contact 
• Replacement required after each operation

Drop-out fuse

In this fuse, the melting of fuse causes the fuse element to drop under gravity about its lower support which is also an additional isolation. Such fuses are generally used for protection of outdoor transformers. When the fuse element is blowing off, the tube can be lifted from hinge by means of a special insulator rod and brought down for the replacement of the fuse element. After replacing the blowing off fuse element, the tube is placed in the hinge as before melting the fuse and the device is closed in a way similar to closing of isolators. Drop out fuse-isolator combination is pole mounted in most cases.

MCCB

MCCB stand for Molded case circuit breaker. For a electrical system, there need a protection system which will helps to protect the system in faulty cases such as short circuit condition, overhead condition etc. For a medium and large system in low voltage distribution, it is required for their protection. Here we will discuss about MCCB (Molded case circuit breakers) || What is MCCB? 

MCCB definition

Molded case circuit breakers are circuit protection device for low voltage distribution systems.  Generally, they will protect the loads in a system against both overloads and short circuits condition. Depending on the load current, we choose the best MCCB as requirement. There is current sensitive device as well as temperature sensitive device so that in short circuit case, if high amount of current is flow beyond its limit, it will trip and also in overload condition, if temperature is exceed beyond its limits then it will trip the circuit. In this way it work and help to protect the circuit, system, device or equipment from damage. Hence it is much more important in electrical system.

MCCBs are composed of five main components. They are:-

a) Frame: The frame is also known as the molded case, which provides an insulated housing to all the circuit breaker components in a MCCB.

b) Operating mechanism: This mechanism handles the opening and closing of the contacts so that supply will pass or cut as requirement. The opening and closing of the contact is independent of movement of handle i.e. fast or slow movement of handle.  When circuit is tripped, the handle will be at mid position. At normal operating condition, the handle will be at up position and down position at off condition.

c) Arc extinguisher: An arc is obtained when a circuit breaker interrupts a current flow. The arc extinguishers will confine and divide the arc so that the arc will be easily extinguish.

d) Contacts: These are used to closed or open the circuit at normal or abnormal condition.

e) Trip unit: The trip unit is used to trip the contacts as the operating mechanism in the event of a short circuit or a preset value of overload current. The protection can be done by combining a temperature sensitive device with a current sensitive electromagnetic device, and both of them will operate mechanically on the trip mechanism. For short circuit protection, electromechanical trip circuit breakers have adjustable magnetic elements and for overload protection, electromechanical trip circuit breakers contain thermal trip elements. Breakers that use a combined of magnetic elements and thermal elements are often called thermo magnetic breakers.

The above diagram is a picture of MCCB (molded case circuit breaker) which is used to protect the circuit. Generally, for a plant it is used as a requirement current level which ranges from 10 to 200 Amperes. For more current rating, we can use other circuit breaker such as oil circuit breaker, vacuum circuit  breaker, air circuit breaker. We should not touch the MCCB while in operation by naked hand (without gloves) because high current will flow on it that may leads the flow of current in your hands too. So don't go near or touch it while it is in operation or work with carefully. 

There are different types of Molded case circuit breakers which can be classified as

1. Type C

2. Type D

3. Type K

4. Type Z

Some points that may come in MCCB

1. Breaking capacity :- The highest amount of current that can flow in the circuit without any damage is called breaking capacity of MCCB. After that point, it leads damage of circuit. While choosing MCCB we need to focus on that. It is always greater than rating value.

2. Rating :- The amount of current that can flow in the circuit through MCCB continuously without damage  is called rating of MCCB.

3. Operating time :- It is the time at which, the MCCB will operate or break the circuit is called operating time of MCCB