In this particular section, we will discuss about fuses, types of fuses and their uses, so lets start with the definition.
A fuse is a short piece of metal, inserted in the circuit which melts and breaks the circuit when excessive current flows through it, thus it protects the circuit.
A fuse is inserted in series with the circuit which is to be protected. Melting point of a fuse material is low as it does allow the normal rated current to flow through it without overheating or melting. But when the current exceeds beyond its rated value because of any faulty condition like short-circuit or overload, the fuse element gets overheating and it starts melting, thus finally it breaks(opens) the circuit which is to be protected.
Fuse element has a inverse time – current relation, as it’s time of operation will be quick if value of current is large. and if the current is low then the operation will be slow. As you can see in the figure below
CHARACTERISTICS OF A FUSE ELEMENT
- Low melting point e.g., tin, lead
- High conductivity
e.g., silver, copper
- No deterioration due to oxidation
- Low cost
e.g., lead, tin, copper.
now, according to above characteristics no material would possess all the qualities like tin has low melting point but it has poor conductivity, silver has good conductivity and no deterioration due to oxidation but is is very expensive. so it is little difficult to choose the appropriate material.
For small currents upto 10A, alloy of tin and lead( 63% tin, 37% lead) is used.
for larger currents, silver is used and copper with a layer of tin( to protect it from oxidation) is used.
As we can see in the graph below that, at origin the fault occurs in the circuit due to any reason like short circuit or overloading, at that time a large asymmetrical faulty current flows in the circuit which can harm the circuit, but we have already connected a fuse in the series so that this large value current will flow through it and the due to overheating, the material of the fuse gets starts melting . during this process the current once reaches its maximum value which is known as cut off current. after melting of fuse element, an arc is developed which has to be extinguished ( we will discuss it in types of fuses below). and after the arc finished , the circuit gets protected . now we will discuss some important terms regarding fuse.
- Current rating of fuse element: the fuse element can normally carry this current without overheating or melting.
- Fusing current: it is the current at which the fuse element melts and disconnects the circuit.
- Fusing factor: it is the ratio of minimum fusing current and current rating of fuse. Its value is always greater than one.
- Prospective current: it is the r.m.s value of first loop of fault current.
- Cut-off current: the point ‘a’ denotes the cut off value of current . it is the maximum value that current reaches during faulty condition.
- Pre-arcing time: It is the time during the material starts melting without arcing.
- Arcing time: It is the time after melting of fuse element , when arc starts and finishes.
- Total operating time: it is the sum of both pre arcing and arcing time.
TYPES OF FUSES:
LOW VOLTAGE FUSES:
a) SEMI ENCLOSED REWIREABLE TYPE:
- It is also known as kit-kat type.
- It is used where low value of fault current are to be interrupted.
It has two parts : 1) a base
2) a fuse carrier
both base and fuse carrier are made up of porcelain, base is fixed while fuse carrier ( which carries the fuse element) is moving, fuse carrier is taken out when the fuse blows out and after replacing it with the new fuse wire , it is again placed over base.
Semi enclosed rewireable fuse are made upto 500A rated current but their breaking capacity is low.
So they are used only for domestic or small loads.
b) HIGH RUPTURING CAPACITY (H.R.C) CARTRIDGE FUSE:
as the breaking capacity of semi- enclosed rewire able fuses is very low , so H.R.C fuse are developed which high breaking capacity. The function of every fuse to trip the circuit as discussed earlier in this section.
- It has a heat resisting ceramic body having metal end caps to which is welded silver current carrying element.
- The space surrounding the element is completely filled with filling powder. E.g. chalk, sand, Plaster of Paris
- filling powder is filled because when the arc generates it has to be extinguished as soon as possible, so this filling powder helps to finish the generated arc.
- They are capable of clearing high and low fault currents.
- No deterioration with age
- No maintenance required
- Reliable discrimination
- High speed of operation.
It has to be replaced and heat produced may affect nearby switches. This is the major disadvantage but still it’s advantages are more valuable than it’s disadvantage.
HIGH VOLTAGE FUSE
- CARTRIDGE TYPE: it has same construction as of the low voltage cartridge type fuse, but it has two wires one is of fuse element which is to melted after fault in the circuit and second one is tungsten wire (which has very high resistance). so when the normal current flows through the circuit, the current goes through silver wire but when the fault occurs in the circuit, the silver wire gets melted and the short circuit current starts flowing through the tungsten wire and due to it’s high resistance , the current finally reaches to zero.
high voltage cartridge type fuse are use upto 33kV with breaking capacity of 8700A at that voltage, rating of the order of 200A at a voltage 6.6kV.
- LIQUID TYPE: In liquid type high voltage fuse, the fuse element is fixed at one side and connected with the spring on other side with a level detector, the spring is enclosed in a glass chamber which is filled with the oil. when the fuse element blows out , the spring shrinks and pulls the level detector inside the glass chamber. so that some oil move upside and this oil will extinguish the arc generated during blowing out of the fuse. for understanding this phenomena in a better way please visit my youtube channel .
They may be used upto about 100A on 132kV systems.
- METAL CLAD TYPE: These also have similar function only difference is that they are enclosed in a metal clad .
so, in this way we have learnt about Types of fuses and their uses, with full detail. if you have any queries regarding this then please ask in the comment section or visit my youtube channel. “ELECTRICAL PAATHSHALA”